Questions on the Michelson Morley Experiment

[Ibix]

Consider the no dragging ether hypothesis (as I understand it the ether should not be dragged by the Earth in its motion around the Sun). Suppose the speed of light depends on the motion of the source. Since we know velocity is relative, with respect to what should the speed of light have a variable value? Thanks.

I don't think an ether hypothesis (light is a wave, with a fixed speed with respect to the ether) and a speed of light that depends on its source (light acts like machine gun bullets, with a fixed speed with respect to its source) are compatible. You have to pick one or the other (or relativity).

 

[cianfa72]

I don't think an ether hypothesis (light is a wave, with a fixed speed with respect to the ether) and a speed of light that depends on its source (light acts like machine gun bullets, with a fixed speed with respect to its source) are compatible. You have to pick one or the other (or relativity).

Thus, as far as I can understand, the second postulate of relativity --- namely from Wikipedia:
as measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body

actually rules out both the ether and the ballistic hypothesis

 

[Ibix]

Thus, as far as I can understand, the second postulate of relativity --- namely from Wikipedia:
as measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body

actually rules out both the ether and the ballistic hypothesis

You can't rule out anything by postulate. But it does say that relativity theory is incompatible with both ether theory (excepting Lorentz' version) and ballistic theory, yes. You then have to make predictions with the theories and do experiments to determine which one matches and which ones are ruled out.

 

[Grasshopper]

You can't rule out anything by postulate. But it does say that relativity theory is incompatible with both ether theory (excepting Lorentz' version) and ballistic theory, yes. You then have to make predictions with the theories and do experiments to determine which one matches and which ones are ruled out.

Is it true that SR is effectively identical with Lorentz ether theory? I’ve not studied LET, but I’ve read in suspect sources that (if I recall correctly) the math is identical, and so are the physical predictions. Those same suspect sources (if I recall correctly) say you can’t get GR from Lorentz theory though, so it is inferior.

 

[vanhees71]

Sounds interesting. Which "suspect sources" are these?

 

[Grasshopper]

Sounds interesting. Which "suspect sources" are these?

Honestly I cannot remember. It’s been years. I honestly have little to no interest in either pseudoscience or stuff way outside the mainstream. It was just a passing fancy, and the discussion here reminded me of it.

 

[PeterDonis]

the math is identical, and so are the physical predictions

Yes, that's correct.

you can’t get GR from Lorentz theory

You can't get GR from standard SR either, so I'm not sure what the point of that particular criticism is. The usual criticism of LET is Occam's razor.

 

[Grasshopper]

Yes, that's correct.
You can't get GR from standard SR either, so I'm not sure what the point of that particular criticism is. The usual criticism of LET is Occam's razor.

Ah, that’s what it was: positing something that was completely unnecessary and undetectable.

Anyway thanks all for the replies

 

[vanhees71]

A very illuminating exercise to think about the Doppler effect of sound in special relativity. Sound of course needs a fluid as a medium and then of course there's a preferred reference frame, namely the rest frame of the fluid (let's assume for simplicity that the fluid is at rest wrt. an inertial frame). Thus the frequency of the sound depends on both the velocity of the observer and the source of sound relative to the rest frame of the fluid. Now specialize the formulae to a wave with phase velocity being the speed of light (in vacuo), of course particularly an em. wave in vacuo. You'll see that for this case the frequency observed by any observer only depends on the relative velocity between observer and source.

The conclusion is that with a "limiting speed", i.e., a Minkowski space-time you cannot observe a preferred reference frame for electromagnetism (aka "the ether rest frame" a la Lorentz or Poincare). That's why indeed Occam's razor should be applied to simply "kill the ether".

 

[mzh62]

In the Michelson-Morrey experiment, the magnitude of the optical path time difference ∆ t (∆ t 'after 90 degrees of rotation) determines the observable interference fringes and the number of fringes in the experiment. Now the key problem is to substitute the actual parameters into the specific situation indicated by the calculation, ∆ t=3.67 × 10^-16s; ∆ t’ = -3.67 × 10^-16s,

the size of the two is the same, and the sign is opposite, and they will correspond to the same interference fringe. That is to say, after the experimental device is rotated 90 degrees, it is reasonable that the interference fringe does not change, which has nothing to do with the problem of "ether" or "speed of light", that is, the "zero" structure of the experiment cannot explain any problem, and therefore no physical conclusions or judgments can be drawn. In the Michelson-Morrey experiment, the difference between the two optical path time differences before and after rotation is subtracted to obtain ∆=∆ t - ∆ t '≈ (l₁+l₂) v²/c³=2 × 3.67 × 10^-16s,

there is no observable experimental result corresponding to this mathematical operation in the experiment, because the interference fringe is not positive or negative.

 

[Nugatory]

there is no observable experimental result corresponding to this mathematical operation in the experiment, because the interference fringe is not positive or negative.

we have a non-zero $\Delta T$ between the two paths - the only way that the two light beams would remain in phase and not display interference effects would be if the frequency were an exact multiple (and in this context "exact" means physically unrealizable) of $\Delta T$.

 

[Ibix]

there is no observable experimental result corresponding to this mathematical operation in the experiment, because the interference fringe is not positive or negative.

There speaks someone who has never actually used an interferometer. If there's any tilt in the system you see multiple fringes as lines on the screen which move laterally as the path difference changes. And if there's any sphericity in your system the zero fringe is straight and the fringes on either side curve away from it. And if your sources aren't perfectly coherent the zero fringe is contrastier. So anybody with half a clue leaves a little sphericity and a little tilt in their system and tracks the path difference by tracking the position of the straightest clearest fringe.

Even if you're stuck with a design that has an eyepiece instead of a screen you can still spot the zero fringe and track it with about five minutes' practice. It's not difficult, even with the battered old one with the sloppy hysteresis-inducing adjustment screws we had.

 

[Ibix]

See the diagram here: https://www.physicsforums.com/threa...michelson-interferometer.1044816/post-6791684