# Can the gravitational field create its own gravity?

1. Apr 4, 2009

### friend

We know matter/energy creates a gravitational field. But energy can be carried away from a system of orbitting bodies in the form of gravitational waves. So if gravitational fields can carry away energy (gravitational wave), then the gravitational field has an energy density in and of itself independent of matter and radiation. It this is so, then can the gravitational field be augmented by its own energy density to an even greater gravitational field? Could this explain dark matter? Thanks.

2. Apr 5, 2009

### Forestman

I was just reading about this today. And yes a gravitational field can create its own gravitational field as well, but it is very weak. In fact the gravity field created by the suns gravity is what causes the shift in Mercury's orbit that Newton's theory of gravity could not explain.

The book I read this in was "The Quantum Zoo" by Marcus Chown on pages 134 and 135.

As for the dark matter I don't know enough to answer your question there.

3. Apr 5, 2009

### A.T.

A gravitational wave has energy. A static gravitational field doesn't.

Orbit precession is caused by the curvature of space, which part of the gravitational field (space-time curvature) in General Relativity:

http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html

There is no "gravity causing gravity" here.

Could you quote exactly what was written there?

4. Apr 5, 2009

### Forestman

I deleted most of this post because I was afraid that it might violate copyright.

I use the terms gravitational field and spacetime curvature interchangeably. I was talking about GR when I was talking about the shift in Mercury's orbit.

Last edited: Apr 5, 2009
5. Apr 5, 2009

### Forestman

A.T. I hope that I did not sound rude in my statements, but I just wanted to be clear.

6. Apr 5, 2009

### A.T.

The spacetime curvature already includes the spatial curvature, which explains the orbit precession. Speaking of gravity causing gravity here seems overcomplicated and confusing to me. But I'm not sure if Chown means orbit precession by the "gravitational tug greater than expected from the inverse square law".

No problem, thanks for the quote. It just seems to me that Chown has a tendency to draw these unjustified cause-and-effect-chains. The gravitational effect in GR is sometimes greater than in Newtons gravity. But that doesn't mean, that there is gravity causing even more gravity. In fact sometimes the acceleration towards a big mass in GR appears to be smaller than predicted by Newton.

Last edited: Apr 5, 2009
7. Apr 5, 2009

### Forestman

I would write more from the book, but there is just so much, but anyway. I believe that he means that the "extra gravity", which is produced by the suns gravity, is responsible for the orbit precession.

8. Apr 5, 2009

### friend

I don't see why that would be the case. If there is energy in the field, then there is energy in the field, right?

Now, if the gravity field itself has an energy density which creates a slight, additional secondary gravitational field, then this secondary field would create a third order effect, which would create an even smaller forth order effect, to infinity. I wonder if the addition of all these higher order effects would accumulate to a significant value.

9. Apr 5, 2009

### Forestman

I read in the book The Quantum Zoo that these effects of additional gravity only add up to something big in a blackhole.

10. Apr 5, 2009

### atyy

In the "complete" version of GR there is
(i) Einstein field equation which relates spacetime curvature and the stress-energy-momentum tensor
(ii) Maxwell and other equations which describe the electromagnetic field, charged particles and other "stuff"
(iii) A prescription for assigning stress-energy-momentum to stuff.

In that language, it is the stress-energy-momentum of stuff which causes spacetime curvature which is gravity.

However, it is also often said that "gravity" causes gravity, by which it is meant that the Einstein field equations are nonlinear in the spacetime metric.

In an "approximate" version of the theory in which gravity waves propagate on flat spacetime, it is possible to assign gravity some energy. However, this energy is not the same energy as the stress-energy-momentum tensor of the "complete" theory.

So there are several different notions which share some English words in their names, and one must be careful to distinguish these different things with similar names.

Anyway, my guess is that none of the above-mentioned notions of gravity causing gravity or of gravity having energy would solve either dark matter or dark energy because they are just different ways of describing completely standard General Relativity.

Last edited: Apr 5, 2009
11. Apr 6, 2009

### cragar

are you asking if a graviton would affect another graviton and I would haft to say yes.
Another, so if every gravity field had a field then wouldn't this go off to infinity ?

12. Apr 6, 2009

### friend

Do gravitons only exist in gravity waves? Or can they exist in static fields as well?

Yes, but each iteration would contribute minutely to the previous one. The question is what does this converge to, and how does this convergence depend on distance?

13. Apr 6, 2009

### lightarrow

Why? Can you elaborate?

14. Apr 6, 2009

### cragar

um im not sure what you mean by static fields but i would think that the graviton would exist in both becuase it is the particle for that field, the gravity waves would haft to be a field .

Last edited: Apr 6, 2009