THE FABRIC OF THE COSMOS: WHAT IS SPACE?

PBS Airdate: November 2, 2011

BRIAN GREENE (Columbia University): We think of our world as filled with stuff, like buildings and cars, buses and people. And nowhere does that seem more apparent than in a crowded city like New York.

Yet all around the stuff that makes up our everyday world is something just as important but far more mysterious: the space in which all this stuff exists.

To get a feel for what I'm talking about, let's stop for a moment and imagine. What if you took all this stuff away? I mean all of it: the people, the cars and buildings. And not just the stuff here on Earth, but the earth itself; what if you took away all the planets, stars and galaxies? And not just the big stuff, but tiny things down to the very last atoms of gas and dust; what if you took it all away? What would be left?

Most of us would say "nothing." And we'd be right. But strangely, we'd also be wrong. What's left is empty space. And as it turns out, empty space is not nothing. It's something, something with hidden characteristics as real as all the stuff in our everyday lives.

In fact, space is so real it can bend; space can twist, and it can ripple; so real that empty space itself helps shape everything in the world around us and forms the very fabric of the cosmos.

CRAIG HOGAN (University of Chicago): You can't understand anything about the world unless you understand space, because that's the world: the world's space with stuff in it.

S. JAMES GATES, JR. (University of Maryland): We're not usually very conscious of space. But then again, I tell people, fish are probably not conscious of water either, and they're in it all the time.

JOSEPH LYKKEN (Fermi National Accelerator Laboratory): Space is not really nothing; it actually has a lot going on inside.

BRIAN GREENE: When most of us picture space, we think of outer space, a place that's far, far away.

But space is actually everywhere. You could say it's the most abundant thing in the universe. Even the tiniest of things, like atoms, the basic ingredient in you and me and everything else we see in the world around us, even they are almost entirely empty space.

In fact, if you removed all the space inside all the atoms making up the stone, glass and steel of the Empire State Building, you'd be left with a little lump, about the size of a grain of rice but weighing hundreds of millions of pounds. The rest is only empty space.

But what exactly is space? I can show you a picture of Spain, of Napoleon, of my uncle Harold, but space, it, looks like this: nothing.

So how do you make sense of something that looks like nothing?

LEONARD SUSSKIND (Stanford University): Why is there space rather than no space? Why is space three-dimensional? Why is space big? We have a lot of room to move around in; how come it's not tiny? We have no consensus about these things.

ALEX FILIPPENKO (University of California, Berkeley): What is space? We actually still don't really know.

S. JAMES GATES, JR.: It is one of the deepest mysteries in physics.

BRIAN GREENE: Fortunately, we're not completely in the dark. We've been gathering clues about space for centuries, some of the earliest came from thinking about how objects move through space.

To get a feel for this, take a look at that skater. As she glides across the rink, she's moving in relation everything around her, like the ice. And when she goes into a spin, not only can she see that she's spinning, she can also feel it, because as she spins, she feels her arms pulled outward.

But now, let's imagine that you could take away all the stuff around her, from the rink to the most distant galaxies, so the only thing left is the skater spinning in completely empty space.

If the skater still feels her arms pulled outward, she'll know she's spinning. But if empty space is nothing, what is she spinning in relation to?

Imagine you're that skater. When you look out, you don't see anything. It's just uniform, still blackness all around you, and yet, your arms are being pulled outwards. So you say to yourself, "What could I be spinning with respect to? Is there something out there I'm not seeing?"

Trying to answer questions like these, scientists came up with a bold new picture of space. And the key was to make something out of nothing.

When you go to the theater, you watch the actors, the scenery, the story.

(As Benedick, Much Ado About Nothing/Production company or theater or File Footage):

Benedick: Lady Beatrice, thou hast wept this whole while.

Beatrice: Yea. And I will weep a while longer.

Benedick: I do confess that I love nothing in the world so well as thee. I protest I love thee.

Beatrice: Well, then God forgive me!

BRIAN GREENE: But there's something important here that you won't find mentioned in the Playbill. Something we hardly ever notice: the stage. It's an absolutely vital part of the show, and yet most of us, we don't even give it a second thought. But Isaac Newton, he did.

This is how the father of modern science pictured space: as an empty stage. To Newton, space was the framework for everything that happens in the cosmos, the arena within which the drama of the universe plays out.

And Newton's stage was passive: absolute, eternal and unchanging. The action couldn't affect the stage, and the stage couldn't affect the action.

By picturing space in this way, Newton was able to describe the world as no one had ever done before. His unchanging stage allowed him to understand almost all motion we can see around us, yielding laws that can predict everything from the way apples fall from trees to the path the earth takes around the sun.

These laws worked so well that we still use them for the things that we do today, from launching satellites to landing airplanes.

And the laws all hinge on one radical idea: space is real. Even though you can't see it, or smell it or touch it, space is enough of a real, physical thing to provide a benchmark for certain kinds of motion. Like that skater: Newton would say that when she spins, her arms splay out because she is spinning with respect to something, and that something is space itself.

S. JAMES GATES, JR.: Philosophers had been debating the nature of space for a very long time. What Newton does is change the terms of the debate, and with that, essentially, modern science gets born.

BRIAN GREENE: Newton's stage was a huge hit. It enjoyed the limelight for over 200 years. But in the early decades of the 20th century, a new set of ideas emerged that shook the stage to its very foundations, ideas put forward by a young clerk, working in a Swiss patent office. His name? Albert Einstein.

Einstein grew up in the late 1800s, at the dawn of the age of electricity. Electric power was lighting up cities, giving rise to all kinds of technologies Newton could never have imagined.

All of these developments tapped into something that had captivated Einstein since he was a child: light. Not light bulbs and street lamps, but the very nature of light, itself. And it was his fascination with one particularly weird feature of light, its speed, that would lead Einstein to overturn Newton's picture of space.

To see how, let's take a ride.

Right now, we're traveling at about 20 miles per hour.

To go faster, all the driver needs to do is step on the gas, and the cab's speed changes. Now, you can feel that change, but you can also see it on the cab's speedometer, or one of those radar speed signs.

Okay, you can slow it down now.

But now imagine that instead of measuring the speed of the cab, you have a radar sign that measures the speed of the light coming off its headlights. That sign would measure the light traveling at an astounding 671,000,000 miles an hour.

Now, when the cab starts moving, you'd think that the speed of the light would increase by the same amount as the car. After all, you'd think that the moving cab would give the light an extra push.

But surprisingly, that's not what happens.

Our radar sign, or any measurement of light's speed, will always detect light traveling at 671,000,000 miles per hour, whether the cab is moving or not. But how could this be? How could all measurements of light's speed always come out the same?

JANNA LEVIN (Barnard College/Columbia University): If you're running at a wall, it's coming at you faster than if you're standing still with respect to that wall. But that's not true with light. The speed of light is the same for everybody. That's really extraordinary.

BRIAN GREENE: So, here's how Einstein made sense of this extraordinary puzzle: knowing that speed is just a measure of the space that something travels over time, Einstein proposed a truly stunning idea: that space and time could work together, constantly adjusting by exactly the right amount, so that no matter how fast you might be moving when you measure the speed of light, it always comes out to be 671,000,000 miles per hour.

JANNA LEVIN: To respect that absolute quality about light, time had to cease to be absolute. Space had to cease to be absolute. And those two had to become relative in such a way that they slosh between each other.

BRIAN GREENE: If space and time being flexible sounds unfamiliar, it's only because we don't move fast enough in everyday life to see it in action. But if this cab could move near the speed of light, the effects would no longer be hidden.

For example, if you were on a street corner as I went by close to the speed of light, you'd see space adjusting, so my cab, it would appear just inches long, and you'd also hear my watch ticking off time very slowly.

But from my perspective inside the cab, my watch would be ticking normally.

But from my perspective, inside the cab, my watch would be ticking normally and space, in here, would appear as it always does.

But when I look outside the cab, I'd see space wildly adjusting, All to keep the speed of light constant.

So, with Einstein, time and space are no longer rigid and absolute. Instead, they meld together with motion, forming a single entity that came to be called "spacetime."

EDWARD "ROCKY" KOLB (University of Chicago): I think, as we live our life every day, we live with a Newtonian picture of space and time. It's something that we are comfortable with. But Einstein was able to make reason conquer sense. That really was the genius of Einstein.

S. JAMES GATES, JR.: This notion that space and time are a unity, to me, is one of the greatest insights that has ever occurred in science. It's so counterintuitive to everything we've ever experienced as human beings.

BRIAN GREENE: And in the hands of Albert Einstein, this new picture of space would solve a deep mystery having to do with the most familiar force in the cosmos: gravity.

Newton knew that gravity is a force that attracts objects to each other. And his laws predicted the strength of this force with fantastic precision. But how does gravity actually work? How does the earth pull on the moon across hundreds of thousands of miles of empty space? They behave as if they are connected by some kind of invisible rope, but everyone knew that wasn't true, and Newton's laws provided no explanation.

ALEX FILIPPENKO: Einstein found that no Band-Aid patches would fix Newtonian gravity. He had to invent the mechanism for it; he had to understand it.

BRIAN GREENE: After puzzling over this problem for more than 10 years, Einstein reached a startling conclusion: the secret to gravity lay in the nature of spacetime. It was even more flexible than he had previously realized. It could stretch, like an actual fabric. This was a truly radical break from Newton.

Think of this table as spacetime, and think of these balls as objects in space. Now, if spacetime were nice and flat, like the surface of this table, objects would travel in straight lines. But if space is like a fabric that can stretch and bend…? Well, this may seem a little strange, but watch what happens if I put something heavy on the stretchy spacetime fabric.

Now if I take my shot again, the ball travels along an indentation in the fabric that the heavier object creates.

And this, Einstein realized, is how gravity actually works. It's the warping of spacetime caused by the objects within it. In other words: gravity is the shape of spacetime itself. The moon is kept in orbit, not because it's pulled to the earth by some mysterious force, but rather because it rolls along a curve in the spacetime fabric that the earth creates.

LEONARD SUSSKIND: With Einstein, space became not only real, but flexible. So suddenly space had properties, suddenly space had curvature. Suddenly space had a flexible kind of geometry, almost like a rubber sheet.

S. JAMES GATES, JR.: It opens up a whole new way of thinking about reality that describes the entire universe. Einstein becomes Einstein, because of that observation.

BRIAN GREENE: Where Newton saw space as passive, Einstein saw it as dynamic: it's interwoven with time, and it dictates how things move. So, after Einstein, space can no longer be thought of as a static stage. It's an actor, and it plays a leading role in the cosmic drama.

Now, it's one thing to think of space as dynamic, active and flexible, like a fabric. But is it really? Is this just a metaphor? Or does it actually describe what space is?

Well, Einstein's theory predicts that one way to find out would be to take a little journey to the edge of a black hole.

Black holes are collapsed stars, massive objects crushed to a fraction of their original size. Gravity around them is so strong that, according to Einstein's math, a spinning black hole can literally drag space along with it, twisting it like an actual piece of cloth.

The nearest black hole is trillions of miles away, making it a challenge to test this prediction.

But in the late 1950s, a physicist named Leonard Schiff began searching for a way to test Einstein's ideas about space, much closer to home. Schiff was inspired by something we usually think of as a child's toy: a gyroscope.

He thought that if space really twists like a fabric, a gyroscope might allow him to detect it.

It was a strange idea, and he chose a strange place to share it with the world: the faculty swimming pool at Stanford. Here, in 1959, Schiff met with two colleagues, William Fairbank and Bob Cannon. He was excited about an ad he had seen for a high-tech gyroscope. Though it looked different, it basically worked the same as the child's toy. Then and there, the three decided to launch a device like this into orbit around the earth.

Normally, a gyroscope's axis points in a fixed direction. But if Earth is actually dragging space, then the gyroscope's axis would be dragged along with it, shifting its orientation in a way that could be measured.

It was a brilliantly simple plan. There was just one problem: Einstein's theories predict that the earth's rotation twists space by only a tiny amount, an amount so small, it would be like trying to measure the height of a penny from 62 miles away.

The team spent more than two years trying to figure out how to make such a precise measurement. They finally devised a plan to attach four freely-floating gyroscopes to a telescope aimed at a distant star. If space twists, then, over time, the gyroscopes would no longer point at the star, since they'd get caught up in the swirl of space.

And in 1962, they applied to NASA for a grant, requesting around a million dollars for what would come to be called "Gravity Probe B."

Members of the team originally thought the project would take about three years. They were just a little optimistic.

With an ever-growing team, Gravity Probe B became one of the longest-running experiments in history. Decade after decade was spent trying to realize the original vision, which meant launching a telescope into space and building gyroscopes that were among the smoothest objects ever created.

BRAD PARKINSON (Stanford University): The technology is just frightening. It was like the carrot on the front of the mule. It was like, it was always five to ten years away when we could do this. And it was five to ten years away for about 35 years.

BRIAN GREENE: Consuming more than four decades and $750,000,000 dollars, the project was nearly cancelled by NASA nine times. Finally, in April of 2004, the team gathered to witness the launch. Of the three men who sat by the pool back in 1959, only one was alive to see it.

ROBERT CANNNON (Stanford University): And there we were watching. It's a terribly exciting moment in your life, just a thrilling experience. It was flawless. Ten thousand things did not go wrong.

BRIAN GREENE: For over a year, Gravity Probe B orbited the earth, while the team nervously monitored its every move, trying to see if the earth would actually twist space.

Finally, the data began to trickle in, and there was a problem: the gyroscopes were experiencing a tiny, unexpected wobble, and to clean up the data would cost millions.

With funds running out, it looked like nearly half a century of work was about to go down the drain.

Then, at almost the last possible moment, two sources of additional funding emerged: the son of original team leader William Fairbank, who made a private donation; and Turki al-Saud, a member of the Saudi royal family with a degree in aeronautics from Stanford, who arranged for a large grant.

Over the next two years, the problem with the data was solved, revealing that the axes of the gyroscopes shifted by almost exactly the amount predicted by Einstein's equations.

BRAD PARKINSON: I think it's the first time that you could actually see Einstein's effect, his drift, with a naked eye.

BRIAN GREENE: This experiment provides the most direct evidence ever found that space is something real, a physical entity, like a fabric. After all, if space were nothing, there would nothing to twist.

But at the same time that Albert Einstein was investigating space on the largest of scales, another band of physicists was probing the universe on extremely tiny scales. And there, they found a completely uncharted realm, where, Einstein's picture of space…? It was nowhere to be found.

To see what I'm talking about, imagine you could shrink billions of times smaller than your current size. This is the realm of atoms and subatomic particles, the fundamental building blocks of everything we can see.

And when you get down to this size, the world plays by a wildly different set of rules, called "quantum mechanics." According to these rules, even if you try to remove every last atom and particle, you'd find that empty space is still far from empty.

In fact, it's teeming with activity: particles are constantly popping in and out of existence. They erupt out of nothingness, quickly annihilate each other and disappear.

LEONARD SUSSKIND: In quantum mechanics, empty space is not that empty. It's full of fluctuating fields, full of all sorts of jittery things going on.

RAPHAEL BOUSSO (University of California, Berkeley): It's a place where particles are constantly fluctuating and annihilating each other and being created again and annihilating.

S. JAMES GATES, JR.: It's a place of chaos and bubbling.

BRIAN GREENE: While the theory predicted this, it wasn't until 1948 that a scientist, named Hendrik Casimir, suggested that even though we can't see these particles, they should cause empty space to do something we can see, and he predicted that if you take two ordinary metal plates and place them extremely close together, say, closer together than the thickness of a sheet of paper, then particles with certain energies would be excluded because, in some sense, they wouldn't fit between the plates.

