Page 40 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
proposed theories that unified this interaction with the electromagnetic force, just as Maxwell had unified electricity and
magnetism about a hundred years earlier. They suggested that in addition to the photon, there were three other spin-1
+
particles, known collectively as massive vector bosons, that carried the weak force. These were called W (pronounced
-
W plus), W (pronounced W minus), and Zº (pronounced Z naught), and each had a mass of around 100 GeV (GeV
stands for gigaelectron-volt, or one thousand million electron volts). The Weinberg-Salam theory exhibits a property
known as spontaneous symmetry breaking. This means that what appear to be a number of completely different particles
at low energies are in fact found to be all the same type of particle, only in different states. At high energies all these
particles behave similarly. The effect is rather like the behavior of a roulette ball on a roulette wheel. At high energies
(when the wheel is spun quickly) the ball behaves in essentially only one way – it rolls round and round. But as the wheel
slows, the energy of the ball decreases, and eventually the ball drops into one of the thirty-seven slots in the wheel. In
other words, at low energies there are thirty-seven different states in which the ball can exist. If, for some reason, we
could only observe the ball at low energies, we would then think that there were thirty-seven different types of ball!

In the Weinberg-Salam theory, at energies much greater than 100 GeV, the three new particles and the photon would all
behave in a similar manner. But at the lower particle energies that occur in most normal situations, this symmetry
between the particles would be broken. WE, W, and Zº would acquire large masses, making the forces they carry have a
very short range. At the time that Salam and Weinberg proposed their theory, few people believed them, and particle
-
+
accelerators were not powerful enough to reach the energies of 100 GeV required to produce real W , W , or Zº particles.
However, over the next ten years or so, the other predictions of the theory at lower energies agreed so well with
experiment that, in 1979, Salam and Weinberg were awarded the Nobel Prize for physics, together with Sheldon
Glashow, also at Harvard, who had suggested similar unified theories of the electromagnetic and weak nuclear forces.
The Nobel committee was spared the embarrassment of having made a mistake by the discovery in 1983 at CERN
(European Centre for Nuclear Research) of the three massive partners of the photon, with the correct predicted masses
and other properties. Carlo Rubbia, who led the team of several hundred physicists that made the discovery, received the
Nobel Prize in 1984, along with Simon van der Meer, the CERNengineer who developed the antimatter storage system
employed. (It is very difficult to make a mark in experimental physics these days unless you are already at the top! )

The fourth category is the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the
protons and neutrons together in the nucleus of an atom. It is believed that this force is carried by another spin-1 particle,
called the gluon, which interacts only with itself and with the quarks. The strong nuclear force has a curious property
called confinement: it always binds particles together into combinations that have no color. One cannot have a single
quark on its own because it would have a color (red, green, or blue). Instead, a red quark has to be joined to a green and
a blue quark by a “string” of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron. Another
possibility is a pair consisting of a quark and an antiquark (red + antired, or green + antigreen, or blue + antiblue = white).
Such combinations make up the particles known as mesons, which are unstable because the quark and antiquark can
annihilate each other, producing electrons and other particles. Similarly, confinement prevents one having a single gluon
on its own, because gluons also have color. Instead, one has to have a collection of gluons whose colors add up to white.
Such a collection forms an unstable particle called a glueball.
The fact that confinement prevents one from observing an isolated quark or gluon might seem to make the whole notion
of quarks and gluons as particles somewhat metaphysical. However, there is another property of the strong nuclear
force, called asymptotic freedom, that makes the concept of quarks and gluons well defined. At normal energies, the
strong nuclear force is indeed strong, and it binds the quarks tightly together. However, experiments with large particle
accelerators indicate that at high energies the strong force becomes much weaker, and the quarks and gluons behave
almost like free particles.

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