# Speed of Gravity/Light

Discussion in 'Physics' started by tbinder3, Nov 26, 2013.

1. ### tbinder3 Thread Starter Member

Jun 30, 2013
30
1
For the past couple years, I have been extremely curious as to whether the speed of gravity is faster/slower/= to the speed of light. Even though theoretically the speed of light is the fastest speed at which energy/matter/etc. can travel.

But what about the speed of gravity? Is the speed of gravity related to speed of light? Is the speed of gravity a measurable constant?

Haven't done any research at all, and if someone can point me to a nice article debating a similar issue, that would be TERRIFIC. Also kinda curious what others thoughts are on this issue.

EDIT: Hope this is in the right section.

2. ### Wendy Moderator

Mar 24, 2008
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2,731
Close enough. My understanding is the speed of gravity is that of light. Of course, I don't really know. We don't have any way of turning gravity on / off.

3. ### tbinder3 Thread Starter Member

Jun 30, 2013
30
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Your saying speed of light = to speed of gravity? Interesting, cause I did a quick Goggle and thats what most theories are concluding as well.

Any good articles/experiments/debates someone can point me to?

4. ### mattbullet Member

Nov 29, 2008
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The way I see it is this. If it was raining straight down and I was running forward, the drops would be hitting me more in the face then the back of my head. The raindrops have a certain speed and so do I.
If I was orbiting a star photons would do the same thing. If I was traveling fast enough I would only see light from the front. The photons have a certain speed.
Now let's say I'm orbiting a black hole. If gravity was particles traveling at some speed I should feel more gravity from my front than my back. I would be accelerating and spiral away from the black hole. That is if gravitrons convey gravity the way photons convey electromagnetism or raindrops convey wetness.
It seems more like warped space then something moving at a speed.

5. ### WBahn Moderator

Mar 31, 2012
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5,646
Though I think there are a number of experiments you could perform if we had sensitive enough instruments; but the gravitational force is so weak compared to the electromagnetic force that getting there is a challenge.

But I was just reading the other day that relativistic time dilation effects have now been measured at speeds as low as 10m/s. Now THAT'S pretty impressive! That puts them in the range where they could measure and report the time dilation experienced by world class sprinters, though I don't know if they could work with an experiment that was that short in duration.

Anyway, we may get there (to where we can measure speed of gravity). It would be interesting to know which experiment to get a reasonably definitive answer is the closest to being realizable and just how far away (in resolution, not in time before getting that resolution) it is.

6. ### Alec_t AAC Fanatic!

Sep 17, 2013
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That's the way it seems to me, too.
Can gravity exist in the total absence of mass? Is gravity a consequence of the existence of mass?

7. ### Wendy Moderator

Mar 24, 2008
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It is also a consequence of linear acceleration. The famous elevator thought experiment by Einstein shows that in a truly closed system it is not possible to tell the difference between gravity and acceleration. As far as I know this is still the case.

8. ### WBahn Moderator

Mar 31, 2012
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Of course, in the total absence of mass, what is accelerating?

But, if we could postulate a massless particle that did not move at the speed of light, then it would be unable to distinguish gravity from a uniformly accelerating reference frame.

9. ### Markd77 Senior Member

Sep 7, 2009
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Light isn't like rain, you will always see light coming from all around, whatever speed you are travelling, and it will appear to you to be travelling towards you from all directions at the speed of light - even if you are travelling at some measurable fraction of it. It's called "Special Relativity".

10. ### Wendy Moderator

Mar 24, 2008
20,993
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If the particle were truly massless, how could it not move at the speed of light? It would be an all or nothing process, where Neuton's Laws do not apply.

11. ### WBahn Moderator

Mar 31, 2012
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It couldn't. Which is why I said "if we could postulate" such a particle.

12. ### #12 Expert

Nov 30, 2010
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Somebody did a proof of this on TV. The answer was, "speed of light" IIRC. Darn senior moments!

The setup: Our star vanishes.
Do the planets begin straight line motion when the sun vanishes or wait until the moment when the lack of light reaches us?
This depends on whether the warp of space-time flattens immediately or takes time. If it flattened immediately, we could send messages by gravity wave instantaneously.

13. ### morrobay New Member

Nov 7, 2013
11
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Then with t = t'/√ 1- v^2/c^2 Where t' is the proper time in the moving frame and t is the elapsed time for stationary observer.
We have: t = t'/√ 1- 100/9*10^16
Then if the runner records 1 hr on his/her clock (proper time). The clock with the stationary observer will show
1.00000000000000555 hr.With 3600sec/hr thats one hour and .00000000001998 seconds
So the stationary observer concludes that the moving clock is running slow by that amount of time.

14. ### WBahn Moderator

Mar 31, 2012
20,057
5,646
It's hard to follow your equations. It would really help if you used some parens.

t = t'/√(1 - (v^2/c^2))

With the timing resolution being taken to increasing absurd levels in some sporting events, it may not be too long before we see claimed resolutions overlapping relativistic effects.

15. ### tbinder3 Thread Starter Member

Jun 30, 2013
30
1
Telling us what #12?

The planet falls into a straight line motion, leaving its gravitational rotation, and when the last of light reaches our planet, we will leave our "original" orbit

But what does this mean, for the gravity/light relations? There =?

Great friggen example!

16. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Eliminating the mass of our sun would release so much energy that our planets would probably be vapourised. So their paths post-elimination is academic .

Aug 17, 2013
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Since the hypothetical elementary particle that carries gravity (the graviton) would have 0 mass, gravity must travel at the the speed of light.

18. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Why does having zero mass mean it must travel at the speed of light?

Aug 17, 2013
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Because according to Einstein's E=m c^2 it is mass that prevents a particle from reaching the speed of light. Therefore, the lower the mass the faster it travels; and since gravitons supposedly have 0 mass, they travel at the speed of light.

20. ### Alec_t AAC Fanatic!

Sep 17, 2013
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I don't see how that follows. The Einstein equation just means that a lower mass will have lower energy.
I accept that the maximum velocity would be c, but see no reason why a mass-less entity couldn't have an arbitrary lower velocity.