There is geometrical symmetry. The human body, for instance, has one kind of geometrical symmetry: The left and right side of our bodies are pretty much the same. A typical starfish has a five-fold symmetry, meaning that if you rotate its body through 1/5 of a circle, it looks like it wasn’t rotated at all. A circle has even deeper symmetry: No matter how much you rotate it, the rotated circle looks just like the old one.
There are many kinds of symmetries, from the left-right symmetry of a face (left), to the five-fold symmetry of a starfish (center), to the complete symmetry of a circle (right). These sorts of symmetries are also seen in physics theories, leading some physicists to describe the equations as beautiful.
Symmetry also has an aesthetic meaning, although this is harder to define; artistic symmetry is beauty found in a pleasing and regular form.
Both of these definitions of symmetry have some place in the meaning used by physicists. Equations are geometrically symmetric if they can be “flip flopped” without changing their meaning. For instance, take the simple sum 3 + 4. If we swap the order, we get 4 + 3. Both of these equations equal 7 and we can thus say that addition is symmetric in this case. Of course, not all equations are symmetric when the order is swapped. For example, in subtraction 4 – 3 isn’t the same as 3 – 4.
These simple symmetries give us an insight into more complex symmetries. These more complex symmetries have a huge impact on theoretical physics. To understand how that is true, we must turn to a physicist who may not be a household name, but should be.
Emmy Noether has been called the most influential woman in mathematics. In an era when women were often expressly forbidden from the academic world, she won the highest respect of leading scientists and mathematicians, including Albert Einstein and David Hilbert.
Before Noether, scientists noticed that certain things, like energy and electrical charge, were “conserved.” That is, the amount of energy in a system is the same before and after an event like a collision. Similarly, electrical charge might move around, but the total charge remains the same. (Note that this only works in “closed” systems, which aren’t gaining or losing energy or charge to external sources.) Exactly why these things were conserved wasn’t understood, but these conservation laws were (and are) taught in all introductory physics classes.
Noether connected these conservation laws with mathematical symmetries that could be expressed in equations. She saw that each symmetry implied a physically conserved quantity. If an equation was unchanged if you swapped it from one point in time to a different point in time, this meant that energy was conserved. If an equation was unchanged if you changed a position with a different position, momentum was conserved.
This observation was a brilliant revelation. Conservation laws weren’t an unexplained phenomenon. They were the measurable manifestation of symmetries in the laws governing the universe. The beauty of the cosmos was the beauty of symmetry.
Noether’s theorem led theoretical physicists to explore the idea of symmetry in natural law more fully, leading to a deeper appreciation of the role of symmetry in the rules that govern the cosmos. Now the symmetry of a particular theory is among the first things physicists consider as they evaluate its merit.
If you talk to a physicist—especially a theoretical physicist—about modern theories and why they are the way they are, the scientist may well wax lyrical about the beauty and simplicity of the equations. Symmetry is the basis for this aesthetic judgment. You need not be a physicist to see the beauty of the stars glittering in a dark midnight sky, the allure of a shimmering rainbow and the delicacy of a snowflake, yet they, too, are inscribed in the symmetry of written formulas, there for all to see—once you know how.
On July 4, 2012, the CERN auditorium was full. That’s not unusual; the room often hosts scientific presentations to packed houses. What was unusual was that this seminar was watched by millions of people worldwide, including reporters from high-impact media outlets like BBC, CNN, and The New York Times.
So what was the announcement that caused a hectic world to briefly pause and listen? A new subatomic particle had been discovered, and its properties were consistent with those predicted for the long-sought Higgs boson. The Higgs boson, if it exists, is the experimental evidence needed to confirm the existence of the Higgs field, which is thought to give fundamental subatomic particles their mass.
Physicists were careful to not claim that they had conclusively discovered the Higgs boson. The Higgs boson was predicted in 1964 to have a litany of very specific properties. Until scientists are able to demonstrate that the newly-discovered particle matches all of the predictions, there remains the possibility that the new particle is something wholly unexpected. Of the properties that had been tested prior to the seminar, all of them pointed to this being the Higgs, which is why scientists said “consistent with the Higgs boson.” Using a metaphor involving the senses, what was found looked and smelled like the Higgs boson, but nobody had been able to taste, feel and touch it. So some uncertainty remained. This uncertainty still remains today, and it will be some time before scientists can definitively state that the observed particle was the Higgs boson.
