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Entry 296, on 2006-03-08 at 16:01:59 (Rating 1, Science)

I now have listened several times to a podcast, produced by the BBC, on the subject of gravitons. The graviton is the theoretical particle which carries the gravitational force. Existing theories of gravity (both Newton's and Einstein's) don't require a carrier particle; they use an unspecified force (in the case of Newton), or curvature of space-time (in Einstein's relativity theory) to explain how gravity works. But the other forces all have carriers, and quantum physics predicts gravity should too.

So why shouldn't the particle associated with gravity have been discovered too? Gravity is very weak. It doesn't seem that way, but in reality it is about 10 thousand billion billion billion billion times weaker than the strong force. Its only because there is so much material generating gravity (for example, the Earth) that it seems strong. Because the force is weak, the particle is hard to detect because of its very weak interaction with other particles.

The question of why gravity is so weak brings up another set of bizarre theories, including the idea that gravity is "diluted" by being spread over extra dimensions we can't detect, and various superstring theories involving 10, 11, or even more, dimensions!

Quantum theory introduces the idea of wave-particle duality. This means waves and particles are two ways of looking at the same thing. Every wave is a particle. Every particle is also a wave. So detecting a graviton will probably be done through detecting gravity waves. Unfortunately the waves are unbelievably small.

A gravitational wave would only cause a displacement of a million million millionth of a meter over a separation of a few kilometers. This is like measuring the distance to a star (tens of trillions of kilometers) to the accuracy of a single hair! I just love the fact that physics will even contemplate trying to detect this sort of subtle phenomenon. What an incredible achievement it will be when its finally done!


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