# Regarding arguments in favor of Principle of Relativity

1. Sep 30, 2012

### A Dhingra

hello..

I am reading "Relativity, the Special and The General theory of relativity by A. Einstein", and i have a few doubts that i wish to clarify.

In one of the section there are two arguments given in favor of the principle of Relativity,one being that laws of classical mechanics apply to celestial bodies to great deal of accuracy, and the other one is first assuming it to be wrong generating a need for a Special system 'absolutely at rest' and then dealing with no such anisotropic properties are revealed in physical state.
Check out this link : http://www.marxists.org/reference/archive/einstein/works/1910s/relative/ch05.htm
(specifically the last two paragraphs)

Is it trying to say that reference frames moving in different directions with respect to the absolute frame should observe any given event differently? And was this tested by the Famous Michelson and Morley experiment(or this one was trying to prove the presence of an absolute frame, The Ether)?
I am quite confused and didn't really understand the second argument. So can you please explain that with a few more examples (and provide more arguments in favor of it, if possible.)

Thanks for any help...

2. Sep 30, 2012

### Staff: Mentor

If the laws of physics depend on the reference frame, we should observe some differences in experiments for different times of the year (and day) and in different directions (quicker/slower relative to the absolute system).
Right (both - they tried to confirm the ether theory and failed).

3. Oct 7, 2012

### A Dhingra

I guess it helps.. but if in case i have any further doubt regarding this I'll come here again.
Thanks.

4. Oct 10, 2012

### harrylin

More or less yes, here's a little more precision. He is specifically discussing "the description of natural phenomena" - that is, observable events. Thus he does not discuss philosophy but the question if any particular frame is preferred for observation.

For example, Newton postulated an "absolute space" which he used as logical foundation for his theoretical development, but it was in no way preferred for natural phenomena. As a result it does not appear in any calculations, and the concept was largely forgotten. The same happened with the stationary ether concept: if such an entity exists, it must conform to the (special) relativity principle, so that all inertial frames are equivalent for the laws of nature. Consequently it plays no role in the transformation equations between such frames.* In his 1916 book he stressed that there is no preference for selecting any inertial frame for applying the laws of nature - that is the (special) PoR. It's a mistake to think that the PoR is in conflict with all ether concepts. IOW, he made sure to not say what you ask he was trying to say, because that would have been unfounded and irrelevant.

Next, indeed he referred to Michelson-Morley and follow-ups. Based on the combination of Newton's mechanics with Maxwell's electrodynamics they expected to find a clearly measurable anisotropy. A detailed description is here, with clear illustrations:
http://en.wikipedia.org/wiki/Michelson–Morley_experiment

That they did not find the expected anisotropy gave a big push for the development of a new theory - it became clear that either Maxwell's electrodynamics or Newton's mechanics needed correction, and the puzzle to solve was how to fix it so that the PoR could work for both mechanics and optics. So far concerning the sound argumentation of that chapter.

However, while you are right that they were trying to detect anisotropy, the purpose of Michelson was not to prove the presence of the ether; instead they wanted to measure their velocity relative to it, based on the stationary ether hypothesis. A positive measurement result (expected based on the validity of Newton's mechanics which they did not question) would have been evidence in favour of that hypothesis. See: http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

* Note that his argument there explicitly excludes rotation and although that is included in GR, a few years later he admitted that rotation is a "weighty argument" in favour of such an entity. http://www.tu-harburg.de/rzt/rzt/it/Ether.html [Broken]

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