To be sure, there is one weak but exciting indication from new results that might be interpreted as signaling a new particle beyond the Standard Model. This is a small excess of collisions which produce pairs of photons that, remarkably, are seen by both of the experiments operating at the LHC. But the statistical significance is not high, given that we are bound to get some signal by random chance in one of the many channels looked at. So this could be a random fluctuation that will go away when more data are taken.
Even if this hint grows into the discovery of a new particle, which would be extremely exciting news, it is too soon to say whether it will lead to a deeper unification or just add complication to the already complicated Standard Model. Luckily, more data can be expected soon.
It’s the same with quantum gravity, the unification of quantum theory with Einstein’s theory of gravity. Many proposals for quantum gravity suggest that at certain very high energy scales we must see new physics. This would indicate that at correspondingly tiny scales space becomes discrete, or new features of quantum geometry kick in. One consequence would be that the speed of light is no longer universal—as it is in relativity theory—but would gain a dependence on energy and polarization visible at certain scales.
In the last decade, this prediction has been tested by sensitive measurements of gamma rays that have traveled for billions of years from extremely energetic events called gamma-ray bursts. If the speed of light depends even slightly on energy, we would see higher-energy photons arriving systematically earlier or later than lower-energy photons; the enormous travel time would amplify the effect. This has been looked for by the Fermi Gamma-ray Space Telescope and other detectors of gamma rays and cosmic rays. No deviations from relativity theory are seen. Thus, our best hope of discovering quantum-gravity physics has been frustrated.
A similar story seems to characterize cosmology. Something remarkable happened in the very early universe to produce a world vast in scale but at the same time extremely smooth and homogeneous. One explanation for this is inflation, a sudden enormous expansion at very early times, but there are competitors. Each of these theories requires delicate fine-tuning of parameters and initial conditions. Once this tuning is done, each predicts a distribution of noisy fluctuations across the smooth universe. The fluctuations show up as a seemingly random distribution of very slightly denser and less dense regions which, over hundreds of millions of years of expansion, amplify and give rise to the galaxies. These fluctuations make bumps that are visible in the cosmic microwave background (CMB) radiation. So far, their distribution is as random, featureless, and boring as possible, and the simplest theories—whether inflation or its alternatives—suffice to explain them.
In each of these domains, we have sought clues from experiments into how nature goes beyond, and solves the puzzles latent in, our incomplete theories of the universe, but we have so far come up with nearly nothing. It’s beginning to seem as if nature is just unnaturally fine-tuned. In my opinion, we should now be seeking explanations for why this might be. Perhaps the laws of nature are not static but have evolved, through some dynamical mechanism, to have the unlikely forms they are observed to have.
One Hundred Years of Failure
Seth Lloyd
Professor of quantum-mechanical engineering, MIT; author, Programming the Universe
The year 2015 marked the hundredth anniversary of Einstein’s announcement of the general theory of relativity. General relativity describes the force of gravity in terms of the curvature of space and time: The presence of matter warps the underlying fabric of the universe, causing light to curve and clocks to slow down in the presence of matter. General relativity supplies us with a physical theory allowing us to describe the cosmos as a whole; it predicts the existence of such exotic objects as black holes; it even supports closed timelike curves that in principle allow travel backward in time. General relativity is a tremendous scientific success story, and its centennial was accompanied by many articles, television shows, scientific conferences, and more to celebrate Einstein’s achievement.
Unmentioned in this celebration was the darker story. As soon as Einstein announced his elegant theory, other physicists began trying to reconcile general relativity with quantum mechanics. Quantum mechanics is the physical theory governing matter at its smallest and most fundamental scales. The last century has seen tremendous advances in its application to the study of elementary particles, solid-state physics, the physics of light, and the fundamental physics of information-processing. Pretty much as soon as the print on Einstein’s papers had dried, physicists began trying to make a quantum theory of gravity. They failed.
The first theories of quantum gravity failed because scientists did not understand quantum mechanics very well. It was not until a decade after Einstein’s results that Erwin Schrödinger and Werner Heisenberg offered a precise mathematical formulation of quantum mechanics. By the beginning of the 1930s, Paul Dirac had formulated a version of quantum mechanics that incorporated Einstein’s earlier—and by definition less general—special theory of relativity. Throughout the next half century, in the hands of physicists such as Richard Feynman and Murray Gell-Mann, this special-relativistic version of quantum mechanics, called quantum field theory, provided dramatic advances in our understanding of fundamental physics, culminating by the mid 1970s in the so-called Standard Model of elementary particles. The Standard Model unifies all the known forces of nature apart from gravity: It has been confirmed by experiment again and again.
What about a quantum theory of gravity, then? After Dirac, when physicists tried to extend the successful techniques of quantum field theory to general relativity, they failed. This time they failed because of a knotty technical problem. One of the peculiarities of quantum field theory is that when you try calculating the value of some observable quantity, such as the mass of the electron, the naïve answer you obtain is infinity.
