Best Understanding of the SR Postulate and the FAQ

strangerep

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It's symmetry that determiners everything really.
Regarding homogeneity of spacetime, you might be interested in the fact that relaxing that requirement leads to a Hubble-like constant, and hence a cosmological constant. These constants then become slightly less of a mystery -- by which I mean they become the same kind of mystery as the speed of light itself. This paper by S. Manida sketches the derivation of the associated extended Lorentz transformation, and shows how a new universal length constant arises.

But... I won't say any more about that here since it's a bit tangential to the topic of this thread, which was about standard SR.
 

Wes Tausend

Gold Member
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...

DaleSpam, ghwellsjr,

I think I understand the caution about measuring lightspeed in less than two directions. I'll offer a wordy Asimov style simplification and see if my intuition is in the ballpark.

These two equations are essentially the same form and the second defines the measurement of lightspeed in opposing directions:

img10.gif
...AND...
img7.gif


Einstein perhaps must do this because he wished to satisfy any remaining proponents of the hidden ether wind (no longer needed) which could possibly skew the one way measurement otherwise.

I think maybe you're missing a big point here. Even if there were an ether wind and it was skewing the one way time for light in a round-trip measurement so that it took more time in one direction than the other, there would still be no way of making that measurement--there would be no way of knowing that the light was being skewed by the either (ether?) wind.
Ok, George. I did miss the point. I don't yet have an idea why Michelson and Morley could measure the speed of light, yet it cannot be done. I thought Einstein was dismissing (assuming) skewing and the ether as irrelevant. Much of the drivel I wrote throughout has to do with my clumsy attempt to say it a different way to see if it meant the same, but somehow made more sense to me. It didn't. I attribute this slowness to senility creeping up. Maybe I will get the point as time goes on.

As a "family" example, in other words, the refraction index of light in air is about 1.000293.
The determination is found by:
7b78f2dd4ecd813134c0de17086445ee.png


Where the ratio "c/v" equals "n" designating the "slowdown" of light speed as it passes through air.

Then it follows perhaps, if light travels against the wind (or ether) it encounters more *atoms of air (or ethereal obstruction) and measures slower than if it is measured with the wind (ether). This may be analogous to sound/air travel differences or the evidence of encountering more raindrops while walking against an unknown wind than not.
I don't know why you are bringing this up. The concept of the ether is that it undulates the light waves at c, relative to itself. If you are traveling with respect to it, any light that you emit would propagate at c with respect to the stationary state of the ether.
If one was traveling with respect to it, yes



Partially, because the ether (invisible wind) effect was not, or could not be measured (detected) in either direction, Einstein was able to make a logical assumption that the directional speeds of light were always the same, at least in a vacuum. In reflection of DaleSpams note that differential measurements could be some irrational sort of average; i.e. he assumed moving objects always took their "calm" with them or better said, there simply was no ether and vacuum is inherently equivalent to a perfect calm that cannot be disturbed by motion. That was concerning measurement.

Separately he determined lightspeed:

1) It is not dependent on frequency or color of the light.

2) It is not dependent on the speed of the source of the light.

This determination reaffirmed his assumption that lightspeed was constant in both directions.
Did you miss my comment that "these two characteristics are also true for the ether theory of light" in which the speed of light would not be constant in both directions for an observer traveling in the ether? I don't understand why you would say that they reaffirmed his assumption to the contrary.
I thought I was saying, "This determination reaffirmed his assumption that lightspeed was constant in both directions", meaning it was only constant in no motion within calm ether. Wrong?


It seems to me the above is essentially true, but a different way to say it. Would that be correct?
I think Einstein's way of saying it in chapter 8 of his 1920 book is perfectly adequate:

Attributed to Einstein said:
That light requires the same time to traverse the path A —> M as for the path B —> M is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own freewill in order to arrive at a definition of simultaneity.”
I agree Einsteins way of saying things is very good. Again, my only purpose was to say it differently for contrast, yet hopefully mean the same thing to clear it in my own mind.

Thanks for your patience up to now, George, and I apologise for my incompetence. It has been some years since I last looked at this stuff.

