Physicists trying to measure gravity have a similar problem. It is more than 300 years since Newton, allegedly inspired by a falling apple, proposed his universal law of gravity. The problem is that the value of G seems to be different every time it is measured. An international meeting will be held in London in November to discuss gravity measurements. If the differences are real this could be really exciting, or even alarming.
Gravity does more than make apples fall to earth.
It performs a range of vital functions from holding the universe together to making the sun shine. Newton's law of gravity enabled him to explain the movements of the planets. Gravitational forces keep them orbiting around the sun and prevent them from flying off into deep space. In this century, cosmologists and astrophysicists have shown that gravity also holds the sun in its orbit around the centre of the Milky Way, and slows the expansion of the universe. It also forms the stars, by bringing together gas and dust, and sets them alight by generating the pressures that set off nuclear reactions in their cores.