# Is the speed of light related to the mass of the universe?

If the mass of the universe were cut in half, would it affect the speed of light?

Would it be twice as fast?

Would it stay the same?

Do we have instruments that are sensitive enough to measure the speed of light at different positions relative to high-mass objects to empirically answer this question?

The speed of light is (something of) a universal constant, but is it really dependent on the universe or on something intrinsic to photons?

EDIT:

Related question:

Since gravity is a relationship between one atom and every other atom in the entire universe, and it takes all the energy in the universe to travel at the speed of light, is there something about the energy/gravity/mass of the universe that "slows" light from going a faster speed?

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It's not possible to define whether c changes from one place to another. It's only possible to define whether a unitless parameter such as the fine structure constant changes. See Duff, 2002, "Comment on time-variation of fundamental constants," arxiv.org/abs/hep-th/0208093 – Ben Crowell May 3 '13 at 18:52
@BenCrowell: is that because (at c speeds) time passes in a noticeably different way that is relative with respect to an observer? – micahhoover May 3 '13 at 18:55
No, it's for the reasons described in the paper that I linked to. – Ben Crowell May 3 '13 at 21:19
Where ever you heard or read "it takes all the energy in the universe to travel at the speed of light", you should not take it as a definition, it's a poetic way of saying you would need arbitrarily large amounts of energy. – dmckee May 3 '13 at 22:04

@micahhoover Ahh, I see. Well, it takes more than all the energy in the universe to accelerate any massive particle to $c$, no matter what universe you are in - it just can't be done. When people say it takes all the energy in the universe, what they mean is that no finite amount of energy will suffice, and the entire universe is the "closest thing to infinity" they care to imagine for the comparison. Things that do move at $c$, like photons, are simply always doing that - nothing ever accelerated them, and nothing will decelerate them. – Chris White May 3 '13 at 19:00
I think it's even misleading to say "it takes [amount of energy] to accelerate a massive object to the speed of light," regardless of what the amount of energy is (yes, even if it's "infinite energy"). It's just impossible to make a massive object move at $c$. No matter how much energy you have, it cannot be done. – David Z May 3 '13 at 19:05