Re: A little challenge for relativists.
- From: John Kennaugh <JKNG@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 2 Nov 2005 21:15:17 +0000
PD wrote:
John Kennaugh wrote:PD wrote: > >John Kennaugh wrote: >> A little challenge for relativists. >> >> >> "As a modern physicist I completely reject the idea of the ether but I >> still believe Einstein was justified in assuming source independence >> because ..........". >> > >1. Experimental evidence of source independence.
There was none in 1905 in fact there is nothing convincing until 1964 and I don't find that any too convincing. Einstein's rival was Walter Ritz and his emission theory which assumed the speed of light was c relative to the source. He was far too good a physicist to ignore evidence if it existed so you can be assured that there was none. The first evidence which was claimed to support source independence was DeSitters interpretation of double star observations which was readily accepted because it was what everyone wanted to here. It was discredited by Fox in 1965.
First let me say what a pleasure it is to find someone who is willing to take part in a discussion in a civilised manner. It is I'm afraid very rare in this NG.
Evidence and "convincing" evidence is a distinction that is in the eye of the beholder.
True
Ritz did not find the evidence convincing. Einstein at least thought the evidence was suggestive enough to explore the implications. This is perhaps what set Einstein apart, the willingness to take a risk without waiting for everyone to be convinced by experimental evidence.
That is perhaps one way of looking at it. As I see it Einstein was very strongly influenced by Lorentz. He described Lorentz as having made the greatest contribution to electrical theory since Maxwell. I would put Einstein very firmly on the classical route:
Wave ether theory/ Maxwell's ether theory/ Lorentz's ether theory/ Einstein.
On that route source independence had been a belief for nearly 200 years.
Apparently in Sir Edmund Whittaker's "The History of Theories of Aether and Electricity", published in 1953, there is a chapter on relativity, entitled, "The Relativity Theory of Poincare and Lorentz." Einstein is mentioned for the first time in a paragraph on the thirteenth page.
"(1905) Einstein published a paper which set forth the relativity theory of Poincare and Lorentz with some amplification, and which attracted much attention. He asserted as a fundamental principle the constancy of the velocity of light, i.e., that the velocity of light in a vacuum is the same in all systems of reference which are moving relatively to each other, an assertion which at the time was widely accepted. In this paper Einstein gave modifications which must now be introduced into the
formulae for aberration and for the Doppler effect."
I leave to others debate as to how much Einstein owed to Poincare my interest is in the statement "....an assertion which at the time was widely accepted." It has in the past puzzled me as to why Einstein made no attempt to justify his second postulate and the answer seems to be that it was very much in line with what everyone thought. Looking back one does not get a feel for what was 'fashionable' at the time.
>2. The explicit inclusion of an absolute velocity in a law of physics >(Maxwell's electrodynamics), such as appears in no other law of >physics.
Maxwell believed his equations described a wave travelling in the ether and that the constant c was the speed relative to the ether. Without an ether Maxwell's equations are at best a solution to an unknown problem. Apart from which it could not be considered a 'law of physics' as it had failed twice by 1905.
Einstein nevertheless took it to be a strong theory. Maxwell, you may recall, was as tentative about the notion of a field as Faraday was. It was a new concept at the time, and it was hard enough to accept it as a real entity whose causative agent was a current or a charge or a lodestone, let alone a real entity that could act to propagate *itself* without further need of the causative agent.
If you believe there is no ether to physically determine the speed at which light travels then you must find an alternative physical process.
Yes, indeed, and this is where Maxwell himself moved from "ether vortices" to the idea that the electromagnetic wave is not a reflection of the propagation of an effect in something else, but is the propagation of the fields *themselves*. He eventually came to realize that there was nothing in his equations that made reference to a disturbance in "something else", as there were no quantities that reflected the properties of that substance other than the value c.
If I have the history right the ether was first proposed long before it was proposed as a conveyer of light, as a means of explaining action at a distance - that a magnet caused a stress in the ether and that this stress acted upon the pin and moved it.
Fast forward to Maxwell and beyond and you are faced with two choices
1/ Although we intuitively desire some sort of connecting rod to transfer a force one might simply accept that our intuition is wrong. Physics teaches us that all force ultimately acts at a distance, that even a nail is driven into wood by the electric field of the atoms of the hammer repelling those in the nail. The first option is therefore to simply accept that action-at-a-distance is the natural way that all forces act.
2/ That there is an ether and an electric field is a stress in that ether.
The problem with option 1 is that it makes an electrostatic field into a metaphysical field - nothing physically real at all. A field of influence mapping the strength and direction of the action at a distance force which *would* act upon a charge *if* a charge was placed there.
Instead, he saw that the spatial development of one field was driven by the time development of the other *field*. And at this point, he put forward the *second* important leap of his thinking, that the fields had an independent and causative reality of their own. Many people found that hard to accept intuitively; many people apparently still do. :>)
OK - That appears to be a 3rd option I hadn't thought of. It cannot be option 1 obviously because a metaphysical field cannot propagate real physical energy. The suggestion seems to be that some sort of physical entity, 'emanation' is produced by (say) a charge who's intensity [analogy] varies according to the inverse square law. If one can contemplate the idea that the entire physical universe is filled with some mysterious physical substance - the ether - then a mysterious physical substance called 'a field' emanating locally from a charge has got to be more believable. A lot less credible ideas are believed these days :o).
I have come across this idea before but was not aware it came from Maxwell. Surely though if you assume this then c is the rate of progression of the spatial development of one field driven by
the time development of the other field. If you imagine a radio transmitter in deep space 1 LY away from anything else then that progression starts at the aerial with a field produced by the currents in the aerial so it must propagate away from the aerial at c relative to the aerial = source dependence. There is nothing else c can be referenced to because there is nothing else.
