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If gravity can bend light, why can't gravity slow light. At least momentarily? Wouldn't that give the illusion of the universe expansion speeding up?

user50238
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1 Answers1

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Light, all light, is composed out of zillions of photons, similar to observational fact that the flow of water in a river is composed out of zillions of molecules.

For light to slow it would mean that individual photons should slow, which is impossible within the known and well validated ( meaning tested a huge number of times and always found to be correct) framework of special relativity. Photons always travel with the speed of light, c, because they have zero mass. The only thing that can happen to a photon is a change in energy, which will either increase its frequency, since for photons E=h*nu, if energy is gained, or decrease it.

So your hypothesis

At least momentarily? Wouldn't that give the illusion of the universe expansion speeding up?

cannot hold.

anna v
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  • In GR the speed of light is only locally $c$, that is a local measurement will always return the value $c$. However the speed of light can have a lower value at a distant location, for example it falls to zero at the event horizon of a Schwarzschild black hole. – John Rennie Jun 10 '14 at 11:51
  • @JohnRennie I cannot find confirmation for this statement of yours . Do you have a link? – anna v Jun 10 '14 at 13:19
  • You need look no farther than this site! See for example Speed of light in a gravitational field?. – John Rennie Jun 10 '14 at 14:12
  • @JohnRennie Well, this seems to be your personal interpretation. I have not found it elsewhere as a claim that the velocity of light changes. It is like an optical illusion. – anna v Jun 10 '14 at 14:57
  • What would you regard as an authority? Wikipedia? – John Rennie Jun 10 '14 at 15:16
  • Or the man himself? According to John Baez in Einstein's 1920 book "Relativity: the special and general theory" he wrote: "... according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [Einstein means speed here] of propagation of light varies with position." – John Rennie Jun 10 '14 at 15:21
  • @JohnRennie Not wikipedia. A book reference would be fine. Actually velocity I am OK with, the vector changes as in lensing. – anna v Jun 10 '14 at 15:45