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

by Why Are We Here (pdf)

  emulsions, and a series of brave researchers dragged their equipment

  up to high elevations to search for possible new signals. Many

  cosmic rays interact and disappear before reaching sea level, so this

  group and others interested in exploring this wondrous new source

  of particles coming from the heavens had no choice but to seek

  higher elevations. Here cosmic rays would have traversed less

  distance in the atmosphere and might be more easily detected.

  The former Italian mountain guide turned physicist Giuseppe

  Occhialini had been invited from Brazil to join a British team

  working on the A-bomb during the war. As a foreign national, he

  couldn’t work on the project, so instead he joined the cosmic-ray

  physics group at Bristol. Occhialini’s mountain training proved

  useful as he dragged photographic emulsions up to the Pic du Midi

  at twenty-eight hundred meters in France. Today you can travel to

  the observatory on top of this peak by cable car, and it is a

  terrifyingly exciting ride. But in 1946 Occhialini had to climb to the

  top, risking his health in the effort to discover signals of exotic new

  physics.

  And he and his team did discover exotic new physics. As Cecil

  Powell, Occhialini’s collaborator at Bristol (and future Nobel

  laureate, while Occhialini, who had done the climbing, did without),

  put it, they saw “a whole new world. It was as if, suddenly, we had

  broken into a walled orchard, where protected trees flourished and

  all kinds of exotic fruits had ripened in great profusion.”

  Less poetically, perhaps, what they discovered were two examples

  in which an initial meson stopped in the emulsion and gave rise to a

  second meson, just as had been suggested by the theorists. Many

  more events were observed with emulsions taken to an elevation

  almost twice as high as Pic du Midi. In October of 1947, in the

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  journal Nature, Powell, Occhialini, and Powell’s student Cesare

  Lattes published a paper in which they named the initial meson the

  pion—which seemed to interact with the nuclear strength

  appropriate to Yukawa’s meson—and the subsequent meson the

  muon.

  It seemed at long last that Yukawa’s meson had been discovered.

  As for its “partner” the muon, which had been confused with

  Yukawa’s meson, it was nothing of the sort. Not spinless, it instead

  had the same spin as the electron and the proton. And its

  interactions with matter were nowhere near strong enough to play a

  role in nuclear binding. The muon turned out to be simply a heavy,

  if unstable, copy of the electron, which is what motivated Rabi’s

  question “Who ordered that?”

  So, the particle that made Yukawa famous wasn’t the particle he

  predicted after all. His idea became famous because the original

  experimental result had been misinterpreted. Fortunately, the Nobel

  committee waited until the 1947 discovery of the pion before

  awarding Yukawa their prize in 1949.

  But, given the track record of errors and mislabeling, it is natural

  to wonder if the pion was in fact the particle Yukawa had predicted.

  The answer is both yes and no. Exchange of charged pions between

  protons and neutrons is indeed one accurate way of trying to

  estimate the strong nuclear force holding nuclei together. But in

  addition to charged pions—the mesons that Yukawa had predicted—

  there are neutral pions as well. Who ordered those?

  Moreover, the theory that Yukawa wrote down to describe the

  strong force, like Fermi’s theory to describe neutron decay, was not

  fully mathematically consistent, as Yukawa had conceded when he

  proposed it. There was, at the time, no correct relativistic theory

  involving the exchange of massive particles. Something was still

  amiss, and a series of surprising experimental discoveries, combined

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  with prescient theoretical ideas that were unfortunately applied to

  the wrong theories, helped lead to more than a decade of confusion

  before the fog lifted and light appeared at the end of the tunnel. Or

  perhaps at the mouth of the cave.

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  C h a p t e r 1 2

  M A R C H O F T H E T I TA N S

  The wolf also shall dwell with the lamb, and the leopard shall lie

  down with the kid.

  —ISAIAH 11:6

  The relationship between theoretical insight and

  experimental discovery is one of the most interesting aspects of the

  progress of science. Physics is at its heart, like all of science, an

  empirical discipline. Yet at times brief bursts of theoretical insight

  change everything. Certainly Einstein’s insights into space and time

  in the first two decades of the twentieth century are good examples,

  and the remarkable theoretical progress associated with the

  development of quantum mechanics by Schrödinger, Heisenberg,

  Pauli, Dirac, and others in the 1920s is another.

