From PhilPapers forum Philosophy of Physical Science:

The Logic of Physics: Some Problematic Concepts
The Michelson Interferometer 
[see also One Slit experiment: Fraunhofer Diffraction (2)]

This ingenious device is supposed to measure the time it takes for a split light beam to reach two different mirrors located at different directions. It was originally supposed to measure the velocity of the ether, but it became the most famous failure in the history of science.
The principle on which it is based is as follows:
The two arms of the interferometer are in a right angle to each other and, unless both parts of the beam are reflected at the same time on the screen, interference fringes will be observed, showing how much out of phase relative to each other both beams are.
I think I have shown that the idea of interference is, to say the least, far from evident. The question now is how to explain the so-called interference fringes when no magnification of the light beams is present. But then, the question is, was there no magnification involved? Let us look more closely at the first experiment by Michelson in 1881 "The relative motion of the earth and the luminiferous ether". I will focus here not so much on the general setup, which I may assume as generally known, but more on the details concerning the light rays.
Here are some interesting quotes:
"The source of light, a small lantern provided with a lens...",
"The lamp being lit, a small hole made in a screen placed before it served as a point of light."
These two quotes make it clear that the setup is exactly the same as the so-called slit experiment. A beam is magnified by a lens, and has to go though a very small opening. Which means that the same remarks I made about Ezekiel's demonstrations can be made here:
1) We are dealing with magnified rays and what we are seeing are the details of those magnification.
2) The sensitivity of the setup is equally explained by the magnification process.

Here is what Michelson had to say about that:
"The first observation showed, however, that owing to the extreme sensitiveness of the instrument to vibrations, the work could not be carried on during the day. The experiment was next tried at night. When the mirrors were placed half-way on the arms the fringes were visible, but their position could not be measured till after twelve o'clock, and then only at intervals. When the mirrors were moved out to the ends of the arms, the fringes were only occasionally visible."
In fact, the perturbations were so great that Michelson moved the whole apparatus from Berlin to the basement of the Astrophysicalisches Observatorium in Postdam. Even then, the set remained extremely sensitive.
What Michelson had apparently built was the first seismograph based on light rays!

The second experiment, with Morley in 1887, tried to solve these problems and others. In this context the way they chose to tackle the sensitivity issue is fundamental, it was done "by increasing, by repeated reflection, the path of the light to about ten times its former value"... And of course by placing the whole apparatus in a bed of mercury.
Ten times the first distance should certainly do the trick of eliminating magnification shakes. What it does not do is eliminate the effects of magnification, that is, blown up details. In fact, the image will be much bigger, showing much more details of the part being magnified.
Also, and that is maybe even more important, it does not stop the small variations from happening, it just makes them invisible to the detector. You must imagine a right triangle with one of the two acute angles representing the distance traveled by small vibrations. The closer you are to the source, the smaller the angle, the greater the vibration will look like, while at the end of the beam, the tiny distance traveled by the vibration will be negligible compared to the angle.
The image will therefore not shake anymore, but will become an intermittent flash: it will seem like the light is turned on and off, and will reinforce the conviction that we are dealing with interference phenomena, that the two light rays are canceling each other out. In fact, what we are seeing is the focus put sometimes on the bright rays, and others on the dark space between them.
The original interferometer comes with a focusing device, the micrometer. This is how Michelson describes the focus operation: "The mirrors c and d were moved as close as possible to the plate b, and by means of the screw m [the micrometer] the distances between a point on the surface of b and the two mirrors were made equal by a pair of compasses."
No doubt the calibration procedure is nowadays much more sophisticated and precise.
Still, the question remains, how accurate is the measurement of both distances? Using a light detector instead of the naked eye, to determine when both rays are in or out of phase, assumes that the transition is abrupt: now you see me, and now you don't.
In other words, how large is the distance, in the width, between the rays forming a single (half) beam, and between both (half) beams? Is it negligible?
On one of Ligo's sites, that of Caltech, an animation is shown of what is advertised as Most precise Ruler ever constructed

But here is the biggest puzzle I am facing. Whatever the interferometer is showing, whether interference is real or imagined, there is a difference between the two states, the one where the light is on, and the other where it is off. The fact that the wave theory of light might be wrong does not make this discrepancy go away.
The fundamental question therefore remains: what is the interferometer measuring?

Let us assume that I am right, and that there is no (destructive) interference of light rays.The only explanation for the intermittence phenomenon remains a change in distance traveled by the light. But then, we have no way of knowing whether both half beams are in fact traveling a different distance since we have no way of distinguishing between them!

In other words, the existence of interference phenomena is essential to the right functioning of the interferometer. Without them we have a very sophisticated flash light.

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