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

by Why Are We Here (pdf)

  that one would never observe a free quark or gluon is valid. This is

  called Confinement because quarks and gluons are always confined

  inside strongly interacting particles such as protons and neutrons

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  and can never break free from them without getting confined in

  newly created strongly interacting particles.

  Since the actual process by which the quarks get confined occurs

  as the forces become stronger and stronger when the quark moves

  farther and farther away from its original companions, the standard

  calculations of quantum field theory, which are valid when the

  interactions are not too strong, break down. So this picture,

  validated by experiment, cannot be fully confirmed by tractable

  calculations at the moment.

  Will we ever derive the necessary mathematical tools to

  analytically demonstrate from first principles that confinement is

  indeed a mathematical property of quantum chromodynamics? This

  is the million-dollar question, literally. The Clay Mathematics

  Institute has announced a million-dollar prize for a rigorous

  mathematical proof that quantum chromodynamics does not allow

  free quarks or gluons to be produced. While no claimants to the

  prize have yet come forward, we nevertheless have strong indirect

  support of this idea, coming not only from experimental

  observations, but also from numerical simulations that closely

  approximate

  the

  complicated

  interactions

  in

  quantum

  chromodynamics. This is heartening, if not definitive. We still have

  to confirm that it is some property of the theory and not of the

  computer simulation. However, for physicists, if not mathematicians,

  this seems pretty convincing.

  One final bit of direct evidence that QCD is correct came from a

  realm where exact calculations can be done. Because quarks are not

  completely free at short distances, I earlier mentioned that there

  should be calculable corrections to exotic scaling phenomena

  observed in the high-energy collisions of electrons off protons and

  neutrons, as originally observed at SLAC. Perfect scaling would

  require completely noninteracting particles. The corrections that

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  one could calculate in quantum chromodynamics would only be

  observable in experiments that were far more sensitive than those

  originally performed at SLAC. It took the development of new,

  higher-energy accelerators to probe them. After thirty years or so,

  enough evidence was in so that comparison of theoretical

  predictions and experiment agreed at the 1 percent level, and

  quantum chromodynamics as the theory of the strong interaction

  was finally verified in a precise and detailed way.

  Gross, Wilczek, and Politzer were finally awarded the Nobel Prize

  in 2004 for their discovery of asymptotic freedom. The

  experimentalists who had first discovered scaling at SLAC, which

  was the key observation that set theorists off in the right direction,

  were awarded the Nobel Prize much earlier, in 1990. And the

  experimentalists who discovered the charmed quark in 1974 won

  the Nobel Prize two years later, in 1976.

  But the biggest prize of all, as Richard Feynman has said, is not the

  recognition by a medal or a cash award, or even the praise one gets

  from colleagues or the public, but the prize of actually learning

  something new about nature.

  • • •

  In this sense the 1970s were perhaps the richest decade in the

  twentieth century, if not in the entire history of physics. In 1970 we

  understood only one force in nature completely as a quantum

  theory, namely quantum electrodynamics. By 1979 we had

  developed and experimentally verified perhaps the greatest

  theoretical edifice yet created by human minds, the Standard Model

  of particle physics, describing precisely three of the four known

  forces in nature. The effort spanned the entire history of modern

  science, from Galileo’s investigations of the nature of moving bodies,

  through Newton’s discovery of the laws of motion, through the

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  experimental and theoretical investigations of the nature of

  electromagnetism, through Einstein’s unification of space and time,

  through the discoveries of the nucleus, quantum mechanics, protons,

  neutrons, and the discovery of the weak and strong forces

  themselves.

  But the most remarkable characteristic of all in this long march

  toward the light is how different the fundamental nature of reality is

  from the shadows of reality that we experience every day, and in

  particular how the fundamental quantities that appear to govern our

  existence are not fundamental at all.

  Making up the heart of observed matter are particles that had

  never been directly observed and, if we are correct, will never be

  directly observable—quarks and gluons. The properties of forces that

  govern the interactions of these particles—and also the particles that

  have formed the basis of modern experimental physics for more

  than a century, electrons—are also, on a fundamental level,

  completely different from the properties we directly observe and on

  which we depend for our existence. The strong interaction between

  protons and neutrons is only a long-distance remnant of the

  underlying force between quarks, whose fundamental properties are

  masked by the complicated interactions within the nucleus. The

  weak interaction and the electromagnetic interaction, which could

  not be more different on the surface—one is short-range, while the

  other is long-range, and one appears thousands of times weaker than

  the other—are in fact intimately related and reflect different facets of

  a single whole.

