Why does special relativity say that c is constant?

As I understand it, special relativity says that all the laws of electromagnetism are the same for observers in all reference points. I also know that this is why the speed of light remains the same for all observers. But what law of electromagnetism states, or predicts, that light has to be exactly the speed it is? I think it has something to do with Maxwell's equations, but I've not been able to connect the dots. Can someone explain in a non-technical way why the speed of light specifically is one of the laws that remains the same for all observers?|||First: That's special relativity, which deals with non-accelerating inertial reference frames. NOT every reference frame, you need general relativity for that. With that in mind...





There's a couple of logical roads to go down here





Maxwell's equations imply that the speed of light is c in a vacuum. You generally think of waves as propagating through SOMETHING, like sound waves propagate through matter. Take away the matter and they don't, hence no sound in the vacuum of space. Maxwell's equations don't say anything about reference frames, basically if you have a changing electric or magnetic field, you get a transverse wave traveling at c.





So just from Maxwell's equations alone there are two conclusions. One is that it means c with respect to an ABSOLUTE reference frame. In other words light IS propagating through something, and that something is something we just can't see or detect, it's everywhere, including a vacuum, and everything is moving through it and the speed of light through this "aether" (as it was called) is what's c. Well they disproved this using the motion of the earth. If you shine light in the same direction the earth is moving, well the earth is moving pretty quick. So the light should appear to be going slower from our POV on earth then if shone the opposite way earth is moving. The experiment proved that in fact it was identical for both cases.





The less obvious conclusion was reached by Einstein, that for ANY inertial observer (which we basically were on earth looking at the light from either side of its motion)it's c.





Einstein himself reached the conclusion by deciding that magnetic fields were really electric fields...but from the POV of the particle in motion(a moving charge being what causes magnetic fields)





It's just not something you can be told directly from Maxwell's equations(though you see that Maxwell's equations don't invalidate them, they're perfectly true)and it's the next logical step. Just like how you simply can NOT derive the Bohr model of the atom (or more sophisticated quantum theories) from Newton's equations. Einstein made the next "jump" in understanding|||speed of light does not change|||The ratio by which it is slowed is called the refractive index of the medium and is always greater than one.* This was discovered by Jean Foucault in 1850.





Why? There is no answer. It just is.|||If you look at the laws that describe electric and magnetic forces at microscopic levels, they only work (and prove) that the speed of light is constant, no matter how fast a person is travelling. this is a consequence of the world we live in and can not be changed. as other's have said, it just is.|||Special relativity says that c is constant because, and this might be unsatisfying, the constancy of c is one of the postulates of special relativity.





There are two postulates that are used to derive all of the laws of special relativity...





1) all reference frames are equivalent





2) the speed of light is the same in all reference frames





The reason that the equations "predict" this "outcome" is that it is one of the first assumptions of the whole theory. This was accepted by Einstein as a postulate because it was the result of experiment.





The speed of light can be predicted (not by the theory of relativity) by using Maxwell's equations.





del E = -(dB/dt) and del X B = (mu)(epsilon)(dE/dT)





The derivatives should be partial, but I don't know how to write partials on yahoo answers.





This reduces to...





v = 1/[(epsilon)(mu)]^(1/2) = c/n





in a vacuum, n = 1, and v = c





Note that this does not rely on the theory of relativity. To say that the theory of relativity predicts this outcome is a bit tautological.|||Its a postulate of special relativity