Exchange Particles

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At the moment there are only four fundamental forces known to exist – gravity, the electromagnetic force, the strong nuclear force and the weak force Other apparently distinct forces are in fact only different manifestations of these fundamental forces. For example

• A moving charge may experience an magnetic force in a magnetic field due to a magnet. There may be no electric field present, so no electric force acts on the charged particle. It may seem from this that the electric and magnetic forces are separate forces, but they are in fact aspects of the same force – the electromagnetic force.

• Tidal forces arise from the force of gravity acting on different parts of a body. Bodies are never zero size, so the force of gravity may have different magnitudes on different parts, causing the body to be deformed or any liquid to move.

• Friction and reaction forces are a result of the electric forces between the surface atoms in different materials.

All four fundamental reactions are mediated by exchange particles. Essentially the exchange particles carry the force from one particle that experiences the force to another particle that experiences the force. Each force has it's own exchange particle, and each exchange particle has a range which is determined by it's mass – the larger the mass, the shorter the range of the force. If the exchange particle has zero mass, the force has infinite range. This is the case for the photon, which mediates the electric force, and the graviton, which mediates the gravitational force. The range of a force decreases with increasing mass of the exchange particle because the energy of the exchange particle must be 'borrowed' from empty space. Since energy increases with mass (), high mass particles have higher energy, which can not be borrowed as long as for short mass particles.

A summary of properties is given in the table below.