With more of this frenetic activity outside the plates than inside, Casimir thought the plates would be pushed together by what we usually think of as empty space itself. And some years later, when the experiment was done, Casimir was proven right. In empty space, the plates were pushed together.

So, on atomic scales, empty space is not empty; it's so flooded with activity that it can force objects to move.

And today, the quest to understand space on the smallest scale is continuing with one of the most expensive science experiments in history.

This is CERN, the European Organization for Nuclear Research, in Geneva. And here, buried a few hundred feet below the ground, is the Large Hadron Collider, the world's most powerful accelerator. With a price tag of about $10 billion, it accelerates subatomic particles to more than 99.99 percent of the speed of light and smashes them into each other.

In the showers of debris produced by these collisions, scientists at places like this have discovered a whole zoo of strange and exotic particles. And right now, they are chasing one of the most elusive: a particle thought to be essential to shaping everything from the atoms in our bodies to the most distant stars. If this particle is found, it will redefine our picture of space and fulfill a quest begun more than 40 years ago.

It all started in 1964, when a young English physicist, named Peter Higgs, suggested something about space that was so radical it nearly ruined him.

PETER HIGGS (University of Edinburgh): I was told that I was talking nonsense, that I couldn't be right. So they clearly hadn't understood what I was saying.

BRIAN GREENE: Higgs and a few others were wrestling with a puzzle which comes down to this: the fundamental particles in the universe all contain different amounts of mass, which we usually think of as weight. Without mass, these particles would never combine to form the familiar atoms that make up all the stuff we see in the world around us. But what creates mass? And why do different particles have different masses?

Try as they might, no one had been able answer this perplexing question. Then, one weekend, after a walk outside Edinburgh, Higgs had a peculiar idea.

Using mathematics, he imagined space in a new way, as something like an ocean. Particles are immersed in this ocean, and gain mass as they move through it.

To see how this works, think of a particle's mass like an actor's fame, and the Higgs ocean is like the paparazzi: some particles, like unknown actors, pass through with ease; the paparazzi simply aren't interested in them. But other particles, like superstars, have to push and press. And the more those particles struggle to get through, the more they interact with the ocean, and the more mass they gain.

Higgs was convinced he'd made a great discovery. But when he submitted his idea to a journal at CERN, it was rejected.

Undaunted, Higgs honed his theory further, until he was offered the chance to present it at Einstein's old haunt: the Institute for Advanced Study, in Princeton.

There, he expected his new idea would meet some of its toughest critics.

PETER HIGGS: I was happily driving up the freeway, and then there was a sign to turn off for Princeton, and that really confronted me with what I was going into. I broke out in a cold sweat, and I started trembling, and I had to pull off the road to recover.

BRIAN GREENE: But Higgs persevered. It was the first in a series of talks that would convince colleagues far and wide that he was onto something profound.

PETER HIGGS: Eventually, I sort of wore them down. I felt I had sort of triumphed, so I enjoyed the parties which followed.

BRIAN GREENE: Today, the idea Higgs pioneered, called the Higgs Field, is crucial to our understanding of space.

JOSEPH LYKKEN: The Higgs Field is everywhere. It's something that even in the emptiest vacuum of space has an effect: it gives you mass. So I think Higgs actually deserves credit for being one of the people that said, "Space is stuff. It has properties in it that are intrinsic, that you can't get rid of. You can't turn them off."

BRIAN GREENE: The only problem? There's no physical proof that the Higgs Field exists, at least not yet. But here at CERN, scientists are attempting to smash particles together with so much energy that they will knock loose a piece of the Higgs Field, producing a tiny particle of its own. It's as if they're trying to chip off a piece of space.

JOSEPH LYKKEN: We think that if we knock into space hard enough, with particle accelerator collisions, that we can actually make a Higgs particle come out of empty space.

LEONARD SUSSKIND: Our whole understanding of matter as we now have it would just fall apart, if the Higgs Field didn't exist.

RAPHAEL BOUSSO: I don't think anybody seriously doubts that we will see it. Certainly, if we don't, that will be an extremely bizarre outcome.

BRIAN GREENE: Finding the Higgs particle would be a major milestone, establishing that the emptiest of empty space has an impact on all of matter.

But it turns out that space contains an ingredient far more elusive than anything Higgs ever imagined, an ingredient that may hold the key to the greatest of all mysteries: the very fate of the cosmos.

It's a mystery that began some 14 billion years ago, in what we call the "Big Bang." In a fraction of a second, the universe underwent a violent expansion, sending space hurtling outward.

Space has been expanding ever since.

For decades, most scientists thought that expansion must be slowing down, thanks to the pull of gravity.

ALEX FILIPPENKO: When I toss an apple up, the gravity of the earth eventually stops it and brings it back. And just like the apple slows down with time, so, too, the universe should have been slowing down in its expansion, because of the gravitational attraction of all matter and energy for all other matter and energy.

BRIAN GREENE: But that raised the question: what is the ultimate fate of the cosmos? Would space go on expanding forever, or would gravity eventually stop space from expanding, causing it to collapse back on itself in a "Big Crunch?"

To solve this mystery, two teams of astronomers set out to measure the slowing of the expansion using a novel tool: exploding stars called supernovas.

ADAM RIESS (Johns Hopkins University): So a supernova is a star that ends its life in a massive explosion. They're extremely luminous; they can be as bright as a billion suns.

SAUL PERLMUTTER (University of California, Berkeley): What make supernovae great is that they are very similar. When they explode, they all get to about the same brightness, and then they fade away in just about the same way.

BRIAN GREENE: Because the explosions are so bright and uniform, the teams reasoned that these supernovas would act as very precise cosmic beacons, allowing them to track how the expansion of space has slowed over time.

The trouble is supernovas are extremely rare. To find enough of them, Perlmutter spent years calling astronomers around the globe, begging for time on their telescopes.

SAUL PERLMUTTER: We needed the biggest telescopes in the world; we needed perfect conditions. And in those perfect conditions, I would be calling people up at the middle of their , when they're trying to do some serious work, saying, "I know that you have a very busy schedule, but, by any chance, if you could just squeeze in this half hour observation, it would really be very interesting to us."

BRIAN GREENE: When they finally had enough data to chart how much the pull of gravity was slowing the expansion of the universe, they were in for a surprise.

SAUL PERLMUTTER: The results looked a little bit strange. They didn't really show any slowing of the universe at all…very surprising, actually…a universe that's actually speeding up.

ADAM RIESS: It was as though space, which we really thought was nothing, actually had an inherent springiness to it. And so space did not want to be compressed, space actually wants to push the universe apart.

ALEX FILIPPENKO: It looked like the universe was expanding faster and faster with time, accelerating rather than decelerating.

ROCKY KOLB: My immediate response was, "I have to figure out why this is wrong. This can't be right!"

BRIAN GREENE: But it was right. And most scientists converged on one explanation: there's something that fills space and counteracts the pull of ordinary attractive gravity, pushing galaxies apart and stretching the very fabric of the cosmos.

This mysterious substance filling space has been dubbed "dark energy," and it's turned our picture of the universe upside down.

ALEX FILIPPENKO: Over the largest distances, dark energy dominates the contents of the universe, and we don't know what it is.

S. JAMES GATES, JR.: If you do sort of a survey, a census of all the energy in the universe, dark energy turns out to be about 70 percent of the universe. And up until a decade ago, nobody even imagined such stuff even existed.

BRIAN GREENE: So, in essence, the weight of empty space itself is 70 percent of the weight of the entire universe. That's roughly the same percentage of Earth's surface that is covered by water. Imagine we didn't know what water is; that's where we stand with dark energy.

JOSEPH LYKKEN: We're really clueless about how to explain it. We have all of this fancy scientific apparatus of quantum mechanics and relativity and particle physics that we've developed in the last hundred years, and none of that works to explain dark energy.

BRIAN GREENE: And the discovery of dark energy held another surprise: the idea that the universe contains such an ingredient had actually been cooked up 80 years earlier. I'll let you in on a little secret: although he didn't call it dark energy, long ago, Albert Einstein predicted that space itself could exert a force that would drive galaxies apart.

You see, shortly after discovering his general theory of relativity, his theory of gravity, Einstein found that, according to the mathematics, the universe would either be expanding or contracting, but it couldn't hover at a fixed size.

This was puzzling because, before they knew about the Big Bang, most scientists, including Einstein, pictured the universe as static: eternal and unchanging.

When Einstein's equations suggested an expanding or contracting universe, not the static universe everyone believed in, he had a problem.

So Einstein went back to his equations and modified them to allow for a kind of anti-gravity that would infuse space with an outward push, counteracting the usual inward pull of gravity, allowing the universe to stand still. He called the modification the "cosmological constant."

Adding the cosmological constant rescued his equations, but the truth is Einstein had no idea if this outward push, or anti-gravity, really existed.

ROCKY KOLB: The introduction of the cosmological constant, by Einstein, was not a very elegant solution to try to find what he was looking for: a stationary universe.

S. JAMES GATES, JR.: It achieves this effect of anti-gravity. It says that gravity sometimes can behave in such a way, not to pull things together, but to push things apart.

ADAM RIESS: Like the clash of two titans, the cosmological constant and the pull of ordinary matter could hold the universe in check and keep it static.

BRIAN GREENE: But about a dozen years later, the astronomer Edwin Hubble discovered the universe is not static. It's expanding due to the explosive force of the Big Bang, 14-billion years ago.

That meant Einstein's original equations no longer had to be altered, and so, suddenly, the need for a cosmological constant went right out the window.

WAITER (Dramatization): Thank you.

BRIAN GREENE (Dramatization): You're welcome.

(Narration) Einstein is said to have called this his biggest blunder. But here's the thing: with the recent discovery that the expansion of the universe is accelerating, scientists are convinced that there is something in space that is pushing things apart. So, 70 years later, Einstein's biggest blunder may rank among his greatest insights.

JOSEPH LYKKEN: It was something that nobody else was thinking about, but it might be that Einstein's cosmological constant is the key to understanding the expansion of the universe, as we see it today.

BRIAN GREENE: Though no one knows what dark energy actually is, it raises an astounding and troubling possibility: Einstein pictured the strength of his anti-gravity as "constant." But is the strength of dark energy constant? What if changes over time?

The answer could overturn everything we thought we knew about the fate of the cosmos.

At the moment, everything in our world, from the molecules making up my body to the molecules making up the moon, is held together by forces that overwhelm the outward push of dark energy, and that's why we don't see things expanding in our everyday lives. But that situation might not last forever.

In one scenario, dark energy will continue to push the galaxies farther and farther apart, until ultimately, they'd be pushed so far apart, the universe would become a cold, dark and lonely place.

In another scenario, the strength of dark energy might increase over time, becoming so strong that it would tear apart everything within the galaxies, from stars, to planets, to matter of all kinds.

ALEX FILIPPENKO: If the dark energy grows with time, then, ultimately, even atoms will get ripped apart, when there's enough dark energy between the nuclei and the electrons to rip space apart—the "Big Rip."

BRIAN GREENE: Our picture of space has gone through a remarkable transformation. Back in Newton's time, space was just the container. It didn't do anything at all. Then, through Einstein, space begins to affect how objects move. Then, with Casimir, literally, objects can be pushed by the activity in empty space. And now through the ideas of Higgs and dark energy, the very expansion of the universe may be coming from the energy of empty space itself.

I don't think anybody would have thought space would have this kind of rich and profound impact on the nature of reality.

But as far as we've come with Isaac Newton's picture of space as something like a stage is not yet finished.

As we examine the fabric of the cosmos ever more closely, we may well find far more surprises than anyone ever imagined. Take me, for example. I seem real enough, don't I?

Well, yes. But surprising new clues are emerging that everything, you and I, and even space, itself, may actually be a kind of hologram.

That is: everything we see and experience, everything we call our familiar three-dimensional reality, may be a projection of information that's stored on a thin, distant two-dimensional surface, sort of the way the information for this hologram is stored on this thin piece of plastic.

Now, holograms are something we're all familiar with from the security symbol you find on most credit cards, but the universe as a hologram? That's one of the most drastic revisions to our picture of space and reality ever proposed. And the evidence for it comes from some of the strangest realms of space: black holes.

LEONARD SUSSKIND: This is a real disconnect, and it's very hard to get your head around: modern ideas, coming from black holes, tell us that reality is two-dimensional, that the three-dimensional world, the full-bodied three-dimensional world, is a kind of image of a hologram on the boundary on the region of space.

S. JAMES GATES, JR.: This is a very strange thing. When I was a younger physicist I would have thought any physicist who said that was absolutely crazy.

BRIAN GREENE: Here's a way to think about this. Imagine I took my wallet and threw it into a black hole. What would happen? We used to think that since nothing, not even light, can escape the immense gravity of a black hole, my wallet would be lost forever, but it now seems that may not be the whole story.

Recently, scientists exploring the math describing black holes made a curious discovery. Even as my wallet disappears into the black hole, a copy of all the information it contains seems to get smeared out and stored on the surface of the black hole, in much the same way that information is stored in a computer.

So in the end, my wallet exists in two places: there's a three-dimensional version that's lost forever inside the hole black and a two-dimensional version that remains on the surface as information.

CLIFFORD JOHNSON (University of Southern California): The information content of all the stuff that fell into that black hole can be expressed entirely in terms of just the outside of the black hole. The idea, then, is that you can capture what's going on inside the black hole by referring only to the outside.

BRIAN GREENE: And, in theory, I could use the information on the outside of the black hole to reconstruct my wallet.

And here's the truly mind-blowing part: space within a black hole plays by same rules as space outside a black hole or anywhere else. So if an object inside a black hole can be described by information on the black hole's surface, then it might be that everything in the universe, from galaxies and stars, to you and me, even space itself, is just a projection of information stored on some distant two-dimensional surface that surrounds us.

In other words, what we experience as reality may be something like a hologram.

LEONARD SUSSKIND: Is the three-dimensional world an illusion, in the same sense that a hologram is an illusion? Perhaps. I think I'm inclined to think yes, that the three-dimensional world is a kind of illusion and that the ultimate precise reality is the two-dimensional reality at the surface of the universe.

This idea is so new that physicists are still struggling to understand it. But if it's right, just as Newton and Einstein completely changed our picture of space, we may be on the verge of an even more dramatic revolution.

For something that's such a vital part of our everyday lives, space remains kind of like a familiar stranger. It's all around us, but we're still far from having unmasked its true identity.

That may take a hundred years, it may take a thousand years, or it may happen tomorrow, but when we solve that mystery, we'll take a giant step toward fully understanding the fabric of the cosmos.

THE FABRIC OF THE COSMOS: THE ILLUSION OF TIME

PBS Airdate: November 9, 2011

BRIAN GREENE (Columbia University): "Once upon a time…," that magical phrase at the beginning of every good story, but what is the story of "time?"

People say that "time flies," that "time is money," we "waste time," we "kill time," we try to "save time." But what do we really know about time?

Well, like this river, time seems to flow endlessly from one moment to the next. And the flow of time seems to always be in one direction: toward the future. But that may not be right. Discoveries over the last century have shown that much of what we think about time may be nothing more than an illusion. Contrary to everyday experience, time may not flow at all. Our past may not be gone; our future may already exist.