But let’s imagine that the discovered particle, which is a boson of mass about 125 times that of the proton, is the Higgs boson. What then?
You’d think scientists would celebrate (and we did…more than a few champagne corks were popped), but once the confetti settled, there were some furrowed brows. Nobody understood why the mass of the Higgs boson was so low. Here’s the source of the conundrum.
A Higgs boson doesn’t always exist as a Higgs boson. Like other quantum particles, it can change forms. For instance, it can briefly convert into a pair of top quarks before coalescing back into a Higgs boson. These evanescent top quarks are called “virtual particles” and are just an example of the several kinds of particles into which a Higgs boson can temporarily fluctuate. So, if you want to predict the mass of the Higgs, you have to take all of these possible forms into account.
Higgs bosons can spontaneously convert into pairs of other subatomic particles. These pairs exist only for a very short time, but their existence will alter the mass of the Higgs boson.
Mathematically, we split the mass of the Higgs into two parts: its “theoretical” mass—that is, the mass it would have if didn’t fluctuate into different particles—plus the effect of the fluctuations. (For the technically brave, I put the equation that describes this in a footnote1.)
To make things even more complicated, the effect of the fluctuations also comes in two pieces. These two terms are multiplied, not added, together. The first term involves the maximum energy for which the Higgs theory applies. This works out to be a huge number, about 1038 GeV2.
The second term is, roughly speaking, the sum of the effect of the bosons (W, Z & Higgs) minus the sum of the effect of the fermions (top quark). Let’s call this the fermion/boson sum.
So, let’s take a birds-eye view of the whole equation. The mass of the Higgs is equal to the theoretical mass plus a monstrously large number multiplied by the fermion/boson sum. Unless the fermion/boson sum is practically zero, the observed mass of the Higgs boson should be huge.
The only way to escape this conclusion is to somehow balance the fermion/boson sum to be exceedingly small. And to have the balance so perfect is utterly unnatural, as if we added up all the monthly paychecks of everyone in the United States and subtracted their monthly bills and those two huge numbers canceled out neatly.
That doesn’t happen in bookkeeping, and it shouldn’t happen in physics, either; unless, that is, there is some new and as-yet-undiscovered physical principle that enforces it. Thus, the small mass of the Higgs boson all but ensures that there is new physics to be discovered. Otherwise, we have to “tune” the masses of these particles to very precise values. Such precise balancing is utterly unnatural in physics theories, leading theoretical physicists to propose a series of ways in which this cancellation could occur naturally.
The most popular is a principle called supersymmetry. At the core of supersymmetry is the idea that, for every known fermion (quarks and leptons), there is a cousin boson (called squarks and sleptons) that we haven’t yet discovered. Similarly, for every known boson (e.g. photon, W, Z, gluon and Higgs boson), there is a cousin, also-undiscovered, fermion (called a photino, wino, zino, gluino and Higgsino). Because every fermion has a cousin boson (and vice versa), the fermion/boson sum is identically zero. Each particle cancels out exactly the effect of the cousin particle predicted by supersymmetry.
There are many technical issues that need to be addressed, not the least of which is that the predicted cousin particles have never been observed. But, so far, scientists can get around that problem. Thus supersymmetry remains an interesting idea.
If the particle found in July of 2012 is the Higgs boson, it definitely brings with it a very puzzling problem. As physicists begin to accept that the Higgs boson has likely been found, they are turning their attention to this most unnatural quandary. The main focus of the LHC is now becoming a search for a natural solution to this difficult question: Why is the Higgs so light?
The actual equation is the following: Mass(Higgs, observed)2 = Mass(Higgs, theoretical)2 + [k Λ]2 × [Mass(Z boson)2 + 2 × Mass(W boson)2 +Mass(Higgs, theoretical)2 – 4 × Mass(top quark)2 ]. k is a technical constant and Λ is the maximum energy that the theory applies.
