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

[Born2bwire]

I am slightly confused.
You say technically yes, but only because the Earth is accelerating.
If the Earth were moving along a constant path then it would not matter since both beams would get the same boost.
Ghwellsir mentions that.

It doesn't matter if the Earth was moving along a constant path because the observer and apparatus are moving with the Earth. In that case, the entire experiment is performed and observed within the same frame. Assuming Galilean transformations, the movement of the light source will impart an extra boost to the light and this would cancel out the effects of the moving apparatus. Only when we consider Lorentz transformations does this cause a difference since the speed of light is the same in all frames. You might as well ask yourself if you threw two baseballs back and forth between two partners whether the exercise would be different if you were stationary or on a uniformly moving platform. In non-relativistic assumptions, there would not be a difference from your point of view regardless of how you observed it, but things would change if the platform was performing a wide turn.

If the Earth is accelerating, then we can see that the apparatus will move while the beams are travelling. So this can cause a slight shift in travel times because the apparatus will have moved while the beams are traveling thus changing the paths that need to be travelled. So we can see that this would affect the experiment even if we assume Galilean transformations.

The thing to remember here is that the experimentalists and theorists were still assuming Galilean transformations.

 

[harrylin]

I am slightly confused.
You say technically yes, but only because the Earth is accelerating.
If the Earth were moving along a constant path then it would not matter since both beams would get the same boost.
Ghwellsir mentions that.
So was this the reason for the null results that for all practicle purposes they were moving at a constant speed and any gains or losses were practicaly canceled out.

Yes - the Michelson interferometer is by design insensitive to rotation, because the enclosed area is nearly zero (in contrast to Sagnac and Michelson-Gale).

 

[ghwellsjr]

I am slightly confused.
You say technically yes, but only because the Earth is accelerating.
If the Earth were moving along a constant path then it would not matter since both beams would get the same boost.
Ghwellsir mentions that.
So was this the reason for the null results that for all practicle purposes they were moving at a constant speed and any gains or losses were practicaly canceled out.

Yes - the Michelson interferometer is by design insensitive to rotation, because the enclosed area is nearly zero (in contrast to Sagnac and Michelson-Gale).

There are actually several different issues going on here:

1) Maxwell, as well as most other scientists of the time, incorrectly believed that his equations supported the detection of a fixed absolute ether rest frame. They did not realize that his equations were Lorentz Transformable and that they actually predicted the null result of MMX.

2) Michelson and Morley designed an experiment, based on their lack of knowledge, that was supposed to detect an ether wind whenever the surface of the Earth (wherever MMX was located) was moving through the ether. But since they didn't know when or if the Earth ever was stationary with respect to the ether, they relied on the fact that the Earth was constantly accelerating, but at an insignificantly low rate, so that if they happened to be stationary in the fixed ether at one point in time, it certainly would be moving through the ether 12 hours later and/or six months later, assuming that the Earth was not dragging the ether along with it.

3) As has been pointed out by Born2bwire earlier in this thread, even if they were moving through the ether, they realized they still would get a null result if their apparatus were aligned so that the two arms of the experiment happened to be at a 45 degree and 135 degree angle with respect to the direction of motion. So they designed their experiment so that it could be rotated very slowly, about one rotation per minute. They then believed that during the course of one revolution of the appartus, they should hit four null spots and two positive peaks and two negative peaks, as long as there was some ether wind. During the course of one revolution, even though the surface of the Earth was accelerating, it wasn't enough to affect the data during one revolution or even several revolutions.

4) Since they always got a null result, in other words, they never could detect an ether wind, Michelson believed that the Earth was dragging the ether along with it. Later, Lorentz discovered that if the ether were causing the lengths of the apparatus to contract along the direction of motion through the ether, this would explain the null result.

5) Finally with the addition of time dilation, the full Lorentz Transformation equations were developed and they realized their original mistake in interpreting Maxwell's equations and they fully understood why MMX produced a null result, all within the context of a continued belief in a fixed, absolute ether that was not dragged by the earth. This interpretation is called the Lorentz Ether Theory and is fully compatible with all experiments. However, they still could not identify the absolute ether rest frame but they believed that their own clocks were dilated and their own rulers were contracted and constantly changing (at a very low rate) as the surface of the Earth was changing direction and speed through the ether on a daily and seasonal basis.

6) Einstein promoted the idea of assuming that any inertial observer was at rest with respect to the ether and everyone else who was moving with respect to that observer was experiencing the time dilation and length contraction. Of course, he didn't word it precisely that way, but that is the equivalent of what he was saying.

 

[harrylin]

There are actually several different issues going on here:

5 x yes

[..]

6) Einstein promoted the idea of assuming that any inertial observer was at rest with respect to the ether and everyone else who was moving with respect to that observer was experiencing the time dilation and length contraction. Of course, he didn't word it precisely that way, but that is the equivalent of what he was saying.

