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Q & A: The Speed of Gravity

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Most recent answer: 10/22/2007
Q:
If it takes a certain number of years for the light from distant stars to reach us and that we are actually viewing those stars hundreds or thousands of years ago, does gravity also have a ’speed’? For example, if a large star were to suddenly appear in space, how long would it take for the surrounding stars to be affected by its gravitational influence? Would they be influenced immediatley, or would there be a delay while the gravity reached out like rays of light?
- Chris
Amersham College, Bucks, UK
A:
That is a very good observation, Chris. As it turns out, gravity does have 'a finite propogation time' which is how textbooks say that gravitational effects are not instantaneous. Gravitational effects move through space at the speed of light. So if a star were to somehow blink into existence out of nowhere, then we would not feel it's gravity until it's light reached us.
One of the ways that it has been proven (or at least, strongly implied) that gravity moves at the speed of light is by observing the orbit of Mercury. In a universe where gravitational effects are instantaneous, the planets would move in almost perfect ellipses (oblong circles). The orbit of Mercury tends to precess, though. That means that oblong orbit itself is very slowly rotating around the sun. This is a small effect, but measurable, and it happens because gravity moves with a speed, not instantaneously.

math dan

(published on 10/22/2007)

Follow-Up #1: Graviton: Obeying the Speed Limit

Q:
In one section, you say that gravity has a finite propagation speed, which is the speed of light. However, gravity is caused by particles called gravitons, as you know, and these are massless. If they are massless, then they travel faster than light i.e. Tachyons, so which do you think they are? Massless or have mass? thank you for your time
- Dave
UK
A:
Dave,
Yes, you are correct that the graviton is thought to be massless (we have never actually observed one, but we are pretty sure that they are there.) But this does not make it a 'tachyon.' There are other examples of massless particles, such as light itself (photons) which travel at the speed of light. In other words, just because something has no mass does not mean it will travel faster than light.

A tachyon (theoretically) would be a particle that travels faster than light. But no tachyons have ever been observed.

So I still hold that gravitons are massless and that they travel at the speed of light.
Thank you for your question.

math dan

(published on 10/22/2007)

Follow-up on this answer.