Looking more closely, you realize that the interactions between the electron and other particles (such as photons, particles of light) have to be taken into account: These interactions “renormalize” the mass of the electron, making it finite. Renormalization works beautifully in the case of quantum field theory, allowing the prediction of quantities such as the mass of the electron to more than six digits of accuracy. But renormalization fails utterly in the case of quantum gravity: Quantum gravity is not renormalizable. Infinity remains infinity. Failure.
More recent decades of failure to quantize general relativity have yielded tantalizing clues. Perhaps the best-known result combining quantum mechanics and gravity is Stephen Hawking’s famous proof that black holes are not absolutely black but in fact emit radiation. Hawking radiation is not a theory of quantum gravity, however, but of quantum matter moving about on a classical spacetime that obeys Einstein’s original non-quantum equations. Loop quantum gravity solves some of the problems of quantum gravity, but exacerbates others: It has a hard time including matter in the theory, for example. Speaking as someone made of matter, I object to theories that do not include it.
One of the primary appeals of string theory is that it naturally contains a particle that could be identified with the graviton, the quantum of gravity. Sadly, even the most enthusiastic followers of string theory admit that it is not yet a fully self-consistent theory but a series of compelling mathematical observations called—with an apparent lack of irony—“miracles.”
Longtime practitioners of quantum gravity have advised me that if one wishes to publish in the field, any advance that claims to improve on one aspect of quantum gravity must be offset by making other problems worse, so that the net effect is negative. If economics is the dismal science, then quantum gravity is the dismal physics.
The last few years have seen a few glimmers of hope, however. Quantum information is the branch of physics and mathematics describing how systems represent and process information in a quantum-mechanical fashion. Unlike string theory, quantum information is in fact a theory: It
proceeds by orderly conjecture and mathematical proof, with close contact to experiment. Quantum information can be thought of as the universal theory of discrete quantum systems—systems that can be represented by bits, or qubits (quantum bits).
Recently, researchers in quantum gravity and quantum information have joined forces to show that quantum-information theory can provide deep insights into problems such as black-hole evaporation, the holographic principle, and the AdS-CFT [anti-de Sitter/conformal field theory] correspondence. (If these subjects sound esoteric, that’s because they are.) Encouragingly, the advances in quantum gravity supplied by quantum-information theory do not yet seem to be counterbalanced by backsliding elsewhere.
We have no idea whether this attempted unification of qubits and gravitons will succeed or fail. Empirical observation of the last century of failure to quantize gravity suggests the latter. With any luck, however, the next hundred years of quantizing gravity will not be so dismal.
Hope Beyond the Higgs Boson
Sarah Demers
Horace Taft Associate Professor of Physics, Yale University
Imagine that a friend you trust tells you a rumor. It’s an unlikely story and they aren’t completely sure of themselves. But a few minutes later, another friend cautiously tells you the same thing. The combination of two similar stories from two reputable witnesses makes you want to explore further. This is what happened on December 15, 2015, when the ATLAS and CMS experiments at CERN announced an initial analysis of their highest-energy run, with the same hint of something interesting in the data. It’s too early to claim a new particle, but the situation in particle physics makes this the biggest news in our recent history.
The Large Hadron Collider’s energy-breaking run was launched with a vengeance in 2010, following an incident that damaged the machine in 2008 and a cautious year in 2009. The 2011–2012 data set delivered the discovery of the Higgs boson. As data streamed in, particle physicists around the world clustered in conversation around espresso machines on this sobering scenario: What if we find the Higgs boson and nothing else? In other words, what if we neatly categorize the particles predicted by the Standard Model, incorporating the Higgs mechanism to provide mass, but make no progress toward understanding the nature of dark matter, dark energy, quantum gravity, or find any clues to explain the 96 percent of the matter/energy content of the universe which isn’t incorporated in the theory?
If physics is a valid framework for understanding the universe, there’s something else out there for us to discover. The prediction of the Higgs, the ultimately successful decades-long campaign to discover it, and the ongoing partnership between experiment and theory to characterize it, gives us confidence in the methods we’re using. But the missing pieces could be beyond our imaginations, the current state of our technology, or both.
The terrifying possibility floating through these “Higgs and nothing else” conversations is that we might reach the end of exploration at the energy frontier. Without better clues to our undiscovered physics, we might not have sufficient motivation to build a higher-energy machine. Even if we convince ourselves, could we convince the world and marshal the necessary resources to break the energy frontier again and continue probing nature under the extreme conditions that teach us about nature’s building blocks?
In 2015, the LHC broke another energy barrier for hadron colliders with a jump from the 8 TeV center-of-mass collisions that produced the Higgs to 13 TeV center-of-mass. The ATLAS and CMS collaborations worked 24/7 to analyze the data in time for a presentation at an end-of-year event on December 15. Both experiments cautiously reported the hint of a new particle with the same signature in the same place. Two photons caught in high-energy collisions can be arranged together to form a mass, as if they originated from the same single particle. Using this technique, both experiments saw a slight clustering of masses—more than what was expected from the Standard Model alone—near 750 GeV/c2. The experimenters are cautious for a good reason. These hints, in the same place for ATLAS and CMS, could disappear with the gathering of data in 2016. If the situation were less dire, this would not be big news. But for me it represents the promise of our current energy-frontier physics program.
