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  The force of gravity was discovered by Newton in the 17th century. He showed that the same force that makes an object fall to the Earth keeps the planets in their orbits. The theory of General Relativity, propounded by Einstein at the beginning of the 20th century, further enhanced our understanding of this force.

Any two objects which have mass will be attracted to each other. This does not agree with our everyday experiences. We do not notice the force of gravity between, say, two tennis balls. However, this is because gravity is very weak, and only very large masses (such as the Earth and the Moon) have an appreciable effect.

Gravity decreases in strength as we move away from the massive object. Again, we do not notice this there is no perceptible lessening of gravity, for example, as we climb up a mountain. However, with sensitive enough equipment, we would be able to tell that the pull of gravity does indeed get less as our distance from the Earth increases.

Our Earth is not large enough to show some of the more spectacular gravitational effects. Only large stars can do this. When a star is large enough, the force of gravity is sufficient to crush the atoms together to make a neutron star. An even larger star forms the ultimate end-product of gravity a black hole. This is so called because its gravity is strong enough to ensure that no light may escape it.

Gravity slows down time, in a similar fashion to relativistic speeds. Sensitive experiments have shown that a clock at high altitude, where gravity is slightly weaker, goes faster than one at sea-level. This effect is tiny upon the Earth, but much larger close to a very massive body like a neutron star. As we approach the surface of a black hole, Einstein\'s theory of General Relativity tells us that time slows and finally stops. JJ

See also relativity.



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