Michelson–Morley experiment: Did it disprove the existence of ether?

[ghwellsjr]

If the interpretation of Lorentz of the MMX were correct (that the experiment frame was moving wrt the ether absolute rest frame and it was experiencing length contraction and time dilation), it would indeed prevent any experiment from succeding measuring a different speed of light than c regardless the motion of the light source (that is actually the case, to this day no experiment has been able to falsify this claim of special relativity). In this sense Einstein and Lorentz interpretation would reach the same predictions and by Ocam's razor the ether would be superfluous.

The only difference between LET and SR is that LET postulates that there exists a single absolute ether rest frame in which the one-way speed of light is the same in all directions and SR postulates that in any inertial frame the one-way speed of light is the same in all directions, (but only one at at time). Ocam's razor prefers SR because it frees you up from trying to find that single illusive ether rest frame--any inertial frame will do.

But could another approach be used to solve this, since measuring the speed of light seems a dead end, perhaps measuring the doppler shift of the Earth with respect to the vacuum?

No, unless you want to abandon SR, which I seriously doubt.

 

[ghwellsjr]

My point is that if Maxwell had the foresight of Einstein's hindsight, he would not have proposed an experiment to measure the speed of the surface of the Earth through the absolute stationary ether because he would have realized that his equations predicted that it was unmeasureable. But I'm not complaining, Maxwell was the most brilliant scientist of the 19th century in my opinion and hindsight is always better than foresight.

I would disagree with that. I don't think Maxwell would have considered it significant that there exists a set of transformations which leave his equations unchanged. In fact, the whole reason he proposed experiments to measure the motion of the aether is because he believed that the aether frame was the only frame in which his equations were exactly correct.

I totally agree with your last sentence but it is at odds with the sentence before it because the Lorentz Transform is how we determine that if a set of equations is correct in one frame, it is correct in all frames. Of course, I'm assuming that Maxwell would have known the significance of the transform.

 

[Born2bwire]

I totally agree with your last sentence but it is at odds with the sentence before it because the Lorentz Transform is how we determine that if a set of equations is correct in one frame, it is correct in all frames. Of course, I'm assuming that Maxwell would have known the significance of the transform.

It's significant to note that Maxwell only lived for another 15 years after he published his treatise (dying in the late 1870's). His original work was very complicated and used cumbersome quarternions. It wasn't until after his death that the field theory and the classic four equations were derived. In addition, experimental work on electromagnetic waves (other than light) did not really take off until after his death as well. Hertz did his famous experiments in the 1880's. Really, I would say that the 1880's is one of the big decades for electromagnetics because that was the decade where electromagnetic experimentation really started and we begin to see the empirical proof of Maxwell's Equations for electromagnetic waves. In the following decade of the 1890's, Lorentz, Poincare and others developed the Lorentz Transformations and then it was in the early or mid 1900's that Einstein published his work.

So given the historical context I do not think one would think any less of James Clerk Maxwell for missing these things since he did not live long enough to see theorists really flesh out his work and for experimentalists catch up with his theories.

 

[ghwellsjr]

So given the historical context I do not think one would think any less of James Clerk Maxwell for missing these things since he did not live long enough to see theorists really flesh out his work and for experimentalists catch up with his theories.

Thanks for that additional historical context and I agree with what you say about Maxwell as I pointed out earlier here:

My point is that if Maxwell had the foresight of Einstein's hindsight, he would not have proposed an experiment to measure the speed of the surface of the Earth through the absolute stationary ether because he would have realized that his equations predicted that it was unmeasureable. But I'm not complaining, Maxwell was the most brilliant scientist of the 19th century in my opinion and hindsight is always better than foresight.

 

[Dale]

Did I misunderstand the purpose of the paper? I thought it was to change the nature of the first postulate so that it was backed up by experimental evidence and no longer just an unsupported postulate.

Not exactly. He just scrapped both postulates, made a completely general theory, and checked to see what restrictions experiments placed on the parameters of the general theory.

