What causes Forces?

In case of Quantum Mechanics, the forces or interactions between matter particles, are all carried by particles of integer spin: 0, 1 or 2. What happens is that actually 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 and this collision changes the velocity of the second particle, just as if there had been a force between the two matter particles, and here the force has just born!

It is an important property of the force-carrying particles that they do not obey the Pauli Exclusion Principle therefore they can exchange as many as particles they want, and so they can give rise to a strong force. However, if the force-carrying particles have a high mass, it would be difficult to produce and exchange them over large distance. So the forces that they carry will have only a short range. On the other hand, of the force-carrying particles have no mass of their own, the forces will be long range. The force-carrying particle exchanged between matter particles are said to be virtual particles because, unlike ‘real’ particles, they cannot be directly detected by a 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 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 a matter particle interact with each other by exchanging virtual force-carrying particles.
For example, the electric repulsive force between two electron is due to exchange of virtual photons, which can never be directly detected; but if one electron moves past another, real photons may be given of, which we detect as light waves.

Force-carrying particles can be grouped into four categories according to strength of the force that they carry and the particle 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 hope to find a unified theory that will explain all forces as different aspects of a single force. Indeed, many would say that this is the prime goal of physics today.

Gravitational Force

The first is Gravitational Force. This force is the universal, that is, every particle feels the force of gravity, according to its mass or energy. Gravity is weakest of the four forces by a long way; it is so weak 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 sometimes 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 physicist would call gravitational waves, which are very weak-and so difficult to detect that they have never yet been observed.

Electromagnetic Force

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 zeroes 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 sun, contains nearly equal number of positive and negative charges. Thus the attractive and repulsive forces between the individual particles nearly cancel out 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 electron and positively charged protons in the nucleus causes the electrons to orbit nucleus of the atom, just as the gravitational attraction causes 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 one allowed orbit to another one nearer to the nucleus, energy is released and a real photo is emitted=which can be observed as visible light by the human eye, if it has the reight wavelength, or by a photon detector such as a photographic film. Equally, if a real photon collides with an atom, it may move an electron from one orbit nearer the nucleus to one farther away. This uses up the energy of the photon, so it is absorbed.

Weak Nuclear Force

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⁺, W^–, and Z^o, and each had a mass of around 100 GeV. The Weinber-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 on fact found to be all the same type of particle, only in different states. At high energies the ball behaves in essentially only one way-it rolls round and round. But as the wheel slows down. The energy of the ball decreases, and eventually the ball drops into one of the thirty-seven slots on 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. W⁺, W, and Z^o would acquire large masses, making the forces they carry have a very short range.

Strong Nuclear Force

The fourth category is strong nuclear force which holds the quark together in the proton and neutron, and hold 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 + blue = 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 particles called glueball.