Fermilab

Illustration of a result from the CMS experiment at the LHC, gathered on May 27, 2012.

The Higgs Boson…or a Higgs Boson?

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 Yesterday, on March 14, 2013—what would have been Albert Einstein’s 134 th birthday—scientists announced another milestone in the hunt for the Higgs boson. After analyzing all the data from the Large Hadron Collider, they have strong evidence that a Higgs boson—and maybe
 
 the
 
 Higgs boson—has been found. First hypothesized in 1964, the Higgs boson is thought to be the phenomenon that gives mass to all fundamental subatomic particles. Without mass, we would live in a very different place. Electrons, protons, and neutrons wouldn’t combine to form atoms, and you, me, and just about everything else simply wouldn’t exist. This latest announcement is just the next chapter in a story that began eight months ago, on July 4, 2012.


 For Americans, Independence Day is a time for fireworks, family, and barbecues. However last year, the really interesting pyrotechnics were of a scientific nature. They didn’t take place in parks across America, either, but at the CERN laboratory just outside Geneva, Switzerland. There, physicists announced that they had discovered a new particle in data collected using the LHC. As a member of one of the two teams making the announcement, my colleagues and I were thrilled.


 Apparently, we weren’t the only ones. Discovering a new particle doesn’t always draw the attention of the international media, but this breakthrough appeared on the home pages of CNN and the BBC and garnered front page, above the fold coverage in the
 
 New York Times
 
 , the
 
 Washington Post
 
 , and more. After a search nearly half a century in the making, scientists claimed that they may have found the elusive Higgs boson.


 The “may” in that last sentence is important. Eight months ago, what scientists had really announced was that a new particle had been discovered. The particle had some of the expected properties of the Higgs boson, but not all of them had been studied. The discovered particle looked and smelled like the Higgs boson, so to speak, but nobody had been able to touch, taste, or listen to it yet. To be sure that the new particle was the Higgs boson, more work was needed. After the announcement last July, the LHC continued to run. Since then, the amount of data it has produced—and allowed us to analyze—has more than doubled.

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<h2>
 The Nature of Discovery
</h2>

 In order to understand the significance of Thursday’s press release, we need to discuss the discovery process in a little more detail. There are two types of discoveries in science. The first kind is when you encounter something totally unexpected, like a leprechaun on the beaches of Maine. The second is when someone conjures up some theory which makes specific predictions. Until these predictions are confirmed by experiments, they are just unsubstantiated musings. The discovery of the Higgs boson is a discovery of the second type.


 The Higgs boson is named after English physicist Peter Higgs. He wrote a paper in 1964 that predicted an energy field in the universe that gives fundamental subatomic particles their mass. His theory also said that if such an energy field existed, there must be a particle that creates it. That particle is the Higgs boson. The Higgs theory predicted all of the boson’s attributes, except its mass.


 Learn how Peter Higgs' ideas about matter transformed our understanding of space.


 That’s what we’ve been looking for with the LHC. Discovering the Higgs boson’s mass is the key to unlocking more information about the particle, including its production and
 <a href="http://www.youtube.com/watch?v=ktEpSvzPROc">
 decay properties
 </a>
 . In addition, the Higgs boson was predicted to be electrically neutral, have no other subatomic particles inside it, and have a quantum mechanical spin of zero. Spin is a distinguishing feature of subatomic particles, just like electric charge. Typically, fundamental particles have a spin of ½ or 1, but the Higgs boson is unique in having a predicted value of zero.

<h2>
 Fruits of More Data
</h2>

 Last July, when we announced we found a new particle, we only were only able to measure some of the ways in which the particle decayed and the rate at which the various decays occurred. It was for this reason that we claimed that we had found a particle which was “consistent with being a Higgs boson.” We simply had not made enough measurements to be sure. It was entirely possible that we had discovered something else.


 Because of the additional data we’ve analyzed since then, along with improved analysis techniques, we’ve been able to make more precise and a greater range of measurements. It is these advanced studies that have given us far more confidence that we’ve found the Higgs boson. So what have we learned about it?


 Our analysis has told us that the particle has a mass of about 125 times the mass of the proton and no electric charge. The rates at which the particle decays into pairs of other subatomic particles—photons, Z bosons, W bosons, tau leptons, and bottom quarks—are statistically consistent with the predictions of Higgs theory. These are simply improvements beyond what was presented eight months ago.

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 If this particle isn’t the Higgs boson, it would be a really bizarre coincidence.

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 However, the really new result comes from studies of the subatomic spin and parity of the discovered Higgs particle. We knew that, given the ways it decayed, the particle discovered in July 2012 had either a spin of zero or two, though we didn’t know which. We also didn’t know the particle’s parity. But we do now.


 The Higgs boson has zero quantum mechanical spin and specific parity properties meaning that if you reflected the particle in a mirror, it would look exactly the same in the reflection. This property is tricky to visualize, but you can understand it by looking at a mirror with both hands raised in the air. The reflection also has both hands in the air. In contrast, if you had your right hand raised and left hand lowered, in the reflection you would have your left hand raised and right hand lowered. The Higgs boson acts like both hands are raised. Technically, we call a particle with this property as having “positive parity.”


 We have also been able to establish, with good precision, that the particle has a positive parity—exactly as predicted by the Higgs theory. When we combine this with the more precise measurements of the decay and production rates, it is now very clear that if this particle isn’t the Higgs boson, it would be a really bizarre coincidence.

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<h2>
 Could There Be More Than One?
</h2>

 But if the observed particle isn’t the Higgs boson, what could it be? There are several possible answers. For instance, the Higgs theory proposed in 1964 had, as a result, a single Higgs boson, hence why people refer to it as
 
 the
 
 Higgs boson. However we know that the
 <a href="http://www.youtube.com/watch?v=XYcw8nV_GTs">
 Standard Model
 </a>
 , which describes the behavior of known subatomic particles, is incomplete. This model doesn’t explain everything. So theoretical physicists have invented new and (they hope) improved explanations to describe a broader amount of phenomena, including why there may be multiple Higgs bosons. One of these is
 <a href="http://www.pbs.org/wgbh/nova/physics/blog/2012/07/supersymmetry-looking-in-natures-mirror/">
 supersymmetry
 </a>
 .