 

[TrickyDicky]

No, unless you want to abandon SR, which I seriously doubt.

Noone seriously would want to abandon SR in its restricted range of application, that's why is called "special" or "restricted" relativity. But we all know we don't live in a "special relativity" world, ours is not Minkowski spacetime.
And some physicists (such as Nobel winner Bob Laughlin in his book "A different universe" in a chapter called The Fabric of Spacetime) claim that in a way Einstein re-introduced a form of ether with GR or the concept of empty space as a medium or substance.
It has also been said here that Maxwell believed that the ether frame was the only frame in which his equations were exactly correct. If we follow Lorentz here this would mean that the ether frame, that is the absolute rest frame, is the only frame where his tranformations hold exactly.

 

[thwle]

I'm still wondering where you got the idea that anybody at the time of MMX thought that two different ways of measuring a distance would give two different results. Can you provide some evidence for that?

The evidence is that the null result is the necessary conseqence of the equivalence and they didn't expect the null result.

 

[thwle]

What non-null result do you think Michelson and Morley expected?

They expected the interference pattern to shift.

 

[ghwellsjr]

Noone seriously would want to abandon SR in its restricted range of application, that's why is called "special" or "restricted" relativity. But we all know we don't live in a "special relativity" world, ours is not Minkowski spacetime.

I didn't know that. I thought I lived in a special relativity world. But if what you are saying is that we don't live in just a special relativity world, we also live in a general relativity world, then I would agree with you.

And some physicists (such as Nobel winner Bob Laughlin in his book "A different universe" in a chapter called The Fabric of Spacetime) claim that in a way Einstein re-introduced a form of ether with GR or the concept of empty space as a medium or substance.

I don't believe anything in GR provides for an absolute ether rest frame.

It has also been said here that Maxwell believed that the ether frame was the only frame in which his equations were exactly correct. If we follow Lorentz here this would mean that the ether frame, that is the absolute rest frame, is the only frame where his tranformations hold exactly.

This doesn't make any sense. The Lorentz Transform is a set of equations that don't know anything about any absolute rest frame. They always "hold" (whatever that means). The issue is whether another set of equations are modified by the LT or remain the same. Maxwell did believe his equations would only work correctly in an absolute rest frame but that has nothing to do with whether or not his equations would come out of the LT in exactly the same form that they went into the LT. Newtonian equations do not come out the same when subjected to the LT so we know they are not frame independent. The fact that Maxwell's equations come out the same means that they apply equally well in any reference frame, not just in an absolute ether rest frame.

 

[Buckleymanor]

What! ?

Let me try again: MMX consists of
(1) a rotatable stone slab on which two orthogonal folded paths are laid out, (two measuring sticks.)
(2) a light source ("argand lamp")
(3) a half silvered mirror at the common endpoint of the two paths to split and recombine the light beam at the beginning and end, respectively, of the round trip along each path.
(4) a telescope to observe the interference pattern in the recombined beam.

(2), (3) and (4) constitute a radar ranging measurement of the measuring sticks, except that rather than a count of wavelengths along the two paths only a count of differences between the two paths is observed. That being zero. It appears that the two methods of length measurment may be equivalent.

I conjecture that the dimensions of a material measuring stick, being dependent on the spacing of its atomic constituents, is dependent upon the forces between those constituents, which (if the forces be propagated or communicated between atoms at light speed) would always establish spacing determined (in effect) by radar ranging.

I hope this clears things up.

thwle

Some, though how do you go about providing evidence of your hypothesis.
It's ok to say the two measurements may be equivalent but if you try to measure one with the other you end up with a null result.
How do you show your stick is longer in one direction to another.

 

[ghwellsjr]

They expected the interference pattern to shift.

But how do you think they expected the interfence pattern to shift? Do you mean it should shift as they rotated the apparatus? Do you mean that it should shift at a constant rate? Do you mean that it shift shift back and forth between two values? Or do you mean that it should only shift as the Earth rotated on its axis and/or revolved around the sun?