  That whole is hidden from us because of the accident of nature

  we call spontaneous symmetry breaking, which distinguishes the two

  weak and electromagnetic interactions in the world of our

  experience and hides their true nature. More than that, the

  properties of the particles that produce the characteristics of the

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  beautiful world we observe around us are only possible because, after

  the accident of spontaneous symmetry breaking, just one particle in

  nature—the photon—remains massless. If symmetry breaking had

  never occurred so that underlying symmetries of the forces

  governing matter were manifest—which in turn would mean that

  the particles conveying the weak force would also be massless, as

  would most of the particles that make us up—essentially nothing we

  see in the universe today, from galaxies to stars, to planets, to people,

  to birds and bees, to scientists and politicians, would ever have

  formed.

  Moreover, we have learned that even these particles that make us

  up are not all that exist in nature. The observed particles combine in

  simple groupings, or families. The up and down quarks make up

  protons and neutrons. Along with them one finds the electron, and

  its partner, the electron neutrino. Then, f
or reasons we still don’t

  understand, there is a heavier family, made up of the charm and

  strange quark on the one hand, and the muon and its neutrino on

  the other. And finally, as experiments have now confirmed over the

  past decade or two, there is a third family, made of two new types of

  quarks, called bottom and top, and an accompanying heavy version

  of the electron called the tau particle, along with its neutrino.

  Beyond these particles, as I shall soon describe, we have every

  reason to expect that other elementary particles exist that have never

  been observed. While these particles, which we think make up the

  mysterious dark matter that dominates the mass of our galaxy and all

  observed galaxies, may be invisible to our telescopes, our

  observations and theories nevertheless suggest that galaxies and stars

  could never have formed without the existence of dark matter.

  And at the heart of all of the forces governing the dynamical

  behavior of everything we can observe is a beautiful mathematical

  framework called gauge symmetry. All of the known forces, strong,

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  weak, electromagnetic, and even gravity, possess this mathematical

  property, and for the three former examples, it is precisely this

  property that ensures that the theories make mathematical sense and

  that nasty quantum infinities disappear from all calculations of

  quantities that can be compared to experiment.

  With the exception of electromagnetism, these other symmetries

  remain completely hidden from view. The gauge symmetry of the

  strong force is hidden because confinement presumably hides the

  fundamental particles that manifest this symmetry. The gauge

  symmetry of the weak force is not manifest in the world in which we

  live because it is spontaneously broken so that the W and Z particles

  become extremely massive.

  • • •

  The shadows on the wall of everyday life are truly merely shadows.

  In this sense, the greatest story every told, so far, has been slowly

  playing out over the more than two thousand years since Plato first

  imagined it in his analogy of the cave.

  But as remarkable as this story is, two elephants remain in the

  room. Two protagonists in our tale could until recently have meant

  that the key aspects of the story comprised a mere fairy tale invented

  by theorists with overactive imaginations.

  First, the W and Z particles, postulated in 1960 to explain the

  weak interaction, almost one hundred times more massive than

  protons and neutrons, were still mere theoretical postulates, even if

  the indirect evidence for their existence was overwhelming. More

  than this, an invisible field—the Higgs field—was predicted to

  permeate all of space, masking the true nature of reality and making

  our existence possible because it spontaneously breaks the symmetry

  between the weak and the electromagnetic interactions.

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  To celebrate a story that claims to describe how it is that we exist,

  but that also posits an invisible field permeating all of space, sounds

  suspiciously like a religious celebration, and not a scientific one. To

  truly ensure that our beliefs conform to the evidence of reality rather

  than how we would like reality to be, to keep science worthy of the

  name, we had to discover the Higgs field. Only then could we truly

  know if the significance of the features of our world that we hold so

  dear might be no greater than that of the features of one random ice

  crystal on a window. Or, more to the point, perhaps, no greater than

  the significance of the superconducting nature of wire in a

  laboratory versus the normal resistance of the wires in my computer.

  The experimental effort to carry out this task was no easier than

  that in developing the theory itself. In many ways it was more

  daunting, taking more than fifty years and involving the most

  difficult fabrication of technology that humans have ever attempted.

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  C h a p t e r 2 0

  S PA N K I N G T H E VA C U U M

  If anyone slaps you on the right cheek, turn to him the other also.