 Supersymmetry isn’t a theory, as is often assumed by non-experts. Rather, it’s a property of a theory. Technically, a theory is supersymmetric if the equation looks the same when you, in every instance, swap certain terms (those that represent the fermions with spins of ½ and terms that represent the integer-spin bosons, to be specific). There are lots of equations that are supersymmetric.

<div class="pullquote">

 The mass of the Higgs boson—if it is
 
 the
 
 Higgs boson—is relatively small.

</div>

 The simplest of the many supersymmetric theories is called the Minimal Supersymmetric Standard Model, known as MSSM. The MSSM simply adds the absolute minimum number of terms to the regular Standard Model equation to make it supersymmetric. In MSSM, the Higgs theory predicts not one Higgs boson, but five! One of those five has the properties of the Standard Model Higgs boson, meaning that the recently discovered particle may be
 
 a
 
 Higgs boson rather than
 
 the
 
 Higgs boson. My colleagues and I are now working to try to figure out which is the case. The answer to that question will have profound implications for our understanding of the universe.


 Suppose that what we’ve discovered is
 
 the
 
 Higgs boson. What have we learned? Well, several things. It suggests that the universe is in a metastable state. That’s a fancy way of saying “pretty stable.” If the universe were in a stable state, it would never change. If it were in an unstable state, it would change quickly. A metastable state is one that persists for a long while, but eventually changes.


 If the universe is indeed in a metastable state, then in the far future it might transition to a more stable state. When that happens, a small bubble of stability will appear somewhere in the universe and sweep across the cosmos at the speed of light. Where the new stable state exists, the laws of physics would change. Matter as we understand it would no longer exist. I wouldn’t worry about it anytime soon—the Milky Way will be burned out before that happens. But it is a prediction of the Higgs theory and the observed mass.


 The other thing we’ve learned is that the mass of the Higgs boson—if it is
 
 the
 
 Higgs boson—is relatively small, only about a hundred times heavier than the proton. That’s not exactly what the Standard Model predicts. Instead of several hundred times heavier than a proton, the Standard Model Higgs boson should be more than a trillion, trillion, trillion times heavier. It will take us quite a while to solve this
 <a href="http://www.pbs.org/wgbh/nova/physics/blog/2013/02/why-is-the-higgs-so-light/">
 huge disagreement between measurement and theoretical prediction
 </a>
 . But when we do, will have a better idea of the ultimate rules that make up the cosmos.

<h2>
 Live To Collide Another Day
</h2>

 Fortunately, we’ll have a few years to mull over our theories and adjust them to the new data. The LHC shut down in December of 2012 for refurbishment and upgrades that are expected to take two years to complete. In late 2014 or early 2015, we expect operations will resume at even higher energy and with much brighter particle beams. My colleagues and I can’t wait. There are a lot of mysteries we hope to solve by studying what is now looking like
 
 a
 
 —if not
 
 the
 
 —Higgs boson.


 Photo © 2012 CERN, for the benefit of the CMS Collaboration

Is it the only one? And how will it change our understanding of the universe?

Sterile Neutrinos: The Ghost Particle's Ghost

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 What do you call the ghost of a ghost?
 

 If you’re a particle physicist, you might call it a “sterile neutrino.” Neutrinos, known more colorfully as “ghost particles,” can pass through (almost) anything. If you surrounded the Sun with five light years’ worth of solid lead, a full half of the Sun’s neutrinos would slip right on through. Neutrinos have this amazing penetrating capability because they do not interact by the electromagnetic force, nor do they feel the strong nuclear force. The only forces they feel are the weak nuclear force and the even feebler tug of gravity.
 
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/xmm-newton_perseus-620.width-990_X1fEBmR.jpg"/>
<figcaption>The Perseus galaxy cluster, one of 73 clusters from which mysterious x-rays, possible produced by sterile neutrinos, were observed. Credit: Chandra: NASA/CXC/SAO/E.Bulbul, et al.; XMM-Newton: ESA
 
 </figcaption>
</figure>

 When
 <a href="http://www.aps.org/publications/capitolhillquarterly/201110/physicshistory.cfm">
 Wolfgang Pauli first postulated neutrinos
 </a>
 in 1930, he thought that his proposed particles could never be detected. In fact, it took more than 25 years for physicists to confirm that neutrinos—Italian for “little neutral ones”—were real. Now, physicists are hunting for something even harder to spot: a hypothetical ghostlier breed of neutrinos called sterile neutrinos.
 

 Today, we know of three different “flavors” of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos (and their antimatter equivalents). In the the late 1960s, studies of the electron-type neutrinos emitted by the Sun led scientists to suspect that they were somehow disappearing or morphing into other forms. Measurements made in 1998 by the
 <a href="http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html">
 Super Kamiokande
 </a>
 experiment strongly supported this hypothesis, and in 2001, the
 <a href="http://www.sno.phy.queensu.ca/sno/first_results/">
 Sudbury Neutrino Observatory
 </a>
 clinched it.
 

 One of the limitations of studying neutrinos from the Sun and other cosmic sources is that experimenters don’t have control over them. However, scientists can make beams of neutrinos in particle accelerators and also study neutrinos emitted by man-made nuclear reactors. When physicists studied neutrinos from these sources, a mystery presented itself. It looked like there weren’t three kinds of neutrinos, but rather four or perhaps more.
 

 Ordinarily, this wouldn’t be cause for alarm, as the history of particle physics is full of the discovery of new particles. However, in 1990, researchers using the
 <a href="http://home.web.cern.ch/about/accelerators/large-electron-positron-collider">
 LEP accelerator
 </a>
 demonstrated convincingly that there were exactly three kinds of ordinary neutrinos. Physicists were faced with a serious puzzle.
 