  Less heralded is another period, from 1954 to 1974, which, while

  not as revolutionary, will, when sufficient time has passed, be

  regarded as one of the most fruitful and productive theoretical

  physics eras in the twentieth century. These two decades took us, not

  without turmoil, from chaos to order, from confusion to confidence,

  and from ugliness to beauty. It’s a wild ride, with a few detours that

  might seem to come from left field, but bear with me. If you find it a

  tad uncomfortable, then recall what I said in the introduction about

  science and comfort. By putting yourself in the frame of mind of

  those involved in the quest, whose frustration eventually led to

  insights, the significance of the insights can be truly appreciated.

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  This tumultuous period followed one in which experimental

  bombshells had produced widespread confusion, making nature

  “curiouser and curiouser,” as Lewis Carroll might have put it. The

  discoveries of the positron and quickly thereafter the neutron were

  just the beginning. Neutron decay, nuclear reactions, muons, pions,

  and a host of new elementary particles that followed made it appear

  as if fundamental physics was hopelessly complicated. The simple

  picture of a universe in which electromagnetism and gravity alone

  governed the interactions of matter made from protons and

  electrons disappeared into the dustbin of history. Some physicists at

  the time, like some on the political right today, yearned for the

  (often misremembered) simplicity of the good old days.

  This newfound complexity drove some, by the 1960s, to imagine

  that nothing was fundamental. In a Zen-like picture, they imagined

  that all elementary particles were made from all other elementary

  particles, and that even the notion of fundamental forces might be

  an illusion.

  Nevertheless, percolating in the background were theoretical

  ideas that would draw back the dark curtains of ignorance and

  confusion, revealing an underlying structure to nature that is as

  remarkable as it is strangely simple, and one in which light would

  once again play a key role.


  It all began with two theoretical developments, one profound and

  unheralded and another relatively straightforward but brilliant and

  immediately feted. Remarkably, the same man was involved in both.

  Born in 1922 to a mathematician father, Chen-Ning Yang was

  educated in China, moving in 1938 from Beijing to Kunming to

  avoid the Japanese invasion of China. He graduated four years later

  from National Southwestern Associated University and remained

  there for another two years. There he met another student who had

  been forced to relocate to Kunming, Tsung-Dao Lee. While they

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  only had a marginal acquaintance with the United States, in 1946

  both of them received scholarships set up by the US government,

  with funds received from China to allow talented Chinese students

  to pursue graduate study in America. Yang had a master’s degree and

  therefore had greater freedom to pursue a PhD, and went with Fermi

  from Columbia to the University of Chicago. Lee had less choice, as

  he did not have a master’s degree, but the only US university where

  he could work directly toward a PhD was also the University of

  Chicago. Yang did his PhD under the supervision of Edward Teller

  and worked directly with Fermi as his assistant for only a year after

  graduation, while Lee did his PhD with Fermi directly.

  During the 1940s, the University of Chicago was one of the

  greatest centers of theoretical and experimental physics in the

  country, and its graduate students benefited from their exposure to a

  remarkable set of scientists—not only Fermi and Teller, but others

  including the brilliant but unassuming astrophysicist Subrahmanyan

  Chandrasekhar. When he was nineteen, Chandra, as he was often

  called by colleagues, had proved that stars greater than 1.4 times the

  mass of the Sun must collapse catastrophically at the end of their

  nuclear-burning lifetime, either through what is now known to be a

  supernova explosion, or directly in what is now known as a black

  hole. While his theory was ridiculed at the time, he was awarded the

  Nobel Prize for that work fifty-three years later.

  Chandra was not just a brilliant scientist but, like Fermi, a

  dedicated teacher. Even though he was pursuing research at the

  Yerkes Observatory in Wisconsin, he drove one hundred miles

  round-trip each week to teach a class to just two registered students,

  Lee and Yang. Ultimately, the entire class, professor included,

  became Nobel laureates, which is probably unique in the history of

  science.

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  Yang moved to the venerable Institute for Advanced Study in

  Princeton in 1949, where he nurtured his budding collaboration

  with Lee on a variety of topics. In 1952 Yang was made a permanent

  member of the institute, while Lee moved in 1953 to nearby

  Columbia in New York City, where he remained for the rest of his

  career.

  Each of these men made major contributions to physics in a

  variety of areas, but the collaboration that made them famous began

  with a strange experimental result, again coming from cosmic-ray

  observations.

  In the same year that Yang moved from Chicago to the IAS, Cecil

  Powell, the discoverer of the pion, discovered a new particle in

  cosmic rays, which he called the tau meson. This particle was

  observed to decay into three pions. Another new particle was

  discovered shortly thereafter, called the theta meson, which decayed

  into two pions. Surprisingly, this new particle turned out to have

  precisely the same mass and lifetime as that tau meson.