 

[TrickyDicky]

I didn't know that. I thought I lived in a special relativity world.

Are you sure? Do we live in a uniform translatory motion universe? According to Einstein's 1905 paper that is the situation in which the first postulate of relativity should rule. Have I read this wrong?

I don't believe anything in GR provides for an absolute ether rest frame.

Ok, but did you read the chapter in the book I mentioned? I think it is in Amazon's preview.

Maxwell did believe his equations would only work correctly in an absolute rest frame but that has nothing to do with whether or not his equations would come out of the LT in exactly the same form that they went into the LT.

I didn't say that. What I said is that if we were to follow Lorentz ether theory, it would make sense that Maxwell equations happen to be Lorentz symmetric, since according to his theory+ the above mentioned Maxwell belief, it is the logical outcome.

 

[thwle]

Some, though how do you go about providing evidence of your hypothesis.
It's ok to say the two measurements may be equivalent but if you try to measure one with the other you end up with a null result.
How do you show your stick is longer in one direction to another.

I don't understand why you say "but", It seems to me "because" would go better -- the null result is the evidence.

Your last sentence baffles me, I have no idea what you mean.

 

[thwle]

But how do you think they expected the interfence pattern to shift? Do you mean it should shift as they rotated the apparatus? Do you mean that it should shift at a constant rate? Do you mean that it shift shift back and forth between two values? Or do you mean that it should only shift as the Earth rotated on its axis and/or revolved around the sun?

On Page 336 (the fourth page of the paper by Michelson and Morley): "If now the whole apparatus be turned through 90 degrees, the difference will be in the opposite direction, hence the displacement of the interference fringes should be 2Dv^2/V^2. ..."

 

[ghwellsjr]

On Page 336 (the fourth page of the paper by Michelson and Morley): "If now the whole apparatus be turned through 90 degrees, the difference will be in the opposite direction, hence the displacement of the interference fringes should be 2Dv^2/V^2. ..."

OK, yes, now I see what you are saying and I agree, they did expect the "radar" method of distance measurement to give a different result than the rod method. In fact, you could say that if they had extremely high precision, they could have done their experiment with just one rod/radar measurement but using two at right angles enables them to get by with less precision. I guess that distinction (two versus one) is what made it difficult for me to see what you were saying but now that I see it, it seems obvious and I can't really understand why I didn't see it the first time you suggested it.

However, I am not agreeing with your statements about emmision theory. I'll post about that later.

 

[thwle]

OK, yes, now I see what you are saying and I agree, they did expect the "radar" method of distance measurement to give a different result than the rod method. ... It seems obvious and I can't really understand why I didn't see it the first time you suggested it.

However, I am not agreeing with your statements about emmision theory. I'll post about that later.

Great!

Honestly, I don't know what to make of emission theory myself. MMX is consistent with it but it raises many questions and I read that other experiments than MMX cast doubt on it.

Thanks

 

[harrylin]

I referred to the paper as I wrote my post.

Yes, emission theory was out of favor, but not disproved. They expected to demonstrate further against emission theory by detecting the velocity of the Earth,

Here you do again not refer to their paper - it's not a coincidence that you don't cite from it (emphasis mine):

"emission theory [..] failed to account for the fact proved by experiment [..].
[...] first, the ether is supposed to be at rest [..] The experimental trial of the first hypothesis forms the subject of the present paper."

but that depended on assumptions that light traveled at a fixed speed relative to a frame of reference (ether) and (unstated) that measuring stick length is not equivalent to radar ranging (not by that name, of course.)
They failed to detect motion of the earth. It seems to me that shows either that emission theory had some truth to it or that there is a greater correspondence of the two methods of length measurement than they had supposed.

See above: emission theory was already disproved. For many people one logical conclusion remained.

