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Light speed - relativity

  1. Sep 27, 2013 #1
    ok, light isn't relative, or is it?

    Its said that galaxy far across the universe appear to be moving away from us at beyond light speed; as since the universe is expanding all around us.

    So, does that mean light within these distant galaxies moves around relative to within its own local space/time?

    Also, since our system is traveling around our galaxy at 30km/s, and our galaxy is apparently moving through space at 600km/s, wouldnt light on earth be slightly red shifted in certain directions?
  2. jcsd
  3. Sep 27, 2013 #2


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    Gold Member

    If I understand correctly, the answer is 'yes'. Light always moves at c Km/s at the place it is - i.e. locally.

    Light from Earth, viewed from elsewhere could be red or blue shifted, depending on the relative motions. Also, light arriving at Earth could be frequency shifted.
  4. Sep 27, 2013 #3


    Staff: Mentor

    As Mentz114 said, the speed of light locally is always c.

    Yes. This has been observed:

  5. Oct 2, 2013 #4
    I also think that the red shifting and blue shifting occurs because spacetime itself is expanding. c is always the same, but when spacetime "stretches", the lightwave stretches, which is what causes the red/blue shift.

    If I understand correctly, you wouldn't notice this if viewing something on earth while also standing on earth as the "stretching" wouldn't be big enough to cause any noticeable effect.
  6. Oct 2, 2013 #5
    No, the Earth isn't expanding eventhough the Universe is.
  7. Oct 2, 2013 #6
    Got it. Matter doesn't expand, but the space-time fabric of the Universe in which matter "sits" does, so no red shifting between two locations on Earth.
  8. Oct 2, 2013 #7
    Is light speed relative to our earth? if not, and since the earth is moving, that would mean it would appear that light on earth travels faster in one direction compared to another.

    I know GPS times have to be slightly adjusted as to account for time/speed difference, but are they also offset to allow for the moving of earth through space?

    I suppose what I'm asking, what space time fabric is light relative to?
  9. Oct 3, 2013 #8
    Light moves at the same speed, relative to everyone. It does not travel faster in one direction than the other, regardless of the movement of earth or anything else.

    Light in a galaxy far across the universe travels at the same speed there, as it does when it reaches us.

    The distance between that galaxy and earth might increase at a rate that is faster than light, but that is because the Universe is expanding.

    The speed of light does not change. It always travels at c relative to everyone everywhere.
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