Wes :frown:
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Wes Tausend

Gold Member
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Yeah, I agree. There are many possible ways of axiomatizing SR, and I can find very little to say in favor of choosing Einstein's 1905 axiomatization. In ch. 2 of my SR book I give a sampling of some approaches: http://www.lightandmatter.com/sr/
Ben,

(Or perhaps I should call you Dr. Crowell. I feel so socially awkward here, with the vast expanse of expertise class between respondents, so I apologise if I offend anyone. Please feel free to advise etiquette.)

Thanks for the book link. In this thread we've gone from extruding proper definitions, on to which physical properties best began to describe SR.

While we are speculating and discussing history on the mind of Einstein and his chosen axioms...

The underlying purpose of my thread here, was that I ironically considered the value of a slightly different axiom as you did and it wasn't a popular idea. I wanted to assign a photon particle to a rest frame and allow a mass particle to do all the moving, just like I would swap any other two non-light slower particles, and that redefinition is not allowed within SR. I assume this is directly because of the desire to retain Einsteins original choice of axiom. I'm not sure why he did that, assumed simplicity perhaps?

Simple interchanging of c-speed particle frames seems to have simple, interesting, and observable, minor geometry consequences, and I'm sure all the re-applied intrinsic math quantities of SR remain totally intact. Paramount to consequence, when Einstein casually chose to eternally set the successful, but restricted Aristotle-like one-direction-c axiom in concrete, like he did, Hubble had not yet discovered that Einstein's beloved Steady-State grid ("space between them is expanding") had moved/was moving, and not stationary. You and I recently both responded to a related thread on this phenomena and you provided https://www.physicsforums.com/showthread.php?t=508610 [Broken].

It isn't really proper to discuss it more here, but if you are interested in a unique review of Relativities, you can PM me.

Thanks for your consideration.

Wes
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ghwellsjr

Science Advisor
Gold Member
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Partially, because the ether (invisible wind) effect was not, or could not be measured (detected) in either direction, Einstein was able to make a logical assumption that the directional speeds of light were always the same, at least in a vacuum. In reflection of DaleSpams note that differential measurements could be some irrational sort of average; i.e. he assumed moving objects always took their "calm" with them or better said, there simply was no ether and vacuum is inherently equivalent to a perfect calm that cannot be disturbed by motion. That was concerning measurement.

Separately he determined lightspeed:

1) It is not dependent on frequency or color of the light.

2) It is not dependent on the speed of the source of the light

This determination reaffirmed his assumption that lightspeed was constant in both directions.
Did you miss my comment that "these two characteristics are also true for the ether theory of light" in which the speed of light would not be constant in both directions for an observer traveling in the ether? I don't understand why you would say that they reaffirmed his assumption to the contrary.
I thought I was saying, "This determination reaffirmed his assumption that lightspeed was constant in both directions", meaning it was only constant in no motion within calm ether. Wrong?
DaleSpam's point was that it is just as rational to assume that there was an ether wind or that an observer is moving in the ether and so the speed of light would not be the same in both directions as it is to assume that there is no ether and no ether wind and so the speed of light would be the same in both directions. A valid theory consistent with experimental results could be built on either assumption. I don't know why you would call it irrational.

Those two experimental determinations cannot affirm that "lightspeed was constant in both directions". That's what this thread is all about: is there any experiment that can measure the one way speed of light apart from an assumption that it is the same in all directions? The answer is no.
 

Wes Tausend

Gold Member
203
23
DaleSpam's point was that it is just as rational to assume that there was an ether wind or that an observer is moving in the ether and so the speed of light would not be the same in both directions as it is to assume that there is no ether and no ether wind and so the speed of light would be the same in both directions. A valid theory consistent with experimental results could be built on either assumption. I don't know why you would call it irrational.

Those two experimental determinations cannot affirm that "lightspeed was constant in both directions". That's what this thread is all about: is there any experiment that can measure the one way speed of light apart from an assumption that it is the same in all directions? The answer is no.
Ok. Now I get it. Thanks. Looking back, "irrational" was a poor choice of words. I meant "irrational" along the lines of not being able to rationalise a difference amongst many theoretical possibilities. Perhaps inserting the phrase "indeterminate of an infinate number of speeds" would have been a better choice.

Anyway Einstein qualifies his assumption, "that it is the same in all directions" by logic.

I hope I have finally made myself more clear.

Thanks George,
Wes
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