Regards,
Harald

 

[harrylin]

If the interpretation of Lorentz of the MMX were correct (that the experiment frame was moving wrt the ether absolute rest frame and it was experiencing length contraction and time dilation), it would indeed prevent any experiment from succeding measuring a different speed of light than c regardless the motion of the light source (that is actually the case, to this day no experiment has been able to falsify this claim of special relativity). In this sense Einstein and Lorentz interpretation would reach the same predictions and by Ocam's razor the ether would be superfluous.

But could another approach be used to solve this, since measuring the speed of light seems a dead end, perhaps measuring the doppler shift of the Earth with respect to the vacuum?

If the theory is right than the PoR cannot be broken, ether or no ether. However:

- Einstein argued in 1920 that the fact that light propagates in space (like a wave) already solved it (a point he had failed to recognize in 1905) and that the properties of space as determined by GRT settled the issue. [Google Einstein ether 1920; he elaborated on that in 1924]

- Ives argued something similar in 1938 when his Doppler experiment established that moving clocks positively suffer retardation by the Lorentz factor due to their speed (a non-null result), which seems to imply the reality of the Lorentz contraction in view of the Kennedy-Thorndyke[sic] experiment. [Google Ives-Stillwell as well as Kennedy-Thorndike]

 

[thwle]

Here you do again not refer to their paper - it's not a coincidence that you don't cite from it (emphasis mine):

"emission theory [..] failed to account for the fact proved by experiment [..].
[...] first, the ether is supposed to be at rest [..] The experimental trial of the first hypothesis forms the subject of the present paper."



See above: emission theory was already disproved. For many people one logical conclusion remained.

Regards,
Harald

I am glad to have my posts responded to even when the respond appears to be adverse.

There seems to be a misunderstanding.

I admit that subsequent work by Willem de Sitter laid emission theories to rest.

When I said I referred to the paper of Michelson and Morley as I wrote my post I
meant that I reread it (not that I quoted from it). The paper begins :

"The discovery of the aberration of light was soon followed by an explanation
according to the emission theory. ... difficulties in this apparently sufficient
explanation were overlooked [and are not specified in the paper] until after an
explanation on the undulatory theory of light was proposed. This new explanation was
at first almost as simple as the former. But it [the undulatory theory] failed to
account for the fact proved by experiment ... "

the reconciliation of undulatory theory with experiment required, they tell us, two
hypotheses:

"first, the ether is supposed to be at rest except in the interior of transparent
media, in which secondly ... The experimental trial of the first hypothesis forms
the subject of the present paper."

They calculated the fringe shift expected under the first hypothesis, but experinent
found no shift instead.

This thread is about the implications of the experiment's outcome.

I detected an unstated assumption in their thinking: that measuring stick length is
not equivalent to radar ranging (that is, echo ranging with light). The essence of the experiment is a comparison of the two methods.

I also think the equivalence stands to reason: If the forces between atoms in a solid
are communicated at the speed of light, then the two methods would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by
Michelson and Morley.

 

[roineust]

The only difference between LET and SR is that LET postulates that there exists a single absolute ether rest frame in which the one-way speed of light is the same in all directions and SR postulates that in any inertial frame the one-way speed of light is the same in all directions, (but only one at at time). Ocam's razor prefers SR because it frees you up from trying to find that single illusive ether rest frame--any inertial frame will do.

No, unless you want to abandon SR, which I seriously doubt.

Hey ghwellsjr,
Still didn't forget the exercise...I will get there soon.

I have a question: What do you mean by "...(but only one at a time)...": one frame at a time, right? if you mean one frame at a time, does this restriction have a name? is it a result of an equation? or of an experiment? what is it called? what is the reason for it? what is the problem with having the speed of light the same at all frames at a once?Thanks,
Roi.

 

[ghwellsjr]

The only difference between LET and SR is that LET postulates that there exists a single absolute ether rest frame in which the one-way speed of light is the same in all directions and SR postulates that in any inertial frame the one-way speed of light is the same in all directions, (but only one at at time). Ocam's razor prefers SR because it frees you up from trying to find that single illusive ether rest frame--any inertial frame will do.

