Page 36 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
CHAPTER 5

ELEMENTARY PARTICLES AND THE FORCES OF NATURE

Aristotle believed that all the matter in the universe was made up of four basic elements – earth, air, fire, and water.
These elements were acted on by two forces: gravity, the tendency for earth and water to sink, and levity, the tendency
for air and fire to rise. This division of the contents of the universe into matter and forces is still used today. Aristotle
believed that matter was continuous, that is, one could divide a piece of matter into smaller and smaller bits without any
limit: one never came up against a grain of matter that could not be divided further. A few Greeks, however, such as
Democritus, held that matter was inherently grainy and that everything was made up of large numbers of various different
kinds of atoms. (The word atom means “indivisible” in Greek.) For centuries the argument continued without any real
evidence on either side, but in 1803 the British chemist and physicist John Dalton pointed out that the fact that chemical
compounds always combined in certain proportions could be explained by the grouping together of atoms to form units
called molecules. However, the argument between the two schools of thought was not finally settled in favor of the
atomists until the early years of this century. One of the important pieces of physical evidence was provided by Einstein.
In a paper written in 1905, a few weeks before the famous paper on special relativity, Einstein pointed out that what was
called Brownian motion – the irregular, random motion of small particles of dust suspended in a liquid – could be
explained as the effect of atoms of the liquid colliding with the dust particles.

By this time there were already suspicions that these atoms were not, after all, indivisible. Several years previously a
fellow of Trinity College, Cambridge, J. J. Thomson, had demonstrated the existence of a particle of matter, called the
electron, that had a mass less than one thousandth of that of the lightest atom. He used a setup rather like a modern TV
picture tube: a red-hot metal filament gave off the electrons, and because these have a negative electric charge, an
electric field could be used to accelerate them toward a phosphor-coated screen. When they hit the screen, flashes of
light were generated. Soon it was realized that these electrons must be coming from within the atoms themselves, and in
1911 the New Zealand physicist Ernest Rutherford finally showed that the atoms of matter do have internal structure:
they are made up of an extremely tiny, positively charged nucleus, around which a number of electrons orbit. He deduced
this by analyzing the way in which alpha-particles, which are positively charged particles given off by radioactive atoms,
are deflected when they collide with atoms.
At first it was thought that the nucleus of the atom was made up of electrons and different numbers of a positively
charged particle called the proton, from the Greek word meaning “first,” because it was believed to be the fundamental
unit from which matter was made. However, in 1932 a colleague of Rutherford’s at Cambridge, James Chadwick,
discovered that the nucleus contained another particle, called the neutron, which had almost the same mass as a proton
but no electrical charge. Chadwick received the Nobel Prize for his discovery, and was elected Master of Gonville and
Caius College, Cambridge (the college of which I am now a fellow). He later resigned as Master because of
disagreements with the Fellows. There had been a bitter dispute in the college ever since a group of young Fellows
returning after the war had voted many of the old Fellows out of the college offices they had held for a long time. This
was before my time; I joined the college in 1965 at the tail end of the bitterness, when similar disagreements forced
another Nobel Prize – winning Master, Sir Nevill Mott, to resign.

Up to about thirty years ago, it was thought that protons and neutrons were “elementary” particles, but experiments in
which protons were collided with other protons or electrons at high speeds indicated that they were in fact made up of
smaller particles. These particles were named quarks by the Caltech physicist Murray Gell-Mann, who won the Nobel
Prize in 1969 for his work on them. The origin of the name is an enigmatic quotation from James Joyce: “Three quarks for
Muster Mark!” The word quark is supposed to be pronounced like quart, but with a k at the end instead of a t, but is
usually pronounced to rhyme with lark.

There are a number of different varieties of quarks: there are six “flavors,” which we call up, down, strange, charmed,
bottom, and top. The first three flavors had been known since the 1960s but the charmed quark was discovered only in
1974, the bottom in 1977, and the top in 1995. Each flavor comes in three “colors,” red, green, and blue. (It should be
emphasized that these terms are just labels: quarks are much smaller than the wavelength of visible light and so do not
have any color in the normal sense. It is just that modern physicists seem to have more imaginative ways of naming new
particles and phenomena – they no longer restrict themselves to Greek!) A proton or neutron is made up of three quarks,
one of each color. A proton contains two up quarks and one down quark; a neutron contains two down and one up. We
can create particles made up of the other quarks (strange, charmed, bottom, and top), but these all have a much greater
mass and decay very rapidly into protons and neutrons.

We now know that neither the atoms nor the protons and neutrons within them are indivisible. So the question is: what
are the truly elementary particles, the basic building blocks from which everything is made? Since the wavelength of light

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