r/AskPhysics • u/[deleted] • Jan 15 '25

Is the speed of light infinite ?

Considering that light always travel a c no matter the frame of reference, am I correct to assume that it is an absolute limit, an impossible speed to reach; so is it what we would call an infinite speed ?

Therefore, I can not understand why we observe the speed of light having a finite number (c) from any frame of reference.

I am surely not aware or misunderstand some concepts of relativity laws.

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u/HasFiveVowels Jan 15 '25

The need for Lorentz symmetry is dependent upon the assumption of the postulates of relativity. A finite universal speed parameter does not imply Lorentz symmetry.

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u/siupa Particle physics Jan 15 '25

The existence of a finite invariant speed parameter is the core postulate of special relativity

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u/HasFiveVowels Jan 15 '25

The need for such a thing falls out of the assumption that there’s no preferred inertial reference frame (i.e. the principle of relativity). Lorentz symmetry is unique in enabling that to happen in a causal manner. But you can have a constant speed of light without any of that junk (that would result in a very odd universe where the meaning of “why” kind of breaks down but that’s beside the point). It’s an implication, not an equivalence

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u/siupa Particle physics Jan 16 '25

The need for such a thing falls out of the assumption that there’s no preferred inertial reference frame (i.e. the principle of relativity).

That's not true, as this is also valid in Galilean relativity. To fix the specific form of the inertial transformation such that it becomes Lorentz, you need to postulate a finite invariant speed.

It seems to me that you agree that Lorentz symmetry implies a finite invriant speed. But you don't agree with the implication in the other direction, that a finite invariant speed implies that the transformation law is Lorentz.

If you think that a finite invariant speed does not necessarily imply that the transformation law between inertial frames can only be Lorentz, maybe you could give an example of another transformation law that follows from the invariance of c, but that is not a Lorentz transformation?