The Greatest Story Ever Told—So Far

by Lawrence M. Krauss

  Then the show began, and in the next forty-five minutes or so spokespeople presented data from both of the two large detectors that compellingly demonstrated the existence of a new elementary particle with mass of about 125 times the mass of the proton. After the initial catastrophe in 2009, the LHC had functioned impeccably—as had both the detectors. I and many of my colleagues were amazed during the early months by the immaculately clean results the detectors displayed regarding known background processes. So we were not surprised that when something new appeared, these detectors could find it, in spite of the unbelievably complicated environment that the detectors were functioning in.

  But more than this, the particle was discovered by looking precisely at the decay channels that had been predicted for a Standard Model Higgs particle. The relative decays into photons (via intermediate top quarks or W’s) versus particles such as electrons (via intermediate Z bosons) agreed more or less with what was predicted, as did the production rate of the new particle in the proton-proton collisions. Of the billions and billions of collisions analyzed by the two detector collaborations up to that point, about fifty potential Higgs candidates had been discovered. Many tests needed to be performed to get a more definitive identification, but if it walked like a Higgs and quacked like a Higgs, it probably was a Higgs. The evidence was good enough that François Englert and Peter Higgs were awarded the Nobel Prize in October of 2013, the first year possible after the claimed discovery.

  In February 2013, the LHC shut down and the machine was upgraded so that it could finally run at its originally designed energy and luminosity. By the final weeks before turnoff, the CERN mass-storage systems had stored more than one hundred petabytes of data, more info than in 100 million CDs. New results continued to roll in from data that had not yet been analyzed before the first announcement (including tantalizing hints of a possible new and unexpected heavy particle, six times heavier than the Higgs, hints that disappeared just as this book was being sent off to press).

  For a real discovery, the more data you have, the better it looks, whereas anomalous results tend to disappear over time. This time things looked good, almost embarrassingly so. If one compared five different predicted decay channels into photons, Z particles, W particles, tau particles (the heaviest known cousin of the electron), and particles containing b quarks, to observation, the predictions of the Standard Model Higgs, with no extra accessories, agreed strikingly well.

  From the angular distribution and energies of the decay products, with a new larger sample of Higgs candidates, the LHC detectors were able to explore whether the particle was indeed a scalar particle, which would make it the first fundamental scalar ever observed in nature. On March 26, 2015, the ATLAS detector at CERN released results that showed with greater than 99 percent confidence that the new particle was a spin 0 particle, with precisely the proper parity assignment to be a Higgs scalar. Nature had shown that it does not abhor scalar fields like the Higgs, as I for one had thought. The existence of such a fundamental scalar changes a great deal about what may be possible in nature, and people, including me, began to consider scenarios we would never before have considered.

  In September 2015, about a month before the first draft of this book was written, the two large detectors ATLAS and CMS combined their data from 2011 and 2012 and presented for the first time a unified comparison of theory and experiment. The result—involving a mammoth computational effort to take into account separate systematic effects in each experiment, involving a total of forty-two hundred parameters—showed with a residual uncertainty of about 10 percent that the new particle had all the properties predicted for the Standard Model Higgs.

  This simple conclusion may seem almost anticlimactic, following as it does a half century of directed effort by thousands of individuals—the theorists who developed the Standard Model and the others who performed the incredibly complex calculations needed to compare predictions with experiments, to determine background rates, and so on, and the thousands of experimental physicists who had built and tested and operated the most complex machine ever constructed. Their story was marked by incredible heights of intellectual bravery, years of confusion, bad luck and serendipity, rivalries and passion, and above all the persistence of a community focused on a single goal—to understand nature at her most fundamental scales. Like any human drama, it also included its share of envy, stubbornness, and vanity, but more important, it involved a unique community built completely independent of ethnicity, language, religion, or gender. It is a story that carries with it all the drama of the best epic tales and reflects the best of what science can offer to modern civilization.

  That nature would be so kind as to actually use the ideas that a small collection of individuals wrote down on paper, inspired by abstract ideas of symmetry and using the complex mathematics of quantum field theory, will always seem to me nothing short of remarkable. It is hard to express the mixture of exhilaration and terror that comes from the realization that nature might actually work the way you are proposing it does when putting the final touches on a paper, possibly late at night, alone in your study. I suppose it may resemble the reaction Plato described that his poor philosophers might have as they are dragged out into the sunlight away from the cave for the first time.

  To have discovered that nature really follows the simple and elegant rules intuited by the twentieth- and twenty-first-century versions of Plato’s philosophers is both shocking and reassuring. It hints that the willingness of scientists to build an intellectual house of cards that could come tumbling down at the slightest experimental tremor was not misplaced. It gives us courage to continue to suppose, as Einstein had once expressed his amazement about, that the universe on its grandest scale is fathomable after all.

