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?
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 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.
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.
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 **.
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.
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.
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.
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.)
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.
