# Gravity, its speed, and how it relates to light?

1. Dec 9, 2004

### Rafajafar

Ok, so I just found out that gravity travels at exactly the speed of light. I had previously thought that it must travel instantaneously because it's directly related to mass, which is an "instantaneous" value, in my mind. I see how I was wrong now.

My question now is why the speed of light? If nothing travels faster than the speed of light, and if a photon (if it could) would feel as though it exists everywhere in the universe at once, what does gravity 'feel'? Does gravity and light take up the same space at the same time from their own perspective? If so, why do we have photons as wave/particles, but gravity is just a curvature in spacetime? Is it possible these curves are nothing more than light waves with extremely large wavelengths? It seems much too coincidental for light and gravity to propagate at the same maximum velocity.

Also, what bothers me is light's speed through a medium. I assume it's some sort of pseudo-friction effect for light to slow through a medium... probably due to electromagnetic fields warping the otherwise straight-line path of light (again, this is only an assumption). What of gravity, though? If light is "slower" through a medium, would gravity be as well? If not, then would gravity be traveling faster than the speed of light in that particular instance? If so, then why do objects of high mass accelerate towards each other. Let's say you have two very dense objects suspended in a translucent non-vacuum medium. The two objects would naturally accelerate towards each other in the suspension, but does that mean that the gravity is actually traveling faster than light through the medium?

I'm really curious.

2. Dec 17, 2004

### ObsessiveMathsFreak

Gravitational waves travel at the speed of light. As for the "speed" of the gravitational force itself, I understand this question is still under debate. (Note :Gravitational waves != gravity). A recent experiment purporting to show that gravity travels at the speed of light was shown to be flawed as it was measuring the speed of light itself. Appareently you can't use light to measure the speed of gravity, and that's why the experiment failed. Oh well.

I don't pretend to understand the argument myself. I believe a solution of the equations of general relativity is required. In fact this question is what gave rise to general relativity in the first place. In short, your not going to find the answers to you questions anywhere on the net. The best you can hope for is an intellectual flame war.! However, you can rest assured that newtonian cosmology, which treats gravity as having an "infinite speed", is an excellent model for nearly all practical purposes.

3. Dec 17, 2004

### magus

actually this occured of interest to me. you said that the force ofgravity traveled at the speed of light. if you could, could you possibly give me a link to a related article?
i have a small book that has many different theories of gravity, and one that in particularily caught my attention was the one that related the effect of gravity to a difference in forces of EM waves. im not really sure how to describe what im trying to say... but if i knew how to insert a pic id gladly draw a diagram. i find the relationship kind of interesting. tell me if you dont know what im talking about, and ill try to be more specific.

4. Dec 17, 2004

### Gonzolo

Slightly edited quote (added numbers) :

1: There is only one "special speed" in physics, which is called c. This c appears in Maxwell's EM equations as well as Einstein's gravity equations.

2: You could say that.

3: Gravity can also be considered as particles, called gravitons. Like photons, they have no mass.

4: Consider that a lightwave with infinite wavelength is called static electricity.

5: The special speed c is not a number within a range, it is a limit. See it as the limiting speed when a mass with fixed kinetic energy tends to 0. There are then only two possible, observable cases: v < c and v = c.

6: Light slows down in matter because it is constantly absorbed and reemitted. Lifetime in the absorbed states fundamentally determines its speed, which remains c in between atoms.

7: Perhaps, but I doubt this will be measured in our lifetimes.

5. Dec 18, 2004