# Speed of Gravity **URL:** \ **Category:** Factual Questions\ **Created:** [January 21, 2016, 10:12pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832 "2016-01-21T22:12:00Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![UncleFred](https://avatars.discourse-cdn.com/v4/letter/u/f0a364/32.png) [@UncleFred](https://boards.straightdope.com/u/UncleFred)\ **Post date:** [January 21, 2016, 10:12pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/1 "2016-01-21T22:12:00Z")
I may not be phrasing this right but; How fast does gravitational attraction travel? Example; Let’s say we’re way out in space and far away from any significant masses. I have two large heavy blocks of lead or something, perhaps a mile apart. Now I somehow get one to move alternately toward and away from the other. This cause an oscillating increasing and decreasing gravitational attraction on the other block. But what is the delay between motion of the first block and the resultant change at the other? Does the change propagate at the speed of light? Or something else? ---
**Author:** ![sevenwood](https://avatars.discourse-cdn.com/v4/letter/s/6de8d8/32.png) [@sevenwood](https://boards.straightdope.com/u/sevenwood)\ **Post date:** [January 21, 2016, 10:18pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/2 "2016-01-21T22:18:09Z")
It is my understanding that gravitational effects travel at the speed of light. Folks who really know should be along shortly. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [January 21, 2016, 10:20pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/3 "2016-01-21T22:20:44Z")
I believe that the theory is that the speed of gravitational waves should be equal to the speed of light in a vacuum. ETA: Ninja’ed. Also, from [Wiki](https://en.wikipedia.org/wiki/Speed_of_gravity): > [@](#): > > Speed of gravity > From Wikipedia, the free encyclopedia > > In classical theories of gravitation, the speed of gravity is the speed at which changes in a gravitational field propagate. This is the speed at which a change in the distribution of energy and momentum of matter results in subsequent alteration, at a distance, of the gravitational field which it produces. In a more physically correct sense, the “speed of gravity” refers to the speed of a gravitational wave, which should be the same speed as the speed of light (c). > > Introduction > > The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c.[1] Within the theory of special relativity, the constant c is not exclusively about light; instead it is the highest possible speed for any interaction in nature. Formally, c is a conversion factor for changing the unit of time to the unit of space.[2] This makes it the only speed which does not depend either on the motion of an observer or a source of light and/or gravity. Thus, the speed of “light” is also the speed of gravitational waves and any other massless particle. Such particles include the gluon (carrier of the strong force), the photons that make up light, and the theoretical gravitons which make up the associated field particles of gravity (however a theory of the graviton requires a theory of quantum gravity). ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [January 21, 2016, 10:21pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/4 "2016-01-21T22:21:52Z")
> [@sevenwood](#): > > It is my understanding that gravitational effects travel at the speed of light. Yep. Everything we know so far seems to indicate that gravity travels at _c_. Actually _detecting_ gravitational waves remains an elusive problem. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [January 21, 2016, 10:29pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/5 "2016-01-21T22:29:19Z")
As far as the theory goes, **sevenwood** is exactly correct: changes in the gravitational field propagate outwards at the speed of light, so the stationary block will not feel the change of influence until a certain amount of time later. If the two blocks are a mile apart, this time delay is about 5 microseconds (i.e., about 1/200 of a second.) That said, we have never actually measured the speed of gravity directly. (There have been a few claims to the contrary, but these claims remain contentious at best.) Gravity is really really weak; for example, if your hypothetical blocks of lead are 1 m x 1 m x 1 m, the force they exert on each other is about the same as the weight of a fruit fly. So either you’re trying to measure ridiculously small forces, or you need to move around a stupidly large amount of mass. Neither option can be done easily. There’s a chance that now that [advanced LIGO](https://en.wikipedia.org/wiki/LIGO) is up and running, we’ll be able to see some highly-energetic event (a supernova, say) and detect its gravitational influence as well; and in that case we could compare the two speeds directly. But so far, we’re just going off of the theoretical predictions. ---
**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:** [January 21, 2016, 10:41pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/6 "2016-01-21T22:41:34Z")
Yup, another confirmation of that (at least as the theory goes, but it’s a _very_ fundamental theory, to the point that, if it’s wrong, we’d have no clue even what building blocks we’d need to use to reconstruct physics). And as an aside, kudos to you for phrasing the question in a physically possible way: Usually, when people ask about this, they posit a mass suddenly coming into existence ex nihilo, which is of course impossible. Wiggling masses, though, no problem, and in fact that’s exactly how you produce gravitational waves. ---
**Author:** ![Robert163](https://avatars.discourse-cdn.com/v4/letter/r/4af34b/32.png) [@Robert163](https://boards.straightdope.com/u/Robert163)\ **Post date:** [January 21, 2016, 10:49pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/7 "2016-01-21T22:49:06Z")
I started a thread about this, it is the same as the speed of light. basically if you have two lead balls the size of tennis balls [A] and \*\* 100 light years apart and then next to [A] you magically introduce [C], it will take 100 years for [A][C] to impact/effect \*\*. ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [January 21, 2016, 10:52pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/8 "2016-01-21T22:52:44Z")
> [@Robert163](#): > > I started a thread about this, it is the same as the speed of light. > > basically if you have two lead balls the size of tennis balls [A] and \*\* 100 light years apart and then next to [A] you magically introduce [C], it will take 100 years for [A][C] to impact/effect \*\*. But if, instead, we built a huge wooden badger… ---