A new eye is now open to the cosmos. The Dark Energy Camera, 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 Dark Energy Survey (DES) collaboration, announced 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 Dark Energy Survey, 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—it was speeding up! 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:
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.
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.
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.
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!
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.
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)
Go Deeper Editor's picks for further reading
Baryon Oscillation Spectroscopic Survey: Baryon acoustic oscillations and dark energy
On this web page, learn more about how baryon acoustic oscillations can be used to study the history of dark energy.
Minute Physics: 2011 Nobel Prize: Dark Energy
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: Cosmic Perspective: Dark Matter
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.
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.
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.
Of Particular Significance: Virtual Particles: What are they? Theoretical physicist Matt Strassler presents an alternative explanation of virtual particles in this blog post.
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 quantum computing. 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 best timekeepers in the world. These new clocks are precise to one part in 1017. 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.
Look out solid, liquid and gas: There’s a new form of matter in town. Actually, this “new” matter isn’t new at all—it is one of the most ancient forms of matter in the universe. Last seen more than 13 billion years ago, just millionths of a second after the Big Bang, this exotic stuff is making a comeback thanks to particle accelerators like the Relativistic Heavy Ion Collider (RHIC) on Long Island and the Large Hadron Collider (LHC) in Europe, where physicists can generate temperatures of more than a trillion degrees centigrade. These enormous temperatures allow scientists to push back the clock of the cosmos and witness matter in the extreme energy environment that existed within microseconds of the Big Bang.
At such high temperatures, the protons and neutrons inside atomic nuclei literally melt, releasing the quarks and gluons inside them and creating a form of matter called a quark-gluon plasma. You can think of the quarks as the “matter” particles and gluons as the particles of force that hold the protons and neutrons together. A reasonable mental image of a proton or neutron would be like a few flecks of Styrofoam (quarks) inside a lottery ball machine. The wind in the lottery machine is analogous to the force field, while the air molecules represent the gluons.
Under ordinary conditions, quarks and gluons are forever locked inside protons and neutrons. They’re like the water held frozen into ice cubes in a glass. But just as ice can be melted into water when energy is added to the system (by pouring hot tea over them, for instance) allowing the molecules from one ice cube to mix with molecules from other cubes, so too it is possible to melt protons and neutrons and have the quarks and gluons scamper around willy-nilly.
To melt protons and neutrons, you need to heat them up to about a trillion degrees. The only way to generate this kind of temperature is to smash together atomic nuclei at high velocities in huge particle accelerators. That’s what physicists are now doing at the LHC and the RHIC, accelerators that take atoms (lead and gold, as well as some others), strip off all of the electrons, and then slam the bare atomic nuclei together. The most violent of these collisions can heat up the nuclear matter enough to free the quarks and gluons to wander as they will. Though experimental calibration issues add some uncertainty to the mix, the current temperature record seems to belong to the ALICE experiment at the LHC, which measured an astounding 5.5 trillion degrees centigrade.
Studying the phase transitions of quark-gluon plasma allows us to understand the behavior of matter in the early universe, just fractions of a second after the Big Bang, as well as conditions that might exist inside neutron stars. The fact that these two disparate phenomena are related demonstrates just how deeply the cosmic and quantum worlds are intertwined. Credit: Brookhaven National Laboratory
While the first lab-made quark-gluon plasma was created in 2000, physicists are only now beginning to understand how this form of matter behaves. At the LHC and RHIC, they are mapping out in more detail the temperatures and pressures at which ordinary matter transforms into quark-gluon plasma. They are also tracing the boundaries between quark-gluon plasmas and even more exotic forms of matter like the stuff of neutrons stars, which is thought to be so dense that, at the center of the stars, quarks get “smooshed” together into an exotic kind of solid.
In fact, physicists believe that there are many different phases of matter involving quarks. While I’ve focused on two states of matter, atomic nuclei and quark-gluon plasma, this just scratches the surface of the possible.
By studying quark-gluon plasma, physicists are able to explore a period in the history of the universe that has thus far eluded us—a period in which protons and neutrons, the basic ingredients of ordinary matter, were coalescing for the first time. Thanks to accelerators like the LHC and the RHIC, we are finally beginning to probe this pivotal chapter in the story of cosmos.
