# How Are Gravitational Waves Produced?

1. Sep 3, 2007

This might seem simple...

I'm having a bit of trouble both finding and interpretting information on how a gravitational wave is produced.
I know they are analogous to EM waves, and that they are produced by accelerating masses, but it's more complicated than that, isn't it?

Can anyone explain to me what a quadrupole moment is, in layman's terms preferably?
How does a binary star system (such as PSR B1913+16) emit gravitational waves? How does it produce an alternating gravitational/gravitomagnetic field, since gravity is never repulsive?
If you could explain everything in classical physics, that'd be great too.

Last edited: Sep 3, 2007
2. Sep 3, 2007

### cesiumfrog

If you made the mass of a star oscillate (monopole moment) the gravitational field elsewhere would also strongly oscillate (gravitational waves), but conservation of mass-energy makes it impossible to do this. (You can't do it for charges either.)

If you shook an isolated star up and down (dipole moment), that would also produce gravitational waves, but conservation of momentum makes it impossible to do this. (You can do this one for electric charges though, since inertia is equal to gravitational rather than electric charge.)

The next weaker term in the mathematical series expansion of the potential gravitational wave equation, namely quadrapole moment, is what results if you vary the displacement between two masses. This is physically possible (they only need orbit each other).

3. Sep 7, 2007