  —MATTHEW 5:39

  As the 1970s ended, theorists were on top of the world,

  triumphant and exultant. With progress leading to the Standard

  Model so swift, what other new worlds were there to conquer?

  Dreams of a theory of everything, long dormant, began to rise again

  and not just in the dim recesses of the collective subconscious of

  theorists.

  Still, the W and Z gauge particles had never actually been

  observed, and the challenge to directly observe them was pretty

  daunting. Their masses were precisely predicted in the theory at

  about ninety times the mass of the proton. The challenge to produce

  these particles comes from a simple bit of physics.

  Einstein’s fundamental equation of relativity, E = mc2, tells us that

  we can convert energy into mass by accelerating particles to energies

  of many times their rest mass. We can then smash them into targets

  to see what comes out.

  The problem is that the energy available to produce new particles

  by smashing other fast-moving particles into stationary targets is

  given by what is called the center-of-mass energy. For those

  undaunted by another formula, this turns out to be the square root

  of twice the product of the energy of the accelerated particle times

  the rest mass energy of the target particle. Imagine accelerating a

  particle to one hundred times the rest mass energy of the proton

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  (which is about one gigaelectronvolt—GeV). In a collision with

  stationary protons in a target, the center-of-mass energy that is

  available to create new particles is then only about 14 GeV. This is

  just slightly greater than the center-of-mass energy available in the

  highest-energy particle accelerator in 1972.

  To reach the energies required to produce massive particles such

  as the W or Z bosons, two opposing beams of particles must collide.

  In this case the total center-of-mass energy is simply twice the

  energy of each beam. If each colliding beam of particles has an

  energy of one hundred times the rest mass of a proton, this then

  yields 200 GeV of energy to be converted into the mass of new

  particles.

  Why, then, produce accelerators with stationary targets and not

  colliders? The answer is quite simple. If I am shooting a bullet at a

  barn door, I am more or less guaranteed to hit something. If I shoot

  a bullet at another incoming bullet, however, I’d have to be a much

  better shot than probably anyone else alive and have a better gun

  than any now made to be guaranteed to hit it.

  This was the challenge facing experimentalists in 1976, by which

  time they took the electroweak model seriously enough that they

  thought it worth the time, effort, and money to try to test it.

  But no one knew how to build a device with the appropriate

  energy. Accelerating individual beams of particles or antiparticles to

  high energies had been achieved. By 1976 protons were being

&nb
sp; accelerated to 500 GeV, and electrons up to 50 GeV. At lower

  energies, collisions of electrons and their antiparticles had

  successfully been carried out, and this is how the new particle

  containing the charmed quark and antiquark had been discovered in

  1974.

  Protons, having greater mass and thus more rest energy initially,

  are easier to accelerate to high energies. In 1976 a proton accelerator

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  at the European Organization for Nuclear Research (CERN) in

  Geneva, the Super Proton Synchrotron (SPS), had just been

  commissioned as a conventional fixed-target accelerator operating

  with a proton beam at 400 GeV. However, another accelerator at

  Fermilab, near Chicago, had already achieved proton beams of 500

  GeV by the time the SPS turned on. In June of that year, physicists

  Carlo Rubbia, Peter McIntyre, and David Cline made a bold

  suggestion at a neutrino conference: converting the SPS at CERN

  into a machine that collided protons with their antiparticles—

  antiprotons—would allow CERN to potentially produce W’s and Z’s.

  Their bold idea was to use the same circular tunnel to accelerate

  protons in one direction, and antiprotons in another. Since the two

  particles have opposite electric charges, the same accelerating

  mechanism would have opposite effects on each particle. So a single

  accelerator could in principle produce two high-energy beams

  circulating in opposite directions.

  The logic of such a proposal was clear, but its implementation

  was not. In the first place, given the strength of the weak interaction,

  the production of even a few W and Z particles would require the

  collision of hundreds of billions of protons and antiprotons. But no

  one had ever produced and collected enough antiprotons to make an

  accelerator beam.

  Next, you might imagine that with two beams traversing the same

  tunnel in opposite directions, particles would be colliding all around

  the tunnel and not in the detectors designed to measure the products

  of the collisions. However, this is far from the case. The cross section

  of even a small tunnel compared to the size of the region over which

  a proton and an antiproton might collide is so huge that the problem

  is quite the opposite. It seemed impossible to produce enough

  antiprotons and ensure that both they and the protons in the proton