It turns out, time itself can speed up or slow down, and events that we think can unfold in only one direction can also unfold in reverse. But how could this be? How could we be so wrong about something so familiar? And if time isn't what we all think it is, then what is it? Did it have a beginning? Will it have an end? Where did it come from?

JANNA LEVIN (Barnard College/Columbia University): We'd like to corner time as a thing, but it defies that completely by being momentary, by, by only having definitions that harken back to the notion of time itself.

DAVID ALBERT (Barnard College/Columbia University): Time is the thing that everyone knows intimately, until you ask them to tell you about it.

ALAN GUTH (Massachusetts Institute of Technology): "What is time?" is really the $64,000 question to physics.

MAX TEGMARK (Massachusetts Institute of Technology): There's basically no aspect of time which I feel we really fully understand.

BRIAN GREENE: So how do you begin to unlock a mystery as deep and elusive as time? Well, one way is to measure it. And using clocks of all different shapes, sizes and kinds, we've been measuring time with ever-greater accuracy for thousands of years.

The first clock was one that you could say ticks just once a day: the rotating Earth. From the repetition of our planet's daily rotation on its axis, to its yearly orbit around the sun, we have always used the predictable, consistent motion of the earth to measure time.

PETER GALISON (Harvard University): We're always looking for things that repeat over and over again. And that repetition, that cycle of things, forms a clock. That's all time becomes is some repetitive process.

BRIAN GREENE: Measuring the earth's motion with a sundial, we divided the day into hours.

WILLIAM PHILLIPS (National Institute of Standards and Technology): The earth rotates once a day, and we tick off the days by looking at the rising and the setting of the sun. You can subdivide that by making a sundial.

BRIAN GREENE: With the swing of a pendulum, we divided hours into minutes and seconds. With the vibration of a quartz crystal, we improved accuracy to the thousandths of a second. But the National Institute of Standards and Technology, in Colorado, is the place to go if you really want to know what time it is.

STEVE JEFFERTS (National Institute of Standards and Technology): This is U.S. official time. It doesn't get any more accurate than this. Here, they measure time with mind-boggling accuracy using one of the smallest objects in the universe: an atom of a rare metal called cesium.

WILLIAM PHILLIPS: Atoms have a natural frequency. And anything that vibrates, that is giving you repetitive motion, can be a clock. The frequency at which the cesium atom ticks is the official timekeeper for the world.

BRIAN GREENE: When a cesium atom is bombarded with energy, it vibrates, or ticks, giving off pulses of light over 9 billion times a second.

STEVE JEFFERTS: We count the ticks of the cesium atom. And the cesium atom ticks at this 9,192,631,770 ticks in a second.

And so every time you count up to that number, one second has gone by. And you get one second, after one second, after one second, after one second.

WILLIAM PHILLIPS: This is just astounding. My watch gains or loses a second every couple of months. We're talking about clocks that would only gain or lose a second in 100 million years.

PETER GALISON: And that kind of story, where we take one measure of time and replace it with something that we decide is more accurate, has been the constant reform process of physics over hundreds of years.

BRIAN GREENE: But no matter how accurate our clocks have become, time remains a mystery. Clocks can tell us what time it is, but they haven't been able to tell us what time itself is. What is it we're actually measuring?

We may not know what time is, but the experience of the passage of time is a fundamental part of our lives. We're always thinking about time, remembering the past, making plans for the future, living our lives within time's constant tick, tick, tick.

I mean, look around any train station, and you can see how time rules our lives. What may not be so obvious is that the rise of train travel played a key role in one of the most startling discoveries about time.

PORTER (Dramatization): Tickets please, sir.

BRIAN GREENE: (Dramatization) Train running on time?

PORTER (Dramatization): Yes sir.

BRIAN GREENE: (Dramatization) Thank you.

(Narration) In the early days of train travel, time posed a unique problem. Back then, each town set their own particular time. Noon was when the sun was directly overhead, you know, more or less. And what time it was in another city, well, you know, that hardly mattered.

And to complicate things even further, trains would carry the time of the city where they began their journey. So, if I were going from Paris to Geneva, I would be on Paris time the whole way, since that's where I started. But were I going the other direction, from Geneva to Paris, I'd be on Geneva time.

PETER GALISON: And as you began to have more and more train lines crossing, and more and more different times located at that interchange, it became a nightmare of confusion.

BRIAN GREENE: The need to coordinate clocks over great distances became a huge issue, especially when the cities were connected by a single track. And here's where the modern story of time begins: as the need for synchronized clocks became ever more critical, a young physicist named Albert Einstein took a job at the patent office in Bern, Switzerland.

PETER GALISON: It was ringside seat to all of the great inventions of the time. The patents showed how…new and exciting ways to synchronize clocks with the exchange of telegraph signals, clocks that were synchronized by radio waves. All made the synchronization of time and what time was and how it was measured, something immediately important and exciting for Einstein.

BRIAN GREENE: Einstein would soon shake up the world with a radical insight into the nature of time. And these mechanical devices provided unexpected inspiration. Einstein realized that these attempts to synchronize clocks, they were much more than merely creative inventions. Instead, he realized that they were revealing a deep crack in our understanding of time itself.

Most people view time in a pretty simple, straightforward way. Time ticks the same for everyone everywhere. It's a common-sense picture established by the father of modern science, Isaac Newton.

S. JAMES GATES, JR. (University of Maryland): Time, for Isaac Newton, is something that is an immutable property of the universe. Time always changes at the same rate.

BILL PHILLIPS: Time just goes along, and there's really nothing we can do about it.

BRIAN GREENE: Sensible as Newton's picture of time may seem, Einstein realized it wasn't right. He discovered that time could run at different rates. As strange as it sounds, this means that time for me may not be the same as time for you. Einstein's discovery smashed Newton's conception of reality.

SEAN CARROLL (California Institute of Technology): Einstein says that time is not just a label on the whole universe; time is experienced individually.

MAX TEGMARK: What Einstein gave us is a much, much richer picture where everybody has their own private time, which runs at their own private rates.

PETER GALISON: There isn't time, in a sense of a universal tick-tock; there were times.

BRIAN GREENE: Einstein came to this shocking revelation by uncovering a hidden connection between space and time.

What Einstein figured out is that there's a profound link between motion through space and the passage of time. Roughly speaking, the more you have of one, the less you have of the other. To see how this works, let's take a little ride.

Right now, I'm heading due north at 60 miles an hour. And that means all my motion is in the northward direction. But let's now turn onto a different road and head northwest.

I'm still going 60 miles an hour, but I'm not making as much progress toward the north as I was a minute ago. And that's because some of my northward motion has been diverted, or shared with, my westward motion. Einstein realized that time and space are linked, in much the same way that north and west are.

And with this surprising insight, Einstein would overthrow the common-sense idea that time ticks the same for everyone.

Here's what I mean. That guy over there would say that I'm not moving at all. But I am. I may not be moving through space, but I am moving through time. I mean after all, my watch just keeps ticking and ticking.

And as long as I'm standing still, that is not moving through space. Einstein says that all of my motion is through time. But look what happens if I walk toward that guy. We've exaggerated it, but because I'm now in motion, he'll perceive my watch ticking slower.

That's because, from his perspective, some of my previous motion through time is being diverted into my motion through space. That is, Einstein realized that if a stationery observer carefully analyzes what he sees, he'll find that time itself elapses more slowly for me than it does for him.

And now that I've stopped moving, the passage of time on our watches once again agrees. And this was Einstein's key insight, that motion through space affects the passage of time.

DAVID KAISER (Massachusetts Institute of Technology): It's mind-blowing that you and I will not agree on measurements of time. Isn't time separate from us, right? Why should my measurement of time depend on how I am moving, or how you're moving? That, that doesn't make any sense.

BILL PHILLIPS: Time itself is running more slowly for the person who's moving. That's amazing. No one before Einstein ever imagined that sort of thing would happen.

MAX TEGMARK: That was uniquely Einstein.

BRIAN GREENE: So why don't we ever see this in everyday life? Well, at the slow speeds we move here on Earth, motion's impact on time is so tiny we don't experience it. But the effect is real and can be measured.

To do this, all you need are a couple of atomic clocks and a jet airplane. And this experiment was carried out in 1971, when scientists flew an atomic clock around the world, and then compared it to one on the ground. As Einstein predicted, the two clocks no longer agreed. They differed by only by a few hundred billionths of a second, but that was very real proof of motion's effect on the passage of time.

BILL PHILLIPS: Einstein's theory has been tested again and again and again. And it all hangs together. It really forms the basis for the way we understand much of the way nature works.

MAX TEGMARK: These effects, which used to be considered sort of obscure and very small, are very in-your-face with today's technology.

BRIAN GREENE: With the discovery of this unexpected link between space and time, Einstein realized that the two could no longer be thought of as separate things. Instead, space and time are fused together in what came to be called "spacetime."

MAX TEGMARK: Einstein unified the idea of space with the idea of time into this four-dimensional structure called spacetime.

BRIAN GREENE: And this fusion of space and time would lead Einstein to perhaps the most mind-bending realization of all: the sharp difference we see between past, present and future may only be an illusion.

In our day-to-day lives, we experience time as a continuous flow. But it can also be useful to think of time as a series of snapshots, or moments, and every event can be thought of as the unfolding of moment, after moment, after moment. And if we picture all moments, or snapshots, lined up, every moment here on Earth, every moment of Earth orbiting the Sun, and every moment throughout the entire universe, we would see every event that has ever happened or will ever happen, from the birth of our universe at the Big Bang, some 14-billion years ago; to the formation of stars in the Milky Way galaxy; to the creation of Earth, four and a half billion years ago; to the time of the dinosaurs; to events happening on Earth today, like me working in my office.

Thinking about spacetime like this led Einstein to overturn our everyday picture of past, present and future.

To get a feel for this, you have to think about the seemingly simple concept of "now." For me, a list of things that I consider to be happening right now might include the tick of noon on my office clock; my cat just now jumping from the windowsill; things happening far away, like a pigeon in Venice taking flight at this very moment; a meteor just now hitting the moon; and the explosion of a star at the far reaches of the universe.

These, and all other events that I think are happening at the same moment in time but in different regions of our universe, make up what I intuitively think of as "now." You can picture them as lying on a single slice of spacetime. Let's call it a "now" slice.

Common sense would say that you and I and everyone else will agree on what's happening or what exists right now, moment after moment after moment. That is, we will all agree on what lies on a given now slice. But Einstein showed that, strangely, when you take motion into account, this common sense picture of time goes out the window.

To see what I mean, think of spacetime as a loaf of bread. Einstein realized that, just as there are different ways to cut a loaf of bread into individual slices, there are different ways to cut spacetime into individual "now" slices. That is, because motion affects the passage of time, someone who is moving will have a different conception of what's happening right now, and so they'll cut the loaf into different now slices. Their slices will be at a different angle.

DAVID KAISER: That person who's moving will, will tilt the knife, will be carving out these slices at a different angle. They won't be parallel to my slices of time.

BRIAN GREENE: To get a feel for the bizarre effect this can have, imagine an alien, here, in a galaxy 10-billion light years from Earth, and way over there, on Earth, the guy at the gas station. Now, if the two are sitting still, not moving in relation to one other, their clocks tick off time at the same rate, and so they share the same now slices, which cut straight across the loaf. But watch what happens if the alien hops on his bike and rides directly away from Earth.

Since motion slows the passage time, their clocks will no longer tick off time at the same rate. And if their clocks no longer agree, their now slices will no longer agree either.

The alien's now slice cuts across the loaf differently. It's angled towards the past. Since the alien is biking at a leisurely pace, his slice is angled to the past by only a miniscule amount. But across such a vast distance, that tiny angle results in a huge difference in time. So what the alien would find on his angled now slice—he considers as happening right now, on Earth—no longer includes our friend at the gas station, or even 40 years earlier when our friend was a baby.

Amazingly, the alien's now slice has swept back through more than 200 years of Earth history and now includes events we consider part of the distant past, like Beethoven finishing his 5th Symphony: 1804 to 1808.

DAVID KAISER: Even at a relatively slow speed we can have, actually, tremendous disagreements on our labeling of "now," what happens at the same time, if we're spread out far enough in space.

BRIAN GREENE: And if that's not strange enough, the direction you move makes a difference, too. Watch what happens when the alien turns around and bikes toward Earth. The alien's new "now slice" is angled to…toward the future, and so it includes events that won't happen on Earth for 200 years: perhaps our friend's great-great-great granddaughter teleporting from Paris to New York.

Once we know that your now can be what I consider the past, or your now can be what I consider the future, and your now is every bit as valid as my now, then we learn that the past must be real, the future must be real. They could be your now. That means past, present, future…all equally real; they all exist.

SEAN CARROLL: If you believe the laws of physics, there's just as much reality to the future and the past as there is to the present moment.

MAX TEGMARK: The past is not gone, and the future isn't non-existent. The past, the future and the present are all existing in exactly the same way.

BRIAN GREENE: Just as we think of all of space as being "out there," we should think of all of time as being "out there" too. Everything that has ever happened or will happen, it all exists, from Leonardo da Vinci laying the final brushstroke on the Mona Lisa; to the signing of the Declaration of Independence; to your first day of school; to events that, from our perspective, are yet to happen, like the first humans landing on Mars.

With this bold insight, Einstein shattered one of the most basic concepts of how we experience time. "The distinction between past, present, and future," he once said, "is only an illusion, however persistent."

But if every moment in time already exists, then how do we explain the very real feeling that time, like this river, seems to endlessly rush forward?

Well, maybe we've been deceived, and time does not flow. Perhaps the river of time is more like a frozen river.

DAVID ALBERT: The most vivid example about the way the world is has to do with this flow of time. Physics does radical violence to this everyday experience of time.

JANNA LEVIN: Our entire experience of time is constantly in the present. And all we ever grasp is that instant moment.

MAX TEGMARK: There is nothing in the laws of physics that picks out one now over any other now. And it's just from our subjective viewpoint that it feels like things are changing.

BRIAN GREENE: Just the way an entire movie exists on celluloid, think of all moments of time as already existing too. The difference is that in the movies, a projector lights up or selects each frame as it goes by, but in the laws of physics, there is no evidence of something like a projector light that selects one moment over another. Our brains may create this impression, but in reality, what we all experience as the flow of time really may be nothing more than an illusion.

But if time, like this frozen river, does not flow, and all of time is "out there," is it possible to travel to the future or the past?

AIRPLANE BOARDING ANNOUNCEMENT (Dramatization): Now departing for 50 years in the future, Flight 24.

BRIAN GREENE: And if we could time-travel, would it be anything like what we all imagine?

(Film Clip): Catapult you through time to a world that has yet to be.

(Film Clip): The time travelers!

(Film Clip): Suppose something goes wrong with the time machine again?

(Film Clip): Throw the switch, Jed.

(Film Clip): Could we go anywhere we want, at any time?

(Film Clip): We're going to attempt time travel.

BRIAN GREENE: No one outside Hollywood has made a working time machine just yet, but surprisingly, time travel might be possible.

AIRPLANE BOARDING ANNOUNCEMENT (Dramatization): Now boarding, Flight 24 to Black Hole Cygnus X-1.

BRIAN GREENE: One way to travel through time is to make use of a strange feature of gravity. The familiar force that keeps our feet planted to the ground can have a profound impact on time.

(Dramatization) Hi.

WOMAN: Hello.

See you later, sir.

BRIAN GREENE: (Dramatization) Right, much later.

