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Question on Michelson Morley experiment and length contraction

  1. Sep 14, 2011 #1
    I'm a lay person who is interested in developing a better understanding of science in general, as well as specific theories.

    I'm just wondering if someone would be able to help me understand how Length contraction explains the null set of the Michelson Morley experiment?

    Hopefully this thread is in the right place and someone can help me with this - cheers.
     
  2. jcsd
  3. Sep 14, 2011 #2

    ghwellsjr

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  4. Sep 14, 2011 #3
  5. Sep 14, 2011 #4
    I had a quick read of the posts, thanks for that, and I will go through them again to try and get a better understanding; but there was just one thing that wasn't immediately clear - to me - and that was how the interference pattern on the interferometer shows that the length is contracted.

    If the light is travelling at the same speed along both arms of the interferometer, would it not have the same wave length and so cause no interference pattern (or the null set, if I have that correct)?
     
  6. Sep 14, 2011 #5

    HallsofIvy

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    Here's an interesting calculation related to this:

    Suppose you are swimming in a river which flows at speed v relative to the bank. You can swim at speed c relative to the water. If you are swimming downstream your speed, relative to the bank is c+ v so you could swim distance L in time L/(c+ v). If you are swimming upstream your speed is c- v so could swim back upstream a distance L/(c- v). You swim both legs in time L/(c- v)+ L/(c+ vf)= (Lc+ Lv)/(c-v)(c+v)+ (Lc-Lv)/(c-v)(c+v)[tex]= 2Lc/(c^2- v^2)= 2L/[c(1- L^2/c^2)][/tex].

    If, instead, you were to swim across the river, you would have to "point" slightly up stream so that your speed across the river would be [itex]\sqrt{c^2- v^2}[/itex] (Imagine a right triangle with hypotenuse of length c and one leg (up river) of length v. The length of the other leg (across the river) is [itex]\sqrt{c^2- v^2}[/itex].) So you go across the river a distance L' and back, for a total distance of 2L' is [itex]2L'/\sqrt{c^2- v^2}[/itex].

    If it takes the same time to swim both laps,
    [tex]2L/c(c^2- v^2)= 2L'/\sqrt{c^2- v^2}[/tex]
    so that
    [tex]L= \frac{L'\sqrt{c^2- v^2}}{c}= L'\sqrt{1- \frac{v^2}{c^2}}[/tex]
     
  7. Sep 14, 2011 #6

    ghwellsjr

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    You always get an interference pattern with a properly constructed and operating interferometer (not a trivial task) but the question is whether that pattern changes as the apparatus is rotated. But Michelson and Morley were expecting the light to take longer along one arm during some part of the day and/or season when rotated to the correct orientation. The fact that it didn't meant their understanding of how light propagated had to be modified.

    Since the scientists prior to Einstein were unwilling to give up on the notion of a fixed ether medium in which the speed of light is exclusively a constant, they explained the null result as the arms changing length as the apparatus was rotated so that the changing speed of light was exactly compensated and made it look like they were stationary in the ether.

    In fact, Michelson thought the earth was dragging the ether along with it to give the null result and suggested that the experiment be repeated at the top of a high mountain to reduce the dragging.
     
  8. Sep 15, 2011 #7

    I'm assuming that c is the speed of light? Does that not mean that the emboldened sentence above could not be possible, because nothing can travel faster than the speed of light?
     
    Last edited: Sep 15, 2011
  9. Sep 15, 2011 #8
    Does that not mean that the concept of Lenght contraction preserves the idea of an aether?

    I'm just having trouble understanding how a non-changing interference pattern indicates, or suggests, that length contraction has occured.

    Could the non-changing interference pattern, or the null result, not be explained by two beams of light [possibly] travelling different distances but arriving in phase, without the necessity of length contraction?

    Is there a difference between this kind of length contraction and that of Special Relativity?
     
  10. Sep 15, 2011 #9

    HallsofIvy

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    No "c" is not the speed of light- it is exactly what I said- the speed with which you swim relative to the water.
     
  11. Sep 15, 2011 #10
    OK, you'll forgive the confusion surely.

    In the equation you posted above, does c not usually represent the speed of light?
     
  12. Sep 15, 2011 #11

    ghwellsjr

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    Yes, that is why those early scientists came up with the concept.
    If they only performed the experiment one time, then they could have jumped to the conclusion that they just happened to be at rest with the aether and that is why their apparatus showed no difference in the round-trip speed of light for all directions. But they weren't that naive and so they repeated the experiment many times during the day and during the year. Since the motion of the Earth's surface is constantly changing and they always detected that they were at rest with respect to the aether, they could only conclude, as Michelson did, that the Earth was dragging the aether along with it, or they could conclude, as the other scientists did, that the appartus was getting modified by the motion of it through the aether so that it only appeared that they were stationary in the aether.
    The issue is whether the physical apparatus is changing its dimensions so that the positioning of the mirrors rigidly attached to it are also changing their distance apart and thereby the path length of the light along the two directions at right angles is changing. So I don't understand how you could suggest that the two beams of light are traveling different distances without the lengths of their path not also changing.
    No difference at all. They are exactly the same. The only difference between the interpretation of those early scientists and Einstein is that they believed in a single absolute aether rest state and that the apparatus was experiencing changing lengths whereas Einstein turned it around and said they could consider their state to be at rest and everyone else moving with respect to them have their lengths contracted. That's what Einstein's concept of a Frame of Reference is.
     
