# ON THE ELECTRODYNAMICS OF MOVING BODIES by A.EINSTEIN

We all know this book... its is basically the SR as we know it.

you can find it here ww.fourmilab.ch/etexts/einstein/specrel/www

what i was looking for was how and on what basis back on 1905 the Albert figures out the speed of light is constant... all I get is this

"Any ray of light moves in the stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence

velocity=light path/time interval

where time interval is to be taken in the sense of the definition in § 1."

I might not have red it through very carefully, but i spent an evening examining and reading it diagonally. It doesn't mention any experiments that prove the quotation, it also doesn't offer any thought experiment that could help us see that the c is realy constant.

is there any document writen by Albert or any other matematician or is there any experiment that proves the light speed constancy (before publishing this book - 1905)???

This is very important to me because lorentz transformation and all the formulas I been calculating mass,time, lenght,energy etc. is based on the consumtion that c is constant.

mathman
The Michelson-Morley experiment (1889?) is the best known example showing the constancy of the speed of light.

Yes so I tried to seek for Albert to mention this experiment in his book... he doesn't mention it....

the eather does not exist
the light emited in this experiment is stationary to the mirrors and to the reciever

so I believe this does not prove the quotation mentioned in the first post

dx
Homework Helper
Gold Member
Its is basically the SR as we know it.

Actually, the modern way of looking at SR is quite different from the one presented in that paper. Two years after Einstein's paper, Minkowski published a paper called "Space and Time". The view presented in that paper is closer to the way we view it now.

dx
Homework Helper
Gold Member
I might not have red it through very carefully, but i spent an evening examining and reading it diagonally. It doesn't mention any experiments that prove the quotation, it also doesn't offer any thought experiment that could help us see that the c is realy constant.

That c is constant was not figured out by thought. It was an experimental fact. Einstein says in the second paragraph, "... together with the unsuccessful attempts to discover any motion of the earth relatively to the light medium suggest that the phenomenon of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest."

That c is constant was not figured out by thought. It was an experimental fact. Einstein says in the second paragraph, "... together with the unsuccessful attempts to discover any motion of the earth relatively to the light medium suggest that the phenomenon of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest."

Maybe you can show me the exact link. how did they get from

there is no absolute rest

to

c is constant

I checked the experiments done on measuring the speed of light, they all show the same results, but they all are done in such a manner that light emiting element and light receiving element are stationary

the phenomenon of electrodynamics ... possess no properties corresponding to the idea of absolute rest.

That means Maxwell's equations do not show any preference for a particular frame. Thus, they hold in any frame. Thus, the wave equation is invariant. Thus, the speed of light in any medium is c. (Which is the inverse of the square root of epsilon-zero*mu-zero).

Ich
Maybe you can show me the exact link. how did they get from

there is no absolute rest

to

c is constant
That link was Maxwell's electromagnetic theory. AFAIK ,the first theory-independent experimental evidence is de Sitter's observation, published 1913.
I checked the experiments done on measuring the speed of light, they all show the same results, but they all are done in such a manner that light emiting element and light receiving element are stationary
Now that's a remarkable claim. You're sure you really checked all experiments? Maybe you overlooked http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#moving-source_tests".

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russ_watters
Mentor
Maybe you can show me the exact link. how did they get from

there is no absolute rest

to

c is constant
What you are missing is that if there was an absolute rest frame, the speed of light would be constant only with respect to that frame. All other frames would show some variation depending on their motion with respect to that frame. That's what the MM experiment was trying to find.
I checked the experiments done on measuring the speed of light, they all show the same results, but they all are done in such a manner that light emiting element and light receiving element are stationary
Stationary with respect to what? Each other, right? That's not the issue - the issue is whether they are stationary with respect to the Universal Rest Frame. If they are not stationary with respect to the Universal Rest Frame, the experiments will show a variation in the speed of light. So what the experiments show is that the Universal Rest Frame doesn't exist - all (inertial) frames are equally valid. That's why the early fixed-apparatus tests were enough to show the postulate is correct.

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Ich
Making parts of the experiment move wrt each other is an unnecessary complication - you don't learn anything useful from it.
I disagree. Everything else is rather indirect evidence, based on theoretical models of the nature of light. If some variation of Ritz's model - where the speed of light depends on the speed of the source - were viable, those experiments would be inconclusive.
Therefore it is necessary that the postulate ("Any ray of light moves in the stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body", as cited by calis) be confirmed directly, which has been done.

