# What is the speed of gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [April 21, 2006, 5:27pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654 "2006-04-21T17:27:18Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![jebert](https://avatars.discourse-cdn.com/v4/letter/j/ec9cab/32.png) [@jebert](https://boards.straightdope.com/u/jebert)\ **Post date:** [April 21, 2006, 5:27pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/1 "2006-04-21T17:27:18Z")
According to this article [http://news.bbc.co.uk/1/hi/sci/tech/4923396.stm](http://news.bbc.co.uk/1/hi/sci/tech/4923396.stm) scientists expect soon to be able to see the first evidence of gravitational waves. > [@](#): > > Simulations on a supercomputer have allowed Nasa scientists to understand finally the pattern of gravitational waves produced by merging black holes. > The work should help the worldwide effort that is currently underway to make the first detection of these “ripples” in the fabric of space-time. > Ultra-sensitive equipment set up in the US and Europe is expected to achieve the breakthrough observation very soon. > The new research will make it easier to recognise the correct signals. > “With these calculations, we are now able to know what will be the distinctive gravitational wave signature that comes out from just outside merging black holes,” commented Professor Peter Saulson, who is part of the Laser Interferometer Gravitational Wave Observatory (Ligo) Scientific Collaboration. > “And by looking for this signal, we will be able to learn whether Einstein’s Theory of General Relativity is correct or whether there is even stranger physics ahead for us in the future.” > Tell-tale sign > Researchers believe their first detection of gravitational waves is imminent. Confirmation would be regarded as a major scientific advance, and would usher in a new way of studying the Universe. > Any accelerating object should send these waves of energy radiating outwards at the speed of light; but only truly massive bodies, such as exploding stars and coalescing black holes, would disturb space-time sufficiently for our technology to pick up the signal. The article states that these waves would travel at the speed of light. Since light waves and the speed of light are predicted as a solution to Maxwell’s equations involving electricity and magnetism, and not gravity, why do we expect gravity waves to travel at the same speed? Why couldn’t the speed of gravity be some other value? ---
**Author:** ![garygnu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garygnu/32/2924_2.png) [@garygnu](https://boards.straightdope.com/u/garygnu)\ **Post date:** [April 21, 2006, 5:38pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/2 "2006-04-21T17:38:11Z")
IIRC, if gravity _isn’t_ the same speed as light, then we’ve got a serious problem on our hands. I do not, however, have the ability to explain any of this. ---
**Author:** ![Don\_t\_Panic](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/don_t_panic/32/398_2.png) [@Don\_t\_Panic](https://boards.straightdope.com/u/Don_t_Panic)\ **Post date:** [April 21, 2006, 5:58pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/3 "2006-04-21T17:58:25Z")
The speed of light, _c_, isn’t really anything that is intrinsic to light or electromagnetic waves - it should perhaps more properly be called “the absolute speed limit of the universe”. In general relativity, gravity is basically identified with curving of spacetime, which creates gravitational waves that propagate at _c_. In most quantum gravity theories, gravity is believed to be transmitted by a massless particles known as the graviton (which will also behave as a wave, depending on how you look at it), although this particle hasn’t yet been observed. Massless particles always travel at the speed _c_, and as gravitons are assumed to be massless, like photons, they will always travel at _c_. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [April 21, 2006, 5:59pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/4 "2006-04-21T17:59:06Z")
This is why I wish _c_ was taught as “Einstein’s constant” rather than the “speed of light in a vacuum”. Yes, _c_ is the speed of electromagnetic propagation as predicted by Maxwell’s equations, but the constant isn’t fundamentally linked to electromagnetism any more strongly than it is to gravitation. Remember, _E_ = _m c_[sup]2[/sup]? No electromagnetism there. ---
**Author:** ![JustAnotherGeek](https://avatars.discourse-cdn.com/v4/letter/j/58f4c7/32.png) [@JustAnotherGeek](https://boards.straightdope.com/u/JustAnotherGeek)\ **Post date:** [April 21, 2006, 6:05pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/5 "2006-04-21T18:05:41Z")
AIUI, the “speed of light” (“c”) is the speed at which disturbances in / on the 4-d surface of the universe can propagate. I.e. it is the speed at which ripples in the electromagnetic fields (light) can move, and it is the speed at which ripples in the spatial domains (gravity) can move. Mind you, I have never been a formal “advanced” physics student. The above may or may not have anything to do with reality. 😉 ---
**Author:** ![vertizontal](https://avatars.discourse-cdn.com/v4/letter/v/ad7895/32.png) [@vertizontal](https://boards.straightdope.com/u/vertizontal)\ **Post date:** [April 21, 2006, 6:10pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/6 "2006-04-21T18:10:47Z")
I seem to recall a Staff Report on this very topic: [What is the Speed of Gravity?](http://www.straightdope.com/mailbag/mgravityspeed.html) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 21, 2006, 7:57pm UTC](https://boards.straightdope.com/t/what-is-the-speed-of-gravity/353654/7 "2006-04-21T19:57:28Z")
Wow, I’d forgotten I’d written that one. Thanks. **Pleonast** , I agree 100% with you on the label attached to _c_. It’s a Fundamental Constant of the Universe, and it’s just incidental that light is (or seems to be) one of the things that travels at that speed. One could say that _c_ is a combination of the electrical and magnetic constants (this is how it comes out of Maxwell’s equations), but one could more elegantly say that the magnetic constant is a combination of the electric constant and _c_, and _c_ is in Maxwell’s equations because it’s in **everything**.