The standard model of particle physics is the best theory currently available to answer the question 'What is the world made of?'. Understandably, the final theory is very complicated and mathematical. However, the basic idea is straightforward. All matter is ultimately made from 12 elementary particles, which interact through four fundamental forces.

Atoms and Molecules

As almost everybody knows, matter is made from atoms, which stick together to form molecules. Simple substances such as air and water are made from simple molecules containing only two or three atoms. More complicated substances, such as mammoths and scientists, are made from more complicated molecules such as proteins and DNA, and contain millions of atoms, which stick together to make cells, tissue, fur and brains. There are around a hundred different types of atom (known as elements), from hydrogen to uranium, catalogued by chemists in the periodic table.

But what are atoms made from? Although for a long time thought to be fundamental, indivisible particles, early 20th Century studies showed that atoms were made up of smaller particles: protons, neutrons and electrons. Protons and neutrons stick together in a tiny point called the nucleus, which is orbited by electrons¹. The hydrogen atom is a single proton orbited by a single electron, whereas a uranium atom has nearly a hundred protons, neutrons and electrons.

Electrons will readily jump from atom to atom, allowing all sorts of interesting chemistry and electronics. However, the protons and neutrons remain stuck together in the nucleus except in nuclear reactions. Nuclear fission occurs when an unstable heavy nucleus (such as uranium) splits into two; this process drives nuclear power stations and atomic weapons. Nuclear fusion occurs when two light nuclei fuse together to form a heavier atom; this energy fuels stars. Unstable or radioactive nuclei may also decay and emit particles due to various processes in the nucleus.

But what are protons, neutrons and electrons made from?

Fundamental Particles of Matter

To answer this question we leave the world of biologists, chemists, and atomic and nuclear physicists, and enter the realm of particle physicists and the standard model of particle physics. Electrons are believed to be fundamental point-like particles with no further constituents. They are a member of a class of particles called leptons. However, protons and neutrons are both made from smaller particles called quarks. There are six different types of quark, which are believed to be fundamental. Only two of these, however, are needed to explain protons and neutrons. The proton is made from two up quarks and one down quark; the neutron consists of one up and two down quarks.

Every part of the universe that we see during our everyday lives can be explained with only these particles. If we look further, however, we realise there must be a lot more. Accordingly, physicists soon realised that in order to explain the radioactive decay of some atoms there must be another particle, as decaying atoms did not appear to conserve energy - a requirement for all physical processes. It was proposed in 1931 that another particle, called the neutrino, was produced in the decay process and carried away some of the energy. Neutrinos are very light and hardly ever interact with ordinary matter. They were not directly detected until 1956.

More exotic particles have also been found in rays of particles, called cosmic rays, which hit the Earth from outer space. These include muons and pions. Unlike the electron or proton, these are unstable particles and decay to electrons or protons². Antiparticles such as positrons (an anti-electron) are also found in cosmic rays.

When physicists built the first particle accelerators, they discovered hundreds of new particles. They soon guessed that not all of these were elementary - although it took much of the 20th Century studying these particles to determine which are fundamental and which are made up of smaller particles, and how they all interact with each other. The standard model is the theory they devised. The model was developed mainly around 1960 to 1980, and was extensively tested in high energy experiments at the end of the century.

In the standard model, matter is made up of two classes of elementary particles: leptons, which include electrons, muons and neutrinos; and quarks, which come in six varieties and stick together in twos or threes to form heavier particles such as protons, neutrons and pions. To make things a little more complicated, antimatter theory predicts each of these particles has a corresponding antiparticle with the same mass, but opposite electric charge and other properties.

Fermions and Bosons

The standard model does not just explain matter. It also explains the forces of nature. Both matter and forces are explained using particles. All these particles fit into two categories: fermions and bosons. Fermions are the quarks and leptons that make up matter; bosons include the photon and other particles associated with forces.

