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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?


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," – 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
up vote 2 down vote accepted

The speed of light is entirely a local concept - it does not care if there are 10 atoms or 10 billion galaxies somewhere in the Universe.

Obviously we can't go to distant galaxies to directly measure the speed of light, so in the absolutely strictest sense this is not directly empirically tested. However, the constancy of the speed of light is one of the most fundamental tenets of physics. In some sense, just about every observation we make in astronomy tests it, for if there were any variation it would manifest in all sorts of crazy ways in every single system we look at.

The confusion seems to stem from the term "universal." The word "universal" means "fundamental" or "unchanging in space and time" or "lies at the heart of our theoretical framework, permeating everything we do." It does not mean "tied to the Universe" or "depends on global properties of the Universe." Along the same lines, a scented candle could be said to have an "earthy" scent, but this has nothing to do with it being located on Earth the planet.

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My interest was not so much the word "universal" as much as the fact that it takes all the energy in the universe to go the speed of light. – micahhoover May 3 '13 at 18:53
@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
@David Zaslavsky... if you were to accelerate an object to the speed of C - planck's constant or C - 1/100planck's constant botch which have enormous but finite energy requirements... could quantum mechanics (ie the combination of uncertainty and fuzziness/quantum jumping) allow you to accelerate to C or beyond it? – frogeyedpeas May 4 '13 at 1:47
Ignore the comment I am going to post as a separate question – frogeyedpeas May 4 '13 at 1:48

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