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 is much
larger than the size of an atom, we cannot hope to “look” at the parts of an
atom in the ordinary way. We need to use something with a much smaller
wave-length. As we saw in the last chapter, quantum mechanics tells us that all
particles are in fact waves, and that the higher the energy of a particle, the
smaller the wavelength of the corresponding wave. So the best answer we can
give to our question depends on how high a particle energy we have at our
disposal, because this determines on how small a length scale we can look.
These particle energies are usually measured in units called electron volts.
(In Thomson’s experiments with electrons, we saw that he used an electric field
to accelerate the electrons. The energy that an electron gains from an electric
field of one volt is what is known as an electron volt.) In the nineteenth
century, when the only particle energies that people knew how to use were the
low energies of a few electron volts generated by chemical reactions such as
burning, it was thought that atoms were the smallest unit. In Rutherford’s
experiment, the alpha-particles had energies of millions of electron volts.
More recently, we have learned how to use electromagnetic fields to give
particles energies of at first millions and then thousands of millions of
electron volts. And so we know that particles that were thought to be
“elementary” thirty years ago are, in fact, made up of smaller particles. May
these, as we go to still higher energies, in turn be found to be made from
still smaller particles? This is certainly possible, but we do have some
theoretical reasons for believing that we have, or are very near to, a
knowledge of the ultimate building blocks of nature. Using the wave/particle
duality discussed in the last chapter, every-thing in the universe, including
light and gravity, can be described in terms of particles. These particles have
a property called spin. One way of thinking of spin is to imagine the particles
as little tops spinning about an axis. However, this can be misleading, because
quantum mechanics tells us that the particles do not have any well-defined
axis. What the spin of a particle really tells us is what the particle looks
like from different directions. A particle of spin 0 is like a dot: it looks
the same from every direction Figure 1-i. On the other hand, a particle of
spin 1 is like an arrow: it looks different from different directions Figure 1-ii. Only if one turns it round a complete revolution (360 degrees) does the
particle look the same. A particle of spin 2 is like a double-headed arrow
Figure 1-iii: it looks the same if one turns it round half a revolution (180
degrees). Similarly, higher spin particles look the same if one turns them
through smaller fractions of a complete revolution. All this seems fairly
straightforward, but the remark-able fact is that there are particles that do
not look the same if one turns them through just one revolution: you have to
turn them through two complete revolutions! Such particles are said to
have spin ½.
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| Figure 1 |
All the
known particles in the universe can be divided into two groups: particles of
spin ½, which make up the matter in the universe, and particles of spin 0, 1,
and 2, which, as we shall see, give rise to forces between the matter
particles. The matter particles obey what is called Pauli’s exclusion
principle. This was discovered in 1925 by an Austrian physicist, Wolfgang Pauli
– for which he received the Nobel Prize in 1945. He was the archetypal
theoretical physicist: it was said of him that even his presence in the same
town would make experiments go wrong! Pauli’s exclusion principle says that two
similar particles can-not exist in the same state; that is, they cannot have
both the same position and the same velocity, within the limits given by the uncertainty
principle. The exclusion principle is crucial because it explains why matter
particles do not collapse to a state of very high density under the influence
of the forces produced by the particles of spin 0, 1, and 2: if the matter
particles have very nearly the same positions, they must have different
velocities, which means that they will not stay in the same position for long.
If the world had been created without the exclusion principle, quarks would not
form separate, well-defined protons and neutrons. Nor would these, together
with electrons, form separate, well-defined atoms. They would all collapse to
form a roughly uniform, dense “soup.” A proper understanding of the electron
and other spin-½ particles did not come until 1928, when a theory was proposed
by Paul Dirac, who later was elected to the Lucasian Professorship of
Mathematics at Cambridge (the same professorship that Newton had once held and
that I now hold). Dirac’s theory was the first of its kind that was consistent
with both quantum mechanics and the special theory of relativity. It explained
mathematically why the electron had spin-½; that is, why it didn’t look the
same if you turned it through only one complete revolution, but did if you
turned it through two revolutions. It also predicted that the electron should
have a partner: an anti-electron, or positron. The discovery of the positron in
1932 confirmed Dirac’s theory and led to his being awarded the Nobel Prize for
physics in 1933. We now know that every particle has an antiparticle, with
which it can annihilate. (In the case of the force-carrying particles, the
antiparticles are the same as the particles themselves.) There could be whole
antiworlds and antipeople made out of antiparticles. However, if you meet your
antiself, don’t shake hands! You would both vanish in a great flash of light.
