Re: A little challenge for relativists.
- From: John Kennaugh <JKNG@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 4 Nov 2005 09:01:11 +0000
Tom Roberts wrote:
John Kennaugh wrote:PD wrote:1. Experimental evidence of source independence.There was none in 1905
Strictly speaking, no. But there certainly was A LOT of evidence that only relative velocities matter for E&M phenomena. Einstein knew of several [quoting from the FAQ]:
When A. Einstein wrote his famous paper: "The Electrodynamics of Moving Bodies" in 1905, he already had experimental support for his new theory:
".... Examples of this sort, together with the unsuccessful attempts to discover any motion of the Earth relatively to the "light medium" suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. They suggest rather that, as has already been shown to the first order of small quantities, the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good..."
What was the experimental support for this claim?
I do not dispute this claim. Relativity is about symmetry. The argument is for symmetry and I do not dispute the strength of those arguments.
The simplest way to achieve symmetry consistent with the facts was to assume that in every FoR "the speed of light is found to be constant w.r.t the source emitting it". It is perfectly consistent with symmetry it is the simplest explanation of MMX and if light is particulate it would be a natural assumption that the speed of such particles would be imparted to them by the source.
Ether was the cause of asymmetry in the contemporary thinking. The MMX was looking for the asymmetry believed to exist because of the ether where every FoR could in theory be uniquely identified by its velocity relative to the ether. It discovered no such asymmetry and there were enough other experiments which indicated the same to conclude that that asymmetry does not exist in nature.
If one assumes this absence of asymmetry is due to there being no ether c cannot be a property of the ether and must be the speed of light relative to the source because there is no possible causality to make it constant w.r.t the observer without the help of an intermediary (ether).
Einstein doesn't go for this obvious approach, source dependence, which automatically gives symmetry he introduces a second postulate which is ".. apparently irreconcilable" with the PoR (symmetry). What he is trying to do is to achieve symmetry *without losing the ether*. He only reconciles it by ditching two axioms of physics.
One has to study the following with care to get the actual meaning:
"The introduction of a 'luminiferous ether' will prove to be superfluous inasmuch as the view here to be developed will not require an 'absolutely stationary space' provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place".
The key word here is 'inasmuch' - He is not saying the ether is unnecessary he is saying that an ether which implies an 'absolutely stationary space' is unnecessary. One can be sure of this interpretation because he clarifies it in his 1920 lecture.
Einstein described Lorentz as having made the greatest contribution to "the theory of electricity" since Maxwell. He had come up with a theory which gave symmetry in the "system of experience" but Einstein was not happy with the asymmetry in the theoretical structure of Lorentz's theory. Lorentz said that because of an observer's motion relative to a special, but undetectable FoR it always appeared to the observer that he was stationary w.r.t the ether. Einstein simplified that by in effect saying that every observer IS stationary w.r.t the ether. Which is exactly what the second postulate describes. That is his way of achieving symmetry. Not get rid of the ether but to make every observer's relationship to the ether equally valid.
The problem is that you need a real physical ether to wrest control of the speed at which light travels from any influence the physical processes taking place in the source might otherwise have in order to get source independence. His second postulate only adds source independence to the first. How can every observer find himself stationary w.r.t it? It is certainly a problem but you do not solve it by giving each observer his own ether and calling it 'the observers FoR' - although he seems to have got away with it.
I know that the ether is very unfashionable these days and that when presenting relativity semantics are carefully chosen, emphasis carefully placed, but if you want a true history then what Einstein did only makes sense if you understand that what he was after was symmetry *without losing the ether* which he achieved only by ditching two axioms of physics. He could have achieved symmetry so much more easily by ditching the ether and assuming source dependence.
His starting point was unashamedly Lorentz's theory. He tried to improve upon LET by envisaging a different type of ether which would remove the unique FoR in the theoretical structure of Lorentz's theory.
"the whole change in the conception of the ether which the special theory of relativity brought about, consisted in taking away from the ether its last mechanical quality, namely, its immobility"
He did not manage to develop that into anything believable and in the end declared his theory to be a 'principle theory' a mathematical model which does not attempt physical interpretation. One could say with justification that Einstein tried to improve on Lorentz's theory and failed but then who cares? No one is interested in physical theory these days only mathematics.
[snip list of experiments supporting a claim I do not dispute]
But at base Einstein was theoretically motivated. By simply considering Maxwell's equations to be a "law of physics" (and Einstein most definitely did so), the postulate of source independence is fully justified.
I showed why logically Maxwell's equations must support source dependence and source independence equally but you snipped it. Apart from which Maxwell's equations are not a comprehensive 'law of physics' they failed to predict MMX they failed to predict that light was quantized indicating that their domain of applicability is somewhat limited. That there was such a limitation was known to Einstein if not its extent.
If one's inclination is to believe in the ether, and Einstein's was, then one is inclined to put greater store on Maxwell's equations than if one doesn't. As I said above the ether is very unfashionable these days and when presenting relativity semantics are carefully chosen, emphasis carefully placed. Emphasis is naturally placed on Maxwell rather than the ether. It has the advantage that few students are likely to understand Maxwell so cannot argue.
-- John Kennaugh "We cannot manufacture a physical medium having physical properties out of nothing merely by coining phrases or by re-defining space" Dr Scott Murray .
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