If that is the equivalent of what he was saying at the time, then it was self-contradictory - some people indeed accused him of that, but not Lorentz who knew him rather well.

I would say that Einstein promoted the idea that we should not introduce phantoms that seem to escape us - the "shut up and measure" approach. Later he changed his opinion and became less positivistic, but his approach of that time had great impact.

 

[ghwellsjr]

If that is the equivalent of what he was saying at the time, then it was self-contradictory - some people indeed accused him of that, but not Lorentz who knew him rather well.

In his 1905 paper, he stated, in effect, that it was apparently self-contradictory, but that it wasn't really.

So are you agreeing with Einstein that it is only apparently self-contradictory or do you think it is actually self-contradictory?

 

[thwle]

The usual interpretation of MMX is biased both by the intended purpose addressing the concept of a luminiferous ether and by the subsequently enunciated special theory of relativity.

I wish contributors to this thread would notice my post, quoted above.

 

[harrylin]

In his 1905 paper, he stated, in effect, that it was apparently self-contradictory, but that it wasn't really.

He stated that the two postulates are only apparently contradictory; and that is obviously correct. That has little to do with your interpretation.

Regards,
Harald

 

[harrylin]

I wish contributors to this thread would notice my post, quoted above.

Actually, the intended purpose was to detect the velocity of the Earth relative to the ether. That's clearly explained in the MMX paper to which I provided the link. Do you think that that basic fact is ignored?

 

[ibreakkidsleg]

No! That is a much different question than your first question - and a poorly phrased one at that. One can never prove anything absolutely, particularly for all situations, so it is nonsensical for that to be the conclusion of an experiment. But the MMX did prove (to a reasonable certainty) that light is always perceived to travel at C...in an MMX.

If you mean that, even in a different inertial frame, the speed of light is measured to be c in an MMX, then you are in error. This is because a corpuscular theory of light is consistent with MMX's.

 

[thwle]

Thanks for responding.
Yes, MMX was intended to provide evidence to help decide between emission theory and ether theory (referred to as undulatory theory in the paper). I am not conversant with emission theory, of which Ritz theory is one variant, but I believe these are pretty much out of favor these days.
Trying to reconcile MMX with ether theory many thought it necissary to conceive of ether as fluid ("ether drag") so these day ether theory is also out of favor because it became untenably complex.
My intention was to point out that MMX is simply explained without complexity and in a way that vitiates its value for the intended purpose -- it can be seen as compatible with both emission theory and ether theory, (though for "ether theory" I prefer to say "the existence of an absolute frame of reference"). It also does not support or refute relativity.

 

[ghwellsjr]

In his 1905 paper, he stated, in effect, that it was apparently self-contradictory, but that it wasn't really.

He stated that the two postulates are only apparently contradictory; and that is obviously correct. That has little to do with your interpretation.

Regards,
Harald

What was it about the two postulates that made them only apparently contradictory?

 

[grav-universe]

If you mean that, even in a different inertial frame, the speed of light is measured to be c in an MMX, then you are in error. This is because a corpuscular theory of light is consistent with MMX's.

Yes, but not wave theory. In order to obtain a null result from the MMX with corpuscular theory, light pulses would have to travel with the added motion of the source. However, experiments with the aberration of light pulses tell us the pulses are observed to travel at the same speed in a particular direction regardless of the motion of the source. The combination of MMX type experiments and experiments with aberration show that light travels as both a wave and a particle, and only Relativity relates them properly.

 

[thwle]

Folks, please check your assumptions. Some posts in this thread assert that MMX measured or compared light speed. It did not because it could not -- there was no calibrated distance measure and no calibrated time measure hence no possibility of measuring speed. The closest thing to a clock in the experiment is the light source which has a frequency, to be sure, but that frequency was not used as a time measure. It wasn't even assumed to be constant -- it didn't need to be.
The light beam was split so the frequency was the same for both paths.
The interference pattern did not shift, so the RATIO of the times for the two paths was constant.

The ONLY thing MMX demonstrates is the correspondence of two methods of length measurement (radar ranging and solid measuring stick) through changes of orientation and velocity in an inertial frame of reference.

 

[ghwellsjr]

Some posts in this thread assert that MMX measured or compared light speed. It did not because it could not -- there was no calibrated distance measure and no calibrated time measure hence no possibility of measuring speed.

MMX was designed to measure a speed, the speed of the surface of the Earth relative to the presumed absolute ether rest frame as a fraction of the speed of light. But since the designers of the experiment did not understand that Maxwell's equations are invariant under the Lorentz Transformation, there was no hope of getting anything but a null result.