No, unless you want to abandon SR, which I seriously doubt.

Hey ghwellsjr,
Still didn't forget the exercise...I will get there soon.

I have a question: What do you mean by "...(but only one at a time)...": one frame at a time, right? if you mean one frame at a time, does this restriction have a name? is it a result of an equation? or of an experiment? what is it called? what is the reason for it? what is the problem with having the speed of light the same at all frames at a once?


Thanks,
Roi.

Great questions, I'm glad I get the chance to answer them. Please note that we are talking here about two theories as opposed to experimental measurements which those theories explain. These two theories have as their foundation the abstract concept of frames of reference which are co-ordinate systems (spatial and time components) to enable us to deal with bigger situations than what we could deal with if we limited ourselves to just the data from experimental measurements.

So now let's lay aside the issue of these two theories and focus on experimental measurements. It turns out that any inertial observer (without any consideration for a frame) who attempts to measure the speed of light will get the constant value c, no matter which direction the measurement is made. Any number of inertial observers moving with respect to each other will get the same result simultaneously for similar experiments. Futhermore, if any number of these observers send similar timing signals between each other, they will each observe that the other one's timers are running slower than their own and by the same amount. If two inertial observers in relative motion happen to be co-located at the moment a flash of light is emitted, they will both observe that they are each located in the center of that expanding flash of light. There are many other experiments that can be performed of this nature that have nothing to do with any frames of reference--it's just the way nature works.

Now we'll look at how frames of reference enter into the picture. In an attempt to understand the apparent strange way that nature works, scientists have come up with theories:

In one of these, Lorentz Ether Theory (LET), it is postulated that there exists only a single absolute frame of reference in which light actually travels in all directions at the speed c and only when you are at rest in that frame will your measurements reflect what is really happening. All other inertially moving observers get the same results because nature is playing tricks on them by adjusting their clocks and rulers in such a way that they get the same results as they would get if they were stationary in the ether. In fact, since we don't know where the ether rest frame is, chances are, all observers are not at rest in the ether frame and so it's a safe bet that we all have our clocks and rulers modified.

Special Relativity, SR, on the other hand, postulates that you can consider anyone of the inertial observers to be at rest in the ether frame of reference where his measurements reflect what is really going on and all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether but the theory says that their clocks and rulers are actually getting modified, according to the one observer which we are considering to be at rest in the ether.

The theories include methods by which we can assign values to locations and clocks that include all observers and objects so that we can transform a scenario defined according to one frame of reference into any other frame of reference. We can even transform or define a frame of reference where there is no observer or object. The important thing to consider here is that you should not use the values for locations or times from two different frames and thereby see a contradiction--that's what all the so-called paradoxes do--you need to use only one frame at a time, it doesn't matter which one.

 

[harrylin]

I am glad to have my posts responded to even when the respond appears to be adverse.

There seems to be a misunderstanding.
I admit that subsequent work by Willem de Sitter laid emission theories to rest.

Yes there is a misunderstanding but I hope that you don't take it personal - at least, I don't.

What appears to have escaped your attention is that at the time of writing Michelson thought that the Fizeau experiment had definitely laid to rest ballistic emission theories - he did not seriously consider them. See next!

When I said I referred to the paper of Michelson and Morley as I wrote my post I
meant that I reread it (not that I quoted from it). The paper begins :

"The discovery of the aberration of light was soon followed by an explanation
according to the emission theory. ... difficulties in this apparently sufficient
explanation were overlooked [and are not specified in the paper] until after an
explanation on the undulatory theory of light was proposed. This new explanation was
at first almost as simple as the former. But it [the undulatory theory] failed to
account for the fact proved by experiment ... "

Ah, now we have found another cause of disagreement...
Aargh! you are right here, I did a too hasty copy-paste without checking the result - sorry for that! Regretfully I thus added to confusion, and I can likely not delete my mis-citation...
Never mind, I'll try to clean it up now!

