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I Why is the medium in the Michelson Morley experiment ignored

  1. Jan 26, 2016 #1
    Normally in explaining the aether model of light it is said that all waves need a medium, so just like sound uses air, light uses the aether. To my understanding sound can travel through gas, liquid and solids just fine without air being partially entrained in the materials. Sound does not use just one medium, why would light? Light can also travel through gas, liquid and solids and just like sound it will travel at different speeds through the different materials.

    In the Michelson and Morley experiment it was done in air, that was not moving relative to the equipment. I could ignore fact that the experiment was done in air if: air did not have a index of refraction, mirages didn't happen, and no there was no visual distortion between different temperatures of air. But it does and they do happen. Even if the experiment was done in a closed vessel under a vacuum, the equipment relative to that vacuum would not be moving.

    My question are:

    Isn't light clearly using air as a medium?

    why is the fact that the light that is traveling through a relative stationary medium is completely ignored?

    How can this experiment be translated into that the speed of light is constant in a vacuum?
     
  2. jcsd
  3. Jan 26, 2016 #2
    Isn't light clearly using air as a medium?

    Yes.

    why is the fact that the light that is traveling through a relative stationary medium is completely ignored?


    Because the experiment is set up so that it cancels out. It doesn't matter how fast light is moving here. It is a test as to whether it is moving at the same speed in all directions.

    How can this experiment be translated into that the speed of light is constant in a vacuum?

    I would say that it can't be translated this way.




    What MM was about was showing that there couldn't be some absolutely stationary medium through which the Earth is moving.
     
  4. Jan 26, 2016 #3

    jbriggs444

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    In the absence of experimental evidence, it would be plausible. The theory you have in mind sounds indistinguishable from complete ether dragging. A negative result for MM is consistent with complete ether dragging. It takes other experiments to falsify such a theory.

    https://en.wikipedia.org/wiki/Aether_drag_hypothesis
     
  5. Jan 26, 2016 #4

    Dale

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    MMX interferometry experiments have been done in air, vacuum, and plexiglass. Also Fizeau interferometer experiments have been done to test the speed of light in moving media. The results are all consistent with SR. The points you bring up are not ignored in any way and have been well tested experimentally and well analyzed theoretically.

    http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html
     
  6. Jan 26, 2016 #5
    Not really, from what i read about complete aether dragging hypothesis is " aether is completely dragged within and in the vicinity of matter". In that model, Aether is the only substance that light can use to travel. I am consider that light is using air as a medium. So experiments like oliver lodge and hammer experiment would produce negative results.

    If i'm understanding you correctly, light is not using air as a medium. How would you explain index of refraction, mirages and visual distortion of light through different temperature gradients, without using wave theory of light where it is assume that light is using air as a medium.
     
  7. Jan 26, 2016 #6
    I have not seen any MMX have been done in open space, where the medium around the interferometer is moving. I tried to find MMX done under a vacuum but i could not. I would assume that the experiment was done here on earth. So it would be in a pocket of vacuum where there would be no relative motion between the vacuum and interferometer. To me it would be like testing the outside wind speed with an anemometer that is inside a closed building.

    I have not found where an explanation of this experiment is given. I also don't see how it's relevant. For example there wasn't an interferometer in and moving with the water, where there would be no relative velocity between it and the water. As in the MMX experiment and the air around the earth.

    I hate to be vague but how? I have never read anything about the MMX experiment that contributes its failure to there being no relative velocity between the interferometer and the current medium that it is in. If we look at the derivation of the equations for the MMX the fact that there is an atmosphere around earth is clearly ignored.
     
  8. Jan 26, 2016 #7

    Dale

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    This is a physically meaningless statement. Vacuum doesn't have a velocity or motion so "relative motion between the vacuum and ..." doesn't make sense.
     
  9. Jan 26, 2016 #8

    russ_watters

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    From this and follow-ups, it doesn't seem like you understand what the medium was supposed to be/how it was supposed to work -- since detecting the medium was the entire point of the experiment!

    The medium on which light was thought to propagate was believed to permeate the universe without interacting with matter.

