FOUR fundamental forces. It is an old refrain in physics. Gravity and
electromagnetism, along with the 鈥渟trong鈥� and 鈥渨eak鈥� nuclear forces, seem to
account for all that happens in the world. Gravity holds the stars together and
keeps our feet on the ground. The electromagnetic force binds electrons into
atoms and drives the complex web of chemical reactions that make our bodies
work. And the strong force glues neutrons and protons together into atomic
nuclei, and stands behind the life-giving energy of the Sun. It鈥檚 a nice
picture, satisfying and complete, and with each year, physicists come a little
closer to wrapping it all up in one ultimate unified theory.
Wait a minute. What about the weak force? Stars, atoms and nuclei are held
together by the other three forces. But where does the weak force come in? Is it
really necessary? And would the world be any different without it?
It was 100 years ago when the French physicist Henri Becquerel stumbled over
the effects of the weak force when he discovered radioactivity. Since then the
weak force has dwelt in relative obscurity, overshadowed by its three more
forthcoming sisters. We physicists now have a detailed theory that explains it,
and we can calculate how it works. But that鈥檚 mathematics. What does the weak
force really do?
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A century is a long time for something so fundamental to remain so
ill-understood. But it doesn鈥檛 have to stay that way. Although the weak
interaction only shows its face in the quantum world, it isn鈥檛 really obscure,
and it affects all of us profoundly. To get to grips with it, you only need to
forget about 鈥渨eak鈥� for a few moments and focus instead on the idea of 鈥渇orce鈥�.
For it is in the odd but beautiful shape into which quantum theory twists that
familiar concept that the nature of the weak force is to be found.
Intuitively, we all think of force as something that causes a change. To the
classical physicist, it has the specific meaning set out by Isaac Newton three
centuries ago: a force changes the velocity of an object. But that simple notion
began to evolve early this century, as it became clear that many 鈥渟elf-evident
facts鈥� about nature weren鈥檛 really facts at all.
First, Einstein鈥檚 theory of relativity revealed a profound and unexpected
relation between mass and energy. As a consequence, the law of conservation of
matter is false鈥攎ass can be created and and destroyed. Ordinarily this
effect is tiny and goes unnoticed, but it is a very important effect indeed in
the microworld.
Next, the quantum theory undermined some of the most basic notions in
physics. Concepts such as 鈥減article鈥�, 鈥渨ave鈥� and 鈥渢rajectory鈥� have no precise
meanings in the microworld. Sometimes light behaves as a wave, and at other
times as a stream of particles called photons moving on paths that are
impossible to pin down. The same goes also for all other particles, even atoms
and molecules.
Quantum creation
The marriage of relativity and quantum mechanics led to an astonishingly
powerful theory鈥攓uantum electrodynamics or QED鈥攖hat explains the
structure and properties of atoms. It also leads to a generalisation of the
concept of force by which particles can not only change their velocities, but
can be created or destroyed. In an atom, negatively charged electrons surround a
tiny positively charged nucleus. The attraction between negative and positive
charges holds the atom together. The intricate mathematics of QED describing
this attraction can be portrayed in terms of simple diagrams invented by the
American physicist Richard Feynman. In Feynman鈥檚 diagrammatic language, the
attraction between an atom鈥檚 nucleus and an electron can be thought of as
arising from a virtual exchange of one or more photons. In QED, photons can act
either as carriers of the basic electromagnetic force or as the particle avatars
of light. Furthermore, the collision of two real photons can result in the
creation of two new particles: an electron and its antiparticle, the positron
(see Figure 1).
Positrons, which were discovered in 1932, were likened by
Feynman to electrons travelling backwards in time.
By the end of the 1930s, physicists had all the tools needed to understand
why copper is red, the sky is blue, diamonds are hard and glue is glue. All
these things follow from the electromagnetic interactions among a large number
of electrons and atomic nuclei. But the concept of force still had some more
growing to do to explain phenomena that quantum electrodynamics could not cope
with.
Back in 1913, Ernest Rutherford of the Cavendish Laboratory at Cambridge
University fired some alpha particles at a gold foil and was astonished when a
few bounced right back at him, as if they had slammed into something immovable.
