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Speed of light constant throughout universe?

  1. Mar 16, 2009 #1


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    Hello all,

    I have just discovered (and joined) the Physics Forums, and want to ask a question which I have been wondering about for a long time (please forgive me if this has been covered here before):

    How do we know that the speed of light is constant throughout the universe if we have not been beyond the edge of the solar system yet (I don't know where the Pioneer probes are nowadays, so that's why I say that)?

    I can see how one would base a result from experiments between the Earth, and say the Moon, Mars, etc... but how do we know how light behaves beyond the solar system? In the event (only for the sake of discussion) that light behaves altogether differently in other regions of space (I couldn't even begin to hazard a guess why), wouldn't that throw off calculations of distances and possibly the age of the universe, by some degree?

    If this has been discussed elsewhere, can someone please post references?

    Thanks for your time (no pun intended),

  2. jcsd
  3. Mar 16, 2009 #2


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    That would throw off virtually every bit of astrophysics. The fact that our physical extrapolations work very well wherever we look validates that extrapolation.
  4. Mar 16, 2009 #3


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    I agree with you on priniciple, but I guess what I am looking for, among other things, is a 'what are we using for a standard' type of avenue.

    To be more precise, how distant have the extrapolations been applied, and held up? If the speed of light cannot be 'tested', say for example, from the nearest star outside the solar system, wouldn't the tests be valid only for the distances that we have so far physically reached?

    Thanks for your reply....

  5. Mar 16, 2009 #4
    I agree that we have not measured the speed of light outside solar system. One way to look a it is the equivalence or symmetry of the usniverse, that is we believe the every spot of the universe should be the same in physical essence. For example, from earth we believe every direction is equivalent, north direction south direction etc.
    In universe expansion, we think every spot is equivalent. Every spot can be considered at rest roughly, but only the relative sidstance is increasing. It could be one way to accept of 'c'.

    We will continue to believe it until an obvious confilicting evidence comes up.
  6. Mar 16, 2009 #5


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    The most distant quasars are use to measure deviations of physical constants, especially the fine structure constant.
  7. Mar 16, 2009 #6


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    v2kkim, Ich,

    Thank you for the replies. I find this material absolutely fascinating.

    So, if you'll allow, let's dig a little further.

    Say I am in a spacecraft that can travel at 1x the speed of light, and it takes me 7 days (just for the sake of discussion) to get to the nearest star. I can now derive the distance to this particular star based on 1x speed of light x elapsed time, because I physically arrived there. I 'created' an origin and an endpoint for my journey.

    What I am having trouble resolving is how we are able to measure the distance to a distant object, if we have never been to that same distant object to justify the calculation.

    I am almost certain I am missing something that would make it all fit into place, but I don't know what that piece is.


  8. Mar 16, 2009 #7
    So what your asking is how do we know things are as far away as we think they are?
  9. Mar 16, 2009 #8


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    Simply put, yes.
  10. Mar 16, 2009 #9
    Your question is more related to SciFi like Star Trek, my favorite, where they know about their travel and location very well. Ideally we can record our velocity continuously as we change speed and direction, in that way we can have our velocity log and a hand carried clock, so we can calculate our location, but as time goes on the error might accumulate so calibration of the speed log should be needed. Also some known stars or galaxies might be a guidance. This method will apply for a short distance travel, in case you travel really far away then your past travel log book need to be update considering universe expansion. Here I did not include the relativistic effect of length contraction.
    Last edited: Mar 16, 2009
  11. Mar 17, 2009 #10
    we can tell how far away stars are by using what they call a standard candle
    if we know how bright it is we can know how far away it is . And they use a spectrometer to get the Spectral lines so they can figure out what element is making up that light. This is how helium was discovered they looked at the sun and saw spectral lines that they had never seen before. and i think photons dim proportional to the inverse square law.
  12. Mar 17, 2009 #11


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    http://en.wikipedia.org/wiki/Cosmic_distance_ladder" [Broken]
    Last edited by a moderator: May 4, 2017
  13. Mar 17, 2009 #12

    Vanadium 50

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    I disagree with that. The speed of light is essentially the ratio of the strength of magnetism to electricity. If you alter the speed of light, you alter (among other things) the energy levels of atoms, but we know through spectra that distant atoms have the same energy levels as terrestrial atoms.
  14. Mar 17, 2009 #13
    how do we know that those distant atoms are atoms we have have on our periodic table?
  15. Mar 17, 2009 #14


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    They are the same because they have the same energy levels.
  16. Mar 17, 2009 #15
    what if it is dark energy emitting the same energy level but different composition then what we think it is.
  17. Mar 17, 2009 #16


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    Some of those elements on our periodic table were first discovered outside of our planet. Look up where helium got its name.

    If it's emitting energy, then it's hardly 'dark'.

    If you want to believe stuff outside of earth, or outside of our solar system, is somehow fundamentally different than the matter we know of, you'll have to come up with a very good reason for it. Because so far this assumption has proven itself valid as many times as we've had opportunities to check it.

    Unprovable 'what ifs' only lead to the old Cartesian doubt formula. "What if the entire world is just a dream?". That'll get you absolutely nowhere as far as Science is concerned.
  18. Mar 17, 2009 #17
    k if u would have read above i already said how helium was discovered. and u think all energy is in the visible spectrum what about gama rays , x-rays, how about infared i think we get the point, ok i give this one 2 u , ur right to the best of our knowledge
    it is matter that we know of .
  19. Mar 18, 2009 #18


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    We do not know what energy levels have distant atoms - we know only that proportions are the same. For example doppler effect shifts apparent spectral lines and the same does gravity.
    Even more from relativity principles one can conclude that change in speed of light is locally undetectable.
  20. Mar 18, 2009 #19
    Can you determine if the speed of light was always the same.

    Could it have been faster or slower in the past?
  21. Mar 18, 2009 #20
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