  After witnessing the announcement of the Higgs discovery on July 4, 2012, I wrote the following:

  The apparent discovery of the Higgs may not result in a better toaster or a faster car. But it provides a remarkable celebration of the human mind’s capacity to uncover nature’s secrets, and of the technology we have built to control them. Hidden in what seems like empty space—indeed, like nothing, which is getting more interesting all the time—are the very elements that allow for our existence.

  By demonstrating this, last week’s discovery will change our view of ourselves and our place in the universe. Surely that is the hallmark of great music, great literature, great art . . . and great science.

  It is too early yet to judge or even fully anticipate what changes in our picture of reality will result from the Higgs discovery at the LHC, or the discoveries that may follow. Yet fortune does favor the prepared mind, and it is at once the responsibility and the joy of theorists such as me to ponder just that.

  While nature may have appeared to be kind to us this time, perhaps it was too kind. The epic saga I have described here may yet provide a dramatic new challenge for physics and for physicists, and an explicit reminder that nature doesn’t exist to make us comfortable. Because while we may have found what we expected, no one really expected to find just that and nothing else. . . .

  Chapter 22

  * * *

  MORE QUESTIONS THAN ANSWERS

  A fool takes no pleasure in understanding, but only in expressing his opinion.

  —PROVERBS 18:2

  In one sense, our story might end here, because we have come to the limits of our direct empirical knowledge about the universe at its fundamental scales. But no one says we have to stop dreaming, even if the dreams are not always pleasant. Before July 2012 particle physicists had two nightmares. The first was that the LHC would see precisely nothing. For if it did, it would likely be the last large accelerator ever built to probe the fundamental makeup of the cosmos. The second was that the LHC would discover the Higgs . . . period.

  Each time we peel back one layer of reality, other layers beckon. So each important new development in science generally leaves us with more questions than answers. But it also usually leaves us w
ith at least the outline of a road map to help us begin to seek answers to those questions. The discovery of the Higgs particle, and with it the validation of the existence of an invisible background Higgs field throughout space, was a profound validation of the bold scientific developments of the twentieth century.

  However, the words of Sheldon Glashow continue to ring true: The Higgs is like a toilet. It hides all the messy details we would rather not speak of. The Higgs field, as elegant as it might be, is within the Standard Model essentially an ad hoc addition. It is added to the theory to do what is required to accurately model the world of our experience. But it is not required by the theory. The universe could have happily existed with a long-range weak force and massless particles. We would just not be here to ask about them. Moreover, the detailed physics of the Higgs is, as we have seen, undetermined within the Standard Model alone. The Higgs could have been twenty times heavier, or a hundred times lighter.

  Why, then, does the Higgs exist at all? And why does it have the mass it does? (Recognizing once again that whenever scientists ask “Why?,” we really mean “How?”) If the Higgs did not exist, the world we see would not exist, but surely that is not an explanation. Or is it? Ultimately to understand the underlying physics behind the Higgs is to understand how we came to exist. When we ask, “Why are we here?,” at a fundamental level we may as well be asking, “Why is the Higgs here?” And the Standard Model gives no answer to this question.

  Some hints do exist, however, coming from a combination of theory and experiment. Shortly after the fundamental structure of the Standard Model became firmly established, in 1974, and well before the details were experimentally verified over the next decade, two different groups of physicists at Harvard, where both Glashow and Weinberg were working, noticed something interesting. Glashow, along with Howard Georgi, did what Glashow did best: they looked for patterns among the existing particles and forces and sought out new possibilities using the mathematics of group theory.

  Remember that in the Standard Model the weak and electromagnetic forces are unified at a high-energy scale, but when the symmetry is spontaneously broken by the Higgs field condensate, this leaves, at observable scales, two separate and distinct forces—with the weak force being short-range and electromagnetism remaining long-range. Georgi and Glashow tried to extend this idea to include the strong force and discovered that all of the known particles and the three nongravitational forces could naturally fit within a single fundamental larger-gauge symmetry structure. They then speculated that this fundamental symmetry could spontaneously break at some ultrahigh energy and short-distance scale far beyond the range of current experiments, leaving two separate and distinct unbroken gauge symmetries left over—resulting in the separate strong and electroweak forces. Subsequently, at a lower energy and larger distance scale, the electroweak symmetry would break, separating that into the short-range weak and the long-range electromagnetic force.

  They called such a theory, modestly, a Grand Unified Theory (GUT).

  At around the same time, Weinberg and Georgi along with Helen Quinn noticed something interesting—following the work of Wilczek, Gross, and Politzer. While the strong interaction got weaker as one probed it at smaller-distance scales, the electromagnetic and weak interactions got stronger.

  It didn’t take a rocket scientist to wonder whether the strength of the three different interactions might become identical at some small-distance scale. When they did the calculations, they found (with the accuracy with which the interactions were then measured) that such a unification looked possible, but only if the scale of unification was about fifteen orders of magnitude in scale smaller than the size of the proton.