The original undulatory theory was disproved and replaced by two competing ones (actually two classes, for there were more): the one of Stokes and the one of Fresnel. The one of Stokes was disproved in favour of that of Fresnel by the Fizeau experiment which Michelson re-enacted before doing the MMX.
However, the results of that same experiment (among others) were also in disagreement with common ballistic emission theories. That is what Michelson meant with:
"The difficulties in this apparently sufficient explanation were overlooked until after an explanation on the undulatory theory of light was proposed."

You can read his 1886 experiment here:
http://en.wikisource.org/wiki/Influence_of_Motion_of_the_Medium_on_the_Velocity_of_Light

Michelson wanted to test two hypotheses of Fresnel's ether theory. He had already obtained positive confirmation in 1886 of the second hypothesis. Therefore he wrote about MMX:
"The experimental trial of the first hypothesis [by Fresnel] forms the subject of the present paper."

Never mind, we seem to agree about their purpose - so let's move on.

the reconciliation of undulatory theory with experiment required, they tell us, two
hypotheses:

"first, the ether is supposed to be at rest except in the interior of transparent
media, in which secondly ... The experimental trial of the first hypothesis forms
the subject of the present paper."

They calculated the fringe shift expected under the first hypothesis, but experinent
found no shift instead.

This thread is about the implications of the experiment's outcome.

I detected an unstated assumption in their thinking: that measuring stick length is
not equivalent to radar ranging (that is, echo ranging with light). The essence of the experiment is a comparison of the two methods.

I also think the equivalence stands to reason: If the forces between atoms in a solid
are communicated at the speed of light, then the two methods would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by
Michelson and Morley.

Yes, that's basically Heaviside-Fitzgerald. And I think that I already showed agreement with your clarification (the one conclusion that remained, according to me, was that there is a greater correspondence of the two methods of length measurement than MMX had supposed).

 

[harrylin]

[..]

Special Relativity, SR, on the other hand, postulates that you can consider anyone of the inertial observers to be at rest in the ether frame of reference where his measurements reflect what is really going on and all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether but the theory says that their clocks and rulers are actually getting modified, according to the one observer which we are considering to be at rest in the ether. [..]

Actually, SRT never postulates "what is really going on": it postulates phenomena that will be observed (measured). In order to eliminate a suggestion of self contradiction I would modify your clarification as follows:

Special Relativity postulates that you can pretend anyone of the inertial observers to be at rest in the ether frame of reference so that all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether; the theory thus says that - according to the one observer which we are considering to be at rest in the ether - their clocks and rulers are actually affected by their motion.
And just as in Newtonian mechanics, any observer can pretend to be in rest so that it's the others that move - but if we pretend all to be at rest at the same time then we mess up!

 

[ghwellsjr]

That's fine, I accept your suggestion.

 

[thwle]

I like the tendency of recent posts so far as relativity is concerned, although I would argue that Occam's principle called for simplicity of theory not of application. Is SR really less simple? (retorical, doesn't belong in this thread.)

This thread being about the implications of MMX, I want to reassert my point of view that MMX demonstrates Lorentz contraction -- nothing more.

 

[harrylin]

I like the tendency of recent posts so far as relativity is concerned, although I would argue that Occam's principle called for simplicity of theory not of application. Is SR really less simple? (retorical, doesn't belong in this thread.)

This thread being about the implications of MMX, I want to reassert my point of view that MMX demonstrates Lorentz contraction -- nothing more.

It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source. Talking about relativity, Einstein put it as follows:

"for a co-ordinate system moving with the Earth the mirror system of Michelson and Morley is not shortened, but it is shortened for a co-ordinate system which is at rest relatively to the sun."
- http://www.bartleby.com/173/16.html

 

[thwle]

It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source. - http://www.bartleby.com/173/16.html

Hmmm. What does that mean?

Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction.