 There were some caveats to the LEP measurement. It was only capable of finding neutrinos if they were low mass and interacted via the weak nuclear force. This led scientists to hypothesize that perhaps the fourth (and fifth and…) forms of neutrinos were
 
 sterile
 
 , a word coined by Russian physicist Bruno Pontecorvo to describe a form of neutrino that didn’t feel the weak nuclear force.
 

 Searching for sterile neutrinos is a vibrant experimental program and a confusing one. Researchers pursuing some experiments, such as the
 <a href="http://www.aps.org/units/dnp/research/lsnd.cfm">
 LSND
 </a>
 and
 <a href="http://www.fnal.gov/pub/science/experiments/intensity/miniboone.html">
 MiniBoone
 </a>
 , have published measurements consistent with the existence of these hypothetical particles, while others, like the Fermilab
 <a href="http://www.symmetrymagazine.org/article/june-2014/minos-result-narrows-field-for-sterile-neutrinos">
 MINOS
 </a>
 team, have ruled out sterile neutrinos with the same properties. Inconsistencies abound in the experimental world, leading to great consternation among scientists.
 

 In addition, theoretical physicists have been busy. There are many different ways to imagine a particle that doesn’t experience the strong, weak, or electromagnetic forces (and is therefore very difficult to make and detect); proposals for a variety of different kinds of sterile neutrinos have proliferated wildly, and sterile neutrinos are even a credible candidate for dark matter.
 

 Perhaps the only general statement we can make about sterile neutrinos is that they are spin ½
 <a href="http://www.particleadventure.org/fermibos.html">
 fermions
 </a>
 , just like neutrinos, but unlike “regular” neutrinos, they don’t experience the weak nuclear force. Beyond that, the various theoretical ideas diverge. Some predict that sterile neutrinos have
 <a href="http://arxiv.org/pdf/1303.6912v3.pdf">
 right-handed spin
 </a>
 , in contrast to ordinary neutrinos, which have only left-handed spin. Some theories predict that sterile neutrinos will be very light, while others have them quite massive. If they are massive, that could explain why ordinary neutrinos have such a small mass: perhaps the mathematical product of the masses of these two species of neutrinos equals a constant, say proponents of what scientists call the “see-saw mechanism”; as one mass goes up, the other must go down, resulting in low-mass ordinary neutrinos and high-mass sterile ones.
 

 Now, some astronomers have proposed sterile neutrinos could be the source of a mysterious excess of x-rays coming from certain clusters of galaxies. Both NASA’s Chandra satellite and the European Space Agency’s XMM-Newton have spotted
 <a href="http://www.nasa.gov/chandra/news/mysterious-xray-signal.html">
 an excess of x-ray emission at 3.5 keV
 </a>
 . It is brighter than could immediately be accounted for by known x-ray sources, but it could be explained by sterile neutrinos decaying into photons and regular neutrinos. However, one should be cautious. There are tons of atomic emission lines in this part of the x-ray spectrum. One such line, an argon emission line, happens to be at 3.62 keV. In fact, if the authors allow a little more of this line than predicted, the possible sterile neutrino becomes far less convincing.
 

 Thus the signal is a bit sketchy and could easily disappear with a better understanding of more prosaic sources of x-ray emission. This is not a criticism of the teams who have made the announcement, but an acknowledgement of the difficulty of the measurement. Many familiar elements emit x-rays in the 3.5 keV energy range, and though the researchers attempted to remove those expected signals, they may find that a fuller accounting negates the “neutrino” signal. Still, the excess was seen by more than one facility and in more than one cluster of galaxies, and the people involved are smart and competent, so it must be regarded as a possible discovery.
 

 It is an incredible long shot that the excess of 3.5 keV x-ray from galaxy clusters is a sterile neutrino but, if it is, it will be a really big deal. The first order of business is a more detailed understanding of more ordinary emission lines. Unfortunately, only time will tell if we’ve truly seen a ghost.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/neutrino/">
 The Ghost Particle
 </a>
 
 Learn more about neutrinos at this companion site to NOVA’s 2006 film “The Ghost Particle.”
 

 Résonaances:
 <a href="http://resonaances.blogspot.com.es/2014/02/signal-of-neutrino-dark-matter.html">
 Signal of neutrino dark matter
 </a>
 
 Particle physicist Adam Falkowski comments on the possible link between sterile neutrinos and the observed galactic cluster x-ray signal.
 

 Scientific American:
 <a href="http://www.scientificamerican.com/article/cosmic-mismatch-hints-at-the-existence-of-a-sterile-neutrino1/">
 Cosmic Mismatch Hints at the Existence of a “Sterile” Neutrino
 </a>
 
 Clara Moskowitz explains how sterile neutrinos could resolve a clash between measurements of the cosmic microwave background radiation and galaxy clusters.
 
</section>

Have astronomers spotted traces of sterile neutrinos, hypothetical particles that could solve some cosmic riddles?

How Blue LEDs Work, and Why They Deserve the Physics Nobel

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 In the early hours of October 7, 2014, while most of America slumbered, members of the Swedish Academy of Sciences were meeting to decide to whom they should award the Nobel Prize in Physics. The Nobel Committee often recognizes grand discoveries, like the prediction of the Higgs boson and the observation of the accelerating expansion of the universe. Thus many were surprised to see the 2014 prize honor a very practical invention: blue light emitting diodes. Yet this year’s awardees, Shuju Nakamura, Isamu Akasaki and Hiroshi Amano, richly deserve the prize.
 
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 The technical achievement
 


 LEDs are one of many useful spinoffs from the basic science of quantum mechanics. In the standard quantum cartoon, electrons orbit the center of atoms, kind of like planets around the Sun. (The planetary model is not completely accurate, but it has many good features that help visualize what is going on in the quantum realm.) Imagine a simple atom with a single electron. According to quantum mechanics, the electron can be in a series of discrete orbits, as if a single planet around our Sun could be in the orbit of Mercury or Venus or any of the planets, but nowhere in between. Electrons near the nucleus have lower energy, while ones farther away have more energy.
 