    The MMX clearly demonstrated that this was not the case. As others have mentioned, people then came up with a lot of "yeah, but what if..." explanations such as aether drag to suggest behaviors of the aether that could fit with the new constraint. Over time, the boxes that the aether may be hiding in have gotten smaller and smaller. They've covered pretty much every "yeah, but what if..."
    Those two statements contradict each other. If air were the medium, it wouldn't propagate in a vacuum at all.
    If it is in a vacuum, there is no known medium.
     
  10. Jan 27, 2016 #9
    Does that mean that a vessel that has a vacuum inside of it does not move? Since the vacuum inside the vessel can't have a velocity or motion the vessel that contains it cannot have velocity or motion or it will brake. I get what the statement is trying to say. You can't move nothing. Since a vacuum is nothing it can't move. but i don't understand how a vacuum here on earth in a vessel would be equivalent to the vacuum in space that earth is moving through. Would not there be relative motion between the earth and vacuum of space but no relative motion between earth and the vacuum in a vessel that is on earth?

    It is also known and accepted that there is no such thing as a perfect vacuum. So there is no instance to where light is traveling through a perfect nothing right?
     
  11. Jan 27, 2016 #10

    PeterDonis

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    No, this is not correct. You are reasoning as though "vacuum" was a thing, that must have some state of motion. It isn't. "Vacuum" is not a thing; the concept of "state of motion" does not even apply to it. Nor can it affect the state of motion of anything else.
     
  12. Jan 27, 2016 #11

    Ibix

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    OP - as Peter says, you seem to be imagining vacuum as a real thing. It's just the absence of anything. What Dale means is that you can only say you are moving relative to some landmark. Since a vacuum is defined as the absence of anything one could conceive of using as a landmark, it is meaningless to talk about moving (or not) with respect to vacuum. Certainly vessels containing a vacuum (or, better said, excluding all matter) can move. But it's meaningless to talk about whether the nothing inside them is moving.
     
  13. Jan 27, 2016 #12
    I do understand that they were trying to detect the medium as to how they understood it and there experiment failed. Which means that there understanding of the medium was wrong. Instead of abandoning the aether model they tried to tweak it a little to work with their understanding.

    I figured this was probably covered at sometime but i can not find any reference to it, nothing. If one example could be given to where there interferometer was moving relative to the medium that it was immersed in and gave a negative result i would be satisfied. Say that an MM interferometer is put in a wind tunnel and the light was not shielded from the wind, meaning nothing around the interferometer to protect it from the wind. if the result came back negative then okay.

    For example the sagnac effect experiment that cames back with a positive result, when there is relative motion between the instrument and the medium that it was immersed in. If it was negative i would of been surprised.

    In the beginning statement i said

    Sound does not use just one medium, why would light. During the MMX the instrument was immersed in air. Air would of been the current medium that light would be using to propagate through. If the experiment was done underwater then water would of been the current medium. If in a vacuum then vacuum. It's not really contradicting, just our understanding of how light works isn't complete.

    For example the two commonly accepted theories of light are light does not need a medium to travel through and wave theory of light. They contradict each other. If i remember right in wave theory, light propagates through a medium no different than sound. Otherwise there would be no way to explain lenses, mirages, index of refraction, total internal reflection, and dispersion.... If we say that air is not a medium that light uses to propagate through. Then wave theory of light could not be used to explain mirages and why air has an index of refraction. It would be contradicting itself, because we would be saying light doesn't use air as a medium, then turn around and do the calculation for mirages like it was using air.
     
  14. Jan 27, 2016 #13

    PeterDonis

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    No, they don't. The wave theory of light based on Maxwell's Equations, which is the current best classical model for light, does not need a medium. It can certainly explain facts about light propagation in an actual material substance, like the index of refraction of air; but it does this by assigning a charge density and current density to the substance, not by postulating any magical "medium".
     
  15. Jan 27, 2016 #14
    Light is electromagnetic radiation, and as it happens, the behaviour of such radiation is exceptionally well understood - both classically ( Maxwell's equations ), and in terms of quantum physics ( quantum electrodynamics ). I probably don't exaggerate when I say that these models are among the most thoroughly studied and tested in all of physics, and needless to say they are in excellent accord with all currently available empirical data.
     
