# What are gravity waves?

1. Feb 20, 2005

### billy_boy_999

what are gravity waves? i am guessing that they are some kind of oscillations in the space-time "medium"?

but gravity waves are not the normal carrier of the gravitational force? otherwise, because its an oscillation, the force wouldn't fall off with the square of the distance. so what special circumstances create gravity waves? where do they come from?

2. Feb 20, 2005

### DB

3. Feb 21, 2005

### pervect

Staff Emeritus
A simple explanation is that gravity waves are to gravity as electromagnetic waves are to the electrostatic columb force between charges. They are related, but separate. Accelerating charges can produce electromagnetic radiation, and accelerated masses can produce gravitational radiation.

here

4. Feb 22, 2005

### gonzo

Taylor and Wheeler give a really simple explanation of gravity waves and why they are predicted.

Imagine heavy balls falling towards each other (by their own gravitational attraction) connected by an ideal massless spring. In theory, the spring should then be able to push the balls apart to their starting position again after they collide. But, the spring discovers that it requires more energy to return them to their starting position.

The idea is if you take a time slice while the balls are moving towards each other and then away from each other you notice something ... they feel the attraction of where the other ball was an instant in the past, not where it currently is, due to the delay for the information of its position to reach the other ball. So as they fall towards each other, they experience a tiny bit less attraction than they "should" because they feel each other as being slightly farther away, and then as they move apart, they feel slightly more attraction then they should for the same reason.

This means that energy has magically disappeared from the system because of this time delay in communication between them (assuming that gravity information travels at less than or equal to C).

The idea is that gravity waves carry this energy away as ripples in the fabric of spacetime.

5. Feb 22, 2005

### jcsd

If you think about it an em wave if you like is where a vector field defined over space is 'changing', simalirly a gravitional wave is where a tensor field defined over space is 'changing'.

6. Feb 22, 2005

### Chronos

Good explanation, Gonzo, in fact it is quite excellent. Gravity waves are a fundamental prediction of GR. Einstein realized this, but was reluctant to accept it. He finally took the very bold step of predicting gravity waves because it was the only apparent solution that satisfied the energy conservancy required under GR. Detecting them would be yet another monumental testimony to the sheer brilliance and depth of GR. Go LISA!

7. Feb 22, 2005

### DB

Ya, just to comment on gonzo's explanation, so a slight amount gravitational energy (attraction) is lost in the creation of a gravity wave?

So, lets say a star forms, its gravitational attraction will be slightly smaller then....what amount of energy??? I hope I've been clear...

Last edited: Feb 22, 2005
8. Feb 22, 2005

### Chronos

No effect on gravitational attraction. A small amount of the kinetic energy of the accelerating/decelerating object is leaked out via gravity waves.

9. Feb 23, 2005

### jgraber

Actually, a little bit of energy has been dispersed in the radiation. This is sometimes expressed that the object's "gravitational mass" is less than its "baryonic mass". This can also be called a "loss" (actually a dispersal) of "binding energy". In the formation of most condensed objects i.e. stars, most of this binding energy escapes as electromagnetic radiation rather than gravitational radiation, except in cases such as inspiralling neutron stars or black holes. In these cases a significant amount of energy is released in the form of gravitational waves.
Hope that helps.
best.
Jim Graber

10. Feb 23, 2005

### jcsd

Are you sure, in GR conservation of energy is not strictly required.

11. Feb 23, 2005

### Chronos

I'm not sure of anything. It leads to poorly conceived wagers. But in cases such as decaying orbits, there is an energy loss [albeit small] that is inexplicable without gravitational waves.

12. Feb 24, 2005

### jcsd

I see now graviational waves conserve energy in the classical limit (gravaitaional waves don't actually have enrgy in some sense though, that is if you look at a small enoguh region of any part of a graviational wave and meausr eit's enrgy it will be zero), so I guess in order for conservation you need to pick a prefered frame and you'd also need asymnpotical flatness.

It still seems odd to me that Einstein would worry about energy conservation edited to add: when, for regions of space the law of enrgy conservation is ruled out as being generally covariant a priori and general covaraincy was one of Einstein's main motivations to formulate the theory of genarl relatiivty.

Last edited: Feb 24, 2005