 When an electron moves from a high-energy orbit to a lower-energy one, it emits energy in the form of light. The color of the emitted light depends on the energy difference between the old orbit and the new one. The bigger the energy difference, the bluer the light.
 

 With a few tweaks, these basic ideas also apply to groups of atoms, like those which make up molecules and solid materials. Which brings us to LEDs. To make an LED, you must fuse together two different types of semiconductor, one with electrons to spare (“n-type”) and one with extra room to hold those electrons (“p-type”). When the proper voltage is placed across the diode, electrons can move and fall into the holes, releasing energy. Early diodes only released a little energy, so they emitted infrared light. To make a diode that could produce visible light, scientists had to widen the energy gap. By the early 1970s, scientists had discovered how to make bright red, yellow, and green light by choosing the proper semiconductor materials and judiciously spiking, or “doping,” them with impurities.
 

 But scientists struggled to make blue light, which requires a very high energy gap. Finally, in 1994, Shuji Nakamura, then employed by the Nichia Corporation, developed high-brightness blue LEDs using indium gallium nitride (InGaN), a mix of gallium nitride and indium nitride. By adjusting the amount of indium in the semiconductor, he tuned the energy gap to produce blue light.
 

 Nakamura wasn’t the first to attempt to use gallium nitride to create LEDs, however most solid state physicists of the era had moved on to different materials. First, nobody knew how to prepare a surface on which gallium nitride crystals could grow, and further, nobody knew how to make p-type layers of GaN. Isamu Asaki and Hiroshi Amano showed it could be done using sapphire as a substrate and were eventually able to create the required p-layer of the material. Quite by accident, they also discovered that a scanning beam microscope increased the brightness of the light emitted by LED.
 

 Nakamura grew his own GaN crystals and developed a simpler method for making the p-type layers using careful heating. He was also the first to understand why the electron beam boosted the light output of LEDs: it was removing hydrogen, just as his own heating technique did.
 

 Modern blue LEDs require a more sophisticated approach, including varying the amount of indium and gallium, although the basic technique is the same as Nakamura’s. Starting with a sapphire substrate, several alternating layers of gallium nitride are added, some doped with indium and others doped with aluminum. These extra elements are key to increasing the efficiency and brightness of the blue LEDs. Further, with the introduction of aluminum, it is possible to make even bluer LEDs—even ultraviolet ones.
 


 The greatest benefit on mankind
 


 To understand why this seemingly mundane development warrants such recognition, one must return to Alfred Nobel’s will, in which he provided the seed money to start the prizes that bear his name. When Nobel’s brother died, a French newspaper mistakenly published Alfred’s obituary. Alfred Nobel was horrified to see himself called a “merchant of death” and a man “who became rich by finding ways to kill more people faster than ever before.” He resolved to repair his legacy by bequeathing a prize to be awarded to “those who…have conferred the greatest benefit on mankind.” Nobel wanted to be remembered as a man who helped make the world a better place.
 

 This year’s award clearly fits the bill. “I really think that Alfred Nobel would be very happy about this prize,” said Per Delsing, head of the Nobel Committee for Physics, in announcing the prize. ”It’s really an invention and it’s really something that will benefit most people.”
 

 The invention of bright blue LEDs brought entirely new industries into existence. Now, blue, red and green LEDs could be combined to make white—or any other color—light. This development led to the power-efficient screens for cell phones, TVs, computers, iPads, and many other electronic miracles of the modern world.
 

 However the real impact of blue LEDs goes well beyond our rainbow-colored gadgets. Today, LEDs are bright enough to use as light sources. Just as Thomas Edison’s original incandescent light bulb revolutionized the early years of the 20
 
 th
 
 century, LEDs are poised to revolutionize the 21
 
 st
 
 .
 

 LEDs can now emit far more light using far less power than incandescent and fluorescent bulbs. For instance, an incandescent light bulb can emit about 16 lumens per watt of electrical power, and a fluorescent light manages about 70 lumens per watt. In comparison, a modern white LED can emit 300 lumens per watt, meaning that it consumes only about 5% of the power of an incandescent. Given that about a quarter of the world’s electricity is used to generate light, the invention of efficient lighting can have considerable economic and environmental impact, potentially reducing the greenhouse gas emissions that are driving anthropogenic global warming. In the developing world, bright, efficient LEDs could provide solar-powered, off-grid energy for homes, hospitals, and more.
 

 This year’s Nobel Prize in physics is very well deserved and reflects Alfred Nobel’s desired legacy of recognizing discoveries and developments that have greatly benefited mankind.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 bockphysics:
 <a href="https://www.youtube.com/watch?v=ksS-FJ3B8Og">
 Primary Colors of Light – Mixing of Colors
 </a>
 
 Watch as blue, green and red light combine to make white light in this video.
 

 HowStuffWorks:
 <a href="http://electronics.howstuffworks.com/led.htm">
 How Light Emitting Diodes Work
 </a>
 
 Discover more about the physics and chemistry of LEDs in this step-by-step introduction.
 

 Nobelprize.org:
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular-physicsprize2014.pdf">
 Blue LEDs – Filling the world with new light
 </a>
 
 Detailed information on the honorees, their work, and its impact, from the Royal Swedish Academy of Sciences.
 
</section>

Discover the physics behind blue LEDs, honored with the 2014 Nobel Prize in Physics.

Journey Into the Dark Realm

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 After nearly a century of observations, astronomers have concluded that the type of matter that makes up you and me amounts to just a scant 5% of the recipe of the universe. A ghostly form of matter called dark matter is five times more common than our familiar atoms. True to its name, dark matter emits no light; we “see” it only indirectly, by measuring its gravitational pull on ordinary atoms. So how do we know it’s really there? To be sure, we need to detect dark matter directly.
 