The recent buzz over the discovery of a new boson that might be the long-sought quantum of the Higgs field has led some to forget that the Large Hadron Collider at CERN isn’t a single-purpose facility. Two large experiments each engage approximately 3,000 physicists in a concentrated effort to better understand the rules that govern the universe in which we live. These collaborations can study many different phenomena. One of the most tantalizing of these phenomena is called supersymmetry—and it could pick up where the Higgs left off.
There are many mysteries remaining in physics. While the Standard Model states that the Higgs boson gives subatomic particles their mass, it is quite silent on the specific masses held by each particle. Further, it doesn’t explain why so many different types of particles exist, nor does it explain why there are three forces and not two or 20.
Subatomic particles have a property called spin which can be usefully (and misleadingly) imagined as each particle being a tiny spinning ball, though the reality is that spin is an inherent property of these particles in the same way that electric charge is. Particles are divided into two classes based on their spin: Particles with half-number spin (1/2, 3/2, 5/2 and so on) are called fermions, and particles with whole-number spin are called bosons.
Supersymmetry proposes a new rule to govern the relationship between fermions and bosons. According to supersymmetry, the equations that describe the universe should work in exactly the same way if all fermion and boson terms are swapped. This implies that, for every particle known in the Standard Model, there should be an as-yet-undiscovered cousin particle. These cousin particles are identical to the known particles in every way except that they have different spin.
If supersymmetry is right, then the existing fermion quarks have cousin bosons called “squarks”; the lepton has a supersymmetric cousin called a slepton. For bosons, the naming convention is a little different: The bosons of the standard model (the gluon, photon and W and Z boson) have supersymmetric fermion cousins called the gluino, photino, wino and zino.
Though none of these particles has yet been observed, their very obscurity does offer us one important insight: If supersymmetry exists, it is not, in fact, symmetric. Recall that I said that the supersymmetric cousins of the familiar particles of the Standard Model were the same in every way except for their spin. This means that the selectron would have the same mass as the familiar electron and the up squark would have the same mass of the up quark. However, were this true, we would have discovered them already. Given that we haven’t, we can categorically say that supersymmetry in its ideal form has already been falsified.
However it could be that supersymmetry is mostly true, but “broken.” In the same way that an imperfect top might spin reasonably well, only to wobble a bit and end with a preferred side always touching the ground when it stops, perhaps the universe might have a supersymmetry that is mostly true. Just what mechanism breaks the symmetry between the Standard Model particles and the supersymmetric cousins is not known, although many ideas have been proposed.
So with this additional consideration, you are to be forgiven if you are suspicious of the whole idea. What is the reason for the interest in the idea of supersymmetry? Why have over ten thousand scientific papers (both experimental and theoretical) been written on the subject?
While there are several reasons to find the idea intriguing, one topical example is the way in which supersymmetry is thought to be linked to the Higgs boson. While we remain unsure if the boson we found in July is the Higgs boson, the new boson has a mass of about 125 GeV, or about 133 times heavier than a proton. This is an utterly unnatural value for the mass of the Higgs boson.
Why unnatural? The Higgs boson gains its own mass (in part) through its interaction with the other subatomic particles: the quarks and leptons and force carrying bosons. These particles should have a huge influence on the mass of the Higgs—on the scale of 1015 GeV. That’s over ten trillion times the observed mass of the new boson. So why isn’t the Higgs weighing in at that enormous mass?
First, we are helped because the contribution from the fermions and bosons are of opposite sign, so they can cancel each other out. But without invoking supersymmetry, it seems pretty suspicious that they would be so close in value. It’s uncanny, like a big bank simultaneously taking in a deposit of about a trillion dollars and making a loan of almost exactly the same amount down to a few bucks.
Supersymmetry can explain this quite easily, though. After all, for each particular fermion (say an electron), there is a corresponding boson (a selectron). Given the symmetry and the fact that fermions and bosons contribute with opposite signs, it is easier to see how these two corresponding particles could cancel each other out exactly. If supersymmetry were in fact perfectly symmetric, they would cancel each other perfectly and mass of the Higgs boson would be caused solely by its interaction with other Higgs bosons.