  13. Sep 16, 2011 #12
    Apologies, I'm sure my questions are coming across as very basic, but I think there are probably a few things which are preventing me from understanding this - not suggesting that it is your job to help me understand, but I do appreciate your taking the time.

    Firstly, is the assumption that some medium is required for the propagation of light waves?

    Could they not simply have concluded that the aether didn't exist, without the need to postulate that the apparatus was getting modified?

    Could the distance not simply be measured, to determine if it is actually changing dimensions?

    Does this mean that with two reference frames moving relative to each other, that from the perspective of both reference frames, the other one would have it's length contracted i.e. both would have their lengths contracted?
     
  14. Sep 16, 2011 #13

    ghwellsjr

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    That's the purpose of this forum--to help people understand. I'm glad to help--as are many other people who have previously helped me.
    Special Relativity is not concerned with the mechanism of light propagation--whether it is photons or waves--what is important is that there is no requirement for a medium, even if it were to exist, it serves no useful purpose in the Theory.
    Just saying the aether doesn't exist doesn't solve the problem of how to reconcile that the speed of light always seems to be c, independent of your motion. In particular, if two observers are measuring the same flash of light, like I show in my animations, how can you understand that they both think they are in the center of the expanding sphere of light when they are in different locations? Can you think of a way to reconcile that without postulating length contraction?
    Yes, but your rulers will get contracted by exactly the same amount so you cannot tell that any dimensions are changing.
    Yes, but only one at a time. It's just like relative speeds. From each one's perspective, the other one is moving but you would never think that the real speed was double the perceived speed. Same with length contraction--you pick a Frame of Reference and any objects moving in that frame are length contracted along the direction of motion and the objects that are at rest in that FoR are not contracted. You can even pick a FoR where both objects/observers are moving at the same speed in opposite directions so they are both contracted by the same amount but not the same amount as when you pick a FoR where one of them is stationary.
     
  15. Sep 21, 2011 #14
    is that definitively the case?
     
  16. Sep 21, 2011 #15

    ghwellsjr

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    Yes, they both will have an identical experience with regard to the expanding sphere of light.
     
  17. Sep 22, 2011 #16
    How do we know this to be the case?
     
    Last edited: Sep 22, 2011
  18. Sep 22, 2011 #17

    ghwellsjr

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    All experiments measuring the round trip speed of light, starting with Michelson and Morley's, indicate that they yield the same constant value c. Do you doubt this?
     
  19. Sep 22, 2011 #18
    I don't doubt the constancy of the speed of light, it's just that when someone posted a video explanation of SR, on another forum, there was another video critiquing the thought experiment used by Einstein. In that video was a link to a website which outlines a critique of SR & the MMX, in which a number of issues are raised.

    Unfortunately, I'm not well up enough on either to determine the accuracy of the respective theories, so I'm in the position of trying to wade through both of them.
     
  20. Sep 22, 2011 #19

    ghwellsjr

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    There's another theory? I'm only aware of Einstein's Theory of Special Relativity and Lorentz's Ether Theory as fitting all the experimental facts. I didn't know anyone questioned the experimental facts. Why would you entertain a critique that questions the experimental evidence? There's a lot of people out there that don't understand relativity and so they bash it without first trying to understand it. I hope you stay motivated to understand it before you try to decide between it and what the bashers are saying.

    Have you gone through my animations in an attempt to understand them? Do you have any questions about them?
     
  21. Sep 22, 2011 #20
    Apologies, I used the term "theory" very loosely there, I should have said an alternative interpretation.

    I have gone through the animations - thanks for those - and I have been discussing it elsewhere. Ultimately I understand it - to a degree - but it ultimately comes down to the issue of length contraction, which the author of the video and the website has questioned. The challenge, to my mind, appears to be relatively sound, but, as mentioned, I'm not really well up enough on either to say for definite.

    Now, I am wary of the fact that there are "bashers" out there and so I try to avoid simply believing what they say, opting instead to try and understand it and see why it is either true or false; the same applies to established scientific theories - I try to opt for understanding over belief, or acceptance.

    There is also, however, the unfortunate issue of confirmation bias, and the author's critique of SR and the MMX confirmed, to a certain extent, what I thought seemed intuitively true. Now, I know "intuition" doesn't form the basis of scientific enquiry, but that doesn't necessarily invalidate it; and finding something which confirms existing beliefs, not doesn't it make true, just as it doesn't necessarily invaldiate it either.

    I find that discussing them is a very useful way of getting to understand them better; I often tend to fluctuate betweeen challenging and questioning because I reach a point where I think I spot a flaw in the reasoning and so challenge, but then there are times when things are more complex and I question to try and build a better pictur for myself.
     
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