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<figcaption>Flickr/luxdarkmatter under a creative commons license
 
 </figcaption>
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 Physicists have been searching for dark matter particles for decades now. Some experiments
 <a href="http://en.wikipedia.org/wiki/Cryogenic_Dark_Matter_Search#Results">
 seem
 </a>
 to have
 <a href="http://www.newscientist.com/article/dn20434-second-experiment-hints-at-seasonal-dark-matter-signal.html#.Ut7Uf_Yo7zI">
 caught them
 </a>
 while other, equally powerful experiments have failed to find any evidence for dark matter. Most recently, the ultra sensitive
 <a href="http://lux.brown.edu/LUX_dark_matter/Home.html">
 LUX detector
 </a>
 , a vat of liquid xenon buried in a mile-deep underground lab, found no evidence for dark matter and ruled out earlier measurements that had reported hints of a signal. Does this mean one or more of these results is wrong? Not necessarily. There are ways for both the LUX measurement and earlier measurements to be true, but this requires that dark matter and ordinary matter interact with each other in very specific, unexpected ways. Scientists are exploring these possibilities.
 

 At the same time, physicists are beginning to think a bit more creatively. Until now, scientists looking for dark matter have imagined that dark matter is very simple. Specifically they imagine that there is just single type of dark matter particle: electrically neutral, experiencing only the weak and gravitational forces and with a mass 10-1000 times that of a proton. This model is popular because it is simple. On the other hand, the universe is not obliged to honor our definition of simplicity.
 

 Suppose someone was studying the behavior of ordinary matter using only gravity as a probe. They’d no doubt construct a simple model of matter as a particle that was something like a neutron. However, we know that our world is very complex, that the neutron is just one member of the particle zoo and that these particles can come together in all sorts of interesting ways. Scientists are beginning to wonder if maybe dark matter might be similar.
 

 Perhaps dark matter isn’t just one particle but a diverse realm of dark matter particles that experience forces that don’t affect ordinary matter. These dark matter particles might interact fairly strongly with each other, but only weakly with ordinary matter. With little experimental evidence to guide them, theoretical physicists are allowed to speculate fairly freely, although there are some constraints imposed by astronomical observations.
 

 One idea postulates a dark equivalent to electrical charge called “dark charge.” Just as ordinary electrons and positrons (antimatter electrons) can interact with each other and emit photons, it is possible that particles carrying dark charge can interact and produce dark photons.
 

 It is crucial to remember that dark charge, if it exists, does not interact with ordinary matter except by way of gravity and maybe the familiar weak force. A dark matter particle carrying dark charge and a familiar particle carrying electrical charge would pass by one another without so much as a “how do you do?”
 

 If a complicated dark sector exists, we can see it only if there is a particle that interacts with both ordinary matter and dark matter. If we could create such a messenger particle and allow it to interact with astronomical dark matter or (more likely) decay into dark matter particles, we might be able to detect it at particle accelerators like the LHC. But there’s a catch: The experimental signature would be “missing” energy in some collisions as the energy flowed into what physicists call the dark sector, the enigmatic realm of dark matter and dark energy. Given that disappearing energy is a fairly common feature of particle collisions (e.g. when neutrinos are created), it would be tricky to pin it on the creation of dark matter messenger particles. But by measuring the distribution of “missing” energy in LHC collisions and comparing it to the predictions of known physics and theoretical models of dark matter particles, it might be possible to catch a glimpse of the dark sector.
 

 Of course, missing energy is just one possible signature of a complicated dark sector. Another possibility invokes the principle of
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/07/supersymmetry-looking-in-natures-mirror/">
 supersymmetry
 </a>
 , which postulates that every known fundamental subatomic particle has a (so far undiscovered)
 <a href="http://www.youtube.com/watch?v=0CeLRrBAI60">
 cousin
 </a>
 with a different quantum spin. Were the LHC to create these theoretical supersymmetric particles in a collision, they would decay into low-mass supersymmetric particles capable of interacting with the complex dark matter sector. After another cascade of decays, a dark matter particle could emit a messenger particle that “sees” both dark matter and ordinary matter and then decay in turn into a matter-antimatter particle pair that could be picked out in the collider data. Because this scenario postulates both supersymmetry
 
 and
 
 complex dark matter, it is even more of a jump into the unknown. But given that we don’t understand a lot of the universe, sometimes wild ideas are required. As Niels Bohr once quipped to Wolfgang Pauli, “We are all agreed that your theory is crazy. The question which divides us is whether it is crazy enough.”
 

 So far, physicists have not found evidence for a complex dark sector, but the search has just begun. Ordinary matter is complex, so it seems very reasonable that the dark sector should be, too. Over the next several years, theorists will begin to flesh out a myriad of dark possibilities, including possibly even dark atoms, just in time for the LHC to turn back on and see if the data supports these interesting ideas.
 


 Go Deeper
 
 

 Author’s suggestions for further reading
 


 arXiv:
 <a href="http://arxiv.org/abs/1311.0029">
 Dark Sectors and New, Light, Weakly-Coupled Particles
 </a>
 
 A technical paper summarizing the motivation for and possible tests of dark sector theory.
 

 Preposterous Universe:
 <a href="http://www.preposterousuniverse.com/blog/2013/03/12/more-messy-dark-matter/">
 More Messy Dark Matter
 </a>
 
 Astrophysicist Sean Carroll blogs about the possibility that dark matter is more “interesting” than we thought.
 

 Sanford Underground Research Facility:
 <a href="http://sanfordlab.org/news/press_release/first-results-lux-experiment-south-dakota">
 First results from LUX experiment in South Dakota
 </a>
 
 A press release outlining the results of the LUX experiment’s first, three-month-long search for dark matter.
 
</section>

Dark matter may not be one type of particle, as physicists long thought, but rather a diverse realm with many varieties.

Quantum Foam, Virtual Particles and Other Curiosities

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 Quantum physicists regularly ask you with a straight face to accept what seems to be complete nonsense. Particles are also waves; cats are alive and dead at the same time. But some of the most incredible creatures of the quantum realm get far less attention than Schrödinger’s famous cat. They’re called virtual particles, and they might be the reason the universe exists in the first place. In the pencast below, I’ll explain the basics of virtual particles. Then read on to learn more.
 
