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

by Why Are We Here (pdf)

  quantum mechanics and relativity, had been built on exploring the

  nature of light. Yet it wasn’t clear how the elegant theoretical edifice

  of quantum electrodynamics could guide considerations of a new

  force. The weak interaction is far removed from direct human

  experience and involves new and exotic elementary particles and

  nuclear transmutations reminiscent of alchemy but, unlike alchemy,

  testable and reproducible.

  The fundamental confusion lay with the nature of the atomic

  nucleus itself and the question of what held it together. The

  discovery of the neutron helped resolve the paradox that had earlier

  seemed to require electrons to be confined in the nucleus to counter

  the charge of additional protons necessary to produce correct

  nuclear masses, but the observation of beta decay—which resulted in

  electrons emerging from nuclei—didn’t help matters.

  ͝͡͝

  The realization that in beta decay neutrons became protons in the

  nucleus clarified matters, but then another question naturally arose:

  Could this transformation somehow explain the strong binding that

  held protons and neutrons together inside nuclei?

  In spite of the obvious differences between the weak forces and

  quantum theory of electromagnetism, QED, the remarkable success

  of QED in describing the behavior of atoms and the interactions of

  electrons with light colored physicists’ thinking about the new weak

  force as well. The mathematical symmetries associated with QED

  worked beautifully to ensure that otherwise worrisome infinities in

  the calculations arising from the exchange of virtual particles

  vanished when making predictions of physical quantities. Would

  something similar work to understand the force binding protons and

  neutrons in nuclei?

  Specifically, if the electromagnetic force was due to the exchange

  of particles, then it was reasonable to think that the force that held

  together the nucleus might also be due to the exchange of particles.

  Werner Heisenberg proposed this idea in 1932 around the time the

  neutron was discovered. If neutrons and protons could convert into

  each other, with the proton absorbing an electron to become a

  neutron, then maybe the exchange of electrons between them might

  somehow produce a binding force?

  A number of well-known problems marred this picture, however.

  First was the problem of “spin.” If one assumed, as Heisenberg did,

  that the neutron was essentially made up of a proton and an electron

  bound together, and since both were spin ½ particles, then adding

  them together in the neutron, it couldn’t have spin ½ as well, since ½

  + ½ can’t equal ½. Heisenberg argued, in desperation, because those

  were desperate times when it seemed all the conventional rules were

  breaking down, that the “electron” that was transferred between

  neutrons and protons, and which bound them together in the

  ͝͡͞

  nucleus, was somehow different from a free electron and had no spin

  at all.

  In retrospect, this picture has another problem. Heisenberg was

  motivated to consider electrons binding together neutrons and

  protons because he was thinking about hydrogen molecules. In

  hydrogen, two protons are bound together by sharing electrons that

  orbit them. The problem with using a similar explanation for

  nuclear binding is one of scale. How could neutrons and protons

  exchange electrons and be bound together so tightly that their

  average distance apart is more than one hundred thousand times

  smaller than the size of hydrogen molecules?

  Here is another way of thinking about this problem that will be

  useful to return to later. Recall that electromagnetism is a long-range

  force. Two electrons on opposite sides of the galaxy experience a

  repulsion—albeit extremely small—due to the exchange of virtual

  photons. The quantum theory of electromagnetism makes this

  possible. Photons are massless, and virtual photons can travel

  arbitrarily far, carrying arbitrarily small amounts of energy, before

  they are absorbed again—without violating the Heisenberg

  uncertainty principle. If the photons were massive, then this would

  not be possible.

  Now if a force between neutrons and protons in nuclei arose due

  to the absorption and emission of virtual electrons, say, then the

  force would be short-range because the electrons are massive. How

  short-range? Well, it works out to be about one hundred times the

  size of typical nuclei. So, exchanging electrons doesn’t work to

  produce nuclear-scale forces. As I say, those were desperate times.

  Heisenberg’s desperate idea about a strange spinless version of the

  electron was not lost on a young Japanese physicist, the shy twenty-

  eight-year-old Hideki Yukawa. Working in 1935 when Japan was just

  beginning to emerge from centuries of isolation, and just before its

  ͟͝͡

  imperial designs ignited the war in the Pacific, Yukawa published the

  first original work in physics to be published by a physicist educated

  entirely in Japan. No one took notice of the paper for at least two

  years, yet fourteen years later he won the Nobel Prize for this work,

  which had by then become noticed, but for the wrong reasons.

