Page 42 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
sufficient energy to make the transition because the uncertainty principle means that the energy of the quarks inside the
proton cannot be fixed exactly. The proton would then decay. The probability of a quark gaining sufficient energy is so
low that one is likely to have to wait at least a million million million million million years (1 followed by thirty zeros). This is
much longer than the time since the big bang, which is a mere ten thousand million years or so (1 followed by ten zeros).
Thus one might think that the possibility of spontaneous proton decay could not be tested experimentally. However, one
can increase one’s chances of detecting a decay by observing a large amount of matter containing a very large number
of protons. (If, for example, one observed a number of protons equal to 1 followed by thirty-one zeros for a period of one
year, one would expect, according to the simplest GUT, to observe more than one proton decay.)

A number of such experiments have been carried out, but none have yielded definite evidence of proton or neutron
decay. One experiment used eight thousand tons of water and was performed in the Morton Salt Mine in Ohio (to avoid
other events taking place, caused by cosmic rays, that might be confused with proton decay). Since no spontaneous
proton decay had been observed during the experiment, one can calculate that the probable life of the proton must be
greater than ten million million million million million years (1 with thirty-one zeros). This is longer than the lifetime
predicted by the simplest grand unified theory, but there are more elaborate theories in which the predicted lifetimes are
longer. Still more sensitive experiments involving even larger quantities of matter will be needed to test them.
Even though it is very difficult to observe spontaneous proton decay, it may be that our very existence is a consequence
of the reverse process, the production of protons, or more simply, of quarks, from an initial situation in which there were
no more quarks than antiquarks, which is the most natural way to imagine the universe starting out. Matter on the earth is
made up mainly of protons and neutrons, which in turn are made up of quarks. There are no antiprotons or antineutrons,
made up from antiquarks, except for a few that physicists produce in large particle accelerators. We have evidence from
cosmic rays that the same is true for all the matter in our galaxy: there are no antiprotons or antineutrons apart from a
small number that are produced as particle/ antiparticle pairs in high-energy collisions. If there were large regions of
antimatter in our galaxy, we would expect to observe large quantities of radiation from the borders between the regions of
matter and antimatter, where many particles would be colliding with their anti-particles, annihilating each other and giving
off high-energy radiation.

We have no direct evidence as to whether the matter in other galaxies is made up of protons and neutrons or antiprotons
and anti-neutrons, but it must be one or the other: there cannot be a mixture in a single galaxy because in that case we
would again observe a lot of radiation from annihilations. We therefore believe that all galaxies are composed of quarks
rather than antiquarks; it seems implausible that some galaxies should be matter and some antimatter.

Why should there be so many more quarks than antiquarks? Why are there not equal numbers of each? It is certainly
fortunate for us that the numbers are unequal because, if they had been the same, nearly all the quarks and antiquarks
would have annihilated each other in the early universe and left a universe filled with radiation but hardly any matter.
There would then have been no galaxies, stars, or planets on which human life could have developed. Luckily, grand
unified theories may provide an explanation of why the universe should now contain more quarks than antiquarks, even if
it started out with equal numbers of each. As we have seen, GUTs allow quarks to change into antielectrons at high
energy. They also allow the reverse processes, antiquarks turning into electrons, and electrons and antielectrons turning
into antiquarks and quarks. There was a time in the very early universe when it was so hot that the particle energies
would have been high enough for these transformations to take place. But why should that lead to more quarks than
antiquarks? The reason is that the laws of physics are not quite the same for particles and antiparticles.

Up to 1956 it was believed that the laws of physics obeyed each of three separate symmetries called C, P, and T. The
symmetry C means that the laws are the same for particles and antiparticles. The symmetry P means that the laws are
the same for any situation and its mirror image (the mirror image of a particle spinning in a right-handed direction is one
spinning in a left-handed direction). The symmetry T means that if you reverse the direction of motion of all particles and
antiparticles, the system should go back to what it was at earlier times; in other words, the laws are the same in the
forward and backward directions of time. In 1956 two American physicists, Tsung-Dao Lee and Chen Ning Yang,
suggested that the weak force does not in fact obey the symmetry P. In other words, the weak force would make the
universe develop in a different way from the way in which the mirror image of the universe would develop. The same
year, a colleague, Chien-Shiung Wu, proved their prediction correct. She did this by lining up the nuclei of radioactive
atoms in a magnetic field, so that they were all spinning in the same direction, and showed that the electrons were given
off more in one direction than another. The following year, Lee and Yang received the Nobel Prize for their idea. It was
also found that the weak force did not obey the symmetry C. That is, it would cause a universe composed of antiparticles
to behave differently from our universe. Nevertheless, it seemed that the weak force did obey the combined symmetry
CP. That is, the universe would develop in the same way as its mirror image if, in addition, every particle was swapped
with its antiparticle! However, in 1964 two more Americans, J. W. Cronin and Val Fitch, discovered that even the CP
symmetry was not obeyed in the decay of certain particles called K-mesons. Cronin and Fitch eventually received the
Nobel Prize for their work in 1980. (A lot of prizes have been awarded for showing that the universe is not as simple as
we might have thought!)

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