Is gravity faster than light?

04 April 2017

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Question

If the sun was to disappear instantly, would the gravitational effect on the Earth be instant? In other words, the effect of gravity being faster than the speed of light?

Answer

Andrew Pontzen illuminated the answer to David's question...

Andrew - This is a beautiful question which I thought about for many years before figuring out what the answer is. Because, in Einstein’s theory of gravity (general relativity), nothing can travel faster than the speed of light. That’s one of the key components that goes into the theory before you even start. But, if you actually start analysing the equations quite carefully, they also have to agree with Newton’s theory of gravity pretty accurately. It has to be the case that since Newton’s theory of gravity works so well for describing things like the solar system and so on that it must be pretty much correct. Certainly in Newton’s theory of gravity, if the Sun were to disappear, then instantaneously we’d be flung out of orbit because there’d be no gravitational tug keeping the Earth in orbit around where the Sun used to be.

It is a bit strange - how does this get reconciled in Einstein's general relativity where nothing should travel faster than the speed of light, not even gravity itself. The answer is actually a bit of a disappointing one. It’s that Einstein's theory of gravity doesn’t even answer that question. It just refuses to answer the question because it has built into it at it’s centre and idea that energy can’t just be destroyed, and neither can mass. So you can’t take something like the Sun and say oh, it’s disappeared what happens next? The theory actually says no, that is just not allowed. So, if you try to make a calculation of what happens next, you’ll get all sorts of contradictory answers and things happening.

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