  Einstein’s visit to Japan in 1922 had cemented Yukawa’s growing

  interest in physics. When Yukawa was still in high school and

  searching for material to help him pass examinations in a second

  foreign language, he found Max Planck’s Introduction to Theoretical

  Physics in German. He rejoiced in reading both the German and the

  physics and was aided by his classmate Sin-Itiro Tomonaga, a

  talented physicist who was his colleague both in high school and

  later at Kyoto University. Tomonaga was so talented that he would

  later share the 1965 Nobel Prize with Richard Feynman and Julian

  Schwinger for demonstrating the mathematical consistency of

  quantum electrodynamics.

  That Yukawa, who had been a student in Japan at a time when

  many of his instructors did not yet fully understand the emerging

  field of quantum mechanics, came upon a possible solution to the

  nuclear-force problem that had been overlooked by Heisenberg,

  Pauli, and even Fermi was remarkable. I suspect that part of the

  problem was a phenomenon that has occurred several times in the

  twentieth century and perhaps before, and perhaps after. When the

  paradoxes and complexities associated with some physical process

  begin to seem overwhelming, it is tempting to assume that some

  new revolution, similar to relativity or quantum mechanics, will

  require such a dramatic shift in thinking that it doesn’t make sense

  to push forward with existing techniques.

  Fermi, unlike Heisenberg or Pauli, was not looking for a

  wholesale revolution. He was willing to propose, as he called it, a

  “tentative theory” of neutron decay that got rid of electrons in the

  ͝͡͠

  nucleus by allowing them to be spontaneously created during beta

  decay. He proposed a model that worked, which
he knew was just a

  model and not a complete theory, but it did allow one to do

  calculations and make predictions. That was the essence of Fermi’s

  practical style.

  Yukawa

  had

  followed

  these

  developments,

  translated

  Heisenberg’s paper on nuclei along with an introduction, and

  published it in Japan, so the problems of Heisenberg’s proposal were

  already clear to him. Then in 1934 Yukawa read Fermi’s theory of

  neutron decay, which catalyzed a new idea in Yukawa’s mind.

  Perhaps the nuclear force binding protons and neutrons was due not

  to the exchange of virtual electrons between them, but to the

  exchange of both the electron and the neutrino that were created

  when neutrons changed to protons.

  Another problem immediately arose, however. Neutron decay is a

  result of what would become known as the weak interaction, and the

  force responsible for it is weak. Plugging in values for the possible

  force that might result between protons and neutrons by the

  exchange of an electron-neutrino pair made it clear that this force

  would be far too weak to bind them.

  Yukawa then allowed himself to do what none of the others had

  done. He questioned why the nuclear force, if it, like QED, results

  from the exchange of virtual particles, had to be due to the exchange

  of one or more of the particles already known or assumed to exist.

  Remembering how loath physicists such as Dirac and Pauli had been

  to propose new particles, even when they were correct, you can

  perhaps appreciate how radical Yukawa’s idea was. As Yukawa later

  described it:

  At this period the atomic nucleus was inconsistency itself, quite

  inexplicable. And why?—because our concept of elementary

  particle was too narrow. There was no such word in Japanese and

  ͝͡͡

  we used the English word—it meant proton and electron. From

  somewhere had come a divine message forbidding us to think

  about any other particle. To think outside of these limits (except

  for the photon) was to be arrogant, not to fear the wrath of the

  gods. It was because the concept that matter continues forever

  had been a tradition since the times of Democritus and Epicurus.

  To think about creation of particles other than photons was

  suspect, and there was a strong inhibition of such thoughts that

  was almost unconscious.

  One of my good physics friends has said that the only time he was

  able to do complicated calculations was after the birth of each of his

  children, when he couldn’t sleep anyway, so he stayed up and

  worked. Thus in October of 1934, just after the birth of his second

  child and unable to sleep, Yukawa realized that if the range of the

  strong nuclear force was to be restricted to the size of a nucleus, then

  any exchanged particle must be far more massive than the electron.

  The next morning he estimated the mass to be two hundred times

  the electron mass. It would have to carry an electric charge if it was

  to be exchanged between neutrons and protons, and it could have no

  spin, so as not to change the proton’s or neutron’s spin when it was

  absorbed or emitted.

  What has all this concern over strong nuclear forces to do with

  neutron decay, the subject that started this chapter and ended the

  last? you may ask. In the 1930s, just as it went against the grain to

  imagine new particles, so too inventing new forces seemed

  unnecessary at best and heretical at worst. Physicists were convinced

  that all the processes that occurred in the nucleus, strong or weak,

  must be connected.