(Narration) So how can gravity be used to make a time machine?

Well, Einstein's theories show that gravity, like motion, can affect time. It's as if gravity can pull on time, slowing its passage. And the stronger the gravitational pull, the more time slows. Here on Earth, the effect is too small to notice, but still very real.

Compared to someone living on the top floor of a skyscraper, someone living on the bottom experiences time elapsing slightly slower because gravity is just a tiny bit stronger closer to the ground. But if you could travel to a black hole, the effect of gravity on time would be huge.

Formed when large stars collapse in on themselves, black holes have immense gravitational pull, millions and even billions of times stronger than the earth's. And if someone watched you travel close to a black hole, they'd see time, for you, slow down dramatically.

JANNA LEVIN: You, near that black hole, will appear to your friend far away to be moving slowly, talking slowly, biologically aging slowly. To them years are passing while for you it might be minutes.

BRIAN GREENE: So depending on the black hole's size and how close I get, if I spend an hour or two in orbit, something like 50 years will have passed back on Earth!

I will have traveled to Earth's future.

WOMAN: Hello, sir.

BRIAN GREENE: (Dramatization) Hi.

WOMAN: Long time, no see. Time travel becomes you.

BRIAN GREENE: (Dramatization) Thank you. Kind of like a fountain of youth.

(Narration) So when I return, I'll find myself in the future. Everyone else will have aged 50 years, but me, I'll have aged only a couple of hours.

Now, time travel to the future is one thing. But what about time travel to the past?

Well, that might be possible too, using something predicted by Einstein's equations, known as a wormhole. If wormholes exist, they would be kind of like, like shortcuts through spacetime, tunnels that link not only one place with another, but also one moment with another.

MAX TEGMARK: A wormhole would connect one part in spacetime to another part in spacetime, which is at an earlier time, like a sort of subway system through time.

BRIAN GREENE: So, let's say I wanted to go back in time and meet myself at the beginning of this program. If a wormhole connected here and there, all I'd need to do is step through.

SHOW-OPEN BRIAN GREENE: Hey, good to see you again.

TIME-TRAVEL BRIAN GREENE: Thanks. Good to be back.

(Narration) Well, that would be kind of weird. But you see, the real problem with time travel to the past is that things would get pretty confusing pretty quickly. I mean, imagine I were to change something about my past, like preventing my parents from meeting. Would that mean I'd never be born?

SEAN CARROLL: If you do travel to the past, you can't change things that we know are true about the past, because they already happened.

MAX TEGMARK: So if you go back and kill who you thought was your grandfather, that must've been some other guy you thought was your grandfather, and everything must somehow become beautifully self-consistent, even if it's in a twisted way.

BRIAN GREENE: And if you can travel to the past, why haven't we been overrun by tourists from the future? I mean, think about it, we haven't seen any intrepid time travelers popping into and out of our world, at least most of us don't think we have, so it's probably safe to assume that time travel to the past just isn't possible, at least not yet.

But since the math hasn't yet ruled it out, we can't dismiss time travel to the past entirely.

BILL PHILLIPS: So it's not at all clear that it could ever be a practical reality, but at least in principle, it doesn't seem to be forbidden.

MAX TEGMARK: My guess is that it's impossible, but it's striking that we've…still haven't been able to rigorously prove that.

BRIAN GREENE: While it seems likely that traveling to the past is out of reach, what about the fact, so common to our everyday experience, that time itself seems to move in only one direction: toward the future? We call this the "arrow of time."

SEAN CARROLL: The arrow of time is probably the most blatant fact about the universe we live in that we don't completely understand. Why we live in a universe that has a directionality to time is a mystery.

JOSEPH LYKKEN (Fermi National Accelerator Laboratory): This is not true of space. In space, I can go from New York to Chicago and then I can change my mind and go from Chicago to New York. So there is a one-way aspect to time that we don't understand at a fundamental level.

WILLIAM PHILLIPS: Why doesn't it go backwards? What does it even mean to say that time goes forward from the past into the future?

BRIAN GREENE: So, what can we say about where the arrow of time comes from? Why do we only see events unfold in one direction? Why don't we ever see them happen in reverse order? Well, it must be the laws of physics. I mean, surely they don't allow something like this to happen.

Well, actually, they do. The laws of physics are the mathematical equations we use to describe everything from the behavior of atoms to the swirl of galaxies. They've been devised and confirmed through centuries of observation and experiment. But surprisingly, there's nothing in the laws of physics that says events have to unfold through the familiar sequence we call "forward in time." According to these equations, events could just as well unfold in reverse order.

S. JAMES GATES, JR.: Most of the equations we use to describe what we see in the universe around us don't have an arrow of time attached to them. They're equations that work equally well moving forward in time or moving in backwards in time.

PETER GALISON: There's this contradiction between the physics, which seems fundamentally reversible, and so much of our life that seems irreversible.

BRIAN GREENE: Though it flies in the face of everyday experience, the laws of physics actually say bizarre things like these are possible! But how could this be?

Well, the answer is not as farfetched as you might think.

Here's why: we all know what will happen if I drop this glass of wine.

Now, the idea that this mess could somehow reverse itself and form back into a solid glass filled with wine seems absurd. But according to the laws of physics, this can happen. All I need to do is reverse the velocities of everything. Every piece of glass, every drop of wine, every molecule and atom in the liquid, glass, table and air: just reverse all their velocities, and, voila!

So, if the laws of physics don't care about whether glasses shatter or unshatter, why don't we ever see them unshatter? How can we square the laws of physics with our everyday experience? Something must be missing in our understanding, but what? What's responsible for the arrow of time?

Like many good mysteries, this one leads us to a graveyard, in our search for clues.

In Vienna, near the final resting places of Beethoven, Brahms, Schubert and Strauss, is 19th century Austrian physicist Ludwig Boltzmann's tombstone. Etched on top is an elegant equation: S=k log W. That's the mathematical formulation of a powerful concept known as entropy.

Entropy is a measure of something that we're all familiar with: disorder, or randomness. And it's an important idea, because there's a tendency of everything in the universe to move from order to disorder. Here's a way to get a feel for the idea.

Take my book, all 569 pages of it. It's very ordered, with the first page followed by the second, followed by the third and so on. But now, let's tear the pages out and let entropy go to work.

As you can see, the pages become very disordered. And the reason is simple: there is only one way for them to land in order, but a huge number of ways for them to land out of order, and so, and so it's much more likely that they'll land in a total mess.

And this is what we experience in our daily lives. Things move from order to disorder; in this case, from a neat, ordered book to pages that are randomly scattered. Everywhere we look, we see examples of entropy, or disorder, increasing with the passage of time: an egg breaks and splatters; ice cubes lose their orderly shape as they melt into water; billowing smoke becomes increasingly disordered.

S. JAMES GATES, JR.: Ordered states become disordered states, and that appears to be, perhaps, the direction of an arrow of time.

DAVID KAISER: We see, sort of, degrees of messiness, a measure of disorder tends to increase in one direction of time.

JANNA LEVIN: And so that, for Boltzmann, begins to create an arc of time.

BRIAN GREENE: So maybe this is the answer. Maybe the arrow of time comes from the tendency of nature to evolve toward ever greater disorder.

This sure seems like progress, but there's just one small problem with this reasoning. Because the laws of physics don't distinguish between the future and the past, entropy should increase not only towards the future, but also toward the past. And that makes no sense.

DAVID KAISER: That's like saying that entropy should increase in either direction that we look. We could look backwards in time and it should increase, we could look forwards in time and it should increase.

BRIAN GREENE: That would mean that, say, the pages of my book in the past would be disordered and then come together to form the neat ordered book in my hands. And when's the last time you saw something like that happen?

How could our everyday experiences be so at odds with the laws of physics? There must be a piece of the puzzle that's missing.

If we're sure the past had to be more ordered, and that everything tends toward disorder, as the equations of entropy tell us, is there something else besides the laws of physics that explains this?

Well, think of hitting a baseball. The laws of physics can help you predict where it will land, but those laws are not the only things you need. Run the film backward, and you can see that you also need the initial conditions, like how hard the ball was hit. Similarly, if the laws of physics can't give us an explanation for the arrow of time, maybe we need to look further, to the initial conditions of the universe. That brings our attention back to the Big Bang.

If the history of the universe were like a movie, and you ran it backwards, you'd see an increase in order the further back in time you go. Gradually, today's universe, with billions of galaxies clumped here and there, would turn back into clouds of gas and dust as everything contracts.

SEAN CARROLL: And these clouds of gas and dust move closer and closer to each other. So, if you get far enough into the past, they're squeezed into a smaller and smaller volume.

BRIAN GREENE: We have now come to the place where the buck finally stops. If this represents all of space at each moment of time, then we can see there simply isn't any more space and time before this single moment. So the ultimate source of order, of low entropy, must be the very beginning of the universe: the Big Bang.

S. JAMES GATES, JR.: The Big Bang is a highly ordered state. It's probably the most ordered event in all of physics. And so, everything that has come after that has been an increase in disorder.

DAVID KAISER: What the Big Bang gives us is a reason why the universe might look different when we look backwards in time versus forward. Moreover, when we go back to early times, the universe should've looked not just different from today but highly ordered.

SEAN CARROLL: Why was the entropy low? We don't know. But at least we know that there was a point that the universe began in, when the entropy was low.

BRIAN GREENE: So our best understanding is that the Big Bang set the arrow of time on its path. You can picture this as something like a wind-up clock. Just as the stored energy of a tightly wound clock is released as it unwinds, the universe has been unwinding since the Big Bang, becoming ever more disordered.

MAX TEGMARK: Our universe started out in a very unusually orderly state. And that's ultimately responsible for the fact that time seems to have a direction.

BRIAN GREENE: We don't yet know why our universe began in a highly ordered state, but the fact that it did means that every time a glass shatters, it's actually carrying forward something set in motion billions of years ago. The glass breaks but doesn't unbreak, because it is following the natural drive from order to disorder that began with the Big Bang.

SEAN CARROLL: We only ever move from the past to the future. And everything we see around us, all the changes, the formation of stars, to our lives, is all little epiphenomena, surfers riding the wave of increasing disorganization in the universe that defines the difference between the past and the future.

BRIAN GREENE: So the Big Bang may have stamped the arrow of time on our universe, and everything that has happened since may simply be the drive towards greater disorder that began with that event 13.7 billion years ago.

But if time had a beginning and disorder is always increasing, does that mean that time will have an end? What will the universe be like in the far, far future?

Recent discoveries are shedding new light on this question. The explosive force of the Big Bang sent space hurtling outwards. And as a result, the universe is still expanding today. Until recently, most people thought that expansion must be slowing down. That is, we thought of space, filled with galaxies, as kind of like a car traveling down a highway.

RADIO ANNOUNCER (Dramatization): You are listening to WUNI, the stellar sounds of the cosmos.

BRIAN GREENE: If the driver takes his foot off the gas, the car gradually slows down. Similarly, we thought the universe was expanding but at a slower and slower rate. But, surprisingly, astronomers found the expansion of the universe is not slowing down. It's accelerating. It's as if someone's not taking their foot off the gas pedal but stepping on it, causing a turbo booster to kick in. And that's making the expansion of the universe speed up more and more.

DAVID KAISER: Our expansion will keep accelerating in the future, not slow down. It goes against everything we had kind of gotten used to thinking about.

BRIAN GREENE: This has some very strange implications for the future. Because the expansion of our universe is accelerating, in the far future, after 100 billion years or so, all of the other distant galaxies will have hurtled out of sight from us. It will appear as if our galaxy in the middle of nothing.

A surprising outcome is that our descendants will be at a terrible loss. Light from distant galaxies has to travel so far to reach us that when we look out at them, we're actually looking back in time. So in the far future, when those distant galaxies are no longer visible, astronomers will find the past, in cosmic terms, is out of reach.

And as for the end of the time, one theory suggests that, eventually, black holes will dominate the cosmos. Then they, too, will evaporate, leaving nothing but random particles drifting through space.

JANNA LEVIN: In a far distant future, where everything has decayed and everything's just sort of smoothed out, there's no change. And without change we don't really have a clear notion of the passage of time.

BILL PHILLIPS: If you don't have events happening, then it's hard to see how you would even imagine that there was time.

MAX TEGMARK: You can't even tell which direction of time is forward and which is backward. In a very real sense, time, itself, will one day lose its meaning.

BRIAN GREENE: About 350 years ago, Isaac Newton, who was one of the first to think about time scientifically, wrote that he did not need to define time, because it is something "well known to all." But in trying to square our experience of "time" with the true nature of time, we've been forced to challenge some of our most deeply held beliefs.

We now know that in every event that goes from order to disorder, there's a link to the Big Bang itself, giving us the arrow of time. The common sense notion that one true time governs the universe has given way to a picture in which time is different for each and every one of us. And the flow of time, which seems to us as real as the flow of a river, may be nothing more than an illusion. Past, present and future may all exist on equal footing.

Our everyday experiences of time will always exert a powerful influence. We will continue to imagine that time is universal, that the past is gone, that the future is yet to be. But because of our scientific discoveries, we can also look beyond experience and recognize that we are part of a far richer and far stranger reality.

THE FABRIC OF THE COSMOS: QUANTUM LEAP

PBS airdate: 11/16/2011

NARRATOR: Lying just beneath everyday reality is a breathtaking world, where much of what we perceive about the universe is wrong. Physicist and best-selling author Brian Greene takes you on a journey that bends the rules of human experience.

BRIAN GREENE (Columbia University): Why don't we ever see events unfold in reverse order? According to the laws of physics, this can happen.

NARRATOR: It's a world that comes to light as we probe the most extreme realms of the cosmos, from black holes to the Big Bang to the very heart of matter, itself.

BRIAN GREENE: I'm going to have what he's having.

NARRATOR: Here, our universe may be one of numerous parallel realities, the three-dimensional world, merely a mirage; the distinction between past, present and future, just an illusion.

BRIAN GREENE: But how could this be? How could we be so wrong about something so familiar?

DAVID GROSS: Does it bother us? Absolutely.

STEVEN WEINBERG (The University of Texas at Austin): There's no principle built into the laws of nature that says theoretical physicists have to be happy.

NARRATOR: It's a game-changing perspective that opens up a whole new world of possibilities. Coming up: the realm of tiny atoms and particles, the quantum realm. The laws here seem impossible...

BRIAN GREENE: There's a sense in which things don't like to be tied down to just one location.

NARRATOR: ...yet they're vital to everything in the universe.

ALLAN ADAMS (Massachusetts Institute of Technology): There's no disagreement between quantum mechanics and any experiment that's ever been done.

NARRATOR: What do they reveal about the nature of reality? Take a Quantum Leap on the Fabric of the Cosmos, right now, on NOVA.

BRIAN GREENE: For thousands of years, we've been trying to unlock the mysteries of how the universe works. And we've done pretty well, coming up with a set of laws that describes the clear and certain motion of galaxies and stars and planets.

But now we know at a fundamental level, things are a lot more fuzzy, because we've discovered a revolutionary new set of laws that have completely transformed our picture of the universe. From outer space, to the heart of New York City, to the microscopic realm, our view of the world has shifted, thanks to these strange and mysterious laws that are redefining our understanding of reality. They are the laws of quantum mechanics.

Quantum mechanics rules over every atom and tiny particle in every piece of matter: in stars and planets, in rocks and buildings, and in you and me.

We don't notice the strangeness of quantum mechanics in everyday life, but it's always there, if you know where to look. You just have to change your perspective and get down to the tiniest of scales, to the level of atoms and the particles inside them.