They had鈥攊t was the atomic nucleus. After the discovery of the neutron in
1932, physicists pictured nuclei as clusters of positively charged protons and
uncharged neutrons. But this left a puzzle. Why should such nuclei stick
together? Gravity is far too feeble a force, and protons repel one another
electrically. A third force was needed, one far stronger than either
electromagnetism or gravity.
Out of line
Copying QED, physicists attributed this 鈥渟trong鈥� force to the exchange of
particles between neutrons or protons. But they found that their concept of
force had to be stretched a bit further. The force to which neutrons and protons
are subject had to do more than change an object鈥檚 velocity鈥攊t also had to
be able to change a neutron to a proton, or vice versa. This meant that there
had to be more than one kind of particle that could be exchanged. For when a
proton changes into a neutron, one unit of charge would be lost, unless it were
carried away by the force-carrying particle. In this case, there were three such
particles鈥攖he pions鈥攚ith charges -1, 0 and +1. Like photons, pions
can act as carriers of the strong force, or can be created as real particles
(see Figure 2).
So far, so good: the strong force explains why nuclei hang together. But
there was a further problem: the strong force couldn鈥檛 explain all the ways by
which nuclei sometimes come apart.
In the type of radioactive disintegration called beta decay, a nucleus
spontaneously spits out an electron and an antineutrino鈥攁 particle that is
like an electron except that it has no charge and no mass. Neither of these
particles is present in the parent nucleus, but both are somehow created in the
decay process. Physicists in the 1940s took a stab at describing beta decays in
terms of novel Feynman diagrams
(see Figure 3). But these were funny (and
troublesome) because they involved four lines coming together rather than just
three, as in those for electromagnetic or nuclear forces. Why? Physicists were
trying to build a theory before its time. There was more going on inside the
black dots of the diagrams than anyone then knew. Learning these details would
finally bring them face to face with the weak force鈥攖he force that causes
beta decay鈥攂ut first they had to learn some other things about subnuclear
physics.
One of the most startling facts that we have learnt since those days is that
neutrons and protons are not elementary. Both types of particle are made up of
three smaller quarks. So the diagrams that show protons and neutrons interacting
only by pion exchange don鈥檛 fully describe what is going on. The proton contains
two 鈥渦p鈥� quarks and one 鈥渄own鈥� quark, while the neutron has two down quarks and
one up quark. There are also four other 鈥渇lavours鈥� of quarks鈥攕trange,
charmed, top and bottom鈥攚hich occur in more exotic particles. All these
quarks have fractional electric charges, and also carry another kind of charge
known as 鈥渃olour charge鈥�.
Not elementary
This colour charge is somewhat analogous to ordinary electrical charge.
However, electrical charge comes in two types鈥攑ositive or
negative鈥攚hile colour charge comes in three. That is, each flavour of
quark may appear in any of three colours. The colour force acting between quarks
binds them together three at a time to make particles such as protons or
neutrons. The attraction is extraordinarily powerful鈥攁bout 30 tonnes of
force holds a pair of quarks together.
As in QED, the colour force is mediated by the exchange of
particles鈥攃alled gluons鈥攂etween quarks. But there is a big
difference. Photons do not carry an electric charge, but gluons carry the colour
charge. Thus, gluons can be created or destroyed by other gluons as well as by
quarks. Because gluons carry colour, quarks that exchange them can change their
own colours in the process. In quark talk, the force that holds protons and
neutrons together in nuclei isn鈥檛 fundamental. Protons and neutrons are
colourless, and it is only a pale residue of the force between coloured quarks
that binds nuclei together. From afar, the force looks like pion exchange
(see Figure 4).
But in the more fundamental theory (quantum chromodynamics or QCD)
the diagrams contain nothing but quarks and gluons.
Now it becomes clear how the weak force fits in. If we think of the strong
force as a mechanism by which quarks can change their colours鈥攂y the
exchange of gluons鈥攖hen the weak force is a mechanism by which quarks can
change their flavours. That鈥檚 just what happens in beta decay, where a down
quark becomes an up quark via the exchange of a particle that is peculiar to the
weak force.
