Inside the atom, particles interact through two forces which are never felt in
the everyday world. But they may hold the key to the Universe
ATOMS, the basic building blocks of life, are mainly empty. If the nucleus of
an atom were a centimetre across, then the electron cloud that surrounds the
nucleus would be a kilometre away. So why, when you bang your fist against a
brick wall, does it hurt? The answer is 鈥渂ecause of the electromagnetic force鈥�
鈥� one of the four fundamental forces of nature that we met in Inside Science
No. 15.
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These four forces are all that physicists need to explain the workings of the
everyday world, the Universe at large, and the inside of atoms. Gravity is the
most familiar force, and holds matter together in the form of planets, stars
and galaxies. The electromagnetic force binds matter on the much smaller
atomic scale. It holds the electrons in place around the nucleus, and it holds
the atoms themselves in place alongside their neighbours. It is
electromagnetic interactions between atoms within a brick that prevent its
compression when you hit it, and transmit the impact back into your hand.
The electromagnetic force is not enough, however, to explain all the behaviour
of matter on the subatomic scale. Two additional forces operate deep within
the atom. Moreover, although these forces are not apparent at larger scales,
they have played a part in establishing the Universe as it is today.
Finding forces
Strong attraction
HOW do we know that these forces exist? One clue comes from the basic
structure of matter. Atoms consist of a cloud of negatively charged electrons
that surround a central positively charged nucleus, which generally contains
two kinds of particle 鈥� positive protons and neutral neutrons.
Each proton carries one unit of positive charge, each electron one unit of negative charge. In an atom, the nucleus
contains the same number of protons as there are electrons in the cloud
outside, making the atom neutral overall. Opposite charges attract, but only
up to a certain point; the electrons cannot 鈥渇all in鈥� to the nucleus because
of quantum effects, so they provide the visible face of the atom, largely
responsible for its chemical properties. But like charges repel one another,
so why does the electric force between protons, packed together in a volume
10-14 of a metre across, not blow the nucleus apart?
The answer in part lies with the neutral neutrons which 鈥渄ilute鈥� the electric
force between protons. But then, how are the neutrons held there?
There must be another fundamental force operating within nuclei, stronger than
the electric force. This holds the protons (and neutrons) together within the
nucleus. But this force has no influence outside the nucleus, so it must,
unlike gravity or electromagnetism, be limited to a very short range 鈥� only
about 10-15 metres. It is called the strong force, and experiments
involving collisions between fundamental particles show that within the
nucleus it is about a hundred times stronger than the electric force between
protons.
Colourful quarks
Fundamental truth
WHERE does the strong force come from? To answer this, physicists have tried
to develop an understanding of the strong force using the same kind of
description that works so well for electromagnetism. The electromagnetic force
is the one that physicists understand best, and which is described by a very
satisfactory theory known as quantum electrodynamics, or QED (see Part I).
This uses quantum field theory 鈥� it describes the 鈥渇ield鈥� about an object in
terms of continual absorption and emission of fundamental particles, known as
field quanta.
In QED, the object is anything that carries electric charge, the field is the
electromagnetic field, and the field quanta are photons 鈥� 鈥減articles鈥� of
light. An electric charge is constantly emitting and absorbing individual
鈥渧irtual鈥� photons. If another charged object absorbs one of these photons,
then the objects have influenced each other 鈥� they have interacted via the
electromagnetic force. By analogy with QED, physicists interpret the strong
force in terms of a property that is equivalent to electric charge, associated
with field quanta that are equivalent to photons. This understanding arose
after experiments revealed a deeper level to the structure of matter.
These experiments showed that protons and neutrons are composed of more
fundamental particles, called quarks (Figure 1). Quarks, like electrons, seem
to be truly fundamental particles that cannot be divided further. They carry
electric charges which are either one third or two thirds the size of the
standard unit of charge on an electron or proton. But they come in
combinations that ensure that the charge always adds up to whole units or
zero. For example, there are three quarks inside every proton and every
neutron. The property of quarks that is like electric charge but involves the
strong force is called their colour charge. This name arose because it appears
to come in three varieties, like the primary colours of light; but it has
nothing to do with colour in the everyday sense of the word. It鈥檚 just that it
is easy to think in familiar concrete terms, so the three types of charge
associated with the strong force are given the names red, green and blue.
Rather as positive and negative electric charge can add up to zero, so the
colour charges of quarks can add up to give no colour. This is what happens in
protons and neutrons. These particles are 鈥渃olourless鈥�, even though they
contain coloured quarks 鈥� the proton, for example, contains one blue quark,
one red and one green. In a similar way, an atom is 鈥渃hargeless鈥�, even though
it contains both positive protons and negative electrons.
So the strong force, which affects only coloured particles, does not operate
directly on protons or neutrons. It operates on the quarks within them. But it
can still hold protons and neutrons together in the nucleus, in the same way
that electromagnetic forces can hold electrically neutral atoms together. The
positively charged nucleus of an atom is only partly screened by its own
electron cloud, and feels the influence of negatively charged electrons in the
cloud surrounding the neighbouring atom, giving rise to Van der Waals forces;
similarly, the coloured quarks inside a proton feel the presence of quarks in
the proton next door.