The question of why there seem to be so many more particles than antiparticles
around us is extremely important, and I shall return to it later in the
chapter. In quantum mechanics, the forces or interactions between matter
particles are all supposed to be carried by particles of integer spin – 0, 1,
or 2. What happens is that a matter particle, such as an electron or a quark,
emits a force-carrying particle. The recoil from this emission changes the
velocity of the matter particle. The force-carrying particle then collides with
another matter particle and is absorbed. This collision changes the velocity of
the second particle, just as if there had been a force between the two matter
particles. It is an important property of ' the force-carrying particles that
they do not obey the exclusion principle. This means that there is no limit to
the number that can be exchanged, and so they can give rise to a strong force.
However, if the force-carrying particles have a high mass, it will be difficult
to produce and exchange them over a large distance. So the forces that they
carry will have only a short range. On the other hand, if the force-carrying
particles have no mass of their own, the forces will be long range. The
force-carrying particles exchanged between matter particles are said to be
virtual particles because, unlike “real” particles, they cannot be directly
detected by a particle detector. We know they exist, however, because they do have
a measurable effect: they give rise to forces between matter particles.
Particles of spin 0, 1, or 2 do also exist in some circumstances as real
particles, when they can be directly detected. They then appear to us as what a
classical physicist would call waves, such as waves of light or gravitational
waves. They may sometimes be emitted when matter particles interact with each
other by exchanging virtual force-carrying particles. (For example, the
electric repulsive force between two electrons is due to the exchange of
virtual photons, which can never be directly detected; but if one electron
moves past another, real photons may be given off, which we detect as light
waves.) Force-carrying particles can be grouped into four categories according
to the strength of the force that they carry and the particles with which they
interact. It should be emphasized that this division into four classes is
man-made; it is convenient for the construction of partial theories, but it may
not correspond to anything deeper. Ultimately, most physicists hope to find a
unified theory that will explain all four forces as different aspects of a
single force. Indeed, many would say this is the prime goal of physics today.
Recently, successful attempts have been made to unify three of the four
categories of force – and I shall describe these in this chapter. The question
of the unification of the remaining category, gravity, we shall leave till
later. The first category is the gravitational force. This force is universal,
that is, every particle feels the force of gravity, according to its mass or
energy. Gravity is the weakest of the four forces by a long way; it is so weak
that we would not notice it at all were it not for two special properties that
it has: it can act over large distances, and it is always attractive. This
means that the very weak gravitational forces between the individual particles
in two large bodies, such as the earth and the sun, can all add up to produce a
significant force. The other three forces are either short range, or are
sometimes attractive and some-times repulsive, so they tend to cancel out. In
the quantum mechanical way of looking at the gravitational field, the force
between two matter particles is pictured as being carried by a particle of spin
2 called the graviton. This has no mass of its own, so the force that it
carries is long range. The gravitational force between the sun and the earth is
ascribed to the exchange of gravitons between the particles that make up these
two bodies. Although the exchanged particles are virtual, they certainly do
produce a measurable effect – they make the earth orbit the sun! Real gravitons
make up what classical physicists would call gravitational waves, which are
very weak – and so difficult to detect that they have not yet been observed.
The next category is the electromagnetic force, which interacts with
electrically charged particles like electrons and quarks, but not with
uncharged particles such as gravitons. It is much stronger than the
gravitational force: the electromagnetic force between two electrons is about a
million million million million million million million (1 with forty-two zeros
after it) times bigger than the gravitational force. However, there are two
kinds of electric charge, positive and negative. The force between two positive
charges is repulsive, as is the force between two negative charges, but the
force is attractive between a positive and a negative charge. A large body,
such as the earth or the sun, contains nearly equal numbers of positive and
negative charges. Thus the attractive and repulsive forces between the
individual particles nearly cancel each other out, and there is very little net
electromagnetic force. However, on the small scales of atoms and molecules,
electromagnetic forces dominate. The electromagnetic attraction between
negatively charged electrons and positively charged protons in the nucleus
causes the electrons to orbit the nucleus of the atom, just as gravitational
attraction causes the earth to orbit the sun. The electromagnetic attraction is
pictured as being caused by the exchange of large numbers of virtual massless
particles of spin 1, called photons. Again, the photons that are exchanged are
virtual particles. However, when an electron changes from one allowed orbit to
another one nearer to the nucleus, energy is released and a real photon is
emitted – which can be observed as visible light by the human eye, if it has
the right wave-length, or by a photon detector such as photographic film.
