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In this wikipedia article it is described how a beam of light, with its locally constant speed, can travel "faster than light". That is to say it travels a distance, which, from a special relativistic point of view, is surprisingly big.

I wonder if a gravitational wave on such a curved spacetime (of which the wave is actually part of) behaves equally.

Does a gravitational wave also ride on expanding spacetime, just as light does? Do the nonlinearities of gravitation-gravitation interaction influence the propagation of a wave (like e.g. a plasma) such that light and gravity are effectively not equally fast?

If I want to send a fast signal in this expanding universe scenario, in what fashion do I decide to I send it?

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Nikolaj-K
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2 Answers2

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It's important to stress, once again, that nothing is actually traveling faster than light in a literal sense. It's just that the universe expands, so that object that sent the signal is, measuring with today's ruler, farther away than the speed of light times the age of the universe. You can't receive signals from objects too far away, though - we can only communicate with objects inside our horizon.

All of the above reasoning also applies to gravitational wave signals, which also travel at the speed of light.

kleingordon
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The speed of light is only locally invariant. For example if you watch a beam of light falling onto a black hole event horizon you'll find it slows down, and if you waited an infinite time you'd see it stop at the event horizon. Whether the speed of light is really changing depends on your point of view. I'd say it's just your choice of co-ordinates that makes it look as if it's changing, but opinions will differ.

Anyhow, the "faster than $c$" motion referred to in your link is similar in that it's a matter of geometry and the co-ordinates you're using. Since it's just geometry gravity waves will be affected in the same way as light, as indeed will everything that propagtes in space including objects with a non-zero rest mass.

John Rennie
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