Down at the quantum level, the laws that govern this tiny realm appear completely different from the familiar laws that govern big, everyday objects. And once you catch a glimpse of them, you never look at the world in quite the same way.

It's almost impossible to picture how weird things can get down at the smallest of scales. But what if you could visit a place like this, where the quantum laws were obvious, where people and objects behave like tiny atoms and particles? You'd be in for quite a show.

Here, objects do things that seem crazy.

I mean, in the quantum world,

BRIAN GREENE 2: There's a sense in which things don't like to be tied down to just one location...

BRIAN GREENE: ...or to follow just one path.

It's almost as if things were in more than one place at time.And what I do here can have an immediate effect somewhere else, even if there's no one there. And here's one of the strangest things of all: if people behaved like the particles inside the atom, then, most of the time, you wouldn't know exactly where they were.

Instead, they could be almost anywhere, until you looked for them.

Hey. I'm going to have what he's having.

So, why do we believe these bizarre laws? Well, for over 75 years, we've been using them to make predictions for how atoms and particles should behave. And in experiment after experiment, the quantum laws have always been right.

ALLAN ADAMS: It's the best theory we have.

SETH LLOYD (Massachusetts Institute of Technology): There are literally billions of pieces of confirming evidence for quantum mechanics.

WALTER LEWIN (Massachusetts Institute of Technology): It has passed so many tests of so many bizarre predictions.

ALLAN ADAMS: There's no disagreement between quantum mechanics and any experiment that's ever been done.

BRIAN GREENE: The quantum laws become most obvious when you get down to tiny scales, like atoms, but consider this: I'm made of atoms; so are you. So is everything else we see in the world around us. So it must be the case that these weird quantum laws are not just telling us about small things, they're telling us about reality.

So how did we discover them, these strange laws that seem to contradict much of what we thought we knew about the universe?

Not long ago, we thought we had it pretty much figured out, the rules that govern how planets orbit the sun, how a ball arcs through the sky, how ripples move across the surface of a pond. These laws were all spelled out in a series of equations called "classical mechanics," and they allowed us to predict the behavior of things with certainty.

It all seemed to be making perfect sense, until about a hundred years ago, when scientists were struggling to explain some unusual properties of light: for example, the kind of light that glowed from gases when they were heated in a glass tube.

When scientists observed this light through a prism, they saw something they'd never expected.

PETER GALISON (Harvard University): If you heated up some gas and looked at it through a prism, it formed lines, not the continuous spectrum that you see projected by a piece of cut glass on your table, but very distinct lines.

DAVID KAISER (Massachusetts Institute of Technology): It wouldn't give out a smear, kind of a complete rainbow of light; it would give out, sort of, pencil beams of light, at very specific colors.

PETER GALISON: And it was something of a mystery, how to understand what was going on.

BRIAN GREENE: An explanation for the mysterious lines of color would come from a band of radical scientists, who, at the beginning of the 20th century, were grappling with the fundamental nature of the physical world.

And some of the most startling insights came from the mind of Niels Bohr, a physicist who loved to discuss new ideas over ping-pong. Bohr was convinced that the solution to the mystery lay at the heart of matter itself, in the structure of the atom.

He thought that atoms resembled tiny solar systems, with even tinier particles called electrons orbiting around a nucleus, much the way the planets orbit around the sun.

But Bohr proposed that, unlike the solar system, electrons could not move in just any orbit, instead, only certain orbits were allowed.

PETER GALISON: And he had a, a really surprising and completely counter physical idea, which was that there were definite states, fixed orbits that these electrons could have, and only those orbits.

BRIAN GREENE: Bohr said that when an atom was heated, its electrons would become agitated and leap from one fixed orbit to another. Each downward leap would emit energy, in the form of light in very specific wavelengths. And that's why atoms produce very specific colors. This is where we get the phrase "quantum leap."

S. JAMES GATES, JR. (University of Maryland): If it weren't for the quantum leap, you would have this schmear of color coming out from an atom as it got excited or de-excited. But that's not what we see in the laboratory. You see very sharp reds and very sharp greens. It's the quantum leap that's the origin and the author of that sharp color.

BRIAN GREENE: What made the quantum leap so surprising was that the electron goes directly from here to there, seemingly without moving through the space in between.

It was as if Mars suddenly popped from its own orbit out to Jupiter.

Bohr argued that the quantum leap arises from a fundamental, and fundamentally weird, property of electrons in atoms: that their energy comes in discrete chunks that cannot be subdivided, specific minimum quantities called "quanta." And that's why there are only discrete, specific orbits that electrons can occupy.

DAVID KAISER:An electron had to be here or there and simply nowhere in between. And that's, that's like nothing we experience in everyday life.

WALTER LEWIN:Think of your daily life. When you eat food, you think your food is quantized? Do you think that you have to take a certain amount of minimum food? Food is not quantized. But the energy of electrons in an atom are quantized. That is very mysterious, why that is.

BRIAN GREENE: As mysterious as it might be, the evidence quickly mounted showing that Bohr was right. Electrons followed a different set of rules than planets or ping-pong balls.

Bohr's discovery was a game changer. And with this new picture of the atom, Bohr and his colleagues found themselves on a collision course with the accepted laws of physics.

The quantum leap was just the beginning. Soon, Bohr's radical views would bring him head to head with one of the greatest physicists in history.

Albert Einstein was not afraid of new ideas. But during the 1920s, the world of quantum mechanics began to veer in a direction Einstein did not want to go, a direction that sharply diverged from the absolute, definitive predictions that were the hallmark of classical physics.

MAX TEGMARK (Massachusetts Institute of Technology): If you asked Einstein or other physicists, at the time, what it was that distinguished physics from all kind of flaky speculation, they would have said it's that we can predict things with certainty. And quantum mechanics seemed to pull the rug out from under that.

BRIAN GREENE: One test in particular, which would come to be known as the double slit experiment, exposed quantum mysteries like no other.

If you were looking for a description of reality based on certainty, your expectations would be shattered.

We can get a pretty good feel for the double slit experiment and how dramatically it alters our picture of reality, by carrying out a similar experiment, not on the scale of tiny particles, but on the scale of more ordinary objects, like those you'd find here in a bowling alley.

But first I need to make a couple of adjustments to the lane.

You'd expect that if I roll a few of these balls down the lane, they'll either be stopped by the barrier or pass through one or the other slit and hit the screen at the back. And in fact, that's just what happens. Those balls that make it through always hit the screen directly behind either the left slit or the right slit.

The double slit experiment was much like this, except, instead of bowling balls, you use electrons, which are billions of times smaller.

You can picture them like this. Let's see what happens if I throw a bunch of these balls.

When electrons are hurled at the two slits, something very different happens on the other side. Instead of hitting just two areas, the electrons land all over the detector screen, creating a pattern of stripes, including some right between the two slits, the very place you'd think would be blocked. So, what's going on?

Well, to physicists, even in the 1920s, this pattern could mean only one thing: waves. Waves do all kinds of interesting things, things that bowling balls would never do. They can split, they can combine.

If I sent a wave of water through the double slits, it would split in two, and then the two sets of waves would intersect. Their peaks and valleys would combine, getting bigger in some places, smaller in others, and sometimes they'd cancel each other out.

With the height of the water corresponding to brightness on the screen, the peaks and valleys would create a series of stripes, in what is known as an interference pattern. So how could electrons, which are particles, form that pattern? How could a single electron end up in places a wave would go?

LEONARD SUSSKIND (Stanford University): Particles are particles; waves are waves. How can a particle be a wave?

S. JAMES GATES, JR.: Unless you give up the idea that it's a particle, and think, "Aha, this thing that I thought was a particle was actually a wave."

LEONARD SUSSKIND: A wave in an ocean, that's not a particle. The ocean is made out of particles, but the waves in the ocean are not particles. And rocks are not waves, rocks are rocks. So a rock is an example of a particle, an ocean wave is an example of an ocean wave, and now somebody's telling you a rock is like an ocean wave. What?

BRIAN GREENE: Back in the 1920s, when a version of this experiment was first done, scientists struggled to understand this wavy behavior. Some wondered if a single electron, while in motion, might spread out into a wave. And the physicist Erwin Schrí¶dinger came up with an equation that seemed to describe it.

STEVEN WEINBERG: Schrí¶dinger thought that this wave was a description of an extended electron, that, somehow, an electron got smeared out, and it was no longer a point, but was like a moosh.

PETER GALISON: There was a lot of argument about exactly what this represented. Finally, a physicist named Max Born came up with a new and revolutionary idea for what the wave equation described.

BRIAN GREENE: Born said that the wave is not a smeared out electron or anything else previously encountered in science. Instead, he declared it something that's really peculiar: a "probability wave." That is, Born argued that the size of the wave at any location predicts the likelihood of the electron being found there.

STEVEN WEINBERG: Where the wave is big, that's not where most of the electron is, that's where the electron is most likely to be.

DAVID KAISER:And that's just very strange, right? So the electron, on its own, seems to be a jumble of possibilities.

PETER FISHER (Massachusetts Institute of Technology): You're not allowed to ask, "Where is the electron right now?" You are allowed to ask, "If I look for the electron in this little particular part of space, what is the likelihood I will find it there?" Well, I mean, that bugs anyone, anytime.

BRIAN GREENE: As weird as it sounds, this new way of describing how particles like electrons move, is actually right. When I throw a single electron, I can never predict where it will land, but if I use Schrí¶dinger's equation to find the electron's probability wave, I can predict, with great certainty, that if I throw enough electrons, then, say 33.1 percent of them would end up here, 7.9 percent would end up there, and so on.

These kinds of predictions have been confirmed again and again by experiments.

And so, the equations of quantum mechanics turn out to be amazingly accurate and precise, so long as you can accept that it's all about probability. If you think that probability means we're reduced to guessing, the casinos of Las Vegas are ready to prove you wrong.

Try your hand at any one of these games of chance, and you can see the power of probability.

Let's say I place a $20 bet on number 29, here at the roulette table. The house doesn't know whether I'll win on this spin or the next or the next.

CROUPIER: One.

BRIAN GREENE: But it does know the probability that I'll win. In this game it's one in 38.

CROUPIER: Twenty-one.

Twenty-nine!

BRIAN GREENE: So, even though I may win now and then, in the long run, the house always takes in more than it loses.

The point is the house doesn't have to know the outcome of any single card game, roll of the dice or spin of the roulette wheel. Casinos can still be confident that over the course of thousands of spins, deals and rolls, they will win. And they can predict with exquisite accuracy exactly how often.

According to quantum mechanics, the world itself is a game of chance much like this.

All the matter in the universe is made of atoms and subatomic particles that are ruled by probability, not certainty.

ED FARHI (Massachusetts Institute of Technology): At base, nature is described by an inherently probabilistic theory. And that is highly counterintuitive and something which many people would find difficult accepting.

BRIAN GREENE: One person who found it difficult was Einstein. Einstein could not believe that the fundamental nature of reality, at the deepest level, was determined by chance.

WALTER LEWIN:And this is what Einstein could not accept. Einstein said, "God does not throw dice." He didn't like the idea that we couldn't with certainty say this happens or that happens.

BRIAN GREENE: But a lot of other physicists weren't so put off by probability, because the equations of quantum mechanics gave them the power to predict the behavior of groups of atoms and tiny particles with astounding precision.

Before long, that power would lead to some very big inventions: lasers, transistors, the integrated circuit, the entire field of electronics.

MAX TEGMARK: If quantum mechanics suddenly went on strike, every single machine that we have in the U.S., almost, would stop functioning.

BRIAN GREENE: The equations of quantum mechanics would help engineers design microscopic switches that direct the flow of tiny electrons and control virtually every one of today's computers, digital cameras and telephones.

ALLAN ADAMS: All the devices that we live on, diodes, transistors...just...that form the basis of information technology, the basis of daily life in all sorts of ways, they work. And why do they work? They work because of quantum mechanics.

STEVEN WEINBERG: I'm tempted to say that without quantum mechanics, we'd be back in the Dark Ages, but I guess, more accurately, without quantum mechanics, we'd be back in the 19th century: steam engines, telegraph signals...

MAX TEGMARK: Quantum mechanics is the most successful theory that we physicists have ever discovered. And yet, we're still arguing about what it means, what it tells us about the nature of reality.

BRIAN GREENE: In spite of all of its triumphs, quantum mechanics remains deeply mysterious.

It makes all this stuff run, but we still haven't answered basic questions raised by Albert Einstein all the way back in the 1920s and 30s; questions involving probability and measurement; the act of observation.

For Niels Bohr, measurement changes everything. He believed that before you measured or observed a particle, its characteristics were uncertain. For example, an electron in the double slit experiment: before the detector at the back pinpoints its location, it could be almost anywhere, with a whole range of possibilities. Until the moment you observe it, and only at that point, will the location's uncertainty disappear.

According to Bohr's approach to quantum mechanics, when you measure a particle, the act of measurement forces the particle to relinquish all of the possible places it could have been and select one definite location where you find it. The act of measurement is what forces the particle to make that choice.

Niels Bohr accepted that the nature of reality was inherently fuzzy, but not Einstein. He believed in certainty, not just when something is measured or looked at, but all the time. As Einstein said, "I like to think the moon is there even when I'm not looking at it."

DAVID KAISER:That's what Einstein was, was so upset about. Do we really think the reality of the universe rests on whether or not we happen to open our eyes? That's just bizarre.

Einstein was convinced something was missing from quantum theory, something that would describe all the detailed features of particles, like their location even when you were not looking at them. But at the time, few physicists shared his concern. And Einstein just thought it was giving up on the job of the physicist. It wasn't bad physics, per se, it just was totally incomplete.

PETER GALISON: That's Einstein's refrain: quantum mechanics is not incorrect, it's, as far as, in so far as it goes, but it's incomplete. It doesn't capture all of the things that can be said or predicted with certainty.

BRIAN GREENE: Despite Einstein's arguments, Niels Bohr remained unmoved. When Einstein repeated that "God does not play dice," Bohr responded, "Stop telling God what to do."

But in 1935, Einstein thought he'd finally found the Achilles heel of quantum mechanics, something so strange, so counter to all logical views of the universe, he thought it held the key to proving the theory was incomplete.

It's called "entanglement."

WALTER LEWIN:The most bizarre, the most absurd, the most crazy, the most ridiculous prediction that quantum mechanics makes is entanglement.

BRIAN GREENE: Entanglement is a theoretical prediction that comes from the equations of quantum mechanics. Two particles can become "entangled," if they're close together, and their properties become linked. Remarkably, quantum mechanics says that even if you separated those particles, sending them in opposite directions, they could remain entangled, inextricably connected.

To understand how profoundly weird this is, consider a property of electrons called "spin." Unlike a spinning top, an electron's spin, as with other quantum qualities, is generally completely fuzzy and uncertain, until the moment you measure it. And when you do, you'll find it's either spinning clockwise or counterclockwise. It's kind of like this wheel. When it stops turning, it will randomly land on either red or blue.

Now, imagine a second wheel. If these two wheels behaved like two entangled electrons, then every time one landed red the other is guaranteed to land on blue, and vice-versa.

Now, since the wheels are not connected, that's suspicious enough. But the quantum mechanics embraced by Niels Bohr and his colleagues went even further, predicting that if one of the pair were far away, even on the moon, with no wires or transmitters connecting them, still, if you look at one and find red, the other is sure to be blue. In other words, if you measured a particle here, not only would you affect it, but your measurement would also affect its entangled partner, no matter how distant.

