what is the relation between gravity and light?

**phy_freak** · Feb8-10, 02:51 PM

why did they have to be the same speed? do you think that this speed was somehow set as a standard speed for such things, i don't know maybe there are other things that are also in the speed of light.. so the speed of light isn't so rare..

**Chronos** · Feb9-10, 01:05 AM

GR sets a universal speed limit on 'communication' between objects in the universe. That limit happens to be the speed of light.

**S.Vasojevic** · Feb9-10, 12:07 PM

Originally Posted by phy_freak

why did they have to be the same speed? do you think that this speed was somehow set as a standard speed for such things, i don't know maybe there are other things that are also in the speed of light.. so the speed of light isn't so rare..

Hipothetical particle responsible for carrying gravity is massless boson called graviton. Massless particles propagate at speed of light.

If graviton is never confirmed to be real, then your question - why gravity propagates at speed of light - is very good question. Simply stating that GR forbids FTL communication, doesn't explain anything, because question is why speed of gravity propagation is exactly same as the speed of light propagation, not why gravity is not faster then light.

**Matterwave** · Feb9-10, 03:43 PM

I think the "speed of gravity" can be worked out using GR...and it turns out to be c...but I've no idea how. :P

**Nabeshin** · Feb10-10, 12:48 AM

Originally Posted by Matterwave

I think the "speed of gravity" can be worked out using GR...and it turns out to be c...but I've no idea how. :P

Related thread:

http://www.physicsforums.com/showthread.php?t=369420

Edit: Also, on the grounds of dimensional analysis, we expect to get c. The only other really conceivable option would be for gravity to propagate instantaneously.

**Ich** · Feb10-10, 02:11 AM

If graviton is never confirmed to be real, then your question - why gravity propagates at speed of light - is very good question.

A graviton simply inherits its properties from the classical wave it is meant to quantize. Whether or not there is a graviton is irrelevant, the speed is derived from linearized gravity, i.e. the wave solution in the weak field approximation.

**Chronos** · Feb10-10, 02:16 AM

Instantaneous speed of gravity has been thoroughly debunked AFAIK.

**phy_freak** · Feb10-10, 09:58 AM

Originally Posted by S.Vasojevic

Hipothetical particle responsible for carrying gravity is massless boson called graviton. Massless particles propagate at speed of light.

If graviton is never confirmed to be real, then your question - why gravity propagates at speed of light - is very good question. Simply stating that GR forbids FTL communication, doesn't explain anything, because question is why speed of gravity propagation is exactly same as the speed of light propagation, not why gravity is not faster then light.

could you please explain what you meant by "GR" and "FTL" ?

**mgb_phys** · Feb10-10, 10:02 AM

Originally Posted by phy_freak

could you please explain what you meant by "GR" and "FTL" ?

GR = general relativity, Einstein's law of gravity
FTL = faster than light