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Michelson-Morley experiment, a pointless exercise when taking SR into account ?

  1. Oct 24, 2011 #1
    Hi all,

    So, I've been trying to get my head straight about a luminiferous aether and have been reading about the experiment of Michelson and Morley. It seems there are many factors of SR that get in the way of this experiment actually producing any plausible results. I assume the whole point of the aether in the case of this experiment is that our measurements of the speed of light within it are in relation to a 'static' or 'stationary' aether itself.

    The first problems are in reference to time dilation, if a body is moving through aether firing beams of light - lets say one straight ahead and one to the right (well... at those slightly off angles used to measure differences in time after reflection) - then:
    a. the beam of light off to the right will have to travel further to reach the reflector, travelling through a stationary aether this will mean that it will take longer
    b. the equipment being used at the time of firing the light will be at a different point in space and time when the light returns, creating artificial interference patterns

    This may do for now! Other problems involve curvature of space-time around large gravitational bodies that could cause a significant amount of interference, or could possibly completely nullify the effects of the experiment (if aether is moving around the Earth in curved space-time). We may be in a nice little bubble of gravity.

    Also, does any body know what the actual results were from the experiment? Or is it all still a bit .... up in the air?!?! :)

    Hope someone has some help and advice and can clear up this problem
    Thanks very much!!

    Last edited by a moderator: Apr 30, 2014
  2. jcsd
  3. Oct 24, 2011 #2


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    Actually, the beam of light off to the right will have a smaller distance to travel through a stationary aether and will take less time than the beam going straight ahead. That's why we need length contraction to bring the front mirror closer than the side mirror so that it takes the same amount of time for both beams to make their round trips.

    I guess if you don't understand how the experiment works, that might account for why you think there is a problem with it, could that be?
  4. Oct 25, 2011 #3
    The beam of light off to the right will also have to travel further... perpendicular to its direction of travel as the whole thing is moving through the aether, so it must have further to travel.
  5. Oct 25, 2011 #4


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    I've already said you're wrong so I guess it's pointless for me to say it again.

    I'll have to try to figure out what you're thinking. Let's see, you must be comparing the absolute distance relative to the aether that the beam going off to the right has to travel compared to the relative distance that the beam off to the front has to travel. But you also said that because the beam off to the right has farther to travel that it will take longer. But you can't be thinking that because the beam off to the front is experiencing a head wind from the aether and that makes it take longer, even if you take into account the fact that its reflector is closer than the one off to the right (because of length contraction).

    So let's see, is there some other way to think about this that would cause you to conclude that the distance the light has to travel to the reflector off to the right is longer than the distance to the reflector in front? Oh, I think I've got it!! You must be comparing the distance off to the right when the apparatus is moving through the aether to the distance off to the front when the apparatus is stationary in the aether, right? Is that what you're comparing?

    Please, I'm trying to help you, ftc , but I'm having a difficult time. I probably haven't been able to figure out what you're thinking so maybe you can tell me exactly what you are comparing the distance off to the right to. Please be precise.
    Last edited by a moderator: May 15, 2014
  6. Oct 25, 2011 #5
    Thanks for your reply! What I mean is that the beam off to the right is also experiencing head wind. How could this not occur?? I am not saying that the right beam will take longer than the front beam! Just that there will be discrepancies in all directions. The beam to the right, if sent perfectly perpendicular to direction of travel, should land further back on the reflector, or if it's landing in the same place, should take a bit longer. What am I missing as I'm sure Michelson and Morley have thought these things through. I'm thinking in terms of simple trigonometry. I am also having a difficult time!
  7. Oct 25, 2011 #6


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    OK, so you're saying that the beam off to the right is taking a bit longer than it would if the apparatus were not moving through the aether, is that what you mean?

    I'll assume so.

    Well M&M believed that what you say is true but the presumed speed of the surface of the earth through the aether was so very slight that they didn't expect there to be any problem with aiming of the light beams at the reflectors.

    But they did figure that the time that it would take for the light to traverse off to the right and bounce back from the reflector to the source would only increase by such a slight amount that it would be too small to measure with any clock available to them at the time.

    So instead of relying on a timing device to make the measurement, they used a second beam placed at right angles to the first beam and caused the two returned beams to interfere with each other which would be very sensitive to any change in the difference in their path lengths (and therefore the time of travel, assuming that the speed of light would be constant with reference to the presumed aether).

    Then, since they didn't know the actual direction of the motion of the earth through the aether, they made their apparatus capable of rotating so that the two beams would exchange places with being at right angles to the motion through the aether.

    In order for you to understand how the experiment works, you need to understand that the change in path lengths for the two light beams will be different along the direction of motion through the aether and at right angles to it. I have created some animations to help illustrate what's going on. They actually show four round reflectors instead of just a pair of flat ones (to avoid aiming issues) with the initial light going in all directions.

    Here is the animation that shows what M&M expected if they were stationary in the aether:


    And here is animation that shows what they expected if they were moving at one-half the speed of light through the aether.


    Can you see how in the first animation, the reflected light from all four mirrors arrives simultaneously at the observer, whereas in the second animation, it arrives first from the two side mirrors and then a short time later from the forward and backward mirrors? (Note that the orientation of my animations is rotated 90 degrees from your description, but that doesn't make any difference. Just be aware that the observer is moving to the right so his side mirrors are above and below him in the animation.)
  8. Oct 25, 2011 #7
    Wow, very cool, thanks for showing me these animations!

