Stephen Hawking wrote:
"Force-carryng particles can be grouped into four catagories according to the strength of the force that they carry and the particles with which they interact......
"The first category is the gravitational force. This force is universal, that is every particle feels the force of gravity, according to its mass or energy. Gravity is the weakest of the four forces, it is so weak that 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 picture as being carried by a particle of spin 2 called the gravition. This has no mass of its own, so the force that carries is long range, The gravitational force between the sun and earth is ascribed to exchange of gravitions 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 gravitions make up what classical physicists would call gravitational waves, which are very weak - and so difficult to detect that they have never yet observed."
page 74, A BRIEF HISTORY OF TIME
Best, Mike B)
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