If there were no weak force, and quarks couldn鈥檛 change their flavours, the
Universe would be a very different place. To see why, just consider the light
that pours through your window on a fine, sunny morning. It comes from the safe
and steady nuclear furnace of the Sun that has burnt for billions of years. As
explained by the German-American physicist Hans Bethe in the 1930s, the Sun鈥檚
gravity provides the hot, dense conditions deep within its core that make
nuclear fusion possible. The strong force releases nuclear energy as small
nuclei assemble into larger ones. Electromagnetism stabilises the process and
sends solar energy to the Earth. But without the weak force, none of this would
happen. The weak force changes protons in the Sun into neutrons, so that helium
nuclei can be made. Nuclear reactions simply would not take place without the
weak force. And with nothing to oppose gravitational attraction, the
Sun鈥攁nd every other star in the Universe鈥攚ould implode
catastrophically.
So there is no arguing that the weak force is irrelevant. Although the strong
force is the source of the Sun鈥檚 energy, the weak force provides the mechanism
by which it is extracted. But how does it work? That is, how does the weak force
enable quarks to change their flavours?
Shattered symmetry
In the 1930s, some physicists imagined the existence of two heavy charged
particles鈥攖he W+ and W鈥�鈥�
that would carry the weak force just as photons carry the electromagnetic force
(see Figure 5). The large masses of
the conjectured particles would account for the weakness of the weak force. The
virtual particles that carry force have to be created from scratch, thereby
seeming to violate the principle of energy conservation. But they do this on
borrowed time: the heavier the particle, the more costly the loan, and the
weaker the force. It was a nice picture, and it could be made to work after a
fashion. The trouble was that it didn鈥檛 quite fit the facts, and wasn鈥檛 quite
consistent mathematically.
A sensible theory had to introduce a third heavy particle which is today
called the Z0 boson. Furthermore, the three heavy bosons had to be linked to
the massless photon in what is called an 鈥渆lectroweak gauge theory鈥�. But
this created another problem. For the linkage seemed to demand that all four
particles鈥攁nd all other particles as well鈥攕hould be massless. The
big problem was鈥攁nd still is鈥攖o understand how particles get their
masses.
But a simple way out was found by Abdus Salam and Steve Weinberg in 1967.
They argued that all particles were indeed born massless, long ago in the
inferno of the big bang. In the beginning, there was perfect symmetry between
the weak and electromagnetic forces, and between the four particles that carry
them. However, this most symmetric world was as unstable as a pencil balanced on
its point. As the Universe cooled, it underwent a process akin to
crystallisation called 鈥渟pontaneous symmetry breaking鈥� by which particles acquired the
masses they have.
To realise this fairy-tale transformation, they made use of a mathematical
trick that had been devised by Peter Higgs of the University of Edinburgh, which
demands the existence of yet another particle: the not yet found Higgs boson.
The W+, W鈥� and Z0 particles of the electroweak theory have been
discovered, however, and are now routinely produced at large accelerators.
They behave just as the electroweak theory says they should.
So the weak force isn鈥檛 so elusive after all. It鈥檚 closely related to
electromagnetism, but has been relegated to a weaker role by historical
accident. But what would things have been like had the pencil not fallen and the
symmetry between these two forces remained intact?
First, the weak force would be just as strong as the electromagnetic force.
Out of the four force-carrying particles, one would serve as the carrier of
electromagnetic radiation, so our world would still have light. And, in one of
several possible theoretical scenarios, protons and neutrons would have
identical masses.
The strong force would still bind neutrons and protons into nuclei, but not
those we are accustomed to. Atoms would consist of nuclei, electrons and
neutrinos, all bearing charges of one sort or another.
A nucleus with P protons and N neutrons would be surrounded
by P electrons and N neutrinos. Stars might form, and perhaps
even shine. Chemical reactions could still take place, so there might be
molecules, rocks, planets, and maybe even life.
But it would be a very strange world in which atoms could not be ionised or
otherwise taken apart. Electric currents could be generated, but macroscopic
charged bodies would not exist. So there would be no TV tubes, no computer
screens, no fluorescent lights, no thunder, no lightning, and no possibility of
getting a shock after walking on a carpet on a dry day. The weak force would not
exist, but electricians would have to work with several different kinds of
electricity. Compared to this imaginary world, our imperfect world of busted
symmetry doesn鈥檛 seem all that bad.