This example is but one in the myriad of phenomena which can be explained by supersymmetry. You should remember that we don’t know that supersymmetry is actually present in the universe; just because it works on paper doesn’t make it real. It makes it a cool idea. However scientists at the Large Hadron Collider are hot on supersymmetry’s trail. If supersymmetry is the answer to why the mass of the Higgs boson is small but not zero, we will find it at the LHC.
Scientific American: Is Supersymmetry Dead?
Davide Castelvecchi asks what it will mean for physics if the LHC does not turn up evidence of supersymmetry.
This week, we’ve come one step closer to understanding the rules that govern the universe. Two days ago, my colleagues at Fermilab announced our final results in a search for the answer to a mystery nearly 50 years old. In an intellectual tour de force, the CDF and my own DZero experiments analyzed a decade of data, combining dozens of hints that together tell an interesting tale. This announcement was an aperitif for an even more dramatic statement made today.
The construction of the CMS detector at the LHC. CMS is one of the detectors involved in the hunt for the Higgs. Credit: Mark Thiessen/National Geographic Society/Corbis
As physicists gathered in Melbourne, Australia, for the International Conference on High Energy Physics, one of the most anticipated conferences of the year, the two large collaborations at CERN made an extraordinary announcement. In back-to-back seminars held at CERN and simulcast to the conference (and the world), the leaders of two different experiments, CMS and ATLAS, gave strong evidence that we found something that can’t be explained by well-understood physics—something which could (and it’s worth emphasizing the “could”) be the Higgs boson.
The Higgs boson is the missing piece in the current best model of the universe, the Standard Model. In the Standard Model, building blocks called quarks and leptons are held together by the four known forces: gravity, electromagnetism and the strong and weak nuclear forces. Using these basic ideas, physicists can explain most of the measurements we have made. But one thing we have not been able to explain is one of the most fundamental and vexing questions in physics: Why do those building blocks have mass?
In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. The search for the Higgs boson is an energetic activity, directly involving as many as six thousand physicists—myself included—and the most powerful particle collider on Earth, the Large Hadron Collider (LHC) at CERN.
One of the fantastic benefits of being a physicist doing research at CERN and Fermilab is that I have been privileged to see this discovery evolve with an insider’s perspective in more than one world-class experiment and in collaboration with some of the finest minds on the planet. Over the past few years, we have searched through the data at both laboratories. Our measurements so far have shown where the Higgs boson isn’t. The results released today may finally show where it is.
The first tantalizing suggestions of the Higgs came in December of 2011, when scientists working with CMS and ATLAS announced that their data contained hints that the Higgs boson might be starting to show its face, and that it could have a mass about 125 times heavier than a proton. However, neither experiment had enough data to claim a discovery—or even to be certain that they were seeing anything at all.
In March, the search picked up again. This time, though, the LHC’s energy level and beam intensity were dialed up. If the LHC had been making Higgs bosons before, it would be making even more of them now—about 25% more, depending on the boson’s mass. The CERN management made their plans for 2012 so that both CMS and ATLAS would have enough “beam time” to independently discover the Higgs boson—if, that is, our hypotheses about its mass and other properties were correct. However, given the intellect and work ethic of the scientists involved, nobody really thought it would take the whole year to see a signal that “looked like” a Higgs boson, although proving anything we found was the actual Higgs boson predicted by the Standard Model could well take the entire years’ worth of data.
By June of this year, both LHC experiments had already recorded as much data in 2012 as in all of 2011. The accelerator and its detectors were performing superbly. Now the race was on to be the first to finish the job and find—or rule out—the Higgs boson.
ATLAS and CMS won’t find the Higgs itself, though; it disappears too quickly, decaying into other subatomic particles. It’s those particles that we’re looking for in the ATLAS and CMS data. Depending on the true mass of the Higgs boson, it could decay in several different ways. Seeing an excess of these decay products is an indication that we might have discovered the Higgs.