Whether a particle is a fermion or a boson is determined by its spin. Spin is a fundamental property that particles possess, in the same way that they have a certain mass or electric charge. Spin is the intrinsic angular momentum of an object. You can picture a particle as a spinning ball, although as fundamental particles are point-like, this is not really true. The spin of a particle is measured in units of 1.06×10⁻³⁴kgm²/s (a value known as the Dirac constant). Bosons have zero or integer spin (ie, 0, 1, 2... multiplied by the Dirac constant), fermions have half integer spin (ie, 1/2, 3/2, 5/2... multiplied by the Dirac constant).

This apparently minor difference has a large effect on the way particles behave. Fermions are only created in particle-antiparticle pairs; they are the fundamental particles of matter. Bosons can be created and destroyed much more easily; they act as force carriers which mediate the interactions between particles.

The cause of this different behaviour is rather complex, and still not entirely understood. It is due to quantum mechanics. Fermions obey the Pauli Exclusion Principle, which only allows one particle in each quantum state. Bosons do not obey the Exclusion Principle, so any number can occupy the same quantum state.

The fermion/boson distinction does not only apply to elementary particles. Particles made up of fermions (such as atoms) can act either as fermions or bosons. Particles of matter acting as bosons can create a great many interesting phenomena at low temperatures, such as Bose-Einstein condensation, and superconductivity.

The table below lists all the fermions in the standard model. (The range of names is due to the varying imagination of physicists.)

There are two classes of fermion: leptons and quarks. Both contain six particles in three generations. The first generation contains the most common particles: electrons, and up and down quarks. Higher generations contain heavier particles, are only produced in high energy processes³ and quickly decay to lighter particles.

Leptons

Leptons include the familiar electron, as well as the muon and tau, which are basically heavier unstable versions of the electron. Each of these particles also has a corresponding neutrino.

Quarks, Baryons and Mesons

Although quarks are fundamental particles, nobody has ever detected a solitary quark. They only exist as part of other particles. Quarks can bind together to form particles in two ways. Baryons consist of three quarks (or three antiquarks) bound together, such as the proton (up-up-down), the neutron (up-down-down) or the Ω^- (strange-strange-strange). Mesons consist of a quark-antiquark pair such as a pion, which comes in three forms: up-antidown, down-antiup, and a mixture of up-antiup and down-antidown.

The fermions act as building blocks for larger particles and ultimately everyday matter. However, the standard model also contains bosons to explain the fundamental forces of nature.

Forces

The great success of the standard model is that it explains how fundamental particles interact with each other. Particles interact by four fundamental forces: gravity, electromagnetism, the strong nuclear force and the weak nuclear force.

All the forces work by exchanging an additional force particle between the particles which experience the force. For example, the electromagnetic force is mediated by the photon, and an electron can emit a photon which is then absorbed by another electron. The photons themselves are electromagnetic waves - light.

Equivalent force carrier particles exist for the other forces. The strong force is mediated by gluons; the weak forces by three particles known as W⁺, W^-, and Z⁰⁴. Gravity is presumably mediated by a graviton - although a successful theory for quantum gravity has yet to be written, so this remains an open question.

Gravity

Although gravity is the most obvious force in everyday life, it is not explained by the standard model. This is not a problem when studying microscopic particles, however, as it is incredibly weak compared to the other forces. We only notice its effect because it is always attractive and has an infinite range.

Electromagnetism

Electromagnetism is the force between electrically charged particles. It causes electrons to stick to atoms, and determines the behaviour of atoms and molecules. Although electromagnetism has an infinite range and is much stronger than gravity, it plays a smaller role at large scales as it can be both attractive and repulsive⁵. The two contributions usually cancel each other out, unless we can separate a lot of charged particles - but this takes a lot of energy.

Strong

The strong nuclear force is what holds the nucleus of an atom together. As the nucleus contains only protons (with positive charge) and neutrons (with zero charge), the electromagnetic force would cause it to fly apart. However, the much stronger strong force holds them together. Unlike gravity and electromagnetism, the strong force has a very short range, so it plays no role in larger scale objects.

