# What is the speed of gravity

1. Jan 19, 2010

### DARKSYDE

say for instance you drop our sun in to the middle of our solar system, how long would it take for pluto to feel its gravitational force.

2. Jan 19, 2010

### hover

According to relativity, the effects of gravity travel at the speed of light. So take the distance of pluto to the sun and divide by the speed of light and you will get your answer.

3. Jan 19, 2010

### SystemTheory

I'm not sure where this paper fits in the mainstream interpretation of GR, but I find it to be a compelling case for the theoretical assumption that gravity is at least much much faster than the speed of light:

http://www.metaresearch.org/cosmology/speed_of_gravity.asp [Broken]

this has to do with the time-lag between the observed position of the Sun (or other body) versus its position within the gravitational calculations being in different positions in space.

Last edited by a moderator: May 4, 2017
4. Jan 19, 2010

### bcrowell

Staff Emeritus
The Van Flandern paper that SystemTheory linked to is kind of strange. When I first started reading it, it seemed obvious to me that it was crank stuff. Then I noticed that he gave a reference to where it was published in Physics Letters A in 1998. I couldn't believe this, so I looked up the reference, and sure enough, there it was.

The standard interpretation of GR is that low-amplitude disturbances in the gravitational field propagate at c. This prediction is infamously difficult to test experimentally in a model-independent way. The problem is that we don't have any viable competing test-theories that predict any other speed. This page http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html [Broken] gives what I think is a standard depiction of the consensus among relativists.

If there were anything seriously wrong with the description of the propagation of gravitational effects as described in GR, then it would be amazing that the rate of gravitational radiation by the Hulse-Taylor pulsar is in such good agreement with experiment. Solar system tests are also in good agreement with GR, although they do not do a good job of making a direct test of propagation at c. What is probably better understood today than in 1998, when Van Flandern got his paper published, is that, as shown by the Kopeikin fiasco, it is not at all easy to get the answer to this question from solar system measurements.

Last edited by a moderator: May 4, 2017
5. Jan 19, 2010

### meopemuk

bcrowell,

please correct me if I am wrong.

All verifiable experimental predictions of GR can be obtained within $$(v/c)^2$$ approximation. (The only exception that I know is the radiation energy loss by double pulsars.) With this accuracy GR equations are equivalent to the Einstein-Infeld-Hoffmann Hamiltonian in which gravitational interaction between bodies is represented by instantaneous position-and-momentum-dependent potentials. So, you are right that the idea of gravity as an action-at-a-distance force cannot be disproved so easily.

Eugene.

6. Jan 19, 2010

### Staff: Mentor

7. Jan 20, 2010

### vin300

http://redshift.vif.com" [Broken]
It says gravitons are virtual photons, cosmological redshift is due to interconversion of graviton energy and photon energy

Last edited by a moderator: May 4, 2017
8. Jan 20, 2010

### bcrowell

Staff Emeritus