Nuclear glue
Ties that bind
ELECTROMAGNETIC forces involve the exchange of photons; the equivalent field
particles that carry the strong force are called gluons, because they 鈥済lue鈥�
particles together. Field lines depict the forces between quarks, in the same
way that they are used to depict forces between electrically charged particles
(Figure 3). Because this whole theory is based on QED, but involves so-called
colour charges, it is known as quantum chromodynamics, or QCD.
Like phtons, gluons have no mass. But there is one particularly important
difference between gluons and photons. Photons are not electrically charged,
and so do not interact with each other through the electromagnetic force.
Gluons, however, carry colour charge, like quarks do 鈥� there are eight
different kinds of gluon, each with its own combination of colours. So gluons
can interact with each other as well as with quarks via the strong force. This
makes that force very different from the electromagnetic force. When two
electrically charged particles are pulled apart, the force between them
decreases. But this does not happen with colour charge. The force between two
coloured quarks does not decrease as the distance between them increases.
Instead, the gluons associated with the quarks pull on each other as well as
on the quarks. This seems to be the reason why a lone quark can never escape
from inside a proton, but must always exist in a colourless combination with
other quarks. However, no one has yet proved theoretically that quarks and
gluons must always be confined in this way.
Even the strong force, however, is not sufficient to explain all the behaviour
of particles in nuclei and outside them. For example, a neutron that is free
from the confines of an atomic nucleus does not live forever. After typically
of a little under 15 minutes, an isolated neutron will spit out an electron
and a particle called an antineutrino, and will become a proton. A neutron
inside a nucleus, however, does not do this, except in unstable radioactive
nuclei. Physicists can describe the 鈥渄ecay鈥� of the neutron in terms of the
emission and absorption of field quanta, and therefore in terms of a
fundamental force. In this case, the force is called the weak force, because
withinthe nucleus it is about 10 000 times weaker than the strong force.
There are three field particles that transmit the weak force. Two of these are
electrically charged. These are called W+ and W-;. The third force carrier, which is
uncharged, is known as the Z掳. So the weak force may change the charge of a particle, as when a neutron decays into a proton,
or it may involve interactions in which there is no charge change.
All three weak force carriers are heavy 鈥� each has a mass roughly a hundred
times that of a proton or a neutron. So how can a neutron emit a virtual W or
Z particle that is much heavier than itself, even if that particle is later
reabsorbed?
The answer lies in quantum uncertainty. Quantum physics shows us that no
quantity, such as the mass of a neutron, can ever be fixed precisely. There is
always some uncertainty in the amount of mass 鈥� strictly speaking, mass-energy
鈥� associated with a particle, or even with a point in empty space. Over a long
period of time, this uncertainty is very small. But over a very short period
of time, the uncertainty is very large. A neutron can create, and emit, a W or
Z particle out of nothing at all, provided that the particle is absorbed by
the neutron or another particle in such a short time that the Universe does
not notice the discrepancy. The time allowed depends on the mass of the
particle (longer if it is lighter) and is fixed by the quantum rules.
Photons, the field quanta of the electromagnetic force, have zero rest mass.
These particles can travel forever across the Universe. But W and Z particles
are heavy, and so cannot travel far from their parents. So, like the strong
force but for a different reason, the weak force has a very short range.
Physicists have been able to create W and Z particles, by colliding beams of
subatomic particles together at such high energies that the amount of mass
available, in line with E=mc2, exceeds the mass of these carriers of the weak force. Then, the
particles are real, not virtual. Their existence can be traced through
detectors such as those at the European research centre CERN, and their masses
can be determined.
Such experiments help us to investigate the role of forces in the early
Universe. When the Universe was very young, just after the Big Bang, it was
hot and dense (see Inside Science Number 1, 22 October 1987). Space was filled
with energetic radiation and particles of all kinds. This energetic radiation
is what we now detect as a weak hiss of radio noise at a temperature of 3 K, coming from all
directions in space 鈥� the cosmic background radiation. In the first moments of
the existence of the Universe, this radiation was at a temperature of billions
of degrees, and under those conditions W and Z particles could be made as
easily as photons.
Two into one
Electroweak force
THEORY suggests, and high-energy experiments confirm, that at high enough
energies electromagnetism and the weak force are parts of a single, unified
electroweak force. They only split into two forces at lower energies 鈥� lower
temperatures. Physicists discovered the connection between the weak and
electromagnetic forces not through experiments, but in their attempt to
develop a quantum field theory for the weak force. Indirect confirmation of
the electroweak theory came first in the early 1970s; but it was only in 1984
that researchers at CERN succeeded in observing the direct production of W and
Z particles. This proved two forces could be combined in one package.
This observation underlies an important discovery. One way to develop a feel
for how two forces can be combined into one is to recall that electricity and
magnetism seem at first sight to be unrelated phenomena, yet they are
manifestations of the same underlying electromagnetic force.
A totally unrelated analogy may help. The 鈥渄ripping鈥� from roast meat appears
as a homogeneous liquid when it is hot, straight from the oven. But as it
cools down to normal room temperature, it separates into two distinct
components 鈥� a fatty, more or less white solid on top and a brown jelly below.
