What physical properties of the universe define the speed of light?

[francoisp]

What I mean by this question is why is the speed of light equal to 299,792,458 m/s and not 400,000,000 m/s? What physical property of the universe is imposing this upper limit?

Or put in other words: if we consider for a moment the possible existence of parallel universes, it is concievable that c could have different values in different universes. So what properties could cause c to be different?

[freeztar]

Originally Posted by francoisp

What I mean by this question is why is the speed of light equal to 299,792,458 m/s and not 400,000,000 m/s? What physical property of the universe is imposing this upper limit?

The speed of light through a vacuum is the upper limit for photons. Nobody can say what is imposing the limit because the limit is limited to theory (although quite sound theory by today's standards).

Or put in other words: if we consider for a moment the possible existence of parallel universes, it is concievable that c could have different values in different universes. So what properties could cause c to be different?

That's a loaded question in a way. Lots of properties I have no capability of experiencing COULD be causing multi-dimensional differential events, but I'm still stuck with the unknowing factor.

If we postulate for cheese and giggles, then we can run the gambit of Sci-Fi. Light could redshift due to warping effects of other dimensions pulling light in and out of reality (ie this dimension). It could all be a kaleidoscope, or a Spanish windmill...

Light is pretty constant though.

[Qfwfq]

It is important to realize that c is a property of space-time, although called "speed of light" for historical reasons. The numerical value of c has no significance, c = 1 unless you use different units for spacelike and timelike lengths. When stating the numerical value in m/s you are really stating the ratio between the second, a very large unit of length, and the metre, a very short one.

One misconception is that Special Relativity states it to be "constant". No, what it actually postulates is that it's a Lorentz scalar and a limit on propagation of causality (this isn't the way Einstein initially put it). Considering it equal at different locations is related to considering space-time to be straight; in General Relativity this is dependent on choice of coordinates. It would be very subtle to discuss the notion of c changing physically across space and time (IOW not just a choice of coordinates).

[francoisp]

Using the sound wave analogy, if I change the density of the medium the sound wave travels through, I can speed it up or slow it down. Since an electromagnetic wave travels *inside* the universe I would assume it is also subjected to its environment. Can we infer that the propagation speed could be manipulated by changing the medium it travels through? And what would those changes be?

[Qfwfq]

An electromagnetic wave propagates at less than c through glass, water or any transparent material. Even in air it is slightly slower than in vacuo. This however does not amount to a reduction in c which is a more fundamental thing.

[Jay-qu]

permeability of free space:

Permittivity of free space:

I just tried this calculation with the values I have for the two constants and it didnt come out right I assume I dont have the right units..

[Qfwfq]

Did you try working it out in natural units?

[Jay-qu]

Originally Posted by Qfwfq

Did you try working it out in natural units?

nope.. I had 8.85*10^-12 and 4pi*10^-7 .. which dont work and Im not sure what units these values are in.

[Qfwfq]

Originally Posted by Jay-qu

Im not sure what units these values are in.

[Little Bang]

Suppose that the speed of light changes with the expansion of the universe, there would be no experiment that we could perform to show that it did or did not?