# How do we know Gravity Moves the Speed of Light?

How do we know that gravity moves at the speed of light? What has been done to show this? How do we know that it isnt instantious.

I did a search on these forums and couldnt find a past post about this, but if it has been answered please direct me..:shy:

## Answers and Replies

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caffeine
The evidence is mostly heuristic and based on logic. We've never measured the speed of gravitational propagation, and we probably won't for a very long time. That doesn't mean we don't have some very good guesses. You might find these links useful:

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

http://www.qedcorp.com/pcr/pcr/speedg.html

We know gravitational effects are not instantaneous for a few reasons, not the least of which is the fact that there's no such thing as "instantaneous". Relativity doesn't allow it. Nature seems to hate zeros and infinities with a passion. Most people point to quantum mechanics as proof of the existence of instantaneous (as in the collapse of a state function into an eigencomponent). However, it's fully expected that wavefunction collapse will be a PROCESS, not an EVENT, in the future theory of quantum gravity. We simply have a hard time watching something that most likely takes $$10^{-40}$$ seconds to finish.
If the above links are difficult to follow and don't seem convincing, it's because, as I said, it's a very complicated question and we have no direct evidence. Only some very good guesses.

Stay tuned for the next 50 years, when (hopefully) someone discovers a viable and testable theory of quantum gravity.

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One prediction of Einstein's theory of relativity, is that the speed at which changes in location of a mass get propagated to other masses (the speed of gravity) ought to be the same as the speed of light.

A couple of years ago, a widely-reported experiment purported to confirm this prediction with actual results. But it quickly got a lot of criticism for apparently measuring the speed of light itself, rather than gravity. I'm not sure if there have been any further experimental data on this lately.

The Wikipedia entry at http://en.wikipedia.org/wiki/Speed_of_gravity is pretty good.

Also see http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

I agree caffeine... existence doesn't allow zero's. Only nothingness would. As long as there is a universe and existence.. there are numbers to be found. But nothing is instantaneous? I heard about some military experimenting... that involved instantaneous transmission.

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It seems that light, gravity, and any other force field, they ride something which speed is equal C. (?) Could this be a theory? Or is a complete nonsense?

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Problem with that, Pippo, is that light is an electromagnetic phenomenon that propagates as a result of the emanation of photons. But gravity is the "bending" of spacetime around mass, a very different thing. Neither one requires the phenomenon to ride on gravitons (or anything else, really). **edit I see you changed your post from 'gravitons' to something generic traveling at velocity c, but i'll just leave mine as is**

If you think of the classic "rubber sheet" analogy, a large ball placed on a taut rubber sheet will bend the sheet a bit, similar to how mass bends spacetime. If you roll that ball across the sheet, the indentation it created will move, too. But the indentation won't move instantaneously, because it takes time for the sheet to react to the changed position of the ball. Similarly, as a bit of matter moves through the universe, its gravitational effect propagates around it not instantaneously, but at a given speed -- the speed at which spacetime reacts to the changed position -- the speed of gravity. It just happens that relativity theory predicts that this speed is the same as the speed of light.

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Hooloovoo said:
Problem with that, Pippo, is that light is an electromagnetic phenomenon that propagates as a result of the emanation of photons. But gravity is the "bending" of spacetime around mass, a very different thing. ....
I fully agree with you this is relativity, but when we say that photons are emanated we also accept that they move, the point is this why they move ? I mean what ìs the reason for which an elettromagnetic fields propagate?

If Gravitational influences propagate like em waves, then they will be subject to the same basic speed limit, but there are other phenomena that appear to make their way instantaneously such as photon entanglement. Field effects may not be governed by the cosmological properties that determine light velocity - The precession of Mercury is usually cited in support of the premise that Gravity travels at c, but several other ideas have been put forth challenging that conclusion. The issue has been raised several times on this forum.

azneternity said:
How do we know that gravity moves at the speed of light? What has been done to show this? How do we know that it isnt instantious.
I did a search on these forums and couldnt find a past post about this, but if it has been answered please direct me..:shy:
lol, gravity doesn't bend the speed of light, it bends light itself. implying an object. this called gravitational lensing. and how do we know? we tested this theory of Einstein's during an eclipse not too long ago.

cd

The best way to show the speed of the 'graviton," is to first invent an anti-gravity shield. Then quickly removing the shield would allow the acting force of gravity to be measured. Maybe someone has ideas on how to do that?!

robert Ihnot said:
The best way to show the speed of the 'graviton," is to first invent an anti-gravity shield. Then quickly removing the shield would allow the acting force of gravity to be measured. Maybe someone has ideas on how to do that?!
Perhaps something can be observed in between the earth and the moon in the point where the effect of the gravity is equal to zero (?).

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We could make a small black hole with photons, although i do not believe it would be physicaly possible at our current technology level.... Hmmm do photons have gravity fields themselves? if they do scrach my idea =)