We inhabit a world where the two components of the electroweak force have
separated out, like dripping, to give the appearance of two distinct forces.
Into the future
Ultimate physics
THE success of unifying two of the four forces in one theory has encouraged
physicists to search for a single 鈥渢heory of everything鈥� describing the
electroweak, strong and gravitational forces as facets of a single underlying
force. Such a theory might answer such questions as 鈥渨hat is mass?鈥� and would
certainly provide insights into conditions in the early stages of the
Universe, when it was so hot that all four forces were indistinguishable.
That may seem far removed from practicalities of our existence today. Yet the
present differences between the four forces are crucial to the state of the
world about us.
The Sun, for example, is fuelled by nuclear reactions that have their basis in
the weak force, but is held together by gravity. If the weak force were a
little stronger, compared with gravity, the Sun would have burned out long
ago; if it were weaker, the Sun鈥檚 output of energy would be more feeble.
Either way, there might then be no life on Earth to ponder the nature of the
four forces.
If a unified theory can be developed, it will tell us more about each of the
four component forces, and therefore about the nature of the world we live in
today.
Weak carriers
When a neutron (n) decays, it converts into a proton (p) by emitting a W
鈥� particle as in a. The W鈥� almost immediately converts
into an electron (e鈥�) and an antineutrino (v炉). The W鈥�
is one of the carriers of the weak force. The outgoing antineutrino
v炉 can be replaced by an incoming neutrino, v, without changing the
physics as in b. This reaction via the weak force then looks very similar to
the reaction between charged particles due to the electromagnetic force, c,
which occurs through the exchange of a photon (&ggr;).
Towards a theory of everything
BECAUSE of the fundamental importance of the basic forces, physicists have an
intense interest in discovering if a fully unified theory, describing all four
forces with one set of equations, does exist. Such a theory has some great
difficulties to overcome. In particular, it must encompass a quantum field
theory of gravity.
The standard theory of gravity is Einstein鈥檚 theory, general relativity. The
theory of relativity links space and time, and within this theory what we are
used to thinking of as the force of gravity is described in terms of the
distortion of space-time caused by the presence of matter. But the space and
time of general relativity are both smooth and continuous. In this sense,
general relativity is a 鈥渃lassical鈥� theory. It does not incorporate the
essential idea from quantum physics that everything comes in discrete units,
or quanta. A physicist trained only in Newtonian theory would find it easy to
understand general relativity, but much harder to understand quantum physics.
The other three fundamental forces are all describable in quantum terms, and
this raises hope that all three can be incorporated into a unified theory. But
gravity must first be quantised before it can be included in the package.
Even the individual field theories have run into problems. In particular, they
were plagued by infinities that emerged from the equations and could only be
removed by a trick known as renormalisation. In effect, renormalisation
amounts to dividing both sides of an equation by infinity, to 鈥渃ancel out鈥� the
unwanted terms.
This is not a very satisfactory state of affairs since, strictly speaking,
dividing one infinity by another could give any answer. But at least it could
be made to work. When gravity was included in the package, however, the trick
could not be made to work. Infinities still emerged from the equations, but
these infinities could not be cancelled. Quantum gravity seemed to be non-
renormalisable.
A breakthrough came when some theorists developed a description of fundamental
particles not as mathematical points but as tiny one dimensional entities, or
strings. This changed the structure of the equations describing interactions
between particle in two important ways. First, the equations now include,
automatically, the description of a quantum field particle that has exactly
the properties required to be the carrier of the gravitational force 鈥� the
graviton. Secondly, in some variations of these equations all the infinities
disappear from the equations automatically, with no need for renormalisation.
The infinities only disappear if gravity is included; gravity can only be made
to fit if the infinities disappear.
This powerful discovery suggests to many physicists that the new ideas, which
go by the name of string theory, are a step on the road to a true theory of
everything. The strings themselves are tiny. It would take 10
20 of them (a 1 followed by 20 zeroes) laid end to end to stretch
across an atomic nucleus. But because they are 鈥渟pread out鈥�, even by such a
small amount, they require a fundamentally different set of equations for
their description. It will take many years for the implications to be worked
out and the complete theory, if one exists, to be found. But then, in a sense,
science will have achieved its ultimate goal.
Further reading
Building the Universe (Blackwell, 1986, 拢9.95) provides an overview of
forces and particles in the form of articles culled from the pages of New
快猫短视频.
Superforce, by Paul Davies (Heinemann, 1984, 拢12.95) and Superstrings: A
theory of Everything? edited by Paul Davies and Julian Brown (Cambridge
University Press, 1988, 拢6.95 ppb) are both readable accounts of the
search for a unified theory.
Feynman Lectures on Physics, by Richard Feynman and others (Addison-Wesley,
拢15.50, 1989).
In Search of Schr枚dinger鈥檚 Cat, by John Gribbin (Corgi, 1984,
拢3.95), explains the underlying quantum principles.
The Particle Connection, by Christine Sutton (Hutchinson, 1984, 拢8.95)
goes into a little more detail.