# Orbital period decay and gravitational waves

1. Jan 6, 2010

### Ranku

We know that the orbital period of binary stars decay due to the emission of gravity waves that carry away energy from the system. What is the form of the energy loss of the system: kinetic energy or potential energy?

2. Jan 7, 2010

### Chalnoth

It's a combination of the two. The potential energy becomes more negative, while the kinetic energy increases.

3. Jan 7, 2010

### Ranku

Can we discern potential and kinetic energy components in the gravitational waves themselves?
When gravitational waves impact an object, say a dust sphere, the effect is not gravitational. That would seem to suggest there is no potential energy component in the gravitational waves themselves, even though the energy loss of the source of gravitational waves may well be a combination of potential and kinetic energy.

4. Jan 7, 2010

### Chalnoth

I'm not sure that question makes a whole lot of sense. The essential thing is that they transport energy away from the binary system, and can, in principle, deposit that energy elsewhere.

What do you mean the effect is not gravitational?

5. Jan 7, 2010

### Ranku

When gravitational waves impact a dust sphere, it alternately ellipses in perpendicular directions transverse to the direction of the waves. Gravitational waves do not cause gravitational motion of the dust sphere toward the source of the gravity waves.

Indeed not only are gravity waves not gravitational, primordial gravity waves could actually be repulsive! http://arxiv.org/abs/0909.1922v1

6. Jan 7, 2010

### Chalnoth

That's not a statement that they're not gravitational. I don't think anybody who studied General Relativity would even begin to think that gravity waves should cause matter to be attracted to a source.

Gravity waves are basically ripples in the fabric of space-time. Part of the gravity wave will be space-time that is sort of squeezed, while another part will be sort of stretched. There is no net curvature to a gravity wave. They're still gravitational in the sense that they are gravity acting on matter. It's just not in an overly-simplistic way.

The basic idea of how gravity waves are produced is this:
Imagine you have some configuration of matter. We could be talking about a solar system, or a galaxy, or just an amorphous blob of dust. The specific bit of matter we're considering is irrelevant.

Now, this bit of matter undergoes a change, for whatever reason (say, a large collision, such as between two black holes). It is impossible for a change in the configuration of this matter to propagate faster than light, and so the gravitational field away from the bit of matter cannot respond instantly to the change. What happens is that the effect of the change ripples outward from the source, until the space-time settles down to match the new configuration. But some of those ripples keep going, and become free-flowing gravitational waves.

Granted, it's not quite as simple as this. Space-time has no trouble keeping up with some sorts of changes to the distribution of matter fields. For example, the gravity wave emission from the Earth-Sun system is completely and utterly negligible. The potential well of the Earth basically moves along with the Earth, and isn't left behind just because the Earth is moving.