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

1. Apr 27, 2006

### 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

2. Apr 27, 2006

### 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.

3. Apr 27, 2006

### clj4

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.

4. Apr 27, 2006

### Tom Mattson

Staff Emeritus
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.

5. Apr 27, 2006

### robphy

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.

6. Apr 27, 2006

### cosmik debris

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"

7. Apr 27, 2006

### robphy

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.