# Historical interpretation of symmetries required by Principle of Relativity

Einstein's 1905 paper discussed electromagnetism and the laws of mechanics. But did not discuss gravity, and of course did not discuss the strong or weak nuclear forces.

• Principle of Relativity - (from translation of Einstein's 1905 paper) "the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good"

1) At what point did physicists interpret this to mean all physical laws should have these symmetries?

It seems like physicists were trying to get a relativistic version of gravity fairly soon after this, so maybe immediately?

But I'm not sure when the symmetry point of view took hold. For example, would physicists back then interpret conservation of energy and momentum as a consequence of special relativity (translation invariance), or would they still consider that a consequence of the individual physical laws? As a particular example, consider the initial confusion around decays involving neutrinos. When it appeared to be a possible violation of energy conservation, did they also view this as a potential violation of relativity? Or did they not think of it in that context?

2) Since the "equations of mechanics hold good" under parity transformations, did physicists historically believe special relativity predicted parity symmetry?

For example, if parity violation was discovered in 1906, would this have been considered a blow to special relativity?

• At what point did physicists interpret this to mean all physical laws should have these symmetries? In 1905, mechanics plus optics constituted all physical laws. By extending relativity from mechanics to optics, Einstein was proposing exactly that all physical laws (as known in 1905) should have the corresponding symmetries. It is also possible to think of the symmetry we're talking about as a symmetry of spacetime itself, and this was done by Ignatowsky in 1911, en.wikisource.org/wiki/… . – Ben Crowell Jan 23 '16 at 20:47
• @BenCrowell there was also gravity at the time, and around this time was the initial steps towards quantum theories, and yes later they learned about more fundamental forces. Did physicists immediately take special relativity to require any new physical theories to have these symmetries? Or did this understanding develop over time? – BuddyJohn Jan 24 '16 at 0:50
• @BuddyJohn such a change in outlook can't have been instantaneous. – vonbrand Feb 4 '16 at 23:35