For Einstein, that kind of weird long-range connection between spinning wheels or particles was so ludicrous that he called it spooky: "spooky action at a distance."

ALAIN ASPECT (Institut d'Optique, Palaiseau): When you have one particle here and one particle there, and they are separated enough that there is no signal able to allow them to communicate, and they still seem to be talking to each other, that is a big mystery.

STEVEN WEINBERG: What's surprising is that, when you make a measurement of one particle, you affect the state of the other particle. You change its state.

DAVID KAISER:There's no forces or pulleys or, you know, telephone wires. There's nothing connecting those things, right? How could my choice to act here have anything to do with what happens over there?

WALTER LEWIN:So there's no way they can communicate with each other, so it is completely bizarre.

Einstein just could not accept that entanglement worked this way, convincing himself that only the math was weird, not reality.

BRIAN GREENE: He agreed that entangled particles could exist, but he thought there was a simpler explanation for why they were linked that did not involve a mysterious long-distance connection. Instead, he insisted that entangled particles were more like a pair of gloves.

Imagine someone separates the two gloves, putting each in a case. Then that person delivers one of those cases to me and sends the other case to Antarctica.

Thanks.

Before I look inside my case, I know it has either a left-hand or a right-hand glove. And when I open my case, if I find a left-hand glove, then, at that instant, I'll know the case in Antarctica must contain a right-hand glove, even though no one has looked inside.

There's nothing mysterious about this. Obviously, by looking inside the case, I've not affected either glove. This case has always had a left-hand glove, and the one in Antarctica has always had a right-hand glove. That was set from the moment the gloves were separated and packed away.

Now, Einstein thought that exactly the same idea applies to entangled particles. Whatever configuration the electrons are in must have been fully determined from the moment that they flew apart.

ALAIN ASPECT: Einstein comes and says, "Look, if there is a strong relation, it means that the direction of the spins were already determined before you do the measurement."

BRIAN GREENE: So who was right?

Bohr, who championed the equations that said that particles were like spinning wheels that could immediately link their random results, even across great distances? Or Einstein, who believed there was no "spooky" connection, but instead, everything was decided well before you looked?

Well, the big challenge in figuring out who was right, Bohr or Einstein, is that Einstein is saying a particle, say, has a definite spin before you measure it. "How do you check that?" you say to Einstein. He says, "Well, measure it, and you'll find the definite spin." Bohr would say, "But it's the act of measurement that brought that spin to a definite state."

No one knew how to resolve the problem. So the whole question came to be considered philosophy, not science.

In 1955, Einstein died, still convinced that quantum mechanics offered, at best, an incomplete picture of reality.

In 1967, at Columbia University, Einstein's mission to challenge quantum mechanics was taken up by an unlikely recruit. John Clauser was on the verge of earning a Ph.D. in astrophysics. The only thing standing in his way was his grade in quantum mechanics.

JOHN CLAUSER (J. F. Clauser & Associates): When I was still a graduate student, try as I might, I could not understand quantum mechanics.

BRIAN GREENE: Clauser was wondering if Einstein might be right, when he made a life-altering discovery. It was an obscure paper by a little known Irish physicist named John Bell. Amazingly, Bell seemed to have found a way to break the deadlock between Einstein and Bohr and show, once and for all, who was right about the universe.

JOHN CLAUSER: I was convinced that the quantum mechanical view was probably wrong.

BRIAN GREENE: Reading the paper, Clauser saw that Bell had discovered how to tell if entangled particles were really communicating through spooky action, like matching spinning wheels, or if there was nothing spooky at all and the particles were already set in their ways, like a pair of gloves.

What's more, with some clever mathematics, Bell showed that if spooky action were not at work, then quantum mechanics wasn't merely incomplete, as Einstein thought, it was wrong.

JOHN CLAUSER: I came to the conclusion that, "My god, this is one of the most profound results I've ever seen."

BRIAN GREENE: Bell was a theorist, but his paper showed that the question could be decided, if you could build a machine that created and compared many pairs of entangled particles.

ALLAN ADAMS: Bell turned the question into an experimental question.

DAVID KAISER:It wasn't just going to be about philosophy or, or trading pieces of paper.

ALLAN ADAMS: And the experiment that he envisioned could be done.

DAVID KAISER:You could really set up an actual experiment to, to force the issue.

BRIAN GREENE: Clauser set about constructing a machine that would finally settle the debate.

JOHN CLAUSER: Now, I was just this punk graduate student at the time. This really seemed like, "Wow!" There's always the slim chance that you will find a result that will shake the world.

BRIAN GREENE: Clauser's machine could measure thousands of pairs of entangled particles and compare them in many different directions. As the results started coming in, Clauser was surprised and not happy.

JOHN CLAUSER: I kept asking myself, "What have I done wrong? What mistakes have I made in this?"

BRIAN GREENE: Clauser repeated his experiments, and soon French physicist Alain Aspect developed some even more sophisticated tests.

In Aspect's test, the only way that measuring one of the particles could directly influence the other would be for a signal to travel between them faster than the speed of light, something Einstein himself had shown impossible. The only remaining explanation was spooky action, and so Aspect's experiment removed virtually all doubt.

ALAIN ASPECT: Quantum mechanics is true, even in the most mysterious and the most weird situation.

BRIAN GREENE: The results of these experiments are truly shocking. They prove that the math of quantum mechanics is right. Entanglement is real. Quantum particles can be linked across space. Measuring one thing can, in fact, instantly affect its distant partner, as if the space between them didn't even exist.

The one thing that Einstein thought was impossible, spooky action at a distance, actually happens.

JOHN CLAUSER: I was again very saddened that I had not overthrown quantum mechanics, because I still had, and to this day, still have, great difficulty in understanding it.

WALTER LEWIN:That is the most bizarre thing of quantum mechanics. It is impossible to even comprehend. Don't even ask me why. Don't ask me—which you're going to—how it works, because it's an illegal question. All we can say is that is apparently the way the world ticks.

BRIAN GREENE: So, if we accept that the world really does tick in this bizarre way, could we ever harness the long-distance spooky action of entanglement to do something useful?

Well, one dream has been to somehow transport people and things from one place to another without crossing the space in between, in other words, teleportation.

STAR TREK CLIP Beam me aboard!

Energize.

Energize!

BRIAN GREENE: Star Trek has always made beaming, or teleporting, look pretty convenient. It seems like pure science fiction, but could entanglement make it possible?

Remarkably, tests are already underway, here on the Canary Islands, off the coast of Africa.

ANTON ZEILINGER (University of Vienna): We do the experiments here, on the Canary Islands, because you have two observatories. And, after all, it's a nice environment.

BRIAN GREENE: Anton Zeilinger is a long way from teleporting himself or any other human. But he is trying to use quantum entanglement to teleport tiny individual particles, in this case, photons, particles of light.

He starts by generating a pair of entangled photons in a lab on the island of La Palma. One entangled photon stays on La Palma, while the other is sent by laser-guided telescope to the island of Tenerife, 89 miles away.

Next, Zeilinger brings in a third photon, the one he wants to teleport, and has it interact with the entangled photon on La Palma.

The team studies the interaction, comparing the quantum states of the two particles. And here's the amazing part. Because of spooky action, the team is able to use that comparison to transform the entangled photon on the distant island into an identical copy of that third photon.

It will be as if the third photon has teleported across the sea, without traversing the space between the islands.

ANTON ZEILINGER: We, sort of, extract the information carried by the original and make a new original there.

BRIAN GREENE: Using this technique, Zeilinger has successfully teleported dozens of particles. But could this go even further?

Since we're made of particles, could this process make human teleportation possible one day?

ATTENDANT: Welcome to New York City.

BRIAN GREENE: Let's say I want to get to Paris for a quick lunch. Well, in theory, entanglement might someday make that possible. Here's what I'd need. A chamber or particles here in New York that's entangled with another chamber of particles in Paris.

ATTENDANT: Right this way, Mr. Greene.

BRIAN GREENE: I would step into a pod that acts sort of like a scanner or fax machine. While the device scans the huge number of particles in my body—more particles than there are stars in the observable universe—it's jointly scanning the particles in the other chamber. And it creates a list that compares the quantum state of the two sets of particles. And here's where entanglement comes in. Because of spooky action at a distance, that list also reveals how the original state of my particles is related to the state of the particles in Paris.

Next, the operator sends that list to Paris. There they use the data to reconstruct the exact quantum state of every single one of my particles.

And a new me materializes.

It's not that the particles traveled from New York to Paris. It's that entanglement allows my quantum state to be extracted in New York and reconstituted in Paris, down to the last particle.

ATTENDANT: Bonjour, Monsieur Greene.

BRIAN GREENE: Hi, there.

So, here I am in Paris, an exact replica of myself. And I'd better be, because measuring the quantum states of all my particles in New York has destroyed the original me.

EDWARD FARHI: It is absolutely required in the quantum teleportation protocol that the thing that is teleported is destroyed in the process. And you know, that does make you a little anxious.

I guess you would just end up being a lump of neutrons, protons and electrons. You wouldn't look too good.

BRIAN GREENE: Now, we are a long way from human teleportation today, but the possibility raises a question: is the Brian Greene who arrives in Paris really me?

Well, there should be no difference between the old me in New York and the new me, here in Paris. And the reason is that, according to quantum mechanics, it's not the physical particles that make me me, it's the information those particles contain. And that information has been teleported exactly, for all the trillions of trillions of particles that make up my body.

ANTON ZEILINGER: It is a very deep philosophical question, whether what arrives at the receiving station is the original or not. My position is that, by "original" we mean something which has all the properties of the original. And if this is the case, then it is the original.

JOHN CLAUSER: I wouldn't step into that machine.

BRIAN GREENE: Whether or not human teleportation ever becomes a reality, the fuzzy uncertainty of quantum mechanics has all sorts of other potential applications.

Here at M.I.T., Seth Lloyd is one of many researchers trying to harness quantum mechanics in powerful new ways.

SETH LLOYD: Quantum mechanics is weird. That's just the way it is. So, you know, life is dealing us weird lemons, can we make some weird lemonade from this?

BRIAN GREENE: Lloyd's weird lemonade comes in the form of a quantum computer.

These are the guts of a quantum computer. This gold and brass contraption might not look anything like your familiar laptop, but at its heart, it speaks the same language, binary code, a computer language spelled out in zeros and ones, called bits.

SETH LLOYD: So the smallest chunk of information is a bit. And what a computer does is simply busts up the information into the smallest chunks, and then flips them really, really, really rapidly.

BRIAN GREENE: This quantum computer speaks in bits, but unlike a conventional bit, which at any moment can be either zero or one, a quantum bit is much more flexible.

SETH LLOYD: You know, something here can be a bit. Here is zero, there is one. That's a bit of information. So if you can have something that's here and there at the same time, then you have a quantum bit, or qubit.

BRIAN GREENE: Just as an electron can be a fuzzy mixture of spinning clockwise and counterclockwise, a quantum bit can be a fuzzy mixture of being a zero and a one, and so a qubit can multitask.

SETH LLOYD: Then it means you can do computations in ways that our classical brains could not have dreamed of.

BRIAN GREENE: In theory, quantum bits could be made from anything that acts in a quantum way, like an electron or an atom. Since quantum bits are so good at multi-tasking, if we can figure out how to get qubits to work together to solve problems, our computing power could explode exponentially.

To get a feel for why a quantum computer would be so powerful, imagine being trapped in the middle of a hedge maze. What you'd want is to find the way out, as fast as possible. The problem is there are so many options.

And I just have to try them out, one at a time. That means I'm going to hit lots of dead ends, go down lots of blind alleys, and make lots of wrong turns before I'd finally get lucky and find the exit.

And that's pretty much how today's computers solve problems. Though they do it very quickly, they only carry out one task at a time, just like I can only investigate one path at a time, in the maze.

But, if I could try all of the possibilities at once, it would be a different story. And that's kind of how quantum computing works.

Since particles can, in a sense, be in many places at once, the computer could investigate a huge number of paths or solutions at the same time, and find the correct one in a snap.

Now a maze like this only has a limited number of routes to explore, so even a conventional computer could find the way out pretty quickly. But imagine a problem with millions or billions of variables, like predicting the weather far in advance. We might be able to forecast natural disasters, like earthquakes or tornados.

Solving that kind of problem right now would be impossible, because it would take a ridiculously huge computer. But a quantum computer could get the job done with just a few hundred atoms. And so, the brain of that computer, it would be smaller than a grain of sand.

There's no doubt, we're getting better and better at harnessing the power of the quantum world, and who knows where that could take us? But we can't forget that at the heart of this theory, which has given us so much, there is still a gaping hole: all the weirdness down at the quantum level, at the scale of atoms and particles, where does the weirdness go?

Why can things in the quantum world hover in a state of uncertainty, seemingly being partly here and partly there, with so many possibilities, while you and I, who, after all, are made of atoms and particles, seem to always be stuck in a single definite state. We are always either here or there.

Niels Bohr offered no real explanation for why all the weird fuzziness of the quantum world seems to vanish as things increase in size. As powerful and accurate as quantum mechanics has proven to be, scientists are still struggling to figure this out.

Some believe that there is some detail missing in the equations of quantum mechanics. And so, even though there are multiple possibilities in the tiny world, the missing details would adjust the numbers on our way up from atoms to objects in the big world, so that

it would become clear that all but one of those possibilities disappear, resulting in a single, certain outcome.

Other physicists believe that all the possibilities that exist in the quantum world, they never do go away.

Instead, each and every possible outcome actually happens, only most of them happen in other universes, parallel to our own. It's a mind-blowing idea, but reality could go beyond the one universe we all see, and be constantly branching off, creating new, alternative worlds, where every possibility gets played out.

This is the frontier of quantum mechanics, and no one knows where it will lead.

MAX TEGMARK: The very fact that our reality is much grander than we thought, much more strange and mysterious than we thought, is to me also very beautiful and awe inspiring.

ED FARHI: The beauty of science is that it allows you to learn things which go beyond your wildest dreams, and quantum mechanics is the epitome of that.

STEVEN WEINBERG: After you learn quantum mechanics, you're never really the same again.

BRIAN GREENE: As strange as quantum mechanics may be, what's now clear is that there's no boundary between the worlds of the tiny and the big. Instead, these laws apply everywhere, and it's just that their weird features are most apparent when things are small.

And so, the discovery of quantum mechanics has revealed a reality, our reality, that is both shocking and thrilling, bringing us that much closer to fully understanding the fabric of the cosmos.

THE FABRIC OF THE COSMOS: UNIVERSE OR MULTIVERSE?

PBS Airdate:November 23, 2011

NARRATOR: Lying just beneath everyday reality is a breathtaking world, where much of what we perceive about the universe is wrong. Physicist and best-selling author Brian Greene takes you on a journey that bends the rules of human experience.

BRIAN GREENE (Columbia University): Why don't we ever see events unfold in reverse order? According to the laws of physics, this can happen.

NARRATOR: It's a world that comes to light as we probe the most extreme realms of the cosmos, from black holes to the Big Bang to the very heart of matter, itself.

BRIAN GREENE: I'm going to have what he's having.

NARRATOR: Here, empty space teems with ferocious activity, the three-dimensional world may be just an illusion, and there's no distinction between past, present and future.

BRIAN GREENE: But how could this be? How could we be so wrong about something so familiar?