I disagree. Everything else is rather indirect evidence, based on theoretical models of the nature of light. If some variation of Ritz's model - where the speed of light depends on the speed of the source - were viable, those experiments would be inconclusive.
Therefore it is necessary that the postulate ("Any ray of light moves in the stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body", as cited by calis) be confirmed directly, which has been done.

I dont understand, you say that RITZ'S model is not viable and then you say that a direct experiment has been done?

apart from that thank you for many answeres I will read some articles through.

Ich
I dont understand, you say that RITZ'S model is not viable and then you say that a direct experiment has been done?
Well, yes, that's what I said. Ritz promoted an alternative model of the nature of light, which has some internal inconsistencies and therefore is not viable. However, one could imagine that some alternative model which does not predict c to be constant be viable, therefore it is better to test the second postulate without relying on additional theories. And that has been done, you find references to the relevant papers in the website linked above.

russ_watters
Mentor
I disagree. Everything else is rather indirect evidence, based on theoretical models of the nature of light. If some variation of Ritz's model - where the speed of light depends on the speed of the source - were viable, those experiments would be inconclusive.
Therefore it is necessary that the postulate ("Any ray of light moves in the stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body", as cited by calis) be confirmed directly, which has been done.
I actually deleted the part of my post you objected to when I saw that you had posted experiments done on moving source (and for practical purposes, time dilation tests with moving clocks do also verify it), so my point there was moot anyway.

Still, it sounds like you are saying that such experiments would be necessary if an otherwise viable competing model were found -- but one hasn't been. So it still sounds unnecessary to me, besides being a pre-emptive strike on a potential competitor. But whatever - very minor quibble.

What you are missing is that if there was an absolute rest frame, the speed of light would be constant only with respect to that frame. All other frames would show some variation depending on their motion with respect to that frame...

Do you agree that if an absolute rest frame had the properties that:

1) Any object moving relative to the absolute rest frame length contracts.
2) Any clock moving relative to the absolute rest frame slows down.

in the manner proposed by Lorentz, then the speed of light would appear to be constant to any inertial observer and the absolute rest frame would be undetectable and the results would be indistinguishable from those of Special Relativity.

russ_watters
Mentor
Do you agree that if an absolute rest frame had the properties that:

1) Any object moving relative to the absolute rest frame length contracts.
2) Any clock moving relative to the absolute rest frame slows down.

in the manner proposed by Lorentz, then the speed of light would appear to be constant to any inertial observer and the absolute rest frame would be undetectable and the results would be indistinguishable from those of Special Relativity.
So what properties would this absolute rest frame have that would be unique to it?

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Ich
Still, it sounds like you are saying that such experiments would be necessary if an otherwise viable competing model were found
No, but as long as a competing model is not strictly ruled out (which is very difficult), you have a model-dependent measurement. It is definitely not satisfactory to base something as profound as SR on Electrodynamics.

is there any document writen by Albert or any other matematician or is there any experiment that proves the light speed constancy (before publishing this book - 1905)???

This is very important to me because lorentz transformation and all the formulas I been calculating mass,time, lenght,energy etc. is based on the consumtion that c is constant.

That the speed of light is constant was already proposed before Einstein in 1904 by Poincaré as a result of Lorentzian electrodynamics.

Poincaré: From all these results, if they were to be confirmed, would issue a wholly new mechanics which would be characterized above all by this fact, that there could be no velocity greater than that of light (because bodies would oppose an increasing inertia to the causes that would tend to accelerate their motion; and when approaching the velocity of light, this inertia would become infinite), any more than a temperature below that of absolute zero. For an observer, participating himself in a motion of translation of which he has no suspicion, no apparent velocity could surpass that of light, and this would be a contradiction, unless one recalls the fact that this observer does not use the same sort of timepiece as that used by a stationary observer, but rather a watch giving the “local time.”

Einstein himself acknowledged in 1909 and 1912 that the he took the postulate of the constancy of light from Lorentz's ether.

Einstein: Superficial consideration suggests that the essential parts of Lorentz's theory cannot be reconciled with the relativity principle. According to Lorentz's theory, if a light beam propagates through space, it does so with a speed c in the resting frame K of the ether, independently of the state of motion of the emitting object. Let's call this the invariance of the speed of light principle. The theorem of the addition of speeds states that the same light beam will not propagate at speed c in a different frame K' moving uniformly relative to the ether. The laws of propagation thus seem to be different in the two frames and, hence, the relativity principle seems to be incompatible with the laws governing light's propagation.

References:
en.wikipedia.org/wiki/History_of_special_relativity