  16. Jan 27, 2016 #15

    Ibix

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    Also - if light is travelling in air not aether, that means that aether is somehow excluded from the apparatus. That was not a feature of aether theories, so far as I am aware. The point of aether was that it was everywhere.
     
  17. Jan 27, 2016 #16
    I am by no means suggesting a single magical "medium". But does by assigning a charge density and current density to a medium suggest that light is using a medium? kind of like how for sound we assign a coefficient of stiffness and density. To calculate how fast it should move through a material.
     
  18. Jan 27, 2016 #17

    mfb

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    The sagnac effect exists in vacuum as well, and has been measured both with a table rotating relative to earth (and the air) and with a table rotating just due to the rotation of earth (no motion relative to air).
    Actually, modern sagnac interferometers are so precise, they are sensitive to the rotation of earth around the sun as well.

    Sound can travel through air, glass, water and so on, but they are all made out of atoms, and sound works by moving those atoms. Remove the atoms and there is no sound any more. Light does not work by moving atoms, otherwise it could not propagate in vacuum.
    GPS is doing this experiment, constantly, with a large range of velocities (exceeding the speed of sound), and with incredible precision.

    If you don't understand things, feel free to ask, but claiming "it has to be wrong because I don't understand it" is not appropriate.
     
  19. Jan 27, 2016 #18

    PeterDonis

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    No, it says that light is affected by the presence of charge and current density.
     
  20. Jan 27, 2016 #19

    russ_watters

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    Yes...though some people did relatively quickly abandon the aether model becasue they realized it wasn't needed.
    You are unlikely to find such an experiment because it would add unnecessary error to the results. Light travels through mediums and it travels without them - and at different speeds in different mediums and without them. Adding a medium with a known impact on light would interfere with the purpose of the experiment: detecing the unknown medium that light was believed to travel on.
    Light doesn't travel through just one medium, it can travel through many. And its behavior in them is very well understood from other experiments (like light refraction). But unlike sound it also can travel without a detectable medium. That's the unusual thing about light that the experiment was trying to detect.
    Again: as far as we can tell, "vacuum" is not a medium, it is an absence of a medium. In either case, the experiments demonstrate that light can travel through a vacuum and there is no medium in a vacuum that impacts its motion (speed). That was/is the entire point of the experiment(s).
    People sometimes get that erroneous impression from the way it is described in school. It's not correct. There is only one accepted theory of light, so nothing to contradict with.

    [edit]
    Also, please don't take this the wrong way, but you are kind of reaching here, making this up as you go, which makes it a little hard to follow. I'm trying to interpret: it sounds like you are suggesting a possible aether that is separate from the others (as originally proposed), but that can be blocked by solid objects such as a vacuum chamber. If that were the case, this medium would still permeate the air and space (not sure about liquids or glass...). That way, if you pump the air out of a vacuum chamber, the aether would still be there.

    The problem with this description is that the aether would still exist with (in) air and would therefore still be detectable outdoors, flowing over the earth as the earth moved around the sun. It would even cause problems if you opened the door to the building, causing the aether to flow through. Also, interacting with matter in that way would cause a host of other problems that would suggest other means for detecting it. For example, a mercury barometer could not contain any aether and therefore you shouldn't be able to see through the glass tube that contains an aetherless vacuum. And flowing around objects would cause pressure variations, leading to a whole field of research of "aether dynamics" with fluid-like properties that would be detectable.
     
    Last edited: Jan 27, 2016
  21. Jan 27, 2016 #20

    Dale

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    Of course not. The vessel can move, but the vacuum inside does not have any state of motion. Vacuum is not a rigid object that moves rigidly along with the container, nor is vacuum some liquid that sloshes around inside a container. Ask yourself, how could you determine if the vacuum inside a vessel is at rest relative to the container or sloshing around? There is no possible physical experiment that can determine that, so the concept is physically meaningless.

    If you had bothered to read the article that I posted you would already know the answer to this. This is already well understood and analyzed and very succinctly explained in the link.

    You are wasting everyone's time. Please read the information already provided and come back when you are prepared to have an informed conversation about the already existing available experimental evidence.
     
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