Lawrence Krauss - The Greatest Story Ever Told--So Far

by Why Are We Here (pdf)

equal number of religions, were required to build the accelerator and

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  the detectors that monitor it—taking almost two decades to

  complete the task. Its scale dwarfs that of all machines constructed

  before it. And it was built for no more practical reason than to

  celebrate and explore the beauty of nature.

  Seen in this perspective, the cathedrals and the collider are both

  monuments to what may be best about human civilization—the

  ability and the will to imagine and construct objects of a scale and

  complexity that requires the cooperation of countless individuals,

  from around the globe if necessary, for the purpose of turning our

  awe and wonder at the workings of the cosmos into something

  concrete that may improve the human condition. Colliders and

  cathedrals are both works of incomparable grandeur that celebrate

  the human experience in different realms. Nevertheless, I think the

  LHC wins, and its successful construction over two decades

  demonstrates that the twenty-first century is not yet devoid of

  culture and imagination.

  Which brings me finally to the road to July 4, 2012.

  By 2011 the LHC was cruising along, as one of the CERN officials

  put it. The amount of data taken by October of that year was already

  4 million times higher than during the first run in 2010, and thirty

  times higher than had been obtained by the beginning of 2011.

  At this point in the collection of data that physicists had been

  waiting forty years for, rumors began to fly in the community. Many

  of these came from the experimenters themselves. I have a part-time

  position at Australian National University in Canberra, and the

  International Conference on High Energy Physics was going to be in

  held in Melbourne in July of 2012. Melbourne has a big LHC

  contingent, and when visiting, I kept hearing how a greater and

  greater possible mass range for the Higgs particle had been ruled out

  by the experiments already.

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  Many experimentalists relish being able to prove theorists wrong.

  So it was in this case. One experimentalist had excitedly told me less

  than six months before the meeting that the entire Higgs mass

  region had been ruled out except for a narrow range between 120

  and 130 times the mass of the proton. She expected that by July they

  would be able to rule out that region too. As one who was skeptical

  of the Higgs, I wasn’t unhappy to hear this. In fact, I was getting a

  paper ready to explain why the Higgs might not exist.

  On April 5, the situation got more interesting as the LHC center-

  of-mass beam energy was increased slightly, to eight thousand times

  the rest energy of the proton. This translated into an increased

  potential for new particle discovery. By mid-June it was announced

  that the leaders of the two main experiments, along with the director

  general of CERN, would not be traveling to Melbourne for the

  meeting, but would be presenting results remotely from a televised

  conference on the morning of July 4 in the main colloquium room at

  CERN—the same room where Rubbia had announced the discovery

  of the W particles.

  On July 4 I was at a physics meeting in Aspen, Colorado. Because

  of the significance of the impending announcement, the physics

  community there had set up a live remote presentation screen—so

  that at 1:00 a.m. we could all sit and watch history unfold. About

  fifteen of us showed up in the dark at the Aspen Center for Physics,

  mostly physicists, but also a few journalists, including Dennis

  Overbye from the New York Times, who knew he was going to have a

  late night writing. As it turned out, so would I. The Times had asked

  me for an essay for the following week’s Science Times section if

  things worked out as expected.

  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

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  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).

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  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 sc
alar. 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

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  —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

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  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. . . .

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  C h a p t e r 2 2

  M O R E

  Q U E S T I O N S

  T H A N

  A N S W E R S

  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 with

  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

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  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 n
ot 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

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  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