# Can SR be derived from other postulates than the constancy of c

#### lalbatros

The usual foundation for deriving SR is the constancy of the speed of light.
Are there other ways to derive SR, and eventually some more general?

Thanks,

Michel

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#### dicerandom

You could postulate some of the results of SR, i.e. that spacetime is described by a Minkowski metric, that transformations between reference frames are described by the Lorentz transformation, and then derive things like the constancy of c and the equivalence of inertial frames. I think that the constancy of c line of reasoning is more physical though.

#### clj4

lalbatros said:
The usual foundation for deriving SR is the constancy of the speed of light.
Are there other ways to derive SR, and eventually some more general?

Thanks,

Michel
There have been several attempts in this direction. They all failed due to errors (that were discovered later). I have a list of all the failures.

#### Tom Mattson

Staff Emeritus
Gold Member
Here's the basic program that Lorentz followed in 1904.

1.) Assume that all frames move with a velocity less than c.

2.) Do a Galilean transform on the equations of electrodynamics.

3.) Notice that they're not in their original form.

4.) Insert time dilation and length contraction by hand so that the original form of the equations is restored.

#### robphy

Homework Helper
Gold Member
Had sufficiently energetic particle accelerators existed back then or if we had precise enough timepieces [or if everyday speeds were at least a significant fraction of the speed of light], one could have experimentally observed many of the "relativistic effects" of SR. Then, one may have deduced the appropriate equations and possibly the existence of a finite limiting velocity.

Along another thread, one might have asked what symmetries underlie the usually-formulated Maxwell's Equations of electromagnetism. Among the symmetries one would find is a group of transformations that preserve it: the Lorentz group, whose eigenvectors in an appropriately defined space are directions corresponding to the lightlike vectors in spacetime.

In some sense, the postulate of the "constancy of the speed of light" [or better, "the existence of a finite limiting velocity of signal propagation"] could be regarded as a [fortunately simple] representative "law of physics" that must be included under the umbrella of the "principle of relativity".

In an earlier post (from an earlier thread), I included an attachment that diagrams numerous attempts which have appeared in the literature. Depending on your scientific philosophy, you may or may not like the various approaches. However, it's clear that [thanks to the numerous symmetries of Minkowski spacetime] there are numerous ways to get to SR.

#### cosmik debris

lalbatros said:
The usual foundation for deriving SR is the constancy of the speed of light.
Are there other ways to derive SR, and eventually some more general?

Thanks,

Michel
If you do a google search on the usenet newsgroups you can find a derivation by Tom Roberts called "A Physicist's Derivation of Special Relativity"

#### robphy

Homework Helper
Gold Member
cosmik debris said:
If you do a google search on the usenet newsgroups you can find a derivation by Tom Roberts called "A Physicist's Derivation of Special Relativity"
Thanks for the tip. I just browsed through it
The first sentence of my linked-post above applies here: This is one of those "Lorentz Transformations without the speed of light" proofs where the value of the speed of light plays its role only in the last step.

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