Understanding forces

What holds matter together then?

Nature's glue is supplied by forces. These are carried by particles that have a sort of ghostlike ethereal existence as they deliver their information from one particle to another. Most of them have a very short life because the matter particles they are acting on are very close together. Some, however, can live for billions of years, such as those brining information to us from quasars on the edge of the known Universe.

Gravity is the most familiar force. In everyday life it seems to be the most important yet it is the weakest of them all. At the other end of the scale is the strong force, the force that acts between the quarks. It is the one responsible for holding the nucleus together. In between are the electromagnetic force and the weak force.

Gravity is 100 000 000 000 000 000 000 000 000 000 000 000 000 times weaker than the strong force, yet it governs the behaviour of the large scale Universe. That is because its range is infinite, it is always attractive and it acts on all particles. Gravity is most important for large objects; planets, stars and people for example. Individual particles hardly notice it's there. Because it is so weak, the particles that carry gravity, called gravitons, remain but a theory. They have not yet been detected experimentally and there is a major effort around the world to find them

The electromagnetic force is the second most powerful force of nature, over 100 times weaker than the strong force. It acts on electrically charged particles, holding electrons in orbit around atomic nuclei and sticking atoms together into molecules. The electromagnetic force also carries light to us from stars and galaxies, brings electricity into our homes and even allows us to see each other. The particles that carry the electromagnetic force are called photons, they are particles of light.

The weak force is so closely related to the electromagnetic force that the same theory is used to describe the two. The main difference between the two forces is the mass of the particles that carry them. Photons have no mass, that is what lets them travel indefinite distances - at the speed of light. The particles that carry the weak force, however, are extremely heavy on the particle scale of things, as massive as a whole nucleus of zirconium. This means that they can't travel very far and their range of influence is confined to the scale of atomic nuclei. The carriers of the weak force are called W and Z. Ws are either positively or negatively electrically charged. Zs are neutral. The weak force is important in stars, it helps them to shine. It is also the force responsible for radioactive beta decay.

The strong force is a little different from the rest. Like gravity and electromagnetism the particles that carry it, gluons, have no mass. But unlike gravity and electromagnetism, however, it does not have infinite range. Rather it is restricted, like the weak force, to the scales of atomic nuclei. The reason for this is that the strong force has a kind of charge associated with it, called colour charge. This is a bit like electric charge except it comes in three kinds, which we call red, blue and green. It is this colour charge that limits the range of the strong force because gluons carry the colour charge. This means that they can attract and interact with each other, which stops them travelling very far.

The strong force needs to be strong because it has to fight against the electromagnetic force to hold all the positively charged protons together in nuclei. You can get a feel for how strong the electromagnetic force is by trying to push like poles of powerful magnets together.

This table of matter particles and forces shows which particles feel which forces. All particles with mass experience gravity as well.

With all those particles and forces, the Standard Model is almost complete. There is just one ingredient we haven't mentioned: antimatter, a sort of "mirror image" of ordinary matter. For every kind of quark, there is an antiquark. For electrons, muons, taus and their associated neutrinos there are positrons, anti-muons, anti-taus and anti-neutrinos. It seems that antimatter does not exist in large quantities in the Universe today. At the Big Bang, however, when the Universe was born, matter and antimatter are believed to have existed in equal amounts. Finding out what has become of the antimatter is one of the biggest puzzles of modern particle physics research.

The Standard Model. Click for a bigger version.

The Standard Model is an extraordinarily successful theory. But we know it is not perfect and physicists are actively looking for a better theory.

Learn more about Search for a better theory
or return to
Introduction to Particle Physics

Particle Physics Education CD-ROM 1999 CERN