How unexpected were the Michelson-Morley experiment results? Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?

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    $\begingroup$ That's more of a question for history of science... whether people were surprised can, if at all, be learned from personal communications etc., but it can't be found in the actual physics results. $\endgroup$
    – CuriousOne
    Commented Apr 14, 2016 at 8:17
  • $\begingroup$ Please don't focus too much on the title: I am trying to understand if the theory did allow physicists to make any prediction for this experiment $\endgroup$ Commented Apr 14, 2016 at 8:31
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    $\begingroup$ There is a History of Science and Mathematics site on this network that may be better posed to answer this question. Are you asking about people's reaction to the result, or about what tools they had available to grapple with it? $\endgroup$ Commented Apr 14, 2016 at 9:18
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    $\begingroup$ As a reminder, we don't migrate questions to other sites unless they're off topic here. So far there seems to be limited support for the idea that this question is off topic. (It has only been an hour though.) $\endgroup$
    – David Z
    Commented Apr 14, 2016 at 9:59
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    $\begingroup$ The theory of total ether drag had been around for a few decades before the experiment, and would have predicted the null result. (Of course, the total ether drag hypothesis has other problems but afaik it was reasonably well known and well regarded at the time.) $\endgroup$
    – Mark A
    Commented Apr 15, 2016 at 1:30

2 Answers 2


As far as I know, the only clue at the time that the speed of light would be invariant were Maxwell's Equations where "something" shows up as a constant. However, speed of light being invariant in all inertial reference frames is very counter-intuitive. One might rather expect physics to be slightly different in different frames, which is what the MM experiment was looking for.

  • $\begingroup$ The MM experiment doesn't prove the "speed of light being invariant in all inertial reference frames". $\endgroup$
    – Geremia
    Commented Aug 21, 2017 at 17:07
  • $\begingroup$ See this question. $\endgroup$
    – Geremia
    Commented Aug 21, 2017 at 17:22

It was a completely unexpected result at the time. The principle of the MM experiment hinged on the hypothesis that Maxwell's equations of electromagnetism were valid only in a special frame of reference called the aether frame.The speed of light was equal to its standard value only in this frame and its speed in any other inertial frame had to be given by the Galilean velocity transformation.It was thought that the aether was an all pervasive medium through which the Earth moved. So the claim was that, by the Galilean velocity transformation, the speed of light measured on the Earth should vary with the direction in which light traveled. This was what the Michelson-Morley experiment failed to establish. The negative result led to the postulate of the special theory of relativity that the speed of light is the same in all inertial frames.

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    $\begingroup$ Didn't physicists already know that maxwell's equation were invariant for Lorentz transformations? $\endgroup$ Commented Apr 14, 2016 at 11:25
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    $\begingroup$ In response to your question about whether physicists knew of the invariance of Maxwell's equations under Lorentz transformations, certainly Lorentz knew about it because it was he who discovered this invariance.Lorentz had all the ingredients for proposing what was later called the special theory of relativity but the physicists of the time found it extremely difficult to abandon the Galilean velocity transformation. $\endgroup$
    – Procyon
    Commented Apr 14, 2016 at 11:48
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    $\begingroup$ @MarcoDisce -- No they did not. The Lorentz transformation did not exist at the time of the MM experiment. Voigt first introduced something Lorentz-like the same year the results of the MM experiment were published, but it was narrow in scope and did not form a mathematical group. Lorentz and Lamour worked with something Lorentz-like eight years later (1895 and 1897, respectfully), but the full impacts weren't seen until 1904 (Lorentz) and 1905 (Poincare, Einstein). $\endgroup$ Commented Apr 15, 2016 at 4:15
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    $\begingroup$ @Marco: Historical accuracy aside, they had this line of thought along these lines: "Maxwell's equations require Lorentz invariance. Lorentz invariance is wrong. Thus we expect to see violations of Maxwell's equations". $\endgroup$
    – user5174
    Commented Apr 15, 2016 at 5:14
  • $\begingroup$ @Procyon I have one question: Did anyone consider the possibility of the ether moving perpendicular to the orbital plane? The result would be null here as well right. I know it's a bit of a long shot but still. $\endgroup$
    – Sidarth
    Commented Apr 22, 2017 at 6:32

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