# Michelsom Morley Experiment on a Grand Scale

Disclaimer first. Albert Michelson was a genius. I collect memorabilia of his life and experiments, especially his speed of light work in southern Ca. However; given our current tech I think we could redo his famous experiment. Please tell me if you think this new version of the famous Michelson Morley Experiment would work. Here it is: Instead of using Michelson's rotating table let's use our rotating Earth. Replace the interferometer mirrors with selected stars. Over the course of a year measure the speed of light from those stars, if necessary we could send light from these stars into Michelson interferometers to see if there are fringe shifts. We could choose stars that at different directions of the Earth's motion around the sun. Physicists and astronomers, would look at the data and see what information it reveals. It could be a great experiment. Observatories from around the world could participate. My question is this: Does this proposed experiment have merit? If so, let's do it!

Note to moderator: I tried to load a Michelson Morley Experiment tag but was unable to do so, if possible please add this tag. Thank you.

• What merit do you see in it? In particular, what merit do you see which you believe could be conveyed to a committee in charge of a grant which could pay for this experiment to be done? – Cort Ammon Nov 21 '17 at 3:56
• It could answer the question is there a measurable cosmic field. – Lambda Nov 21 '17 at 3:58
• Comments are not for extended discussion; this conversation has been moved to chat. Please use comments only to critique and improve the question. – ACuriousMind Nov 23 '17 at 19:29

Using mutually incoherent light sources such as separate stars will not work - you'll not get interference!

The idea of using long arms in the Michelson does have merit, and is very much used in modern replications of this experiment. However, if you pass those arms through the atmosphere, with its winds and clouds, you'll probably reach a limit in accuracy for any Earth sized interferometer that is coarser than accuracies achieved by modern Michelson Morley experiments.

At present, the highest accuracy achieved by the MM experiment is about $\Delta c/c\leq 10^{-16}$, where $\Delta c$ is the putative difference between lightspeeds in the two arms. The state of the art is not to use physically hugely long arms but to increase their effective lengths through optical resonance. A modern Michelson Morley experiment is done with a resonant Michelson interferometer of almost the same architecture as the LIGO interferometers. That is, its arms are resonant cavities of huge finesses.

By keeping the physical length small, one keeps the area of the experiment that must be under strict environmental control small. That is, with a physically smaller interferometer it is easier to keep temperature differences small, easier to evacuate the arms and it is easier to keep vibration under control. Precision air bearings with closed loop levitation of the interferometer nowadays replace Michelson's lethal pool of quicksilver and are hugely better at vibration isolation.

• What about just measuring the speed of light from these stars. That’s really what the interferometers are doing anyway. – Lambda Nov 21 '17 at 4:01
• @Lambda Do you mean the use of the light from one star as a source for the interferometer? The interferometer is measuring a phase difference between the two interferometer arms and the idea of the MM experiment is that the phase of the arms would change with their orientation if the aether were present, hence the fringe pattern would change as the instrument rotated. So it's not measuring the speed of light directly. The star's spectrum can influence this interference, and a sudden Doppler shift change in the light is detectable by an interferometer with highly unbalanced arms, .... – WetSavannaAnimal Nov 21 '17 at 4:09
• ....but aside from these effects, the common path from the star has no influence on it at all. – WetSavannaAnimal Nov 21 '17 at 4:09
• I was thinking of just measuring the speed of light from each selected star. Then for a closer look use stellar interferometry. The diameter of the of the mirror would essentially be the length of the interferometer arm. By joining telescopes that can be quit large, something that was not available to Michelson. – Lambda Nov 23 '17 at 3:26
• check out this link: pumas.nasa.gov/files/09_21_05_2.pdf – Lambda Nov 23 '17 at 5:06

Your comments indicate that the interferometer is not the important part of the experiment to you, but rather the use of stars on a galactic scale is. This experiment was already done, over a hundred years before MM:

In 1728 James Bradley made another estimate by observing stellar aberration, being the apparent displacement of stars due to the motion of the Earth around the Sun. He observed a star in Draco and found that its apparent position changed throughout the year. All stellar positions are affected equally in this way. (This distinguishes stellar aberration from parallax, which is greater for nearby stars than it is for distant stars.) To understand aberration, a useful analogy is to imagine the effect of your motion on the angle at which rain falls past you, as you run through it. If you stand still in the rain when there is no wind, it falls vertically on your head. If you run through the rain, it comes at you at an angle, and hits you on the front. Bradley measured this angle for starlight, and knowing the speed of the Earth around the Sun, he found a value for the speed of light of 301,000 km/s.

• Right, I understand stellar aberration. What about just using the raw speed of light from selected stars a to see if there are unequal variations in the speed of light. Unequal being the key differences here. – Lambda Nov 21 '17 at 4:13
• @Lambda So you're proposing an assessment of stellar aberration statistics to test the null hypothesis that $c$ is the same for them all? This can of course be done, as the motion that gives rise to the aberration (i.e. that of Earth) is common to all. I am sure that this would have been thoroughly done already! – WetSavannaAnimal Nov 21 '17 at 4:17
• Has it? I don’t think it has. – Lambda Nov 21 '17 at 4:36
• How do you plan to measure the speed of light? – Cort Ammon Nov 21 '17 at 4:38
• @Lambda At best, would that not test whether light travels at one speed locally, within the apparatus? – Cort Ammon Nov 21 '17 at 6:14