DAVID GROSS (University of California, Santa Barbara): Does it bother us? Absolutely.

STEVEN WEINBERG (The University of Texas at Austin): There's no principle built into the laws of nature that says theoretical physicists have to be happy.

NARRATOR: It's a game-changing perspective that opens up a whole new world of possibilities. Coming up: what if new universes were born all the time…

ALEX VILENKIN (Tufts University): In this picture, the big bang is not a unique event.

NARRATOR: …and ours was one of numerous parallel realities?

BRIAN GREENE: Somewhere there's a duplicate of you and me and everyone else.

NARRATOR: Are we in a universe or a multiverse? The Fabric of the Cosmos, right now on NOVA.

BRIAN GREENE: New York City: they say there's nowhere else like it, home to 8,000,000 people, countless structures, monuments and landmarks, every one of them unique. Or so we think.

Uniqueness is an idea so familiar, we never even question it. Experience tells us people and objects are one of a kind. Why else would we visit museums and collect great masterpieces?

Yet a new picture of the cosmos is coming to light, in which nothing is unique. Not that the world's great masterpieces are fakes, instead, I'm talking about something far more profound: a new picture of the cosmos that challenges the very notion of uniqueness, one in which duplicates are inevitable.

And that's just the beginning. There might be duplicates not just of objects, but of you and me and everyone else. But if this new picture is right, where are these duplicates? And why haven't we ever seen them?

The answer may lie outside our universe. There was a time when the word "universe" meant "all there is," everything. The notion of more than one universe, more than one "everything," seemed impossible. But perhaps, if we could go beyond our solar system, beyond the Milky Way, even beyond other distant galaxies, past the end of the observable universe, we'll find that there's more, a lot more, that our universe is not alone. There may be other universes. In fact, there might be new ones being born all the time. We may actually live in an expanding sea of multiplying universes, a "multiverse."

If we could visit these other universes, we'd find that some might have basic properties of nature so foreign that matter, as we know it, couldn't exist. Others might have galaxies, stars, even a planet that looks familiar but with some surprising differences.

And if there are an infinite number of universes in the multiverse, somewhere there's a place where almost everything is identical to ours, except for the slightest details. Like maybe there's another Brian Greene who ends up in a different line of work.

STEVEN WEINBERG: If the multiverse is indeed infinite, then one is going to have to confront a lot of possibilities that are very hard to imagine.

ALAN GUTH (Massachusetts Institute of Technology): There will be other places where there will be Alan Guths who look and think and act exactly like me, as well as many where there will be Alan Guths who look and think almost exactly like me, but with some small differences.

LEONARD SUSSKIND (Stanford University): Is it science? Is it a part of metaphysics? Is it just philosophy? Is it religion? Physicists tend not to ask those questions, they just say, "Let's follow the logic." And the logic seems to lead there.

BRIAN GREENE: However unfamiliar and strange the multiverse might seem, a growing number of scientists think it may be the final step in a long line of radical revisions to our picture of the cosmos.

That is, there was a time when we thought that the earth was at the center of the cosmos, and that everything else revolved around us. Then, along came scientists like Galileo and Copernicus. And they showed us that it's the sun, not the earth that's at the center of our solar system. And our solar system? It's just a little neighborhood in the outskirts of a gigantic galaxy. And our galaxy? It's one of hundreds of billions of galaxies that make up our universe. Now, all of these ideas sounded outrageous when they were first proposed, but today, we don't even question them.

The idea of a multiverse may be similar. It simply may require a drastic change in our cosmic perspective. On the other hand, some scientists think that the multiverse is nothing but a dead end for physics.

ANDREAS ALBRECHT (University of California, Davis): I'm very uncomfortable with the multiverse. To become solid science it's got a lot of growing up to do.

DAVID GROSS: You know, it exists in the same way that, you know, angels might exist.

STEVEN WEINBERG: We have to make our bets, and I think, right now, the multiverse is a pretty good bet.

ALAN GUTH: I think there's a good chance that the multiverse is real, and that a hundred years from now people might be convinced that it's real.

BRIAN GREENE: So where did this idea come from, and what's the evidence for it? Well, several surprising discoveries suggest we really may be part of a multiverse.

The first of these discoveries has to do with the generally accepted theory of the origin of our universe: the Big Bang.

According to this theory, our universe began some 14 billion years ago in an intensely violent explosion. Over billions of years, the universe cooled and coalesced, allowing the formation of stars, planets and galaxies. As a result of that explosion, the universe is still expanding today. But if you could run the history of our universe in reverse, all the way back to the beginning, you'd find that the Big Bang theory tells us nothing about what sent everything hurtling outward in the first place.

ALAN GUTH: It's called the Big Bang theory, but the one thing it really says nothing about is the bang itself. It says nothing about what banged, why it banged, or what happened before it banged.

BRIAN GREENE: So what fueled that violent explosion? What force could have driven everything apart?

The quest to figure that out would bring scientists face to face with the multiverse.

One physicist whose work unexpectedly helped lay the foundation for the multiverse idea is Alan Guth. Today, he's a professor at M.I.T., but back in 1979, Guth and a colleague, Henry Tye, were pursuing a new idea about how particles might have formed in the early universe.

ALAN GUTH: Henry suggested to me that we should maybe look at whether or not this new process we were thinking of would influence the expansion rate of the universe.

BRIAN GREENE: Guth and Tye hadn't set out to investigate the expansion rate of the universe in the first moments after the Big Bang, but Henry Tye's question caused Guth to review their calculations one more time.

ALAN GUTH: I stayed up quite late that night and went over the calculations very carefully, trying to make sure everything was correct.

BRIAN GREENE: As the night wore on, Guth discovered something extraordinary in the equations describing how new particles might have formed in the early universe.

ALAN GUTH: I came to the shocking conclusion that these new-fangled particle theories would have a tremendous effect on the expansion rate of the universe. The kind of process Henry and I were talking about would drive the universe into a period of incredibly rapid exponential expansion.

BRIAN GREENE: What Guth found in the math was evidence that in the extreme environment of the very early universe, gravity can act in reverse. Instead of pulling things together, this "repulsive" gravity would repel everything around it, causing a huge expansion.

ALAN GUTH: I immediately became very excited about it and scribbled out the calculation in my notebook. And then at the end I wrote: "spectacular realization" with a double box around it, because I realized that, if it was right, it could be very important.

BRIAN GREENE: By discovering this repulsive gravity, Alan Guth had unintentionally shed light on the very beginning of the Big Bang.

Described mathematically, this force was so powerful it could take a bit of space as tiny as a molecule and blow it up to the size of the Milky Way galaxy, in less than a billionth of a billionth of a billionth of a blink of an eye. After this incredibly short outward burst, space would continue to expand more slowly, and cool, allowing stars and galaxies to form just as they do in the Big Bang theory.

Guth called this short burst of expansion "inflation," and he believed it explained what set the universe expanding in the first place. The powerful, repulsive gravity of inflation was the bang in the Big Bang.

But despite having made a momentous breakthrough, Alan Guth had an even more pressing concern.

ALAN GUTH: I had no idea what my employment might be. I was really looking for a more permanent job. The inflationary universe scenario looks very exciting, so I went on, actually, a pretty long trip, giving talks about this.

STEVEN WEINBERG: Suddenly this idea caught on.

ANDREAS ALBRECHT: Talks about inflation were packed with people from all areas of physics.

STEVEN WEINBERG: Lots of astrophysical theorists, including me, got very enthusiastic.

ANDREAS ALBRECHT: It was a very, very exciting time.

STEVEN WEINBERG: If you have a really good idea that allows other people to move the field forward, people are going to pay attention.

ALAN GUTH: …an amazing feeling that, suddenly, I had crossed that gap from being an unknown post-doc to being one of the major players. And it was very hard to absorb, but it certainly felt good.

BRIAN GREENE: One reason inflation was so exciting was that it made predictions that could be tested through observation. Scientists realized that, if the theory were correct, evidence for it should be found in the night sky.

Imagine that we could shut off the sun and take away all the stars. If our eyes could detect the rest of the energy that's still there, we'd see a warm glow everywhere in the cosmos. This sea of radiation is called the cosmic microwave background. It's the last remnants of heat from the Big Bang itself.

Theory predicted that the violent expansion of space during inflation would leave an imprint on this radiation. These telltale "fingerprints" would form a precise pattern of temperature variations—slightly hotter spots and slightly colder spots—that would look something like this.

But it would be about 10 years before the technology was sensitive enough to test this prediction. Then, in 1989, NASA launched the Cosmic Background Explorer satellite, followed by a second satellite, W.M.A.P., in 2001 that would put inflation to the test.

The missions measured the radiation with tremendous precision, and the results were stunning. The temperature variations found in the cosmos were an almost identical match with the predictions of the theory of inflation.

It's just a theory, mathematics on the page, until it makes predictions that are confirmed. W.M.A.P. found what the math of inflation predicted. That is enormously convincing.

ANDREAS ALBRECHT: So inflation has had a number of chances, now, to fail. It made predictions, data came in, and inflation has come through with flying colors.

BRIAN GREENE: Guth's work on inflation, along with that of other physicists, was hailed as a milestone toward understanding the origin of the universe. But soon, two Russian physicists would discover that the equations of inflation held a shocking secret: our universe may not be alone.

One of these Russian physicists was Andrei Linde, who had already made pivotal contributions to inflationary theory. The other was Alex Vilenkin, who happened to attend one of the talks Alan Guth gave during his road trip.

ALEX VILENKIN: He gave a wonderful talk. I hadn't met him before, but what I heard was rather unexpected. In one shot, inflation explained very well, many features of the Big Bang, and was quite remarkable—why the universe is the way it is. So I went home greatly impressed.

BRIAN GREENE: Alex Vilenkin was so impressed that, for months afterward, he couldn't stop thinking about inflation.

ALEX VILENKIN: Usually, I have my thought of the day in the shower, which I tend to take long.

BRIAN GREENE: The more Vilenkin considered the process of inflation, the more he wondered about what would make it stop. How would a region of space transition out of inflation? What exactly would happen at the moment inflation ends?

ALEX VILENKIN: As I thought about it, it turns out that the end of inflation doesn't happen everywhere at once.

BRIAN GREENE: Vilenkin suddenly realized that if inflation doesn't end everywhere at once, then there's always some part of space where it's still happening.

ALEX VILENKIN: So, in this picture, the Big Bang is not a unique event that happened. There were multiple bangs that happened before ours, and there will be countless other bangs that will happen in the future.

BRIAN GREENE: It was a striking and unexpected new picture, in which inflation would stop in some regions but always continue somewhere else. New big bangs are always occurring, and new universes are always being born, yielding an eternally expanding multiverse.

Linde and Vilenkin in particular pushed the idea that inflation might never end, that this ballooning process could happen over and over again, giving one universe after another after another.

So was this a revolution in science or just a theory that's full of holes?

The idea became known as "eternal inflation." And you can picture it something like this. Imagine that this block of cheese is all of space, before the formation of stars and galaxies. Now, according to inflation, space is uniformly filled with a huge amount of energy. And that energy causes space to expand at an enormous speed. As it does, here and there the energy discharges, sort of like a spark of static electricity.

But this is like static electricity on a cosmic scale, and when it discharges, all that energy is rapidly transformed into matter, in the form of tiny particles. That process is the birth of a new universe, what we have traditionally called the Big Bang.

Inside these new universes, which are like holes in the cheese, space continues to expand, but much more slowly. And sometimes, galaxies, stars and planets form, much as we see in our universe, today.

Meanwhile, outside of these new universes, the rest of space is still full of undischarged energy and still expanding at enormous speed. And more expanding space means more places where the energy can discharge into more big bangs and create more new universes.

And that means this process could go on forever. In other words, when it comes to eternal inflation, that cheese is more like Swiss cheese, in which new universes endlessly form, creating a multiverse.

The multiverse: a profound implication of eternal inflation. But as Alex Vilenkin would soon learn, one that would not be easily accepted.

ALEX VILENKIN: I thought I had realized something important about the universe, and I wanted to share this with my fellow physicists. And one of the first, of course, had to be Alan Guth.

Now we know that quantum fluctuations are different in different regions of space…

I thought he would be excited about it, but this encounter didn't go as planned.

…inflation will last longer than in others.

As I was describing to him my new picture of the universe, inflating regions and so forth, ahem, expansion, I noticed that Alan is beginning to doze off a little bit. Actually, I was, of course, very unhappy about that, so I thought that I probably should go.

BRIAN GREENE: One problem with the concept of a multiverse was that there seemed to be no way to detect it. Not only is each universe expanding, but so is the space in between them. That means that nothing, not even light, can travel from any of the other universes to reach us.

ALEX VILENKIN: Physicists did not really respond very well to this idea of eternal inflation. Once I said that I'm going to tell them something about things beyond our horizon that cannot, in principle, be observed, most of them just lost interest right there.

BRIAN GREENE: Alex Vilenkin thought he was on to something big, but others were skeptical. So Vilenkin reluctantly tried to put his work on eternal inflation out of his mind.

ANDREAS ALBRECHT: Who wants to talk about a universe you're never going to see? The multiverse can't make predictions, it can't be tested. You could make the case that it's not really science.

STEVEN WEINBERG: How can you ever be confident of it when you can't see the other parts of the multiverse? We can only see our little patch, our little expanding cloud of galaxies. How are we ever going to know?

PAUL STEINHARDT (Princeton University): You can't prove the multiverse exists. It's not wrong. You can't prove that it doesn't exist. So why should we believe it?

BRIAN GREENE: Alex Vilenkin tried to stop thinking about the multiverse. With no hard evidence to support it, the idea seemed to have hit a dead end.

ALEX VILENKIN: Many people thought it's just not science to talk about things that you cannot observe. So I did not return to the subject for almost ten years.

BRIAN GREENE: Meanwhile, Vilenkin's Russian colleague, Andrei Linde, kept the flame alive. He had independently come up with his own version of eternal inflation, but unlike Vilenkin, he would not be deterred.

ANDREI LINDE (Stanford University): Maybe I am a little bit more arrogant. When I got the idea for this multiverse, I understood that this may be the most important thing which I ever do in my life. And then, if somebody doesn't want to hear it, that's their problem.

BRIAN GREENE: Linde published more than a dozen papers, but his work would meet an equally chilly reception. It seemed no one wanted to hear about the idea of a multiverse.

If the equations of eternal inflation were the only clues pointing to the multiverse, that's where the story might have ended, but the multiverse idea would gain some unexpected support from two completely unrelated areas of science.

One was an idea called string theory, designed to explain how the universe works at the tiniest scales. The other was an astounding discovery made by astronomers exploring the universe on the largest scale, a discovery that's utterly mysterious if there's only one universe. But if we're part of a multiverse, it's a whole new ballgame.

It has to do with the expansion of the universe, and it's easy to explain using a baseball. Now, if I toss this ball up in the air, we all know what will happen. As it rises, it slows down because of gravity.

Now, astronomers knew that the universe was expanding. And they assumed that the expansion would slow down because of the gravitational pull of stars and galaxies, just as the ball slows down because of the gravitational pull of the Earth.

But when they actually did the measurements, they found something astonishing, something that rocked the foundations of physics. They found that the expansion is not slowing down. It's speeding up.

It's as if I took this baseball, and when I throw it, instead of slowing down as it rushes away, it speeds up. Now, if you saw a ball do that, you'd assume there's some invisible force that's counteracting gravity, pushing on the ball, forcing it to speed away ever more quickly.

