Regarding arguments in favor of Principle of Relativity

[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...

 

[mfb]

Is it trying to say that reference frames moving in different directions with respect to the absolute frame should observe any given event differently?

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).

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)?

Right (both - they tried to confirm the ether theory and failed).

 

[A Dhingra]

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

 

[harrylin]

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...

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

 

[sardar]

Hello,

The Principle of Relativity, as proposed by Albert Einstein, states that the laws of physics should be the same for all observers in uniform motion. This means that the laws of physics should not depend on the observer's frame of reference. The arguments in favor of this principle are based on the observations and experiments conducted by scientists, including the famous Michelson-Morley experiment.

The first argument states that the laws of classical mechanics, which were developed by Isaac Newton, apply to celestial bodies with a high degree of accuracy. This suggests that these laws are independent of the observer's frame of reference and support the idea of a universal set of laws that apply to all observers.

The second argument is a bit more complex. It suggests that if we assume the principle of relativity to be wrong, it would lead to the need for a special frame of reference that is "absolutely at rest." This would mean that there is one particular frame of reference that has a special status and all other frames of reference are moving relative to it. However, no such anisotropic properties (properties that depend on the direction of motion) have been observed in physical systems. This supports the idea that there is no absolute frame of reference and all frames of reference are equally valid.

The Michelson-Morley experiment was actually designed to test the presence of an absolute frame of reference, known as the luminiferous ether. However, the results of the experiment showed no evidence of the ether, providing further support for the principle of relativity.

To understand this concept better, let's consider an example. Imagine a person standing on a train platform and another person on a moving train passing by. The person on the platform observes the person on the train moving at a certain speed, while the person on the train observes the platform moving at the same speed in the opposite direction. According to the principle of relativity, both observers are equally valid and the laws of physics should be the same for both of them.

In addition to the arguments mentioned, there are other supporting evidence for the principle of relativity such as the Lorentz transformation, which describes how physical quantities change when observed from different frames of reference, and the fact that the speed of light is constant for all observers, regardless of their relative motion.

I hope this helps to clarify your doubts. The principle of relativity is a fundamental concept in modern physics and has been confirmed by numerous experiments and observations. I would suggest further reading and research to