Another unique property of the strong force between two particles is that its strength increases as they move apart (while they are still within its range). In contrast the strength of the electromagnetic or gravitational force decreases with distance. This explains one of the properties of quarks: they are only ever seen within another particle (eg, within protons, neutrons or pions). In fact, nobody has ever seen a lone quark or managed to isolate one. This is because as we try to separate two quarks, the force pulling them together increases, so an ever-increasing amount of energy is needed to continue pulling them apart. Eventually there is enough energy present to create a quark-antiquark pair out of the vacuum, and the two original quarks separate - now as quark-antiquark particles.

Weak

The weak nuclear force was the last fundamental force to be identified. It was introduced to explain the radioactive decay of atoms in which a neutron changes to a proton and emits an electron and an antineutrino (or a positron and a neutrino). Electrons and neutrinos (and other leptons) are not affected by the strong force, so an additional force was needed to explain this process. The weak force also has a very small range.

Unification of the Forces

It is believed that in the incredibly high energy conditions of the Big Bang, there was a single superforce governing all particle interactions. As the universe cooled, this force split into the four 'fundamental' forces listed above. Therefore, at high enough energies the particle interactions for the different forces should behave the same, and the different forces are really just low energy manifestations of a single force.

It is an ultimate goal of particle physics to produce a theory of a superforce, which also explains all four forces seen at low energies. Developing this 'theory of everything' is an enormous challenge, and it is unlikely to be achieved for a long time. The standard model takes a first step, however, and includes electroweak theory, which unifies the electromagnetic and weak nuclear forces.

Testing the Standard Model

As theoretical physicists were developing the standard model, the experimentalists were designing and building new experiments to test their ideas. So far every experiment test of the standard model has confirmed the predictions of the theory.

Scattering electrons off protons has confirmed that they are made up of quarks. Colliding particles and antiparticles at high energies has produced all the particles listed in the tables above (the top quark, the heaviest and last to be discovered, was found in 1994). The values of physical constants have been measured and found to match the predicted values - sometimes to a ridiculous number of decimal places.

There are still some areas of the standard model which have not been fully tested. Neutrinos are not fully understood, with one theory suggesting that they can change from one type to the other (this still has to be tested). The standard model may be able to explain CP violation - the asymmetry between matter and antimatter - believed to explain why the universe is made solely of the former, but this is not yet confirmed. And there is one missing particle - the Higgs.

The Higgs mechanism is a theory which explains the masses of particles. The idea is that the particles acquire mass as they move through the Higgs field. This is a vital part of the standard model as without it the theory suggests all particles would be massless. To prove this theory experiments are trying to detect the Higgs Boson - a quanta of the Higgs field. This, the final piece of the standard model, will probably be discovered in a few years.

Despite all its successes, nobody believes that the standard model is the final theory of particle physics. It is generally believed to be part of a final theory which unifies all the forces. Apart from being more elegant, this unification is required by cosmology theories to explain how the universe evolved from the big bang to today. There are a great many untested theories of physics beyond the standard model. Some, such as supersymmetry, will probably reach the experimental stage soon. Others, such as string theories are more speculative. It is possible that a grand unified theory of the strong, weak and electromagnetic forces may be achieved in a few decades. However, taming gravity may well take much longer, and is the ultimate challenge for theoretical physicists (and something experimentalists don't even talk about). It defeated Faraday, Einstein and many other scientists. One thing is certain, though: there will be a lot of work for scientists for a long time to come.

External Links

Since particle physicists are ultimately paid by taxpayers, they're always happy to explain to the general public how interesting and important their field of study is. Here are a few websites which do this:

• The Particle Adventure - An excellent site explaining the standard model in eight languages.

• Microcosm - Produced by the CERN particle physics laboratory.

• The Virtual Visitor Center - For the SLAC laboratory in California.

Related BBC Links

• Find out about the Large Hadron Collider.

• Does the Higgs boson really exist?

• Learn about the mystery of the missing solar neutrinos.