**Author:** ![Robert163](https://avatars.discourse-cdn.com/v4/letter/r/4af34b/32.png) [@Robert163](https://boards.straightdope.com/u/Robert163)\ **Post date:** [January 21, 2016, 10:54pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/9 "2016-01-21T22:54:44Z")
> [@XT](#): > > But if, instead, we built a huge wooden badger… i’m missing the context here… monty python? ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [January 21, 2016, 11:00pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/10 "2016-01-21T23:00:11Z")
Yeah, sorry…for some reason juxtaposing your post with Chronos said Monty Python in my head. My brain sometimes works in mysterious ways. Carry on. 🙂 ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [January 21, 2016, 11:08pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/11 "2016-01-21T23:08:53Z")
Just remember that c isn’t “the speed of light”. Rather, c is the speed of causality. It just so happens that the fastest causality can travel is c, and so things like photons are demonstrated to travel at c, and things like gravitons, if they exist, are expected to travel at c. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [January 22, 2016, 1:34am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/12 "2016-01-22T01:34:51Z")
> [@Chronos](#): > > … > And as an aside, kudos to you for phrasing the question in a physically possible way: Usually, when people ask about this, they posit a mass suddenly coming into existence ex nihilo, which is of course impossible. Wiggling masses, though, no problem, and in fact that’s exactly how you produce gravitational waves. > [@Robert163](#): > > …basically if you have two lead balls the size of tennis balls [A] and \*\* 100 light years apart and then next to [A] you magically introduce [C], it will take 100 years for [A][C] to impact/effect \*\*. Took all of 8 minutes for the spell of physicality to be broken. That’s the same time interval as \*c \*from Sun to Earth. Coincidence? I think not. 🙂 ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [January 22, 2016, 2:10am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/13 "2016-01-22T02:10:45Z")
I was curious if gravity waves slow down as they pass through matter like EM waves do. Apparently the answer is “nope”, but they can be bent by passing near another massive object. > ---
**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:** [January 22, 2016, 2:27am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/14 "2016-01-22T02:27:01Z")
In principle, matter should have a “gravitational index of refraction”, for much the same reasons that there is for light… but in practice, gravitational interactions with matter are so phenomenally weak that there would be no chance of detecting the difference, except possibly in the earliest moments of the Universe. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [January 22, 2016, 6:13am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/15 "2016-01-22T06:13:36Z")
> [@MikeS](#): > > As far as the theory goes, **sevenwood** is exactly correct: changes in the gravitational field propagate outwards at the speed of light, so the stationary block will not feel the change of influence until a certain amount of time later. If the two blocks are a mile apart, this time delay is about 5 microseconds (i.e., about 1/200 of a second.) You’re missing three 0’s there; 1/200000 of a second ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 22, 2016, 6:41am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/16 "2016-01-22T06:41:06Z")
So, if we get a pair of 1m[sup]3[/sup] lead cubes on silly strong tether 1km apart, and spin them at 200,000 times a second, we could probably get a nice gravity wave going there. Nice in that you might be able to measure it with enough resolution on its phase that you could tell how fast the wave was traveling. Clearly we can only imagine doing this in space. Maybe at a Lagrange point. Obviously the engineering is simply an exercise for the reader. (I’ll be free for my trip to Stockholm anytime you ask.) ---
**Author:** ![coremelt](https://avatars.discourse-cdn.com/v4/letter/c/4491bb/32.png) [@coremelt](https://boards.straightdope.com/u/coremelt)\ **Post date:** [January 22, 2016, 8:44am UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/17 "2016-01-22T08:44:28Z")
I’m amazed we can’t detect gravity waves from this: > **[Star Caught Circling Black Hole at Record-Breaking Speed](https://www.space.com/20303-black-hole-star-speed-record.html)** > > Astronomers have spotted a star in our galaxy that races around a black hole at a breakneck speed, orbiting once every 2.4 hours. Red Dwarf orbiting a black hole at 2 million km / h (!!!) ---
**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:** [January 22, 2016, 4:58pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/18 "2016-01-22T16:58:07Z")
The frequency of that system (1 orbit per 2.4 hours) is far too low to be detected by LIGO or similar detectors, and far too high to be detected via pulsar timing arrays, which are the only two methods currently being used to attempt to detect gravitational waves. It probably could be detected, and easily, by something like LISA (a space-based gravitational wave detector), but unfortunately LISA got cancelled some years back. There are actually a fair number of known systems that LISA would have been able to detect nearly immediately after being activated, which was one of the big selling points for the project: By comparison, while there are many hypothetical events which should be detectable by LIGO, we don’t really have a good handle on how common any of those events are, nor where or when they are, so it takes a lot more data to pick them out of the noise even if they do occur. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [January 22, 2016, 7:11pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/19 "2016-01-22T19:11:32Z")
> [@Chronos](#): > > they posit a mass suddenly coming into existence ex nihilo, which is of course impossible. Just a nitpick but mass appearing ex nihilo is one theory for the Big Bang and also the mechanism for Hawking radiation / decay. ---
**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:** [January 22, 2016, 7:13pm UTC](https://boards.straightdope.com/t/speed-of-gravity/743832/20 "2016-01-22T19:13:20Z")
There are no theories for the origins of the Big Bang. There aren’t even any hypotheses. At best, there are vague handwavy guesses. And Hawking radiation isn’t ex nihilo; the mass comes from the black hole. [Next page](https://boards.straightdope.com/t/speed-of-gravity/743832.md?page=2)