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 While the Big Bang theory explains how the universe has expanded and cooled since it began, it is quite silent on what “pulled the trigger,” so to speak. We simply don’t know what started the process. How there could be nothing at one moment and an entire baby universe the next?
 

 It turns out that getting something from nothing is just business as usual for virtual particles. The most straightforward way to explain virtual particles is by an example. Consider a particle collision in which one electron hits another and the two scatter. In the classical view, the electric field from one electron interacts with the other and the two feel a repulsive force. However, this approach neglects Einstein’s Nobel Prize realization that light—and, by extension, every electromagnetic field—is quantized. So a quantum treatment of electron scattering needs to include not only the quantum nature of the electrons, but also the quantum nature of the photon. We now treat electron scattering as the two quantized electrons exchanging a quantized photon and, in the process, changing their directions.
 

 So how do virtual particles enter in? Well, you can calculate the properties of the photon that must be emitted to scatter the electrons. Simple energy and momentum conservation considerations tell us what the energy and momentum of the photon must be. However, when you do the calculation, you find that the photon has a mass! Since photons are massless particles, this seems to invalidate the whole idea. It sure sounds like physicists are pulling your leg, just to see how long it will be before somebody is willing to say that the subatomic Emperor has no clothes.
 

 As crazy as this seems, it is true. To see how, we need to invoke another hard-to-swallow axiom of quantum mechanics: the Heisenberg Uncertainty Principle, named after its inventor, Werner Heisenberg. In classical physics, energy and momentum are always conserved. But Heisenberg spotted a loophole in this rule: in the quantum realm, energy and momentum don’t have to be conserved, as long as the non-conservation doesn’t persist for very long. It’s kind of like having a shady accountant. If you audit the books, the amount of money you send him has to agree exactly with the amount of money he uses to pay your bills. But, while he has your funds, he is free to temporarily lend or borrow money so that momentarily he will have the “wrong” amount of money. Further, the larger the amount of money loaned or borrowed, the shorter the period of time it will occur. Similarly, in the quantum realm, energy and momentum can briefly be “wrong,” but the larger discrepancy, the shorter the period of time for which it is allowed.
 

 So in our example of electrons scattered by exchanging photons, the photon can briefly have the “wrong” amount of energy and momentum. Now, it is understandable if you find this a bit hard to take; perhaps an instance of physicists making stuff up to save their theories. And, truth be known, that would be my reaction if there were not an extensive list of experimental measurements that demonstrate that virtual particles exist. In fact, virtual particles play a critical role in most of the experiments performed at large particle physics laboratories like CERN, Fermilab and many other similar facilities.
 

 While I’ve described the idea of a single virtual particle, the idea is actually much richer than that. Virtual particles also exist in association with real particles. For instance, suppose you have an ordinary, garden-variety, electron. A reasonable mental image of the electron would be a little subatomic marble, carrying electrical charge, mass and spin. Anyone with even a cursory understanding of quantum mechanics know that image is a bit dodgy, as electrons exhibit lots of crazy quantum behavior.
 

 The life of an electron is much more complex than that, though. In addition to the usual quantum craziness, where an electron is both a particle and a wave and the position of the electron is generally indeterminate, electrons are surrounded by virtual particles. For instance, an electron can briefly emit a photon. That photon will be reabsorbed quickly in such a way that the energy and momentum conservation laws aren’t violated. But it gets crazier than that. The virtual photon can also turn into a virtual electron/positron pair. Thus, for a brief moment, what was once just an electron becomes an electron plus an additional electron and positron. As long as the virtual particles coalesce before the universe notices, it’s all within the rules. Indeed an electron never exists as a single “bare” electron. Rather, it is always enshrouded in an ephemeral cloud of virtual particles, flickering in and out of existence, and vastly complicating what an electron “really” is.
 

 It might seem far-fetched, but experiments can actually detect the presence of this cloud. That is because every electron acts like a mini-magnet. We can calculate exactly how strong the magnet should be. But when we make very precise measurements of its strength, we find that the measured magnetic moment is about 0.1% off from the simple prediction. It turns out that when you take into account the virtual cloud around the electron, it exactly matches this small 0.1% discrepancy, showing that the cloud is definitely present. Further, the data and prediction exactly match to nine digits!
 

 If your mind isn’t blown, wait…it gets crazier still. Empty space—that is, space that contains nothing—no energy, no charge, no matter, nothing—is filled with a writhing, active population of virtual particles that physicists call “the quantum foam,” with bubbles appearing and popping in wild abandon. At the subatomic level, space is never truly empty.
 

 You’d think that if empty space were filled with a constant roiling boil of quantum activity, you’d see it. The fact that you don’t could give you yet more reason to disbelieve, yet the effects of the quantum foam have been directly observed.
 

 The first observation of the quantum foam came from tiny disturbances in the energy levels of the electron in a hydrogen atom. A second effect was predicted in 1947 by Hendrik Casimir and Dirk Polder. If the quantum foam was real, they reasoned, then the particles should exist everywhere in space. Further, since particles also have a wave nature, there should be waves everywhere. So what they imagined was to have two parallel metal plates, placed near one another. The quantum foam would exist both between the plates and outside of them. But because the plates were placed near one another, only short waves could exist between the plates, while short and long wavelength waves could exist outside them. Because of this imbalance, the excess of waves outside the plates should overpower the smaller number of waves between them, pushing the two plates together. Thirty years after it was first predicted, this effect was observed qualitatively. It was measured accurately in 1997.
 

 Quantum foam also has astrophysical implications. In 1974, Stephen Hawking was thinking about quantum mechanics and black holes. He realized that the quantum foam would exist near the event horizon of the black hole. If an electron/positron virtual pair popped into existence just outside the event horizon, one of the two particles might spiral down and get trapped in the black hole, while the other would escape. As it happens, more energy would escape than be captured, so the energy of the black hole would get slightly smaller. Over the eons, this “Hawking Radiation” would cause the black hole to evaporate until it totally disappeared.
 

