What is the relation between gravity and light?

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Discussion Overview

The discussion revolves around the relationship between gravity and light, specifically focusing on the speed at which gravity propagates and its connection to the speed of light. Participants explore theoretical implications, potential particles involved, and the principles of general relativity (GR).

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question why gravity and light must propagate at the same speed, suggesting that there may be other phenomena that also travel at this speed.
  • One participant states that general relativity sets a universal speed limit for communication in the universe, which is the speed of light.
  • Another participant introduces the hypothetical graviton, a massless boson that would carry gravity, and notes that massless particles propagate at the speed of light.
  • Some participants express uncertainty about how the speed of gravity can be derived from general relativity, acknowledging a lack of understanding.
  • There is a mention of dimensional analysis supporting the conclusion that gravity should propagate at the speed of light, with instantaneous propagation being a less likely alternative.
  • One participant asserts that the idea of instantaneous speed of gravity has been thoroughly debunked.
  • Clarifications are requested regarding the abbreviations "GR" (general relativity) and "FTL" (faster than light).

Areas of Agreement / Disagreement

Participants express a range of views on the relationship between gravity and light, with no consensus reached on the reasons behind the speed of gravity's propagation or the implications of the graviton's existence.

Contextual Notes

Some discussions rely on assumptions about the existence of the graviton and the applicability of general relativity, which may not be universally accepted or confirmed.

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

https://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.
 
Last edited:
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.
 
Instantaneous speed of gravity has been thoroughly debunked AFAIK.
 
S.Vasojevic said:
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" ? :confused:
 
phy_freak said:
could you please explain what you meant by "GR" and "FTL" ? :confused:
GR = general relativity, Einstein's law of gravity
FTL = faster than light
 

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