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B Experiments with light in a vacuum

  1. Jul 11, 2016 #1
    I am getting confused as to which experiment proved that the speed of light was invariant in a vacuum. At first I thought it was the Michelson-Morley experiment but that does not have a moving observer.
     
  2. jcsd
  3. Jul 11, 2016 #2

    Dale

    Staff: Mentor

    The MM experiment was attached to the earth, so it was moving at some point during the day and year. However, the MM experiment only detected the Isotropy of c. For a more complete account of the relevant experiments you should read the sticky thread at the top of the forum.
     
  4. Jul 11, 2016 #3
    Hi, I have gone to the top of this forum but I am not sure of which sticky thread you are referring to.
     
  5. Jul 11, 2016 #4

    Dale

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    Last edited: Jul 11, 2016
  6. Jul 11, 2016 #5
    Just wondering, also, how we do we know what light is doing in a vacuum because we don't see light until it reaches our atmosphere?
     
  7. Jul 11, 2016 #6

    Dale

    Staff: Mentor

    We pull a vacuum and do experiments.
     
  8. Jul 11, 2016 #7
    What kind of experiments?
     
  9. Jul 11, 2016 #8

    Dale

    Staff: Mentor

    This is described in detail in the link above.
     
  10. Jul 11, 2016 #9
    we cannot create very large vacuums and light travels incredibly fast. i cannot see any specific experiment that refers to this type of scenario.
     
  11. Jul 11, 2016 #10

    Dale

    Staff: Mentor

    what type of a scenario?

    There are many listed experiments done in vacuum. I don't know what being large or small has to do with anything. Just search with the word vacuum.
     
  12. Jul 11, 2016 #11

    Ibix

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    Also, you can put mirrors at each end of a vacuum chamber and let the light bounce multiple times to increase the effective length of the chamber - as LIGO does with its 4km long high-vacuum arms.
     
  13. Jul 11, 2016 #12

    Drakkith

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    Staff: Mentor

  14. Jul 11, 2016 #13
    The scenario I am referring to is one which would show that the speed of light is invariant. So we would have a vacuum and a moving observer and a light wave. Could you please tell me of a known experiment of that type of scenario.
     
  15. Jul 11, 2016 #14

    Drakkith

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    I provided a link to several in my post above.
     
  16. Jul 11, 2016 #15

    Nugatory

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    Every experiment performed on earth involves a moving observer, as every point on the surface of the earth is moving. Do you mean an observer moving relative to the source of the light wave?

    If so, the correct operation of the GPS system is an example. All the GPS satellites are moving relative to one another and the surface of the earth while continuously exchanging light (at radio wavelengths) signals. Furthermore, different points on the surface of the earth are moving in different directions at different speeds (because of the earth's rotation). GPS positions are calculated using a constant speed of light, and would be way off if the motion of either the transmitters or receivers affected the speed of light.
     
  17. Jul 11, 2016 #16

    Dale

    Staff: Mentor

    All of the experiments in sections 3.1-3.4 do that.

    The phrase "a moving observer" doesn't mean anything because every observer is at rest in one frame and moving in all other frames. I think that you mean "an observer moving relative to the source". This would include all of the experiments in 3.3. Be sure to read the explanation of "optical extinction", which is the key criticism regarding whether an experiment has a good enough vacuum.
     
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