 Virtual particles and the quantum foam are one of the craziest of the quantum phenomena. They have no classical analog and they certainly seem like something that physicists dreamed up to save the counterintuitive world of quantum mechanics. Borrowing from the movie “The Maltese Falcon,” quantum mechanics is said to be the dreams that stuff is made of, but virtual particles are no dreams. They have been experimentally observed, and indeed it could be that a quantum fluctuation similar to virtual particles was the thing that pulled the trigger on the creation of the universe itself: a crazy start for a universe where, we’re learning, the bizarre is the norm and dreams are reality.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


<a href="http://www.amazon.com/Alice-Quantumland-Allegory-Quantum-Physics/dp/0387914951">
 Alice in Quantumland: An Allegory of Quantum Physics
 </a>
 
 Join Alice on an illustrated adventure in quantum mechanics. By physicist Robert Gilmore.
 

 Of Particular Significance:
 <a href="http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/">
 Virtual Particles: What are they?
 
</a>
 Theoretical physicist Matt Strassler presents an alternative explanation of virtual particles in this blog post.
 

<a href="http://www.amazon.com/QED-Strange-Princeton-Science-Library/dp/0691125759/ref=dp_ob_title_bk">
 QED: The Strange Theory of Light and Matter
 </a>
 
 Physicist Richard Feynman’s classic introduction to quantum electrodynamics offers a more advanced description of the field.
 
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The Dark Energy Survey: What Happens When You Can See Forever?

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 A new eye is now open to the cosmos. The
 <a href="http://www.darkenergysurvey.org/DECam/camera.shtml">
 Dark Energy Camera
 </a>
 , which saw first light on September 12, 2012, is a spectacular new scientific facility with the grandest of goals: no less than understanding the evolution and fate of the entire universe.
 

 For every telescope, “first light” is the moment when the optics and camera are assembled into a single instrument and turned to the night sky for the first time. But first light is just the beginning. While it often yields a spectacular photo or two, single photographs rarely lead to substantive results. Modern measurements require a subtle understanding of the equipment’s idiosyncrasies and the operators and scientists must spend a while familiarizing themselves with their instrument’s performance. After the facility has been put through its paces, real research begins. On January 9, Joshua Frieman, leader of the
 <a href="http://www.darkenergysurvey.org/index.shtml">
 Dark Energy Survey
 </a>
 (DES) collaboration,
 <a href="http://astro.uchicago.edu/~frieman/Talks/Frieman-DES-AAS-press-briefing-1-2013.pptx">
 announced
 </a>
 at the 221st meeting of the American Astronomical Society in Long Beach, California, that the team is wrapping up this getting-to-know-you phase, known as the commissioning period. They have already made interesting scientific observations, including discovering distant supernovae and clusters of galaxies.
 

 The 570 megapixel Dark Energy Camera is hooked up to the venerable four-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory, located in the Chilean Andes. Together, they will complete a study of the sky called the
 <a href="http://www.darkenergysurvey.org/">
 Dark Energy Survey
 </a>
 , which may bring us closer to an answer to one of the deepest mysteries in cosmology: What is dark energy?
 

 This question has been vexing astronomers since 1998, when astronomers discovered that, contrary to their expectations, the expansion of the universe wasn’t slowing down—
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html">
 it was speeding up
 </a>
 ! Cosmologists accounted for this surprising behavior by invoking a form of repulsive gravity first imagined by Einstein. But Einstein abandoned the idea when Hubble’s observation of the expanding universe made it seem unnecessary. Today, in the absence of a specific explanation, astronomers describe it with the generic term “dark energy.”
 

 The Dark Energy Survey will help scientists probe the nature of dark energy. Over the course of 525 nights over five years, astronomers will survey a quarter of the southern sky to a depth of billions of light years, revealing the how the cosmic expansion rate has changed over nearly nine billions of years.
 

 The Dark Energy Survey studies the universe in four distinct ways:
 
<ol>
<li>
 It looks for 4,000 distant supernovae. By comparing their distance (determined by simultaneously observing their brightness and their redshift, the change in their color due to the expansion of the universe, and comparing these two numbers), astronomers will get a good handle on the cosmic expansion history.
 </li>
<li>
 The camera will also study patterns in the spatial distribution of galaxies that are set by a phenomenon called baryonic acoustic oscillations. When the universe was smaller and hotter, the explosion of the Big Bang caused the universe to ring like a bell as the sound of the Big Bang rippled across the cosmos. About 370,000 years after the Big Bang, the universe cooled below a critical temperature, freezing these vibrations into patterns we can still see in microwave radiation and distribution of galaxies that are blazoned across the sky. This process is analogous to flash freezing the ripples on the surface of a pond. By comparing the apparent size of the ripples with their initial size, which can be calculated using information about the conditions that prevailed in the early cosmos, astronomers can provide crucial data on the shape of space itself: whether it is flat or curved and, if curved, exactly how.
 </li>
<li>
 The camera will also have the capacity to study the size and makeup of vast clusters of galaxies. Because the properties of dark energy help determine how and when these clusters formed, by studying their history, we can gain new insight into dark energy.
 </li>
<li>
 Finally, the Dark Energy Camera will see how light from distant clusters of galaxies is being bent by mass between those clusters and our telescopes. This information will tell us more about how dark energy has shaped the distribution of matter throughout the universe by studying the size and shape of clusters of galaxies over time. In total, the camera will be able to track three hundred million galaxies!
 </li>
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 Through these four distinct strategies—each with different strengths and weaknesses—the survey will provide independent measurements of the dark energy of the universe.
 

