[fsh]

There is no way you could repeat the Michelson-Morley experiment with that small and floppy Thorlabs EDU kit. The experiment from 1887 had an arm length of 11 m and was interferometrically stable (typical length fluctuations much smaller than the wavelength) while rotating. That would still be a considerable engineering challenge today.

Modern Michelson-Morley experiments [1, 2] don't use Michelson interferometers anymore. Instead, they compare the lengths of crossed ultrastable high-finesse cavities (in vacuum, of course). The big innovation is that, with lasers and electronics, we can measure the cavity resonance frequencies (and therefore also the cavity lengths) to something like 15 digits of accuracy. This corresponds to less than a tenth of the diameter of a Proton, and is something like 100 million times more accurate than you can achieve with a simple Michelson interferometer.

[1] https://doi.org/10.1103/PhysRevLett.103.090401

[2] https://doi.org/10.1103/PhysRevD.80.105011

[dekhn]

Thanks. I'm not an optics expert, although my friend (the one who said they built a michelson interferometer in a day in their physics lab) is. Since I don't want to mislead anybody, could you explain what the Thorlabs kit can do, and is it technically a Michelson interferometer? The labs that feature the kit all seem to measure the wavelength of light.

It would seem odd that Thorlabs (generally well respected) would sell something that is not what it really is, or misrepresented its capabilities. my guess is that you're sayting the kit itself couldn't reproduce the original experiments, but that it still is a Michelson interferometer in design, which can be used to carry out less demanding experiments, but not demonstrate the (non) existence of aether?

[fsh]

The Thorlabs kit looks like a very decent Michelson interferometer that can be used for a lot of demonstrations such as measuring wavelengths and studying the coherence properties of light sources.

However, repeating the Michelson-Morley experiment is not easy since the expected signal is very small. If there was a stationary aether, the relative length difference for the optical path along the earth's motion compared to the path perpendicular would be (v/c)^2 ≈ 1E-8, where v is the orbital velocity of earth (3E4 m/s), and c is the speed of light (3E8 m/s). The arm length of the Thorlabs kit is just a few cm, so the shift would be on the order of one nm, or one five-hundreth of a (green) wavelength. Thermal drifts and vibrations of optics on a typical optical table are much larger than that, especially when trying to rotate the setup. Michelson and Morley achieved the necessary stability by constructing their interferometer on a solid stone slab, and made a near-frictionless bearing by floating it on mercury. The resulting stability is still impressive by modern standards, but the construction technique is not very practical. Nowadays, large and passively stable optics setups (for example telescope mirrors or laser gyros) are usually made from massive pieces of Zerodur which has near-zero thermal expansion.

[acqq]

"the one who said they built a Michelson interferometer in a day in their physics lab"

Maybe you should check with them too: it's possible that they have also built "a" Michelson interferometer (just like Thorlabs kit features one) but maybe their setup was in spite of that insufficient to perform the needed measurements in the way needed for the valid execution of a Michelson-Morley experiment?

Historically, Michelson constructed his first interferometer in 1881 in Potsdam, Germany:

https://arxiv.org/abs/2111.12176

Inventing it was obviously necessary but not sufficient for a valid Michelson-Morley experiment, which was correctly finished only during 1887 in Cleveland, Ohio.

[dekhn]

I think they must have said they built a michelson interferometer and not that they ran an MM experiment. Basically same as the thorlabs kit (in fact it probably was the equivalent of that kit, but cobbled from edmunds, since thorlabs wasn't really big at the time).