 

[harrylin]

Thanks for responding.
Yes, MMX was intended to provide evidence to help decide between emission theory and ether theory (referred to as undulatory theory in the paper). I am not conversant with emission theory, of which Ritz theory is one variant, but I believe these are pretty much out of favor these days.

?? Are you sure you read the paper? They start by explaining how emission theory had been disproved. Here it is again, see the first paragraph:

http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether

Emission theory was pretty much out of favour then. Their aim was to measure the velocity of the Earth.

Trying to reconcile MMX with ether theory many thought it necissary to conceive of ether as fluid ("ether drag") so these day ether theory is also out of favor because it became untenably complex.

The fluidic dragged ether theory was also already disproved, in part by Michelson's earlier experiments - see paragraph 2! You can read his Fizeau experiment here:

http://en.wikisource.org/wiki/Influence_of_Motion_of_the_Medium_on_the_Velocity_of_Light

My intention was to point out that MMX is simply explained without complexity and in a way that vitiates its value for the intended purpose -- it can be seen as compatible with both emission theory and ether theory, (though for "ether theory" I prefer to say "the existence of an absolute frame of reference"). It also does not support or refute relativity.

Indeed no single experiment addresses all theories. The MMX is part of a package that started with Michelson's Fizeau experiment. For completeness he performed another experiment to demonstrate detection of the rotation of the Earth:

http://adsabs.harvard.edu/abs/1925ApJ...61..140M

Regards,
Harald

 

[harrylin]

[..]
The closest thing to a clock in the experiment is the light source which has a frequency, to be sure, but that frequency was not used as a time measure. It wasn't even assumed to be constant -- it didn't need to be.
The light beam was split so the frequency was the same for both paths. [..]

Actually the frequency did have to be constant: it was used as a time measure. The frequency directly affects the phase shift from which they hoped to measure the velocity of the earth. Based on their assumptions they concluded that "the relative velocity of the Earth and the ether is probably less than one sixth the Earth's orbital velocity, and certainly less than one-fourth".

 

[ibreakkidsleg]

Yes, but not wave theory. In order to obtain a null result from the MMX with corpuscular theory, light pulses would have to travel with the added motion of the source. However, experiments with the aberration of light pulses tell us the pulses are observed to travel at the same speed in a particular direction regardless of the motion of the source. The combination of MMX type experiments and experiments with aberration show that light travels as both a wave and a particle, and only Relativity relates them properly.

Right. MMX is supposed to eliminate preferred-reference-frame theories (ether), and other experiments are supposed to eliminate source-velocity-dependence theories. Although to conclude by process of elimination that ``theory XYZ of light`` is true, one sort of sweeps the problem of unconceived alternatives under the rug.

 

[Buckleymanor]

Folks, please check your assumptions. Some posts in this thread assert that MMX measured or compared light speed. It did not because it could not -- there was no calibrated distance measure and no calibrated time measure hence no possibility of measuring speed. The closest thing to a clock in the experiment is the light source which has a frequency, to be sure, but that frequency was not used as a time measure. It wasn't even assumed to be constant -- it didn't need to be.
The light beam was split so the frequency was the same for both paths.

It did compare light speed as you pointed out.

The light beam was split so the frequency was the same for both paths.
The interference pattern did not shift, so the RATIO of the times for the two paths was constant.

The clock was the frequency and any shift would show that one path of the light took a longer or shorter time to travell a given distance(one of the paths).
So the assumption was that if both beams of light arrived back to the observer in phase the speed was constant throughout the journey that the light took along the two paths.

 

[Dale]

Without assuming Einstein's two postulates you can determine that the metric in a frame using the Einstein synchronization convention in a small region of flat spacetime is of the form:

ds² = -g0²c²dt² + g1²dx² + g2²(dy² + dz²)

Then rather than assuming Einstein's postulates you can simply determine the functions g0, g1, and g2 by experiment. The MMX fixes g1=g2. In other words, the speed of light is isotropic.

See: http://authors.library.caltech.edu/11476/1/ROBrmp49.pdf

 

[roineust]

Hey Dalespam,

The link you attached above, to a 1949 paper, is very interesting in my opinion (the textual part of that paper i mean).

Question: Why at the end of page 378, and at the beginning of page 379, the writer claims: " ...The fundamental measurement of one kind of interval is not to be reduced to that of the other with the aid of postulated constancy of the velocity of light, as would for example..." - Without naming the reason for that restriction...?

Thanks,
Roi.

 

[thwle]

MMX was designed to measure a speed, the speed of the surface of the Earth relative to the presumed absolute ether rest frame as a fraction of the speed of light. .

MMX was intended as you say. But it did not because of a faulty assumption.

But since the designers of the experiment did not understand that Maxwell's equations are invariant under the Lorentz Transformation, there was no hope of getting anything but a null result.

And we believe "that Maxwell's equations are invariant under the Lorentz Transformation" because...? (Is it not an interpretation of MMX itself?)