    So I see now that they were looking for patterns in the light caused by phase interference as a result of time differences in receiving the beams. And I see that the beams off to the left and right are a tad slower, but extremely negligible at Earth's speed.

    But I don't see why this would be any different without the aether? Especially looking at these animations, I can't see why this wouldn't happen in a space without aether too. I understand that the speed of light is measured equally for all observers, but the beams will still have different distances to travel...

    Might you be able to explain how this brought an end to concepts of the aether? Also, do you know if there are incompatibility issues with relativity and aether?

    Thanks for your help!!!!
  9. Oct 25, 2011 #8


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    First off, the change in the difference of the travel times (and the distances) at Earth's speed is not extremely negligible when measured with an interferometer. If light and aether and physics behaved the way M&M expected it to, they would have measured the aether wind.

    But since they didn't, an explanation was needed. Michelson thought the aether was being dragged along by the Earth so that no aether wind could be measured and he proposed repeating the experiment at the top of a high mountain in hopes that the drag would be less.

    However, other scientists came up with an alternate explanation which was that lengths contract along the direction of motion, putting the forward and rearward mirrors just close enough together that the reflected beams would take the same amount of time as the ones from the side mirrors. And then other scientists came up with the idea that clocks would appear to run slower so that the measured round-trip speed of light would always yield the same value of c. This theory was then promoted by Lorentz and called the Lorentz Ether Theory or LET.

    So do you see how if lengths contract along the direction of motion through the ether, they could understand how MMX could not detect an ether wind? And do you see how if time seems to contract for moving clocks that this would explain how any measurement of the speed of light through the moving ether would still give the same answer for the round-trip speed of light?

    Keep in mind that these pre-Einstein scientists all believed in an absolutely fixed stationary ether, they just had no way to identify where it was. They believed that their clocks would slow down and their rulers would contract along the direction of motion through the ether but they believed that nature was operating on an absolute basis as far as time and space were concerned.

    I want to make sure you understand how LET is a viable scientific theory that explains all the facts of any experiment that SR can explain but to answer your last two questions, the concepts of ether (LET) are ended because they don't provide anything useful and no, there are no incompatibility issues between Special Relativity and LET.
  10. Oct 25, 2011 #9


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    http://en.wikisource.org/wiki/On_the_Relative_Motion_of_the_Earth_and_the_Luminiferous_Ether" [Broken] is a link to the paper written by M&M, including their results.
    Last edited by a moderator: May 5, 2017
  11. Oct 25, 2011 #10
    Ah! This is amazing, thank you so much! So I think I was probably thinking about it correctly.... I apologise for not explaining things very well as I'm fairly new to all this discussion stuff.

    The main problem with these pre-Einstein physicists then, from what I understand, is that their concept of 'aether' (as opposed to Lorentz 'ether?') was mainly that it was static and caused a wind. But a post-Einstein ether on the other hand may be understood as simply a fabric in which energy and matter and all the forces exist. Does this sound right?

    I have some ideas which require this LET you see, and I was beginning to think these might be dashed by some kind of proof that there is in fact no ether (or aether!). So, if Lorentz' concept of ether is still a viable theory and that it would provide something useful, it should be an easy concept to revive?

    You have been incredibly helpful - very much appreciated! I wish I could return the assistance...

  12. Oct 25, 2011 #11


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    The search for the stationary state of the presummed ether began with Maxwell when he developed his equations describing the interplay between charges and electromagnetic fields and he discovered that the effect of moving charges would create a disturbance in the field that would propagate out at a speed that just so happened to be very close to the previously accepted value for the speed of light. Since his equations lent credance to a fixed physical field extending throughout all of space, he assumed that it's presence could be measured and he indirectly inspired Michelson to devise his experiment to find it. (He died before learning of MMX.)

    So I would say that it was the pre-Lorentz physicists that had the wrong concept of aether, not so much having to do with the field itself, but with the matter embedded in the field. But it didn't take long for that to get straightened out by Lorentz, et al.

    I wouldn't try to characterize a LET ether as anything other than a presummed "preferred" Frame of Reference with no allusions to anything like a fabric or a medium. I think it can be a useful tool in helping to learn Special Relativity but I wouldn't try to revive it as something that would displace Special Relativity. There's no hope or point in that because they both would fall together. They are identical except for the philosophical notion of whether or not there could be a single preferred Frame of Reference. Even if you believed there were, you would still have no way of identifying it, and to be consistent, you really should do all your science in some presummed preferred Frame of Reference which means you would have to transform any rest frame you were in to this preferred one using the Lorentz Transform in order to carry out your scenario, analysis or whatever, and then transform everything back to the rest frame you were in. It's a lot of work and doesn't amount to anything because you could have done everything in whichever rest frame you choose.

    Any Einsteinian Frame of Reference you choose will behave exactly like a LET absolute ether rest frame.
  13. Oct 25, 2011 #12
    I am very relieved to hear of LET and relativity being... kind of intrinsic properties(/models?) of each other. It is quite an entertaining thought to imagine being a physicist around the time of MMX to just want to go straight out and create an experiment to measure it :)

    Indeed I should learn not to refer to such an entity as a fabric, but it is quite difficult to describe it in a truly meaningful way, maybe we just don't have the words!

    ghwellsjr, you are brilliant - this sentence sums everything up absolutely perfectly!

    Thank you!!!
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