• Question: What exactly are gravitational waves, and how do they arise? Why are there different frequencies, and what about the object producing the waves determines these frequencies - is it related to the mass of the object, or does this only affect the amplitude? How, and why, is it possible to locate where the waves originated from - do they vary over distance in a predictable way? Can we tell from experiment the direction a wave has come from, or two opposite directions it comes from, by looking at the effect of the waves on coherent light? How strong are these gravitational waves - wouldn't they need to be quite strong to bend light, or is the effect they have not the same as gravitational lensing? Thank you!

    Asked by strangeness to Arttu, Ceri, James_M, Monica, Philip on 21 Jun 2011.
    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 21 Jun 2011:

      In general relativity spacetime is flexible. It gets curved in the presence of energy, and we experience that as radiation. These deformations of spacetime can also propagate like waves, and these are gravitational waves. They arise when a massive object is moving is a suitable way: If it moves along a straight line, or if the mass distribution changes in a spherically symmetric way, no waves are emitted, but in most other cases they are. The more massive and less symmetric the source, the higher the amplitude, so the best sources of gravitational waves are mergers of very massive objects such as black holes or neutron stars.The frequency of the waves depends on the way they were formed, for example how the merging black holes rotate, but this in turn depends on their mass.

      Experiments have been going on for some time to detect gravitational waves, but so far they have not been successful. However, that does not mean the theory is wrong because the experiments are only sensitive enough to detect waves from a violent event relatively near us, and we just have not had any recently. However, we have detected the waves indirectly by measuring how rotation periods of binary pulsars change, and it agrees perfectly with the theory.

      In principle we can also measure the direction the waves are coming from, but it would need a more complex experimental setup. A single experiment compares the lengths of two very long orthogonal laser beams, because a gravitational wave would change this by a tiny amount. To detect the direction, we need more beams in different directions. This is done by combining data from two different experiments in different place.

      Gravitational waves are extremely weak, which is why they have not been detected. In the best current experiment, LIGO, the beam length is 4km and they should be able to detect changes of 10^(-18)m in length, and they still have not detected anything.

      Gravitational waves are not the same as gravitational lensing, which is caused by stationary gravitational field. So in term of spacetime, gravitational lensing is caused by the curvature, but gravitational waves by ripples propagating on it.