Astronomers came to the same conclusion about the universe: that some kind of energy in space must be pushing all the galaxies apart, causing the expansion to speed up. Because we don't see this energy, the astronomers called it "dark energy."

RAPHAEL BOUSSO (University of California, Berkeley): It's among the most important experimental discoveries ever, in the history of science.

ANDREAS ALBRECHT: It took most of us completely by surprise.

CLIFFORD JOHNSON (University of California, Berkeley): And so, we're still trying to come to grips with that.

BRIAN GREENE: Discovering that dark energy is pushing every galaxy in our universe away from every other, at an accelerating rate, was shocking enough. But even more surprising was the strength of that dark energy.

For over a decade, scientists have been unable to explain why such a peculiar amount of it exists in empty space. But that mystery seems easier to resolve if we're part of a much larger multiverse.

Now, the idea that space contains any energy at all sounds strange. But our theory of small things, like molecules and atoms, the theory called quantum mechanics, tells us that there's a lot of activity in the microscopic realm, activity that can contribute an energy to space.

And according to the math, the amount of energy generated by that microscopic activity is enormous.

The problem is, when astronomers measured the amount of energy that's actually out there, the amount of energy required to force the galaxies apart at the accelerating rate that's observed, they get a number like this: A decimal point followed by 122 zeroes, and then a one! An incredibly tiny amount, very close to zero, and nothing at all like what the theory predicted. In fact, it's trillions and trillions and trillions and trillions of times smaller, a colossal mismatch.

LEONARD SUSSKIND: We have tried everything to explain why the dark energy is as small as it is. We have tried everything, and everything fails.

STEVEN WEINBERG: Hopeless. I once called this the worst failure of an order of magnitude estimate in the history of science.

DAVID GROSS: Does it bother us? Absolutely!

BRIAN GREENE: Finding that the amount of energy in space is so much less than our theory predicts is not just an academic problem. The precise strength of that repulsive gravity, well, that has profound implications for all of us. For example, if I were to increase the strength of the dark energy just a little bit, by erasing four or five of these zeroes, I still have a tiny number, but the universe would be radically different. That's because a slightly stronger dark energy would push everything apart so fast that stars, planets and galaxies would never have formed. And that means we simply would not exist.

And yet, here we are.

So, why is the amount of dark energy so much less than our theory predicts and also just right to allow the formation of galaxies, stars, planets and life? We just don't know. The mismatch between the theoretical predictions of dark energy and what astronomers have observed is one of the great mysteries that science faces today.

But consider this: if we do live in a multiverse, then the mystery of dark energy might not be so mysterious after all. In fact, if we're part of a multiverse, the value of dark energy we've measured might actually make total sense.

Hi. Reservation for Greene.

To see how the multiverse might solve the dark energy puzzle, imagine you're checking into a hotel, and you get a room number like this: Ten-million-and-one.

Hmmm. Thanks.

DESK CLERK: Enjoy your stay.

BRIAN GREENE: Ten-million-and-one would seem like a pretty strange room number. And getting a room number like this would be surprising, much as the value of dark energy in our universe is a number that scientists have found surprising.

But here's the thing: if this hotel had a huge number of rooms, say, billions and billions, then getting room rumber ten-million-and-one wouldn't be so surprising. In a hotel this big, you expect to find a room with that number.

Similarly, if we're part of a multiverse with a huge number of universes, each with a different value of the dark energy, then you'd expect to find one with the value as small as what we've measured.

If you think of each of these rooms as a universe, and each universe has a different value for the dark energy, then most of these universes won't be hospitable to life as we know it.

The reason is the value of the dark energy wouldn't allow the formation of galaxies, stars and planets. Universes with much less dark energy than ours would just collapse in on themselves, and universes with much more dark energy than ours would expand so fast, that matter would never have the chance to coalesce into clumps and form stars and galaxies.

So, of course, we find ourselves in a universe where the value of the dark energy is hospitable to life. Otherwise, we wouldn't be here to talk about it.

So if we're part of a multiverse, the mystery of dark energy becomes not-so-mysterious. But there's a piece of the puzzle missing. How do we know if there's enough diversity within the multiverse so that every value for dark energy, including the strange value we observe in our universe, can be found somewhere?

The answer would emerge from an entirely different area of physics. I'm talking about a ground-breaking theory that comes from investigating the universe on the tiniest scale.

We know that inside atoms are even tinier bits of matter, protons and neutrons, which are made of still smaller particles called quarks. But physicists realized this might not be the end of the line. These sub-atomic bits might actually be made of something even smaller: tiny vibrating strands or loops of energy called strings. This set of ideas, called "string theory," says everything that exists is made of this one kind of ingredient.

And just as a single string on a cello can produce many different notes depending on how it vibrates, strings can take on different properties depending on how they vibrate, creating many kinds of particles.

From this theory came the promise of elegant simplicity: a single master equation that would explain everything we see in the world around us.

LEONARD SUSSKIND: String theory would be beautiful, it would be elegant; and calculation from that very simple theory would produce the world as we know it.

BRIAN GREENE: But for this beautiful theory to work, there was a catch: the math of string theory required something that defies common sense: a feature that would open the door to the multiverse: extra dimensions of space.

We're all familiar with three dimensions of space: height, width and depth. But the math of string theory says these aren't the only dimensions.

JOSEPH POLCHINSKI (University of California, Santa Barbara): The mathematics works only if the strings move and vibrate, not just in the three directions that we see, but in those and say, six more, nine space dimensions in all.

BRIAN GREENE: So if string theory is right, where are these extra dimensions? And why can't we see them?

Think about the cable supporting a traffic light. From a distance, it looks like a line: one-dimensional. But if you could shrink down to, say, the size of an ant, you'd find another dimension exists, a circular dimension that curls around the cable. And string theory says that if we could shrink down billions of times smaller than that ant, we'd find tiny extra dimensions like this are curled up everywhere in space.

LEONARD SUSSKIND: At every point of space, there's extra dimensions of space that are curled up into little tiny knots that you can't see because they're too small.

BRIAN GREENE: And the shape of those extra dimensions determines the fundamental features of our universe. Just the way the air streams that are going through an instrument, like a French horn, have vibrational patterns that are determined by the shape of the instrument, the shape of the extra dimensions determines how the little strings vibrate. Those vibrational patterns determine particle properties, so all the fundamental features of our universe may be determined by the shape of the extra dimensions.

LEONARD SUSSKIND: The way those extra dimensions of space are put together is, in many respects, like the D.N.A. of the universe. They determine the way the universe is going to behave, just exactly the same way as D.N.A. determines the way an animal is going to look.

BRIAN GREENE: The problem was the more string theorists looked, the more ways they found that extra dimensions could be curled up.

And the math provided no clues as to which shape was the right one corresponding to our universe.

SHAMIT KACHRU (Stanford University): I think the consensus, right now, is that that number seems to be astronomical. There are published papers suggesting upwards of 10-to-the-500—that's 10 followed by 500 zeroes—different possible shapes.

BRIAN GREENE: Ten-to-the-five-hundred different possible shapes for the extra dimensions, each appearing equally valid. It seemed preposterous, especially for a theory that was looking for one single master equation to describe our universe.

But then it occurred to some string theorists that, perhaps, there was a different way to look at the problem, and this different perspective would breathe new life into the idea of a multiverse.

LEONARD SUSSKIND: Ten-to-the-500 different string theories—this sounded like a complete disaster. What good is it to have a theory that has 10-to-the-five-hundred solutions? You can't find anything in there! Well, that left string theorists somewhat unhappy, somewhat depressed.

My own reaction to it at the time is, "This is great. This is fantastic. This is exactly what the cosmologists are looking for: enormous diversity of possibilities. Don't be unhappy about this. This says that string theory fits extremely well with cosmology and with all the interesting ideas about multiverses."

BRIAN GREENE: Turning what seemed like a vice into a virtue, some string theorists became convinced that the multiple solutions of string theory might each represent a real and very different universe. In other words, string theory was describing a multiverse and an extremely diverse one at that.

CLIFFORD JOHNSON: To everyone's surprise, string theory was actually quite readily describing huge numbers of different kinds of solutions which…each of which corresponds to a possible universe.

ANDREI LINDE: So we just got this multiverse for free.

DELIA SCHWARTZ-PERLOV (Tufts University): Both from string theory and from inflation, you have these universes that are produced. These different universes would all naturally have different amounts of dark energy.

BRIAN GREENE: In fact, according to the math, the amount of dark energy would span such a wide range of values from universe to universe that the strange amount we've measured would surely turn up.

RAPHEL BOUSSO: String theory, without even trying, solved that problem.

BRIAN GREENE: So, over a decade after Linde and Vilenkin had come up with their ideas about eternal inflation, the multiverse was revived.

Three lines of reasoning were now all pointing to the same conclusion: eternal inflation, dark energy and string theory. Just the way it takes three legs to support a stool, these three ideas, taken together, support the argument that we may live in a multiverse.

When different lines of research all converge on one idea, that doesn't mean it's right; but when all the spokes of the wheel are pointing at one idea, that idea becomes pretty convincing.

Today the multiverse is hotly debated. Many critics remain. David Gross is going to tell us, "No, no, no." But multiverse advocates, like Alex Vilenkin, Alan Guth and Andrei Linde are no longer alone.

ALEX VILENKIN: The tide appears to be turning. And now these ideas are accepted to a much larger degree.

ANDREI LINDE: The genie is out of the bottle. You cannot put it back.

BRIAN GREENE: So, what would it be like, if we could travel to some of these other universes? What would we see? Some would be vastly different from our own, with properties unlike anything we've ever seen. In fact, some universes in the multiverse might not have light or matter or anything recognizable at all. And there might be other universes with features not unlike the familiar ones we know, but where life takes a completely different form, perhaps communicating in ways we'd find utterly bizarre.

And the math shows that if we were able to visit enough of these universes, we might, eventually, find ones like ours, with a Milky Way galaxy, a solar system and an Earth, except with some slight differences.

In one, maybe the asteroid that killed off the dinosaurs 65 million years ago missed, and evolution charted a new course.

In another, there might be an Earth with people similar to us but better at multitasking.

But there's something even stranger. Somewhere out there, we should find exact copies of our universe, with duplicates of everything and everyone.

How could this be? How could there be exact duplicates of ourselves out there in the multiverse?

To see how, take this deck of cards. It's made up of 52 different cards. And, if I deal them, everyone will get a different hand. But, over the course of many, many rounds, eventually some of the combinations will start to repeat.

SECOND BRIAN GREENE: That's because, with 52 cards, there's a limited number of different hands you can deal.

BRIAN GREENE: So, if you deal the cards an infinite number of times, then repeating hands are inevitable.

THIRD BRIAN GREENE: And in the multiverse, a similar principle applies.

BRIAN GREENE: That's because, according to the laws of nature, the fundamental ingredients of matter, or particles, are kind of like a deck of cards: in any region of space, they can only be arranged in a finite number of different ways.

So if space is infinite, if there are an infinite number of universes, then those arrangements are bound to repeat. And since each one of us is just a particular arrangement of particles,…

SECOND BRIAN GREENE: ...somewhere there's a duplicate of you and me…

ALL THREE BRIAN GREENES: ...and everyone else.

ALAN GUTH: This can be shocking.

SHAMIT KACHRU: It could be that in another universe I was a rock star, and my life is much better—or much worse, depending on your opinion of rock stars.

CLIFFORD JOHNSON: That means all those things that I've never found time to do are maybe being done by some copy of me somewhere else.

ALEX VILENKIN: I was rather depressed actually. This picture robs us of our uniqueness.

LEONARD SUSSKIND: It is a consequence of the ideas, and the ideas seem very well motivated.

BRIAN GREENE: Yet, critics argue the multiverse is just too convenient an explanation for things we don't understand, like the tiny value of dark energy in our universe and the huge number of possible shapes for the extra dimensions in string theory.

PAUL STEINHARDT: The problem with that kind of reasoning is that it doesn't explain why the dark energy is the way it is. It just says it's random chance.

DAVID GROSS: I don't find that satisfactory. You can apply this kind of reasoning anytime you don't have a better explanation.

BRIAN GREENE: On the other hand, supporters of the multiverse point out that sometimes a better or deeper explanation for the way things are simply does not exist.

Take for example, the earth's orbit around the sun. We find ourselves at a distance of 93-million miles, perfect for life. If we were much closer to the sun, our planet would be too hot for life, as we know it, to exist. And if we were much farther from the sun, it would be too cold for life.

So why are we in this sweet spot? Well, starting in the late 1500s, the famous astronomer Johannes Kepler asked that very question. And he spent years trying to find a physical reason, some law of nature that requires the earth to be 93-million miles from the sun. But Kepler never found it, because it doesn't exist. There isn't any physical law requiring the earth to be 93-million miles from the sun.

It's simply one possibility of the many you'd expect to find in a universe we know is full of solar systems.

LEONARD SUSSKIND: You might think it was an extraordinary accident. It's not. It's just that there are a lot of planets out there.

BRIAN GREENE: Similarly, some suggest that the true explanation for many of the fundamental features of our world will elude us, if we don't consider the possibility that we live in a multiverse.

ALAN GUTH: Clearly, if we had a good physical reason, that would be great, and we would understand it. We'd be much happier.

STEVEN WEINBERG: We may have to live with that. There's no principle built into the laws of nature that say that theoretical physicists have to be happy.

LEONARD SUSSKIND: It's a hypothesis. It's the leading hypothesis, because nobody has another hypothesis which makes as much sense.

BRIAN GREENE: The multiverse: a tantalizing possibility. But with no experimental evidence, should you believe it?

We can't believe in anything until there's observational or experimental support. But what we have found over the last few centuries is that mathematics provides a sure-footed guide to the nature of things that we haven't yet been able to see, observe or experiment with.

Math predicted things like black holes and certain subatomic particles long before we ever observed them. And math is suggesting that there may be these other universes. That doesn't mean it's right, but often it's leading you to a deeper understanding of reality.

CLIFFORD JOHNSON: If you choose not to believe it, that's perfectly fine, because we have not given you any evidence yet. And one of the wonderful things about science is that it's about evidence; it's not about belief.

BRIAN GREENE: And some scientists now think we might just be able to find evidence for a multiverse. They propose that if our universe and another were born close together, the two might have collided. That collision could have left its own fingerprint: ripples in the cosmic background radiation, the heat left over from the Big Bang.

LEONARD SUSSKIND: My guess is yes, that in 100 years, we will know one way or another whether these ideas are right.

STEVEN WEINBERG: A hundred years from now it may be an amusing historical episode. We don't know. But if you only work on the things that are already well-established, you're not going to be part of the next big excitement.

BRIAN GREENE: If we do verify the multiverse, it would change our perspective, much as Copernicus did 500 years ago, when he showed that the earth is not the center of the cosmos. And some might say that if our universe is just one of many, our descent from the center would be complete.

DELIA SCHWARTZ-PERLOV: Regardless, I think it's more important is that we're so lucky that we can understand the universe.

ANDREAS ALBRECHT: I think it's a great ride, and I think it's really good for physics that we have this tension. I don't know where we're going to end up.

BRIAN GREENE: So what does this all mean? Are there infinite duplicates of you and me and everything existing right now in an infinite number of other universes?

Is the multiverse the next Copernican revolution? We don't know, at least not yet. But if the idea that we live in a multiverse proves true, we'd be witnessing one of the most exciting and dramatic upheavals to our understanding of the fabric of the cosmos.