 The portion of the sky that the DES will study in detail is observable from Chile from September to February. Since first light, the collaboration has put their equipment through its paces. To get an early glimpse at a complete set of data, the DES collaboration will spend the rest of the 2012-2013 observation season studying a little under 5% of the region they will eventually explore. Using this strategy, they will have as good a measurement on a small portion of the sky after just a few nights of observation as they will over their entire target after five years. This will allow a relatively quick analysis of a subset of the sky and the caliber of this small study will already be world-class. The final shakedown is expected to be completed in February and in September of 2013, the survey will start in earnest, hopefully leading to new insights into the nature of dark energy.
 

 Stay tuned. This is a very exciting time.
 
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 The center of the Milky Way galaxy lends its awesome beauty to the skyline above the telescope domes at Cerro Tololo. The Greater and Lesser Magellanic Clouds grace the upper left corner of the photo. (Photo credit: Reidar Hahn)
 
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 Go Deeper
 
 

 Editor’s picks for further reading
 


 Baryon Oscillation Spectroscopic Survey:
 <a href="http://astro.berkeley.edu/~mwhite/bao/">
 Baryon acoustic oscillations and dark energy
 </a>
 
 On this web page, learn more about how baryon acoustic oscillations can be used to study the history of dark energy.
 

 Minute Physics:
 <a href="http://www.youtube.com/watch?v=v6o2bUPdxV0">
 2011 Nobel Prize: Dark Energy
 </a>
 
 In this video explainer, guest narrator Sean Carroll explains dark energy and cosmic expansion in honor of the 2011 Nobel Prize in physics.
 

 NOVA scienceNOW:
 <a href="http://www.youtube.com/watch?feature=player_embedded&amp;v=6ypfFz38Lg4#!">
 Cosmic Perspective: Dark Matter
 </a>
 
 In this short video, astrophysicist Neil deGrasse Tyson muses on just how much we don’t know about the mysterious components of the universe, dark energy and dark matter.
 
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The Nobel Prize in Physics: Quantum Mechanics Comes of Age

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 Quantum mechanics is one of the most devilishly confusing theories ever devised. Cats that are simultaneously alive and dead, objects that are both particles and waves, subatomic particles that know whether you are looking at them or not—and, most bafflingly, these quantum effects can be erased when individual atoms, electrons, and photons interact with their environment.
 

 That is what makes Serge Haroche and David Wineland’s Nobel Prize-winning work in physics so remarkable: They have achieved mastery of the microrealm. Both of them have spent decades trying to generate systems in which a single atom or a single photon can be studied.
 

 David Wineland, at the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, is an expert at trapping individual atoms using electric fields and by keeping them in an ultra-high vacuum. The mastery of individual atoms is possible by artfully employing laser beams and laser pulses. The laser beams can cool the motion of the atoms and even can transfer quantum information about the atom’s location to the location of electrons inside the atom. This is an extraordinary achievement.
 

 Serge Haroche, of the Collége de France and the Ecole Normale Supérieure, Paris, essentially does the opposite. He uses atoms to study individual photons. Using superconducting niobium, he creates the two most reflective mirrors ever achieved. With the mirrors placed about an inch apart, he introduces a single photon, which bounces back and forth for over a tenth of a second until it eventually hits an imperfection in the mirror and is absorbed. While the photon is bouncing, it travels a distance equivalent to circling the entire globe.
 

 In order to measure the photon, Haroche fires single rubidium atoms through his equipment. These atoms are of a special class called “Rydberg atoms,” in which the electrons “orbit” very far from the atomic nucleus. (Though we now know that atoms do not operate as mini solar systems, the analogy of orbits can still be a useful one.) By measuring the configuration of the Rydberg atom before and after it travels through his apparatus, Haroche can determine if there is a photon inside his equipment without absorbing or altering the photon.
 

 These techniques have made it possible to probe quantum mechanics in more detail than ever before, with Haroche’s work making it possible to effectively make a movie of the transition of a photon from one state to another, a process that scientists call “the collapse of the wave function.” But these two scientists’ work has more practical applications. For instance, if we are able to put equipment into a quantum state and read that state without destroying it, this opens the possibility of
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/02/herding-schrodingers-cats/">
 quantum computing
 </a>
 . The potential power of quantum computing is enormous and if we are able to actually accomplish it, this will change computing in the same degree that ordinary computing has changed the world since the 1940s. Quantum computing is still a ways in the future, but Haroche and Wineland’s work has brought it closer to fruition.
 

 Wineland’s work has also made possible a new generation of clocks that are 100 times more accurate than the
 <a href="http://www.pbs.org/wgbh/nova/physics/amazing-atomic-clock.html">
 best timekeepers in the world
 </a>
 . These new clocks are precise to one part in 10
 
 17
 
 . To give some context, if these clocks were started when the universe began 13.7 billion years ago, by now they would be off by a mere four seconds. Such accuracy is useful for communication and navigation, and could also enable even more stringent tests of Einstein’s theory of general relativity, which states that time runs slower in stronger gravitational fields. When people think about this effect, they usually invoke the mind-bending gravitational fields surrounding black holes, but these new clocks are so precise that the effect of time dilation due to gravity would be obvious if one of them were raised a mere foot off the surface of the Earth.
 

 Haroche and Wineland’s work is of the highest caliber, with potential society-changing implications. In this year’s Nobel Ceremony on December 10, they will rightfully join their peers in the pantheon of great scientists.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 Minute Physics:
 <a href="http://www.youtube.com/watch?v=2dRr-fnPCwM">
 2012 Nobel Prize: How Do We See Light?
 
</a>
 In this video, learn more about how Serge Haroche uses atoms to study individual photos.
 

 Nobel Prize:
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/popular-physicsprize2012.pdf">
 Particle control in a quantum world
 </a>
 
 Explore Haroche and Wineland’s Nobel Prize-winning work in this popular-level article.
 

 Nobel Prize:
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2012/advanced-physicsprize2012.pdf">
 Measuring and manipulating individual quantum systems
 </a>
 
 Explore Haroche and Wineland’s Nobel Prize-winning work in this technical-level article.
 
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Don Lincoln

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