# Do gravity waves exist?

1. Jun 26, 2011

I can't really see how gravity waves work, I mean, how does a gravity wave attract mass and electromagnetism?

2. Jun 26, 2011

### ardie

the description of gravity waves emerges from application of feynmann diagrams in an attempt to quantise gravity. they are predicted to exist by the theory and are in good agreement with general relativity.
the feynmann approach does not follow a classical picture and as such cannot describe the intuitive newtonian or einstein's picture but a more complicated but mathematically rigorous description can be found in advanced quantom mechanics textbooks.
im quoting this part...
"Feynman suggests that a distinction between spin 0 and spin 2 can be made on the basis of the fact that the gravitational attraction between masses of a hot gas is greater than for a cool gas; i.e. that energy is an effective form of gravitational mass. This observation corresponds to a velocity-independent gravitational potential between two massive bodies, which, because requires an interaction energy. "
in basic terms, the graviton (which is a massless particle) interacts with matter to establish the gravitational force. it travels at the speed of light
the effect of light ray deflection is simply a collision between a gravity wave with a photon, and is described by the Feynman rules.

Last edited: Jun 26, 2011
3. Jun 26, 2011

### ardie

as to wether they exist or not, they require some very precise measurements, not currently achievable

4. Jun 26, 2011

### WannabeNewton

Could you elucidate as to what you mean by "attract" mass and electromagnetism? I have never heard of that terminology i.e. that of a wave "attracting" a fundamental force. Low amplitude gravitational waves are perturbations of the metric that propogate at c and have been indirectly verified by observing the continuous decrease in orbital period of the Hulse - Taylor binary system because of energy loss attributed to the emission of gravitational waves.

5. Jun 26, 2011

### Polyrhythmic

That's not true. Gravitational waves arise out of a linearized approach to standard General relativity, originally, they have nothing to with any kind of quantization.

What you are talking about is the hypothetical graviton, which is a virtual particles that might appear in a consistent perturbative approach to quantum gravity, but this area is a huge work-in-progress sector.

A gravitational wave is in principle a small perturbation of spacetime that propagates as a result of a change of the mass/energy configuration of a system. If for example a star moves from point A to point B, the curvature of spacetime has to change accordingly. That change, which propagates according to equations that can be extracted out of the "linearized gravity" approach, is considered a gravitational wave. A possible quantized version of that wave is usually called graviton, in analogy to the quantization of EM waves, which is quantized in terms of photons. Here lies a source of confusion: there are two kinds of gravitons appearing in discussions of quantum gravity: wave-gravitons and virtual gravitons, in analogy to actual photons and virtual photons.

In this sense, a gravitational wave itself doesn't attract anything, it merely propagates a change in spacetime.

6. Jun 26, 2011

### pervect

Staff Emeritus
The 1993 prize in Nobel physics was awarded to Hulse and Taylor for their discovery of a binary pulsar system.

The binary pulsar system is observed to be slowly spiraling into each other, based on the small changes in their mutual orbital period. This can only happen if they are loosing energy. GR predicts that such a system should loose energy in the form of gravitational waves, which are emitted by the spiarling bodies.

GR's prediction of the rate of the orbital decay is acacurate to within less than a percent.

This is good, though indirect, evidence that gravitational waves exist (carry energy) and that the details of the emission process are understood.

See for instance http://nobelprize.org/nobel_prizes/physics/laureates/1993/press.html

7. Jun 27, 2011

### ardie

yes it is a special and general relativity forum and im not talking about special or general relativity, so get over it.
I know very well in what context gravity waves arise in GR, but do they attract EM and mass? no quite. attraction can also mean interaction, and gravitons are certainly hypothesised to interact with photons and matter. this interaction can sometimes be attractive.
in the context of general relativity the intuitive picture is that gravitic information travels at lightspeed, giving rise to a gravitational waves, regions where gravity is different momentarily. it arises much the same way that is used in electrodynamics in gauge theories.

8. Jun 27, 2011

### Polyrhythmic

So why are you talking about Feynman diagrams? In GR, there is no such a thing.

9. Jun 27, 2011

### ardie

gravitational waves have not been proven to exist, both theories have merit in explaining their existance and have similar predictions. my personal believe is, in the quantum world the classical picture will eventually break down or need adjustment

10. Jun 27, 2011

### WannabeNewton

Yes they have. Did you not read Pervect's post?

11. Jun 27, 2011

### bcrowell

Staff Emeritus
You made a mistake. Polyrhythmic corrected it. The hostile tone of your response is unwarranted. Please take ordinary care in checking your facts before posting. If a mistake is pointed out, the correct response is not to huff and puff about it and continue posting the same mistake.