This is where I always end up. If you try to take away the ether you end up with source dependence because there is nothing else c can be referenced to.
With no ether the source is surrounded by nothing which can take part in any physical process so the only game in town is the physical process taking place in the source. Without an ether one has to assume that there is a fundamental physical process taking place in the source sends light on its way at c relative to the source - as Ritz assumed.
The truth is that Einstein's starting point was not an assumption of no ether.
That is correct, AFAIK. He was actually deliberately neutral on this point. Maxwell recognized that his theory was different than all other physical theories in existence, by virtue of the apparently absolute velocity c, whereas no other physical law had a dependence on an absolute velocity. He perhaps felt, however, that this was permissible, given the new ingredient of a field.
I don't follow that. We are talking pre-MMX (Maxwell died pre MMX). If there is an old fashioned ether (not Einstein's later unworkable mobile ether) then the speed of light is constant w.r.t it and it can act as a reference for absolute velocity. As I argue above the 'self propelling field system' without an ether concept implies source dependent relative velocity not absolute velocity. You cannot take away the absolute reference (the ether) and still have absolute velocity if you see my point.
Einstein took a different tack, proposing that there should be no fundamental difference between electromagnetism and other physical laws, in terms of how they behaved in different inertial frames. For Einstein, the central issue was not the new aspect of the fields, but rather the character of physical laws in general.
I have argued - I think successfully, that the source dependent self propelling field idea implies source dependency. This would make it fundamentally identical to all other physical laws and is completely consistent with the PoR. "In every FoR the speed of light is measured to be c relative to the source creating it" is what you would naturally expect if you no longer accept the ether.
There is only a source an observer and the space in between. If you assume there is nothing in the space in between controlling the speed then it must be constant w.r.t the source - there is no possible causality for it to be constant w.r.t the observer.
The second postulate is simply the first plus source independence and AE described the second postulate as 'apparently irreconcilable' with the PoR and he succeeded in reconciling it mathematically only by ditching two axioms of physics. Anyone but Einstein would have calculated that the time was different for two observers and tore it up as wrong. To me what he did and the acceptance by others of what he did would only make sense if there was overwhelming evidence of source independence which had to be somehow accommodated. There was no such overwhelming evidence only 'widely accepted' belief apparently a result of believing in the ether for 200 years. Do you see my point?
He was actually trying to improve on Lorentz's ether theory. The difference between the two theories is that while Lorentz said that the speed of light was controlled by the ether but Lorentzian distortions made it *appear* to every observer that they were stationary w.r.t. the ether, Einstein assumed that every observer *is* stationary w.r.t the ether.
This is certainly one way to look at it, although I would have said that Einstein *concluded* from his postulates that, if there were an ether, then all observers are stationary with respect to the ether.
I'm not sure you can legitimately 'conclude' from an assumption :o)
And this leads to the immediate question: How can two observers in relative motion *both* be at rest with respect to an ether? The conundrum then leads one to question whether the ether is *really* required at all.
You do not resolve the problem by renaming the ether the observer's FoR so that in effect each observer can have his own ether to be stationary w.r.t - but he seems to have got away with it :o)
Einstein looked again at electrodynamics and finally concluded that, no, he could not see a demand for an ether in those physical laws after all. He did this tentatively at first, but more convincingly in the years following 1905.
I have to accept that the texts I have read written by Einstein are limited by what I have found on the Internet plus a few quotes from other people but based only on that a consistent picture appears. You say "He was actually deliberately neutral on this [the ether] point".
Maybe he was being patted on the back and hailed as a hero for getting rid of the ether when he didn't really believe he had - so kept his head down on that point.
Having had 15 years to ponder the question he gave a lecture ETHER AND THE THEORY OF RELATIVITY - 1920. A few quotes:
"It may be added that 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."
"...the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it",
"the hypothesis of ether in itself is not in conflict with the special theory of relativity. Only we must be on our guard against ascribing a state of motion to the ether."
"there is a weighty argument to be adduced in favour of the ether hypothesis. To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view."
In another thread someone claimed Einstein had said there was no ether but in the end he could not substantiate it with a quote. I am willing to be convinced but as far as I can see the idea that Einstein 'got rid of' the ether doesn't stand up. On the contrary relativity owes its entire existence to a refusal to abandon the ether and its source independent property.
He assumed source independence because he assumed ether. He placed great store on Maxwell and described Lorentz as having achieved the most important advance in the theory of electricity since Maxwell.
My question was not why did Einstein think source independence was so important (he ditched two axioms of physics in order to retain it) that is quite straight forward - he never rejected the idea of the ether - my question was why a modern physicist, who doesn't believe in the ether thinks it was justified.
I guess I still don't understand the question. In 20/20 hindsight, a modern physicist has a lot more information than Einstein had in 1905. At the time, Einstein operated from strong hunches. Today, the basis for those hunches are considerably more solid than they were then.
If as I believe the basis of his strong hunches was an underlying belief in the ether then the basis of those hunches is not so much strengthened as not believed at all. The logic is that his theory has proved extremely successful, therefore his hunches must have been right therefore the basis of those hunches must be right so there must be an ether - but no one wants to go there. If they did they would have to conclude that Einstein did not get his mobile ether concept to work and Lorentz has the copyright on the only ether which does. No a very fashionable idea.
The modern approach, which I do not accept, of which Bilge is an extreme example says "who cares what Einstein thought, Physical description is in any case only a construction of the human mind, the proper way to describe things is mathematically and we have mathematics which works. Mathematics which works = correct physical theory. Understanding physics and understanding the maths is one and the same thing".
-- John Kennaugh "The nature of the physicists' default was their failure to insist sufficiently strongly on the physical reality of the physical world." Dr Scott Murray .
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