Does gravity propagate faster than light?

Question

Does gravity propagate faster than light?

Answer 1

An extremely interesting question without an easy answer.

If gravity was limited to the speed of light, the aberration of the sun's gravity affecting earth would speed earth up and we would have been sent out away from the sun long ago.

It seems likely to me that this will only be resolved with a complete reworking in the way we think of 'gravity' and what 'gravity' actually is.

Answer 2

The short answer is that it's complicated. Most scientists now belive that gravitational propagation (ie the speed of gravity) occurs at several orders of magnitude (add a whole bunch of zeros to the end) faster than the speed of light.

The upper limit is tough, if not impossible, to calculate. An infinite speed of gravity opens up many paradoxes of causality (think time travel paradoxes) to the point that even Newton himself called the idea of instantaneous gravity "absurd." There should be an upper limit, but we just don't know. All Newtonian physics (which we use to calculate orbits, etc.), by the way, assume instantaneous gravity.

A common upper limit that gets passed around is the speed of light. Special relativity concludes that nothing can travel faster than the speed of light, and that would have to include gravity. At first glace this would seem to make sense - we would see the universe as we felt it. This, however, does not fit with our observations of the universe - ie how can gravity escape black holes while light cannot, how can binary stars predict eachother's orbit, etc. General relativity tries to get around this with curved space time, then perturbations propogate at the speed of light, gravitational radiation, etc etc blah blah blah. And while I can agree with curved space time, the rest of the math just gets way too messy. The anwer to all of that is that relativity simply is wrong. It's close - but in the end it is a theory, not a fact. It is good to describe many things, just not this. Even the Earth and Moon's orbits would not be possible if we accounted for an 8.3 minute gravitational delay (same as light) from the Sun - they would be unstable to the point of collapse. It is true that "gravitational waves" are bound by the speed of light, but they are altogether something different from gravity itself (think of ripples in the space time fabric caused by a quickly rotating neutron star with a large mountain on it) - since "gravitational waves" has the word gravity in it, I think that must be what causes this misconception that gravity travels as slowly as light.

Since we don't yet have the math to describe an upper limit (special relativity fails miserably at this), we can at least deduce a lower limit on the speed through scientific observation. Some recent numbers I've come across peg the speed of gravity to be at least 2*10^20 c. Where c is the speed of light, and 10^20 is a 10 with 20 more zeros after it. It is an absolutely huge number, though infinitely far away from infinity...

Answer 3

Most scientists generally assume it propagates at the same speed as light, but nobody has been to either prove or disprove that idea. Yet.

Answer 4

Difficult question. Current conventional theory says no. The classic example is what would happen if the Sun suddenly disappeared? Current physics says nothing would happen until the event arrived at the speed of light.

String theory may have a different take on it. I can't say for sure. The source below (The Elegant Universe - Brian Greene) is fantastic. You can watch the program for FREE online - it's great!

If you are pressed for time, watch the second link - it addresses your question pretty closely.