  This was good news if the unified theory was the one proposed by Georgi and Glashow—because if all the particles we observe in nature got unified in this new large-gauge group, then new gauge bosons would exist that produce transitions between quarks (which make up protons and neutrons), and electrons and neutrinos. That would mean protons could decay into other lighter particles. As Glashow put it, “Diamonds aren’t forever.”

  Even then it was known that protons must have an incredibly long lifetime. Not just because we still exist almost 14 billion years after the Big Bang, but because we all don’t die of cancer as children. If protons decayed with an average lifetime smaller than about a billion billion years, then enough protons would still decay in our bodies during our childhood to produce enough radiation to kill us. Remember that in quantum mechanics, processes are probabilistic. If an average proton lives a billion billion years, then if one has a billion billion protons, on average one will decay each year. A lot more than a billion billion protons are in our bodies.

  However, with the incredibly small proposed distance scale and therefore the incredibly large mass scale associated with spontaneous symmetry breaking in Grand Unification, the new gauge bosons would get large masses. That would make the interactions they mediate be so short-range that they would be unbelievably weak on the scale of protons and neutrons today. As a result, while protons could decay, they might live, in this scenario, perhaps a million billion billion billion years before decaying. No problem.

  • • •

  With the results of Glashow and Georgi, and Georgi, Quinn, and Weinberg, the smell of grand synthesis was in the air. After the success of the electroweak theory, particle physicists were feeling ambitious and ready for further unification.

  How would one know if these ideas were correct, however? There was no way to build an accelerator to probe an energy scale a million billion times greater than the rest mass energy of protons. Such a machine would have to have a circumference of the Moon’s orbit. Even if it was possible, considering the earlier debacle over the SSC, no government would ever foot the bill.

  Happily, there was another way, using the kind of probability arguments I just presented that give limits to the proton lifetime. If the new Grand Unified Theory predicted a proton lifetime of, say, a thousand billion billion billion years, then if one could put a thousand billion billion billion protons in a single detector, on average one of them would decay each year.

  Where could one find so many protons? Simple: in about three thousand tons of water.

  So all that was required was to get a tank of, say, three thousand tons of water, put it in the dark, make sure there were no radioactivity backgrounds, surround it with sensitive phototubes that can detect flashes of light in the detector, and then wait for a year to see a burst of light when a proton decayed. As daunting as this may seem, at least two large experiments were commissioned and built to do just this, one deep underground next to Lake Erie in a salt mine, and one in a mine near Kamioka, Japan. The mines were necessary to screen out incoming cosmic rays that would otherwise produce a background that would swamp any proton decay signal.

  Both experiments began taking data around 1982–83. Grand Unification seemed so compelling that the physics community was confident a signal would soon appear and Grand Unification would mean the culmination of a decade of amazing change and discovery in particle physics—not to mention another Nobel Prize for Glashow and maybe some others.

  Unfortunately, nature was not so kind in this instance. No signals were seen in the first year, the second, or the third. The simplest elegant model proposed by Glashow and Georgi was soon ruled out. But once the Grand Unification bug had caught on, it was not easy to let it go. Other proposals were made for unified theories that might cause proton decay to be suppressed beyond the limits of the ongoing experiments.

  On February 23, 1987, however, another event occurred that demonstrates a maxim I have found is almost universal: every time we open a new window on the universe, we are surprised. On that day a group of astronomers observed, in photographic plates obtained during the night, the closest exploding star (a supernova) seen in almost four hundred years. The star, about 160,000 light-years away, was in the Large Magellanic Cloud—a small satellite galaxy of the Milky Way observable in the s
outhern hemisphere.

  If our ideas about exploding stars are correct, most of the energy released should be in the form of neutrinos, despite that the visible light released is so great that supernovas are the brightest cosmic fireworks in the sky when they explode (at a rate of about one explosion per hundred years per galaxy). Rough estimates then suggested that the huge IMB (Irvine-Michigan-Brookhaven) and Kamiokande water detectors should see about twenty neutrino events. When the IMB and Kamiokande experimentalists went back and reviewed their data for that day, lo and behold IMB displayed eight candidate events in a ten-second interval, and Kamiokande displayed eleven such events. In the world of neutrino physics, this was a flood of data. The field of neutrino astrophysics had suddenly reached maturity. These nineteen events produced perhaps nineteen hundred papers by physicists, such as me, who realized that they provided an unprecedented window into the core of an exploding star, and a laboratory not just for astrophysics but also for the physics of neutrinos themselves.

  Spurred on by the realization that large proton-decay detectors might serve a dual purpose as new astrophysical neutrino detectors, several groups began to build a new generation of such dual-purpose detectors. The largest one in the world was again built in the Kamioka mine and was called Super-Kamiokande, and with good reason. This mammoth fifty-thousand-ton tank of water, surrounded by 11,800 phototubes, was operated in a working mine, yet the experiment was maintained with the purity of a laboratory clean room. This was absolutely necessary because in a detector of this size one had to worry not only about external cosmic rays, but also about internal radioactive contaminants in the water that could swamp any signals being searched for.