Over the next few years, we’ll gather ten times the data at 13 TeV than we currently have, and we have theoretical reasons to expect something right around the corner. If the 750 GeV hint disappears with increased statistics, we’ll keep searching for the next hint that could break open our understanding of nature. We move forward with the next step potentially within our reach, determined to find it, if it’s there.
An Unexpected, Haunting Signal
Gerald Holton
Mallinckrodt Research Professor of Physics, professor of the history of science, emeritus, Harvard University; author, Victory and Vexation in Science: Einstein, Bohr, Heisenberg, and Others
The big news for many in the physics community worldwide last December was that at the collider at CERN an unexpected, haunting signal of a possible new particle was found, one that would not fit into any part of the current high-energy theory. An entirely new region in experiment and theory might open up, making the finding of the Higgs an old story.
But just after this finding, the collider was shut down, as usual, until spring. There ensued a tantalizing wait, in a way analogous to the wartime event when the first nuclear reactor, in Chicago, under Enrico Fermi, got to the brink of criticality late one morning. But instead of continuing, Fermi asked everyone there to wait until he would return from his midday siesta.
News About How the Physical World Operates
Leonard Susskind
Felix Bloch Professor of Theoretical Physics, Stanford University; co-author (with Art Friedman), Quantum Mechanics: The Theoretical Minimum
I’ll try to report the news from the physics front that I think may prove to be important. When I say “important,” I mean to someone interested in how the physical world operates.
First of all, from the experimental front there is news from the CERN Large Hadron Collider—evidence of a new particle. What “new particle” means at this stage is a small bump in a data distribution. It could be real or it could be a statistical fluke, but if real it does represent something new. Unlike the Higgs particle, it is not part of the Standard Model of particle physics. In fact, to my knowledge the new particle does not fit neatly into any theoretical framework, such as supersymmetry or technicolor, and it’s not a black hole or a graviton. So far, it just seems to be an extra particle.
If it’s real and not just a fluke, then there will probably be more particles uncovered, and not only new particles but new forces—perhaps a whole new structure on top of the Standard Model. At the moment, no one has a compelling idea of what it means. It might be connected to the puzzle of dark matter, the missing matter in the universe that seeded the galaxies. The new particle is not itself dark matter—it’s too short-lived—but other, related particles could be.
From the more theoretical side, what I find most interesting is new ideas that relate gravity, the structure of space, and quantum mechanics. For example, there is gathering evidence (all theoretical) that quantum entanglement is the glue holding space together. Without quantum entanglement, space would fall apart into an amorphous, unstructured, unrecognizable thing.
Another idea (Full disclosure: It’s my idea) is that the emergence of space behind the horizons of black holes is due to the growth of quantum complexity. This is too technical to explain here, except to say that it’s a surprising new connection between physics and quantum-information science. It’s not a completely far-fetched idea that these connections may not only teach us new things about fundamental physics problems but also be tools for understanding the more practical issues for building and using quantum computers. Stranger things have happened.
Unpublicized Implications of Hawking Black-Hole Evaporation
Frank Tipler
Mathematical physicist, cosmologist, Tulane University; author, Th
e Physics of Immortality
In 1974, Stephen Hawking proved that black holes were not black. Rather, quantum mechanics required that black holes would slowly lose their mass via a really neat mechanism: The gravity of a black hole would create a pair of particles outside the black hole, one particle with negative mass and the other with positive mass; the former would fall inside the black hole and the latter would move away from the black hole. The net effect would be to decrease the mass of the black hole, and its mass would eventually go to zero.
Hawking realized that a zero-mass black hole was a big no-no, because such an entity could only be a naked singularity that destroyed the information inside the black hole. One of the fundamental principles of quantum mechanics—the same theory that tells us black holes evaporate—is “unitarity,” which means that information is conserved. But if a black hole destroys information in the final stages of its evaporation, then information cannot be conserved. Unitarity would be violated if a black hole were to evaporate completely.
Hawking then made a mistake: he argued that we have to accept a violation of unitarity in black-hole evaporation. But unitarity is a fundamental principle of quantum physics. Unitarity has many implications, one of which is that if unitarity is violated, so is the conservation of energy. And a little violation of unitarity is like being “just a little pregnant”; it has a tendency to get larger—very much larger. Leonard Susskind of Stanford University pointed out that a tiny violation of unitarity would give rise to a disastrous positive feedback of violation of energy conservation: If one were to turn on a microwave oven, so much energy would be created out of nothing—conservation of energy does not hold, remember—that the Earth would be blown apart!
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