Meanwhile, interest in a related astrophysical neutrino signature also reached a new high during this period. The Sun produces neutrinos due to the nuclear reactions in its core that power it, and over twenty years, using a huge underground detector, Ray Davis had detected solar neutrinos, but had consistently found an event rate about a factor of three below what was predicted using the best models of the Sun. A new type of solar neutrino detector was built inside a deep mine in Sudbury, Canada, which became known as the Sudbury Neutrino Observatory (SNO).
Super-Kamiokande has now been operating almost continuously, through various upgrades, for more than twenty years. No proton-decay signals have been seen, and no new supernovas observed. However, the precision observations of neutrinos at this huge detector, combined with complementary observations at SNO, definitely established that the solar neutrino deficit observed by Ray Davis is real, and moreover that it is not due to astrophysical effects in the Sun but rather due to the properties of neutrinos. At least one of the three known types of neutrinos is not massless—although it has a small mass indeed, perhaps a hundred million times smaller than the mass of the next-lightest particle in nature, the electron. Since the Standard Model does not accommodate neutrinos’ masses, this was the first definitive observation that some new physics, beyond the Standard Model and beyond the Higgs, must be operating in nature.
Soon after this, observations of higher-energy neutrinos that regularly bombard Earth as high-energy cosmic-ray protons hit the atmosphere and produce a downward shower of particles, including neutrinos, demonstrated that yet a second neutrino has mass. This mass is somewhat larger, but still far smaller than the mass of the electron. For these results team leaders at SNO and Kamiokande were awarded the 2015 Nobel Prize in Physics—a week before I wrote the first draft of these words. To date these tantalizing hints of new physics are not explained by current theories.
The absence of proton decay, while disappointing, turned out to be not totally unexpected. Since Grand Unification was first proposed, the physics landscape had shifted slightly. More precise measurements of the actual strengths of the three nongravitational interactions—combined with more sophisticated calculations of the change in the strength of these interactions with distance—demonstrated that if the particles of the Standard Model are the only ones existing in nature, the strength of the three forces will not unify at a single scale. In order for Grand Unification to take place, some new physics at energy scales beyond those that have been observed thus far must exist. The presence of new particles would not only change the rate at which the three known interactions change with scale so that they might unify at a single scale of energy, it would also tend to drive up the Grand Unification scale and thus suppress the rate of proton decay—leading to predicted lifetimes in excess of a million billion billion billion years.
As these developments were taking place, theorists were driven by new mathematical tools to explore a possible new type of symmetry in nature, which became known as supersymmetry. This fundamental symmetry is different from any previous known symmetry, in that it connects the two different types of particles in nature, fermions (particles with half-integer spins) and bosons (particles with integer spins). The upshot of this (many other books, including some by me, explore this idea in detail) is that if this symmetry exists in nature, then for every known particle in the Standard Model at least one corresponding new elementary particle must exist. For every known boson there must exist a new fermion. For every known fermion there must exist a new boson.
Since we haven’t seen these particles, this symmetry cannot be manifest in the world at the level we experience it, and it must be broken, meaning the new particles will all get masses that could be heavy enough so that they haven’t been seen in any accelerator constructed thus far.
What could be so attractive about a symmetry that suddenly doubles all the particles in nature without any evidence of any of the new particles? In large part the seduction lay in the very fact of Grand Unification. Because if a Grand Unified Theory exists at a mass scale of fifteen to sixteen orders of magnitude higher energy than the rest mass of the proton, this is also about thirteen orders of magnitude higher than the scale of electroweak symmetry breaking. The big question is why and how such a huge difference in scales can exist for the fundamental laws of nature. In particular, if the Standard Model Higgs is the true last remnant of the Standard Model, then the question arises, Why is the energy scale of Higgs symmetry breaking thirteen orders of magnitude smaller-scale than the scale of symmetry breaking associated with whatever new field must be introduced to break the GUT symmetry into its separate component forces?
The problem is a little more severe than it appears. Scalar particles such as the Higgs have several new quantum mechanical properties that are unlike those of fermions or spin 1 particles such as gauge particles. When one considers the effects of virtual particles, including particles of arbitrarily large mass, such as the gauge particles of a presumed Grand Unified Theory, these tend to drive up the mass and symmetry-breaking scale of the Higgs so that it essentially becomes close to, or identical to, the heavy GUT scale. This generates a problem that has become known as the naturalness problem. It is technically unnatural to have a huge hierarchy between the scale at which the electroweak symmetry is broken by the Higgs particle and the scale at which the GUT symmetry is broken by whatever new heavy scalar field breaks that symmetry.
The brilliant mathematical physicist Edward Witten argued in an influential paper in 1981 that supersymmetry had a special property. It could tame the effect that virtual particles of arbitrarily high mass and energy have on the properties of the world at the scales we can currently probe. Because virtual fermions and virtual bosons of the same mass produce quantum corrections that are identical except for a sign, if every boson is accompanied by a fermion of equal mass, then the quantum effects of the virtual particles will cancel out. This means that the effects of virtual particles of arbitrarily high mass and energy on the physical properties of the universe on scales we can measure would now be completely removed.