And that’s what we found! In the shrapnel of the LHC’s powerful collisions, the CMS experiment detected more pairs of photons and Z bosons than we can explain without some new kind of physics appearing. CMS also looked for supporting evidence in predicted decays to bottom quarks, W bosons and tau leptons. The ATLAS experiment also found an excess of events decaying into two photons and two Z bosons, but the ATLAS did not announce the results of their investigations into other decay modes.
To be certain that we didn’t adjust our analysis techniques to produce a preconceived result, we did the searches “blind,” meaning that we designed the analysis before we looked at the relevant data. This was especially important given that we saw hints in December 2011. We didn’t want that information to bias our searches in any way. That way, if the 2012 data told the same story as that of 2011, it would tell us something about the universe and not ourselves.
When all of our results are combined, CMS claims to have found a new boson with a mass of 125 GeV (or about 133 times heavier than a proton) and a statistical significance of about five sigma (which means that this result could happen 1 time in 3.5 million by accident), while ATLAS’ measurement indicates the existence of a particle with about the same mass (126 GeV) and the same statistical significance. While both experiments’ results are significant individually, the fact that both experiments are announcing similar observations and the 2011 and 2012 measurements are compatible lends tremendous credence to today’s announcement.
It is very important to stress that neither experiment team has claimed to have observed the Higgs boson. They have observed something without a doubt, but the Standard Model Higgs boson is a very specific thing. To be sure we’re seeing the Higgs boson and not a lookalike, we need to see it in all of the predicted decay modes.
For instance, the Higgs theory makes specific predictions about the relative probabilities of the Higgs decaying into pairs of bottom quarks, tau leptons and a whole myriad of possibilities. If all of the predicted possibilities aren’t seen, or aren’t seen in the right ratio, it might be that what we’re observing isn’t the Higgs boson after all. Furthermore, the Higgs boson is predicted to have exactly zero quantum mechanical spin. Until those and other properties are confirmed, it is possible that the experiments might be picking up traces of something entirely different. So, although what has been observed is consistent with being a Higgs boson, these measurements cannot rule out some other possibilities. In fact, this announcement is not the end of the story but rather the very beginning.
An invisible army of black holes? A cosmic graveyard of burned-out stars? A swarm of rogue planets that roam the depths of interstellar space? While examples of objects like these have been observed, we now know that they can’t account for the enormous mass of dark matter required to explain why galaxies rotate so fast. Following Sherlock Holmes’ dictum that once you have ruled out the impossible, whatever remains, however improbable, is the answer, scientists have been forced to conclude that dark matter is an entirely new form of matter, never before observed.
Here is what we think: Every galaxy is engulfed by a cloud of dark matter particles that extends far beyond that galaxy’s visible edge. Each dark matter particle is electrically neutral and has a mass tens or thousands of times that of the familiar proton. Finally, there is a lot of this dark matter. Our best estimate is that there is about five times as much dark matter as there is luminous matter, making our visible universe a thin frosting on a dark matter cake.
But physicists will need to observe dark matter first-hand before anyone should believe it is real. Our search for dark matter takes three distinct approaches: direct, indirect, and production—that is, actually making our own dark matter particles.
The search for dark matter rests on a three-legged stool, with direct, indirect and collider experiments all promising approaches to find it. Credit: Don Lincoln/Fermilab
The direct approach starts with a detector cooled to more than 459 degrees below zero Fahrenehit, so close to absolute zero that the atoms that make up the detector are nearly stationary. The detector is buried as much as a mile underground to protect it from ordinary cosmic rays, high-energy particles that are constantly bombarding the Earth. Though these detectors can’t actually “capture” a dark matter particle, should one happen to pass through and collide with the nucleus of an atom inside the detector, the detector will ring like a bell and the passage of the dark matter particle will be observed.
There are dozens of experiments underway using this approach, including one, called the DAMA (DArk MAtter) experiment, that has made a provocative finding. Scientists think that dark matter flows past the solar system like a wind, so DAMA uses the motion of the Earth around the Sun to winnow out a dark matter signal. For half a year, the Earth is moving into the dark matter wind, and for the other half, it is moving with the wind. Therefore, we expect to see an annual variation in the number of dark matter hits. This is exactly what DAMA has seen for many years now.