 

[ghwellsjr]

Without assuming Einstein's two postulates you can determine that the metric in a frame using the Einstein synchronization convention in a small region of flat spacetime is of the form:

ds² = -g0²c²dt² + g1²dx² + g2²(dy² + dz²)

Then rather than assuming Einstein's postulates you can simply determine the functions g0, g1, and g2 by experiment. The MMX fixes g1=g2. In other words, the speed of light is isotropic.

See: http://authors.library.caltech.edu/11476/1/ROBrmp49.pdf

Is this paper concerned with both of Einstein's postulates or just the first one?

The Introduction refers to the first postulate (principle of relativity) as a general postulate and the second postulate (the constancy of the one-way speed of light) as a specific postulate.

On page 380, right-hand column, is the following:

"...Einstein's synchronization insures as a matter of definition the equality of the forward and backward velocity along any given line..." (emphasis in the original).

And then later:

"Alternate synchronizations could have been agreed upon...But...they cannot in practice be carried out as they involve a non-operational appeal to the hypothetical rest-system..."

And right after that they summarize the result of MMX:

"M-M: The total time required for light to traverse, in free space, a distance l and to return is independent of its direction."

And finally, in the Conclusion:

"We have with this completed the task of replacing, so far as possible, Einstein's relativity postulate by facts drawn from experience."

So shouldn't you restrict your final statement to "the round-trip speed of light is isotropic"?

 

[thwle]

?? Are you sure you read the paper? They start by explaining how emission theory had been disproved. ... Emission theory was pretty much out of favour then. Their aim was to measure the velocity of the Earth. ...
Regards,
Harald

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, 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.

 

[ghwellsjr]

MMX was intended as you say. But it did not because of a faulty assumption.

And what was that faulty assumption?

And we believe "that Maxwell's equations are invariant under the Lorentz Transformation" because...? (Is it not an interpretation of MMX itself?)

No, it has nothing to do with any experiment, it is pure mathematical manipulation. Maxwell's equations are, well, a set of equations. Lorentz's Transformation are a set of equations. When you do the operation of Lorentz's Transformation on Maxwell's equations, the question is, do Maxwell's equations come out the same? And yes they do.

Now if you are asking if the Lorentz Transformation is an interpretation of MMX, it was derived from that and several other experiments, but it was also derived by Einstein purely on theoretical grounds based only on his two postulates.

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.

 

[thwle]

Actually the frequency did have to be constant: it was used as a time measure. The frequency directly affects the phase shift from which they hoped to measure the velocity of the earth. Based on their assumptions they concluded that "the relative velocity of the Earth and the ether is probably less than one sixth the Earth's orbital velocity, and certainly less than one-fourth".

OK. If there had been a phase shift, a time difference could have been inferred. There was not and would not have been even if frequency changed.

 

[Vanadium 50]

And we believe "that Maxwell's equations are invariant under the Lorentz Transformation" because...?

Because when you apply the Lorentz transformation to Maxwell's equations you recover Maxwell's equations. After many pages of algebra.

 

[thwle]

It did compare light speed as you pointed out.

The clock was the frequency and any shift would show that one path of the light took a longer or shorter time to travell a given distance(one of the paths).
So the assumption was that if both beams of light arrived back to the observer in phase the speed was constant throughout the journey that the light took along the two paths.

Beg pardon! There is a difference between comparing light phase and comparing light speed.

There was neither comparison nor measure of light speed in MMX.

 

[thwle]

Lorentz's Transformation are a set of equations. When you do the operation of Lorentz's Transformation on Maxwell's equations, the question is, do Maxwell's equations come out the same? And yes they do.

Because when you apply the Lorentz transformation to Maxwell's equations you recover Maxwell's equations. After many pages of algebra.

Oops. I defer on that. I read "Lorentz transformation" and reacted without understanding what you said. (my bad.)

 

[thwle]

And what was that faulty assumption?
QUOTE]


The faulty assumption specifically: that length measure by measuring stick and length measure by electromagnetic echo ranging are not equivalent.

MMX does not show that they are equivalent but seems to suggest that they may be.

I would like you to consider the possibility that the spacing of atoms in a solid measuring stick may be determined by forces that are communicated across those spaces at light speed, in which case measuring stick length becomes dependant on echo ranging.

 

[ghwellsjr]

And what was that faulty assumption?

The faulty assumption specifically: that length measure by measuring stick and length measure by electromagnetic echo ranging are not equivalent.

MMX does not show that they are equivalent but seems to suggest that they may be.

I would like you to consider the possibility that the spacing of atoms in a solid measuring stick may be determined by forces that are communicated across those spaces at light speed, in which case measuring stick length becomes dependant on echo ranging.

Where did you get the idea that anybody made the assumption that any two methods for measuring distance would yield different results?