  Yukawa envisaged a clever way to do this, connecting ideas of

  both Fermi and Heisenberg, and also generalizing ideas from the

  successful quantum theory of electromagnetism. If instead of

  ͢͝͡

  emitting a photon, neutrons in the nucleus emitted a new, heavy,

  spinless charged particle, which Yukawa originally called a mesotron

  —until Heisenberg corrected Yukawa’s Greek and the name was

  shortened to meson—then that particle could be absorbed by

  protons in the nucleus, producing a force of attraction whose

  magnitude Yukawa was able to calculate using equations that were

  extrapolated from, you guessed it, electromagnetism.

  The analogy with electromagnetism could not be exact, however,

  because the meson is massive and the photon is massless. Yukawa

  took the attitude that Fermi might have, if he had thought of it. Yes,

  the theory wasn’t complete, but Yukawa was willing to ignore the

  other aspects of electromagnetism that this theory couldn’t

  reproduce. Damn the torpedoes, full speed ahead.

  Yukawa ingeniously—and ultimately incorrectly—connected this

  strong force to observed neutron decay by suggesting that mesons

  might not always simply be exchanged between neutrons and

  protons in the nucleus. A small fraction of the mesons emitted by

  neutrons might decay en route into an electron and neutrino before

  they could be reabsorbed, causing neutron decay. In this case, the

  neutron decay would not be described by something like the figure

  below and on the left, where the decay and the emission of the other

  particles all occur at a single point. It would appear like the figure on

  the right, where the decay gets spread out and a new particle, shown

  by the dashed line (which represents Yukawa’s meson), travels a

  short distance after emission before decaying into the electron and

  neutrino. With the new intermediate particle, the weak interaction

  mediating neutron decay begins to look more like the

  electromagnetic interaction between charged particles:

  ͣ͝͡

  Yukawa had proposed a new intermediate particle, a heavy

  meson, which made neutron decay look similar to the earlier picture

  of photon exchange in electromagnetism—which had motivated his

  thinking in the first place—but with significant differences. In this

  case the intermediate particle was both massive and electrically

  charged, and also unlike the photon it had no spin angular

  momentum.

  Nevertheless, Yukawa was able to show that for a heavy meson

  his theory would be indistinguishable from Fermi’s point interaction

  describing neutron decay—at least for predicting the details of

  neutron decay. In addition, Yukawa’s theory offered the possibility of

  reducing all of the strange properties of the nucleus—from beta

  decay of neutrons inside the nucleus to the strength of the

  interaction binding together protons and neutrons—to merely

  understanding the properties of a single new interaction, due to the

  exchange of a new particle, his meson.

  However, if this new heavy meson existed, where was it? Why

  hadn’t it yet been seen in cosmic rays? Because of this, and also

  because Yukawa was an unknown entity working in a location far

  from all the action, no real attention was paid to his proposal to

  explain both the strong interaction between nucleons and the

  weaker one that appea
red to be responsible for neutron decay.

  Nevertheless, his proposal, unlike those of Heisenberg and others

  (including Fermi), was simpler and made more sense.

  All of this changed in 1936, less than two years after Yukawa’s

  prediction, when Carl Anderson, the discoverer of the positron,

  together with collaborator Seth Neddermeyer, discovered what

  ͤ͝͡

  appeared to be a new set of particles in cosmic rays. The

  characteristics of the tracks of these new particles in cloud chambers

  implied that they produced too little radiation in traversing matter to

  be protons or electrons. They were also more massive than electrons

  and appeared to be sometimes negative and sometimes positive.

  Before long the new particles were determined to have a mass in the

  range that Yukawa had predicted—about two hundred times the

  mass of the electron.

  It is remarkable how quickly the rest of the world caught on.

  Yukawa published a short note to point out that his theory predicted

  just such particles. Within weeks the major physicists in Europe

  began exploring his model and incorporating his ideas in their work.

  In 1938, in the last major conference before the Second World War

  interrupted essentially all international collaborations in science, of

  the eight main speakers, three dealt with Yukawa’s theory—citing a

  name they would have been unfamiliar with a year or two before.

  While much of the rest of the physics world celebrated the

  apparent discovery of Yukawa’s meson, this discovery was not

  without its own problems. In 1940 the decay of a meson to an

  electron, predicted by Yukawa, was observed in cosmic-ray tracks.

  However, over the years 1943 to 1947 it became clear that the

  particles Anderson and Neddermeyer had discovered interacted

  much more weakly with nuclei than Yukawa’s particle should have.

  Something was wrong.

  Three of Yukawa’s Japanese colleagues suggested that mesons

  were of two different sorts, and that a Yukawa-type meson might

  decay into yet another, different and more weakly interacting meson.

  But their articles were in Japanese and didn’t appear in English until

  after the war, by which time a similar proposal had been made by

  the US physicist Robert Marshak.

  ͥ͝͡

  This delay proved fortuitous. New techniques were being

  developed to observe the tracks of cosmic rays in photographic