The problem is that other experiments which are nominally more sensitive don’t see this annual variation. This has led to considerable confusion and it will take additional work to understand if DAMA has seen the first hints of dark matter or merely an unexplained measurement artifact.
Indirect searches exploit the notion that dark matter might consist of both a matter and antimatter component. If so, occasionally a pair of matter and antimatter dark matter particles might meet and annihilate each other in a flash of gamma rays or matter/antimatter pairs that can be observed by satellites that are designed to detect gamma rays or antimatter in the cosmos. In fact, two such experiments, PAMELA and GLAST, have observed signals that could be the signature of dark matter, but could also have more prosaic explanations. Meanwhile, other experiments see no such signals.
Rather than waiting for dark matter to come to us, though, some physicists are hoping to make their own dark matter right here on Earth. Currently the only particle accelerator capable of making dark matter is the Large Hadron Collider at CERN. By exploiting Einstein’s famous equation E = mc2, we hope to convert the kinetic energy of the beams directly into dark matter. Because dark matter is electrically neutral, it would escape our detectors undetected, but upon adding up the energy contained in all the particles that we can detect, we would notice that the energy books are unbalanced and that some energy is missing.
The scientists working on two of the LHC’s detectors, the ATLAS (A Toroidal Large Apparatus) experiment and my own CMS (Compact Muon Solenoid), are now searching their data tirelessly, looking for collisions with these characteristics. The situation is evolving rapidly as the LHC delivers a torrent of particles to the detectors.
It’s a race between the three different approaches to see which one will be the first to observe a reliable signature of dark matter. No one should be the slightest bit convinced until at least two of the approaches begin to tell a consistent story. One thing is certain; with five times as much dark matter as ordinary matter, the race is on for discovery and Nobel Prize glory.
Go Deeper Editor's picks for further reading
CERN Courier: Shedding Light on Dark Matter
In this article, learn how scientists are using the CMS instrument at the LHC to look for signs of dark matter.
NOVA scienceNOW: Dark Matter Mystery
In this video, host Neil deGrasse Tyson reports from a half mile underground in an abandoned mine, where scientists are using special detectors to look for dark matter particles.
Scientific American: Instant Egghead: Dark Matter
Scientific American editor George Musser explains dark matter in 60 seconds using a coffee cup, some crumbs, and a compact disk.
Astronomers have a pretty sweet job. They are paid to stare at the heavens and wonder. Some of their observations are pretty ordinary, but some observations are revolutionary—like the measurements of galaxy rotation that convinced astronomers that our universe is studded with invisible mass called dark matter. In this pencast, I will explain how that apparently simple observation led astronomers to such an extraordinary conclusion.
When astronomers watch rotating galaxies and compare their observations with predictions based on Newton’s laws of gravity, they find something strange. Stars near the center of galaxies are well behaved and move as expected. However stars farther from the center are rebellious. They move far faster than the laws of physics predict they should; so fast, in fact, that these galaxies shouldn’t exist: They should be ripped apart. Since we know that galaxies have existed for billions of years, this is a glaring paradox.
This conundrum nagged at scientists for over half a century. Astronomers proposed many solutions, from suggestions that our understanding of inertia is wrong to new ideas of how gravity works. But the likeliest explanation is that galaxies contain more matter than we see.
When I say “see,” I don’t mean just “seeing” with our eyes or even with the familiar telescopes that are sensitive to visual light. I mean “seeing” with any and every kind of telescope in our arsenal, including the huge antennas that pick up radio emission from the vast clouds of hydrogen that typically make up most of the mass of galaxies.
To acknowledge the fact that this proposed extra matter is invisible to our ordinary methods of detection, we call it “dark matter.” We know it’s out there, but what is it? Come back next week for more about the quest to capture traces of dark matter here on Earth.
Go Deeper Editor's picks for further reading
American Museum of Natural History: Vera Rubin and Dark Matter In this profile, learn how astronomer Vera Rubin's galaxy observations helped establish the presence of dark matter.
NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.
Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His book "The Quantum Frontier" tells the tale of the Large Hadron Collider, the world’s preeminent particle accelerator, while his other book "Understanding the Universe" introduces the armchair scientist to particle physics and cosmology and tells how the two fields are intertwined.
