# Why is the speed of light absolute?

• I
Galilean relativity doesn't have an invariant speed and it has spatial isotropy and homogenity. Galilean boosts in one axis are also a 1-parameter Liegroup (don't they?). So I don't see how only using the postulates you mention you get an invariatn speed, since those postulates are also in the galilean relativity.
The "invariant speed" in Galilean relativity turns out to be ##\infty##.

In the more general derivation, one actually derives a constant with dimensions of inverse speed squared. SR corresponds to the choice of ##1/c^2## for this constant. Galilean relativity corresponds to the choice ##0##, which is equivalent to letting ##c\to\infty##.

andresB
That makes more sense.

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Heh, you forgot de Sitter.
It's an interesting question, why de Sitter doesn't also follow from these symmetry assumptions. I guess it's because it's not time-translation invariant.

I'm referring to the derivation of the Galilei and Lorentz transformation given here:

https://doi.org/10.1063/1.1665000

It's an interesting question, why de Sitter doesn't also follow from these symmetry assumptions. I guess it's because it's not time-translation invariant.
It's because the homogeneity assumption adopted by Berzi+Gorini (and many others) insists that finite intervals are preserved under spatio-temporal translations. That forces the denominator in the more general fractional-linear transformations to become trivial, resulting in linearity.

vanhees71
stefanbanev
Summary:: To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?

To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?

Well, why experiment may give us a particular result or "Why any observation is possible at all?" ;o) You may consider an observation act as some predicate in some axiomatic thus, any observable reality must be consistent otherwise you can not have definite results of experiments. So, the question "why speed of light is absolute?" is similar to "why it happens to get into existence in this particular reality?" Because otherwise you would have a different set of "why" questions for different realities arrangements. Axiomatic of any reality can not be completely defined it remains open so its expansion/extension is filtered/selected/restricted by requirement to ensure the possibility for its observer to observe definite observations (to provide a local consistency)...

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PeterPeter
Does this FermiLab video help?
"Why can't you go faster than light?" by Fermilab’s Dr. Don Lincoln

The OP and the answers are slightly inaccurate. The correct answer is:

The speed of light in vacuum is constant and given by $c_{0}=\frac{1}{\sqrt{\mu_{0}\epsilon_{0}}}$.

The speed of light in other cases is given by $c=\frac{1}{\sqrt{\mu \epsilon}}$. The speed of light in glass (for example) is about $\frac{2c_{0}}{3}$. This is the reason why prisms and lenses work...

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The OP and the answers are slightly inaccurate. The correct answer is:

The speed of light in vacuum is constant and given by $c_{0}=\frac{1}{\sqrt{\mu_{0}\epsilon_{0}}}$.

The speed of light in other cases is given by $c=\frac{1}{\sqrt{\mu \epsilon}}$. The speed of light in glass (for example) is about $\frac{2c_{0}}{3}$. This is the reason why prisms and lenses work...

The key point about the speed of light in vacuum is that it is invariant. The speed of light in other media is not invariant, but constant relative to the medium.

The speed of light now is taken to be exactly ##299,792,458 m/s##, which defines the metre.