 

[thwle]

Talking about relativity, Einstein put it as follows:

"for a co-ordinate system moving with the Earth the mirror system of Michelson and Morley is not shortened, but it is shortened for a co-ordinate system which is at rest relatively to the sun."
- http://www.bartleby.com/173/16.html

This is not helpful. What shall be the standard of measurement. In MMX, there was no observer moving relative to the apparatus. The Lorentz contraction is not revealed by a measurment but by an invariance.

 

[thwle]

... [two] theories:

In one of these, Lorentz Ether Theory (LET), it is postulated that there exists only a single absolute frame of reference in which light actually travels in all directions at the speed c and only when you are at rest in that frame will your measurements reflect what is really happening. All other inertially moving observers get the same results because nature is playing tricks on them ... In fact, since we don't know where the ether rest frame is, chances are, all observers are not at rest in the ether frame and so it's a safe bet that we all have our clocks and rulers modified.

Special Relativity, SR, on the other hand, postulates that you can consider anyone of the inertial observers to be at rest in the ether frame of reference where his measurements reflect what is really going on and all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether but the theory says that their clocks and rulers are actually getting modified, according to the one observer which we are considering to be at rest in the ether.

So is the only difference that in SR we decide not to care that "we don't know where the ether rest frame is."

 

[harrylin]

This is not helpful. What shall be the standard of measurement. In MMX, there was no observer moving relative to the apparatus. The Lorentz contraction is not revealed by a measurment but by an invariance.

I find Einstein's clarification very helpful, as some people forget that MMX doesn't take the non-inertial Earth as reference. The standard reference system to which MMX refers is the solar system in which the Earth orbits - that's rather common in astronomy.

 

[harrylin]

[I wrote: "It's an indirect demonstration of Lorentz contraction - based on our assumption that light propagation is fully unaffected by its motion wrt the source.]

Hmmm. What does that mean?

Either:
(1) MMX demonstrates that speed of light relative to the MMX apparatus was the same in every direction.
or
(2) MMX demonstrates that solids undergo Lorentz contraction as they move relative to the frame of reference in which the speed of light is the same in every direction.

That's by far not all:

3) Alternatively if for example transverse light propagation in the ether is reduced by the Lorentz factor by its motion wrt the source, then MMX demonstrates that solids undergo isotropic contraction - I think of that example because it's a proposed theory (by H. Varcollier, 1942 and P. Dive, 1950) that is not compatible with the relativity principle, but still close enough.

That's why we prefer instead of null results, positive evidence of the kind that Michelson was looking for - the kind we already got for the speed of light in moving media (but only well tested along the same direction), and for time dilation.

Cheers,
Harald

 

[harrylin]

So is the only difference that in SR we decide not to care that "we don't know where the ether rest frame is."

Even stronger: SR does not care if an ether exists. On purpose an ether is not part of its postulates and it's not required for the (re)derivation of the Lorentz transformations. So, the physical model is replaced by a set of rules. And if one still wants to make intuitive or physical sense of it, then one may use for example the stationary ether model or, alternatively, a physical interpretation of Minkowski Spacetime.

However, IMHO that is cheating a little, as the light postulate summarizes the consequences of the stationary ether model.

 

[thwle]

That's by far not all:

3) Alternatively if for example transverse light propagation in the ether is reduced by the Lorentz factor by its motion wrt the source, then MMX demonstrates that solids undergo isotropic contraction - I think of that example because it's a proposed theory (by H. Varcollier, 1942 and P. Dive, 1950) that is not compatible with the relativity principle, but still close enough.

That's why we prefer instead of null results, positive evidence of the kind that Michelson was looking for - the kind we already got for the speed of light in moving media (but only well tested along the same direction), and for time dilation.

Cheers,
Harald

I have to suppose you understand what your said. Have you understood the things I have said?