  This might not seem that strange. Might they be the same

  particle, simply observed to decay in two different ways? Remember

  that in quantum mechanics, anything that is not forbidden can

  happen, and as long as the new particle was heavy enough to decay

  into either two or three pions—and the weak force allowed such

  decays—both should occur.

  But, if it were sensible, the weak force shouldn’t have allowed

  both decays.

  Think for a moment about your hands. Your left hand differs

  from your right hand. No simple physical process, short of entering

  through the looking glass, can convert one into the other. No series

  of movements, up or down, turning around, or jumping up and

  down, can turn one into the other.

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  The forces that govern our experience, electromagnetism and

  gravity, are blind to the distinction between left and right. No

  process moderated by either force can turn something such as your

  right hand into its mirror image. I cannot turn your right hand into

  your left hand merely by shining light on it, for example.

  Put another way, if I shine a light on your right hand and look at it

  from a distance, the intensity of reflected light will be the same as it

  would be if I did the same thing to your left hand. The light doesn’t

  care about left or right when it is reflecting off something.

  Our definition of left and right is imposed by human convention.

  Tomorrow we could decide that left is right and vice versa, and

  nothing would change except our labels. As I write this on an

  airplane, flying economy class, the person to my right may be quite

  different from the person to my left, but again that is just an accident

  of my circumstances. I don’t expect that the laws governing the flight

  of this plane are different for the right wing than for the left wing.

  Think about this in the subatomic world. Recall that Enrico Fermi

  found that, given the rules of quantum mechanics, the mathematical

  behavior of groups or pairs of elementary particles depends on

  whether they have spin ½, i.e., are fermions. The behavior of groups

  of fermions is quite different from the behavior of particles such as

  photons, which have a spin value of 1 (or any integer value of spin

  angular momentum, i.e., 0, 1, 2, 3, etc.). The mathematical “wave

  function” that describes a pair of fermions, for example, is

  “antisymmetric,” while one describing a pair of photons is

  “symmetric.” This means that if one interchanges one particle with

  another, the wave function describing fermions changes sign. But for

  particles such as photons, the wave function remains the same under

  such an interchange.

  Interchanging two particles is the same as reflecting them in the

  mirror. The one on the left now becomes the one on the right. Thus

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  an intimate connection exists between such exchanges and what

  physicists call parity, which is the overall property of a system under

  reflection (i.e., interchanging left and right).

  If an elementary particle decays into two other particles, the wave

  function describing the “parity” of the final state (i.e., whether the

  wave function changes sign or not under left-right interchange of

  the particles) allows us then to assign a quantity we can call parity to

  the initial particle. In quantum mechanics if the force that governs

  the decay is blind to left and right, then the decay will not change the

  parity of the
quantum state of the system.

  If the wave function of the system is antisymmetric under

  interchange of the particles after the decay, then the system has

  “negative” parity. In this case the wave function describing the initial

  quantum state of the decaying particle must also have negative parity

  (i.e., it would change sign if left and right were interchanged).

  Now, pions, the particles discovered by Powell and hypothesized

  by Yukawa, have negative parity, so that the wave function that

  describes the quantum state of their mirror image would change sign

  compared to the original wave function. The distinction between

  positive and negative parity is kind of like considering first a nice

  spherical ball, which looks identical when reflected in the mirror,

  and hence has positive parity:

  Versus, say, your hand, which changes character (from left to right)

  when reflected in a mirror and could therefore be said to have

  negative parity:

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  These somewhat abstract considerations made the observed data

  on the decays of the new particles that Powell discovered perplexing.

  Because a pion has negative parity, two pions would have positive

  parity, since (−1)2 = 1. A system of three pions, however, would, by

  the same consideration, have negative parity, since (−1)3 = −1.

  Therefore if parity doesn’t change when a particle decays, a single

  original particle cannot decay into two different final states of

  different parity.

  If the force responsible for the decay behaved like all the other

  known forces at the time, such as electromagnetism or gravity, it

  would be blind to parity (it would not distinguish between right and

  left), so it shouldn’t change the original parity of the system after the

  decay, just as shining a light on your right hand will not cause it to

  look like your left hand.

  Since it seemed impossible for a single type of particle to decay

  sometimes into two, and sometimes into three, pions, the solution

  seemed simple. There must be two different new elementary

  particles, with opposite parity properties. Powell dubbed these the

  tau particle and theta particle—one of which could decay into two

  pions, and one into three pions.

  Observations suggested that the two particles had precisely the

  same masses and lifetimes, which was a bit strange, but Lee and