Equally, if a real photon collides with an atom, it may move an electron from
an orbit nearer the nucleus to one farther away. This uses up the energy of the
photon, so it is absorbed. The third category is called the weak nuclear force,
which is responsible for radioactivity and which acts on all matter particles
of spin-½, but not on particles of spin 0, 1, or 2, such as photons and
gravitons. The weak nuclear force was not well understood until 1967, when
Abdus Salam at Imperial College, London, and Steven Weinberg at Harvard both 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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| Figure 2 : a proton and an antiproton collide at high energy, producing a couple of almost free quarks |
Figure 2
shows a photograph of a collision between a high-energy proton and antiproton.
The success of the unification of the electromagnetic and weak nuclear forces
led to a number of attempts to combine these two forces with the strong nuclear
force into what is called a grand unified theory (or GUT). This title is rather
an exaggeration: the resultant theories are not all that grand, nor are they
fully unified, as they do not include gravity. Nor are they really complete
theories, because they contain a number of parameters whose values cannot be
predicted from the theory but have to be chosen to fit in with experiment.
Nevertheless, they may be a step toward a complete, fully unified theory. The
basic idea of GUTs is as follows: as was mentioned above, the strong nuclear
force gets weaker at high energies. On the other hand, the electromagnetic and
weak forces, which are not asymptotically free, get stronger at high energies.
At some very high energy, called the grand unification energy, these three
forces would all have the same strength and so could just be different aspects
of a single force. The GUTs also predict that at this energy the different
spin-½ matter particles, like quarks and electrons, would also all be
essentially the same, thus achieving another unification. The value of the
grand unification energy is not very well known, but it would probably have to
be at least a thousand million million GeV. The present generation of particle
accelerators can collide particles at energies of about one hundred GeV, and
machines are planned that would raise this to a few thousand GeV. But a machine
that was powerful enough to accelerate particles to the grand unification
energy would have to be as big as the Solar System – and would be unlikely to
be funded in the present economic climate. Thus it is impossible to test grand
unified theories directly in the laboratory. However, just as in the case of
the electromagnetic and weak unified theory, there are low-energy consequences
of the theory that can be tested. The most interesting of these is the
prediction that protons, which make up much of the mass of ordinary matter, can
spontaneously decay into lighter particles such as antielectrons. The reason
this is possible is that at the grand unification energy there is no essential
difference between a quark and an antielectron. The three quarks inside a
proton normally do not have enough energy to change into antielectrons, but
very occasionally one of them may acquire 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!)
There is a
mathematical theorem that says that any theory that obeys quantum mechanics and
relativity must always obey the combined symmetry CPT. In other words, the
universe would have to behave the same if one replaced particles by
antiparticles, took the mirror image, and also reversed the direction of time.
But Cronin and Fitch showed that if one replaces particles by antiparticles and
takes the mirror image, but does not reverse the direction of time, then the
universe does not behave the same. The laws of physics, therefore, must change
if one reverses the direction of time – they do not obey the symmetry T.
Certainly the early universe does not obey the symmetry T: as time runs forward
the universe expands – if it ran backward, the universe would be contracting.
And since there are forces that do not obey the symmetry T, it follows that as
the universe expands, these forces could cause more antielectrons to turn into
quarks than electrons into antiquarks. Then, as the universe expanded and
cooled, the antiquarks would annihilate with the quarks, but since there would
be more quarks than antiquarks, a small excess of quarks would remain. It is
these that make up the matter we see today and out of which we ourselves are
made. Thus our very existence could be regarded as a confirmation of grand
unified theories, though a qualitative one only; the uncertainties are such
that one cannot predict the numbers of quarks that will be left after the
annihilation, or even whether it would be quarks or antiquarks that would
remain. (Had it been an excess of antiquarks, however, we would simply have
named antiquarks quarks, and quarks antiquarks.) Grand unified theories do not
include the force of gravity. This does not matter too much, because gravity is
such a weak force that its effects can usually be neglected when we are dealing
with elementary particles or atoms. However, the fact that it is both long
range and always attractive means that its effects all add up. So for a
sufficiently large number of matter particles, gravitational forces can
dominate over all other forces. This is why it is gravity that determines the
evolution of the universe. Even for objects the size of stars, the attractive
force of gravity can win over all the other forces and cause the star to
collapse. My work in the 1970s focused on the black holes that can result from
such stellar collapse and the intense gravitational fields around them. It was
this that led to the first hints of how the theories of quantum mechanics and
general relativity might affect each other – a glimpse of the shape of a
quantum theory of gravity yet to come.
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