If, however, supersymmetry is itself broken, then the quantum corrections will not quite cancel out. Instead they would yield contributions to masses that are the same order as the supersymmetry-breaking scale. If it was comparable to the scale of the electroweak symmetry breaking, then it would explain why the Higgs mass scale is what it is. And it also means we should expect to begin to observe a lot of new particles—the supersymmetric partners of ordinary matter—at the scale currently being probed at the LHC.
This would solve the naturalness problem because it would protect the Higgs boson masses from possible quantum corrections that could drive them up to be as large as the energy scale associated with Grand Unification. Supersymmetry could allow a “natural” large hierarchy in energy (and mass) separating the electroweak scale from the Grand Unified scale.
That supersymmetry could in principle solve the hierarchy problem, as it has become known, greatly increased its stock with physicists. It caused theorists to begin to explore realistic models that incorporated supersymmetry breaking and to explore the other physical consequences of this idea. When they did so, the stock price of supersymmetry went through the roof. For if one included the possibility of spontaneously broken supersymmetry into calculations of how the three nongravitational forces change with distance, then suddenly the strength of the three forces would naturally converge at a single, very small-distance scale. Grand Unification became viable again!
Models in which supersymmetry is broken have another attractive feature. It was pointed out, well before the top quark was discovered, that if the top quark was heavy, then through its interactions with other supersymmetric partners, it could produce quantum corrections to the Higgs particle properties that would cause the Higgs field to condense at its currently measured energy scale if Grand Unification occurred at a much higher, superheavy scale. In short, the energy scale of electroweak symmet
ry breaking could be generated naturally within a theory in which Grand Unification occurs at a much higher energy scale. When the top quark was discovered and indeed was heavy, this added to the attractiveness of the possibility that supersymmetry breaking might be responsible for the observed energy scale of the weak interaction.
All of this comes at a cost, however. For the theory to work, there must be two Higgs bosons, not just one. Moreover, one would expect to begin to see the new supersymmetric particles if one built an accelerator such as the LHC, which could probe for new physics near the electroweak scale. Finally, in what looked for a while like a rather damning constraint, the lightest Higgs in the theory could not be too heavy or the mechanism wouldn’t work.
As searches for the Higgs continued without yielding any results, accelerators began to push closer and closer to the theoretical upper limit on the mass of the lightest Higgs boson in supersymmetric theories. The value was something like 135 times the mass of the proton, with details to some extent depending on the model. If the Higgs could have been ruled out up to that scale, it would have suggested all the hype about supersymmetry was just that.
Well, things turned out differently. The Higgs that was observed at the LHC has a mass about 125 times the mass of the proton. Perhaps a grand synthesis was within reach.
The answer at present is . . . not so clear. The signatures of new supersymmetric partners of ordinary particles should be so striking at the LHC, if they exist, that many of us thought that the LHC had a much greater chance of discovering supersymmetry than it did of discovering the Higgs. It didn’t turn out that way. Following three years of LHC runs, there are no signs whatsoever. The situation is already beginning to look uncomfortable. The lower limits that can now be placed on the masses of supersymmetric partners of ordinary matter are getting higher. If they get too high, then the supersymmetry-breaking scale would no longer be close to the electroweak scale, and many of the attractive features of supersymmetry breaking for resolving the hierarchy problem would go away.
But the situation is not yet hopeless, and the LHC has been turned on again, this time at higher energy. It could be that, in the year between the time I write these words and the book going into its tenth printing, supersymmetric particles will be discovered.
If they are, this will have another important consequence. One of the bigger mysteries in cosmology is the nature of the dark matter that appears to dominate the mass of all galaxies we can see. As I have briefly alluded to earlier, there is so much of it that it cannot be made of the same particles as normal matter. If it were, for example, the predictions of the abundance of light elements such as helium produced in the Big Bang would no longer agree with observation. Thus physicists are reasonably certain that the dark matter is made of a new type of elementary particle. But what type?
Well, the lightest supersymmetric partner of ordinary matter is, in most models, absolutely stable and has many of the properties of neutrinos. It would be weakly interacting and electrically neutral, so that it wouldn’t absorb or emit light. Moreover, calculations that I and others performed more than thirty years ago showed that the remnant abundance today of the lightest supersymmetric particle left over after the Big Bang would naturally be in the range so that it could be the dark matter dominating the mass of galaxies.
In that case our galaxy would have a halo of dark matter particles whizzing throughout it, including through the room in which you are reading this. As a number of us also realized some time ago, this means that if one designs sensitive detectors and puts them underground, not unlike, at least in spirit, the neutrino detectors that already exist underground, one might directly detect these dark matter particles. Around the world a half dozen beautiful experiments are now going on to do just that. So far nothing has been seen, however.
