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Proof: Velocity of Light can't depend on velocity of body emitting the light

  1. Jul 5, 2011 #1
    Hello All,

    I am reading a book about relativity and early on in the book I read the following:

    "...By means of similar considerations based on observations of double stars, the Dutch
    astronomer De Sitter was also able to show that the velocity of propagation of light
    cannot depend on the velocity of motion of the body emitting the light..."

    The author does not go into any detail about this experiment. My question is, could someone please either explain:

    1) The experiment mentioned in the quotation above with some detail.

    2) Or, explain the idea above using a different experiment (assuming one was made) which leads us to the same conclusion.

    Thank you for your help.
  2. jcsd
  3. Jul 5, 2011 #2


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    Hi, Haitham,

    Welcome to PF!

    It's always helpful if you can tell us the source you're quoting from. I found the source by googling: http://www.marxists.org/reference/archive/einstein/works/1910s/relative/ch07.htm I think the preceding sentence pretty much describes the idea. If c wasn't independent of the velocity of the source, then we'd see strange time-lags in observations of an eclipsing binary, since one star is approaching us while the other is receding.
  4. Jul 5, 2011 #3
    Hello bcrowell,

    That's the correct quotation. The book is simply called Albert Einstein: Relativity. You are referring to the sentence:

    "At all events we know with great exactness that this velocity is the same for all colours, because if this were not the case, the minimum of emission would not be observed simultaneously for different colours during the eclipse of a fixed star by its dark neighbour"

    I guess I am a little confused with this concept. I understand the part that if the colours had different velocities we should seem the colours arrive at different times (which is not the case obviously), but how does the eclipse of a fixed star by its dark neighbour have to do with explaining this concept?
  5. Jul 5, 2011 #4


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    Take a look at the graph and animation here:

    If different photons had different velocities, then they would all take different amounts of time to reach us. The graph would get scrambled and distorted.
  6. Jul 5, 2011 #5
    Yes, I know see it. Truly amazing. The graph simplified everything for me.

    Thank you bcrowell. Please let me know if there is anything you need. You helped me a lot. :)

    PS: I wonder if an experiment could be done in the lab to show students.
    Last edited: Jul 5, 2011
  7. Jul 5, 2011 #6
    The earliest experiment of that sort, I guess, should be Michelson-Morley, stating that though sources are moving with the earth, speed of light doesn't change.
  8. Jul 5, 2011 #7


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    But in the MM experiment, the source and the receiver are at rest relative to one another.
  9. Jul 6, 2011 #8
    That's the DeSitter-double-star-experiment, which rule out the emission (or ballistic) theory of light, in which the speed of light depends on the velocity of the source, and in which light acts like a cannon ball:


    While the Michelson-Morley experiment is compatible with Emission theories, there are many other experiments refuting this model, for example the Sagnac effect. It shows, that the speed of light is independent of the motion of the Sagnac interferometer, that is, while the source/receiver is moving, the light path becomes longer for one beam, and shorter for the counter-propagating beam. According to emission theory, this effect shouldn't exist.

    So all of those experiments confirm special relativity, and refute emission theories. See also

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