Admitting my uncertainty as to the meaning of the bold (above), I suppose many other interpretations are possible if enough new postulates be accepted to cobble a correspondence. If one honors Occam's principle, I maintain that only the two possibilities are available. I think we agree the first of the two can be ruled out based on evidence external to MMX. I daresay, they are logically an exhaustion of possibility.

Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

 

[ghwellsjr]

... [two] theories:

In one of these, Lorentz Ether Theory (LET), it is postulated that there exists only a single absolute frame of reference in which light actually travels in all directions at the speed c and only when you are at rest in that frame will your measurements reflect what is really happening. All other inertially moving observers get the same results because nature is playing tricks on them by adjusting their clocks and rulers in such a way that they get the same results as they would get if they were stationary in the ether. In fact, since we don't know where the ether rest frame is, chances are, all observers are not at rest in the ether frame and so it's a safe bet that we all have our clocks and rulers modified.

Special Relativity, SR, on the other hand, postulates that you can consider anyone of the inertial observers to be at rest in the ether frame of reference where his measurements reflect what is really going on and all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether but the theory says that their clocks and rulers are actually getting modified, according to the one observer which we are considering to be at rest in the ether.

So is the only difference that in SR we decide not to care that "we don't know where the ether rest frame is."

Actually, I like harrylin's suggested alternative to my second paragraph quoted above where he basically substituted the word "pretend":

Special Relativity postulates that you can pretend anyone of the inertial observers to be at rest in the ether frame of reference so that all the other inertially moving observers are getting their clocks and rulers modified by nature in such a way that they get the same result as they would if they were at rest in the ether; the theory thus says that - according to the one observer which we are considering to be at rest in the ether - their clocks and rulers are actually affected by their motion.
And just as in Newtonian mechanics, any observer can pretend to be in rest so that it's the others that move - but if we pretend all to be at rest at the same time then we mess up!

So it's not so much that "we decide not to care", it's that we can pretend that any inertial reference frame will behave exactly like the illusive ether frame. It's so much easier to do science this way than to be concerned about where that pesky ether resides.

 

[ghwellsjr]

...
Please tell me what you think of my reasoning in my previous post (here quoted):

"I also think the equivalence [of radar ranging and measuring stick measuring] stands to reason: If the forces between atoms in a solid are communicated at the speed of light, then they would be equivalent. Only if the forces were instantaneously communicated would they be different as calculated by Michelson and Morley."

Think of a Hydrogen molecule. What communication of forces between the two hydrogen atoms determines the distance between them? What is the speed of that communication and relative to what frame of reference is that speed? Isn't this interaction between atoms rather like echo ranging?

I came across this passage from Hermann Bondi's book, Relativity and Common Sense:

"THE VELOCITY OF LIGHT
On this basis, then, we can look at time standards as primary and at distance standards as quite secondary and of little importance. This does seem to be a sound procedure, particularly when we think about what our inch tapes and measuring rods are actually made of. We know that they are composed of atoms whose structure is kept in shape by electric forces. We know that these atoms have certain periods of vibration and we know that, in the materials we call very rigid, it is as a consequence of the particular periods of vibration of the atoms that different atoms keep a definite distance apart in the structure of the rod. Thus we can argue that the length of a rod is really determined by the period of the oscillation of the atoms of which it is composed, this being translated, in the usual way, through the velocity of light into distance. If we argue, as we well may, that the distances between the atoms in what we call rigid materials are the distances corresponding to the oscillations of the atoms, then we could say that those distances, too, are effectively determined by radar methods. On that basis, then, distance becomes a purely secondary quantity, time is the primary thing, and the velocity of light is in natural units necessarily equal to unity. But if we are so perverse as to choose to measure distances in feet rather than in light millimicroseconds, then we have to introduce a conventional factor of conversion which effectively defines the foot, and this we call the velocity of light."

He was not discussing length contraction at this time but his argument seems to be applicable to what you are suggesting.

However, I don't think too many scientists these days concern themselves with the mechanism of length contraction, I believe they are content to regard it as a natural consequence of Special Relativity.