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B What medium does light move in?

  1. Oct 19, 2016 #1
    Waves on water require water to move, sound waves require a medium of the like. My question is what do light waves move in. Empty space contains not much more then low density hydrogen, what medium could light possibly be moving in.
     
  2. jcsd
  3. Oct 19, 2016 #2

    Doc Al

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    Light does not require a medium.
     
  4. Oct 19, 2016 #3

    jtbell

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    One could say that the electromagnetic field that permeates the universe is the "medium" for light.
     
  5. Oct 19, 2016 #4
    Nope, doesn't need a medium or an astrologer:) Just a good storyteller like Einstein:)
     
  6. Oct 19, 2016 #5
    Classically? The electro-magnetic field.
    Quantum mechanically? It is its own medium...okay, slight fib, still electro-magnetic field, but there is no requirement for anything but light for it to move. Essentially, free space is enough.
     
  7. Oct 19, 2016 #6
    It was once thought that there was a medium, called the aether, that was required for light to propagate. The famous Michelson-Morley experiment failed to find evidence of this.
    https://en.wikipedia.org/wiki/Luminiferous_aether
     
  8. Oct 19, 2016 #7

    Dale

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    As have all subsequent experiments with ever greater precision and a multitude of techniques.
     
  9. Oct 19, 2016 #8

    BillTre

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    Could one say the medium through which light moves is space-time?

    Without space-time, there would be nothing for the light to do movement-wise.
    Also a warped space-time affects the movement of the light.
     
  10. Oct 19, 2016 #9
    It would seem to me that space time is this fundamental layer on which all the rest of the fields exist. While true that light does get affected by space-time, but so is everything else regardless if charged, neutral, magnetic or relativistic. Anything that has energy gets affected by space-time.
    In conclusion: The electro-magnetic field rests on top of space time, and that field gives rise to photons.
     
  11. Oct 19, 2016 #10

    Nugatory

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    No. Although it makes sense to speak of moving in space (the position changes with time), it is not at all clear what could be meant by moving in space-time - what's changing with respect to what?
     
  12. Oct 19, 2016 #11

    BillTre

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    Perhaps my understanding of medium is limited.
    How is it defined?
     
  13. Oct 19, 2016 #12
    wasn't that called the aether 100 years ago?
     
  14. Oct 19, 2016 #13

    davenn

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    as a substance for the/allows the wave to propagate ( supports the wave propagation)
    eg a sound wave wont propagate with out a medium/substance ( liquid, gas, solid etc)

    Light and any other E/M will happily propagate where there is no medium eg a vacuum
     
    Last edited: Oct 19, 2016
  15. Oct 19, 2016 #14

    BillTre

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    So why not space-time?
    It allows light to propagate.
     
  16. Oct 19, 2016 #15

    davenn

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    @Nugatory already answered that .... he is welcome to expand on his comment if it wasn't enough for you
     
  17. Oct 19, 2016 #16

    russ_watters

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    How? What are it's properties that are harnessed for propagation of light? For example, air has mass/density, pressure and elasticity (compressibility) that enables sound propagation.
     
  18. Oct 19, 2016 #17
    It's properties are 'none'
    Light travels at the speed of light....time stands still at the speed of light...there is no such thing as 'time' for light....there is for us
    It follows that there is no such thing as 'distance' for light.....light simply links things together.
    No properties of any medium are required by light. !!!
     
  19. Oct 19, 2016 #18

    BillTre

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    Thanks @russ_watters and @lychette.

    So, sounds like you guys are saying the defining property of a medium, in this sense, is that the medium is involved in the movement of a wave itself (a propagated displacement), such as water or air waves move through a medium of water or air because the water or air molecules are actually moving (up and down or back and forth) instead of just something it moves through.

    In which case it seems to me that space-time would be the medium for gravity waves but not light waves, but no one has said that (at least in a way that I understand).
    Seems to be required for propagation, affects the propagation of light (as ether was claimed to do).
     
  20. Oct 19, 2016 #19

    Dale

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    One of the key characteristics of a medium is that it has a rest frame. Spacetime does not. If you were to make such a statement then you could only do it by redefining the term "medium" to get rid of that key feature.
     
  21. Oct 20, 2016 #20

    There is NO medium necessary for light....what else is there to add?....how do you go from this point?
     
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