One revolutionary aspect of relativistic motion, put forth by Einstein but previously built up by The faster you move relative to someone at rest, the greater your lengths appear to be contracted, while the more time appears to dilate for the outside world.
This picture, of relativistic mechanics, replaced the old Newtonian view of classical mechanics, but also carries tremendous implications for theories that aren't relativistically invariant, like Newtonian gravity.
According to Einstein, there's a big problem, conceptually, with Newton's gravitational force law: the distance between any two objects is not an absolute quantity, but rather is dependent on the motion of the observer. If you're moving towards or away from any imaginary line you draw, distances in that direction will contract, depending on your relative velocities. For the gravitational force to be a calculable quantity, all observers would have to derive consistent results, something that you cannot get by combining relativity with Newton's gravitational force law.
Therefore, according to Einstein, you'd have to develop a theory that brought gravitation and relativistic motions together, and that meant developing General Relativity: a relativistic theory of motion that incorporated gravity into it. Once completed, General Relativity told a dramatically different story. An animated look at how spacetime responds as a mass moves through it helps showcase exactly how, Note that spacetime can only be described if we include not only the position of the massive object, but where that mass is located throughout time.
Both instantaneous location and the past history of where that object was located determine the forces experienced by objects moving through the Universe. In order to get different observers to agree on how gravitation works, there can be no such thing as absolute space, absolute time, or a signal that propagates at infinite speed.
Instead, space and time must both be relative for different observers, and signals can only propagate at speeds that exactly equal the speed of light if the propagating particle is massless or at speeds that are below the speed of light if the particle has mass.
In order for this to work out, though, there has to be an additional effect to cancel out the problem of a non-zero tangential acceleration, which is induced by a finite speed of gravity. This phenomenon, known as gravitational aberration, is almost exactly cancelled by the fact that General Relativity also has velocity-dependent interactions. As the Earth moves through space, for example, it feels the force from the Sun change as it changes its position, the same way a boat traveling through the ocean will come down in a different position as it gets lifted up and lowered again by a passing wave.
Gravitational radiation gets emitted whenever a mass orbits another one, which means that over long Before the first black hole ever evaporates, the Earth will spiral into whatever's left of the Sun, assuming nothing else has ejected it previously. Earth is attracted to where the Sun was approximately 8 minutes ago, not to where it is today.
What's remarkable, and by no means obvious, is that these two effects cancel almost exactly. The fact that the speed of gravity is finite is what induces this gravitational aberration, but the fact that General Relativity unlike Newtonian gravity has velocity-dependent interactions is what allowed Newtonian gravity to be such a good approximation. There's only one speed that works to make this cancellation a good one: if the speed of gravity equals the speed of light.
So that's the theoretical motivation for why the speed of gravity should equal the speed of light. If you want planetary orbits to be consistent with what we've seen, and to be consistent for all observers, you need a speed of gravity that equals c , and to have your theory be relativistically invariant. There's another caveat, however. In General Relativity, the cancellation between the gravitational aberration and the velocity-dependent term is almost exact, but not quite.
The opportunity to do this arose in September , when Jupiter passed in front of a quasar that emits bright radio waves. From that they worked out that gravity does move at the same speed as light. Their actual figure was 0. Their result, announced on Tuesday at a meeting of the American Astronomical Society meeting in Seattle, should help narrow down the possible number of extra dimensions and their sizes.
But experts say the indirect evidence that gravity propagates at the speed of light was already overwhelming. The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. Are there future prospects for a direct measurement of the speed of gravity? One possibility would involve detection of gravitational waves from a supernova.
The detection of gravitational radiation in the same time frame as a neutrino burst, followed by a later visual identification of a supernova, would be considered strong experimental evidence for the speed of gravity being equal to the speed of light.
But unless a very nearby supernova occurs soon, it will be some time before gravitational wave detectors are expected to be sensitive enough to perform such a test. Hawking and W. Israel, editors Cambridge Univ. Press, Carlip, "Aberration and the Speed of Gravity," Phys. Feynman, R. Leighton, and M. The collision was so extreme that it caused a wrinkle in space-time — a gravitational wave.
That gravitational wave and the light from the stellar explosion traveled together across the cosmos. They arrived at Earth simultaneously at a.
Eastern on August But it was also the first-ever direct confirmation that gravity travels at the speed of light. We all know light obeys a speed limit — roughly , miles per second. Nothing travels faster. But why should gravity travel at the same speed?
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