So, we are in potentially the best of times or the worst of times. A race is going on between the detectors at the LHC and the underground direct dark matter detectors to see who might discover the nature of dark matter first. If either group reports a detection, it will herald the opening up of a whole new world of discovery, leading potentially to an understanding of Grand Unification itself. And if no discovery is made in the coming years, we might rule out the notion of a simple supersymmetric origin of dark matter—and in turn rule out the whole notion of supersymmetry as a solution of the hierarchy problem. In that case we would have to go back to the drawing board, except if we don’t see any new signals at the LHC, we will have little guidance about which direction to head in order to derive a model of nature that might actually be correct.
Things got more interesting when the LHC reported a tantalizing possible signal due to a new particle about six times heavier than the Higgs particle. This particle did not have the characteristics one would expect for any supersymmetric partner of ordinary matter. In general the most exciting spurious hints of signals go away when more data are amassed, and about six months after this signal first appeared, after more data were amassed, it disappeared. If it had not, it could have changed everything about the way we think about Grand Unified Theories and electroweak symmetry, suggesting instead a new fundamental force and a new set of particles that feel this force. But while it generated many hopeful theoretical papers, nature seems to have chosen otherwise.
The absence of clear experimental direction or confirmation of supersymmetry has thus far not bothered one group of theoretical physicists. The beautiful mathematical aspects of supersymmetry encouraged, in 1984, the resurrection of an idea that had been dormant since the 1960s when Nambu and others tried to understand the strong force as if it were a theory of quarks connected by stringlike excitations. When supersymmetry was incorporated in a quantum theory of strings, to create what became known as superstring theory, some amazingly beautiful mathematical results began to emerge, including the possibility of unifying not just the three nongravitational forces, but all four known forces in nature into a single consistent quantum field theory.
However, the theory requires a host of new space-time dimensions to exist, none of which has been, as yet, observed. Also, the theory makes no other predictions that are yet testable with currently conceived experiments. And the theory has recently gotten a lot more complicated so that it now seems that strings themselves are probably not even the central dynamical variables in the theory.
None of this dampened the enthusiasm of a hard core of dedicated and highly talented physicists who have continued to work on superstring theory, now called M-theory, over the thirty years since its heyday in the mid-1980s. Great successes are periodically claimed, but so far M-theory lacks the key element that makes the Standard Model such a triumph of the scientific enterprise: the ability to make contact with the world we can measure, resolve otherwise inexplicable puzzles, and provide fundamental explanations of how our world has arisen as it has. This doesn’t mean M-theory isn’t right, but at this point it is mostly speculation, although well-meaning and well-motivated speculation.
Here is not the place to review the history, challenges, and successes of string theory. I have done that elsewhere, as have a number of my colleagues. It is worth remembering that if the lessons of history are any guide, most forefront physical ideas are wrong. If they weren’t, anyone could do theoretical physics. It took several centuries or, if one counts back to the science of the Greeks, several millennia of hits and misses to come up with the Standard Model.
So this is where we are. Are great new experimental insights just around the corner that may validate, or invalidate, some of the grander speculations of theoretical physicists? Or are we on the verge of a desert where nature will give us no hint of what direction to search in to probe deeper into the underlying nature of the cosmos? We’ll find out, and we will have to live with the new reality either way.
No matter what curveballs nature may throw at us, the recent discovery of the Higgs, the latest and one of the greatest experimental and theoretical achievements of the remarkable Standard Model of p
article physics, has beautifully capped more than two millennia of intellectual effort by brave and determined philosophers, mathematicians, and scientists to uncover the hidden tapestry that underlies our existence.
It also suggests that the beautiful universe in which we find ourselves may not only resemble, at least metaphorically, an ice crystal on a windowpane, it may be almost as ephemeral.
Chapter 23
* * *
FROM A BEER PARTY TO THE END OF TIME
For the fashion of this world passeth away.
—1 CORINTHIANS 7:31
My own research focus for much of my career has been the emerging field of cosmology called particle astrophysics. Following the flood of theoretical developments of the 1960s and 1970s, it was difficult for terrestrial experiments, limited as they are by our abilities to build complex machines such as particle accelerators, to keep up. As a result, a number of us turned to the universe for guidance. Since the Big Bang implies that the early universe was hot and dense, conditions existed then that we might never achieve in laboratories on Earth. But if we are clever, we can look for remnant signatures of those early times out in the cosmos, and we may be able test our ideas about even the most esoteric aspects of fundamental physics.
My previous book, A Universe from Nothing, described the revolutions in our understanding of the evolution of the universe on large scales, and over long times. Not only have our explorations revealed the existence of dark matter, which, as I have described, is likely composed of new elementary particles not yet observed in accelerators—although we may be on the cusp of doing so—but far more exotic still, we have discovered that the dominant energy of the universe resides in empty space—and we currently have no idea how it arises.
The Greatest Story Ever Told—So Far Page 27