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Is Light Constant in GR

  1. Apr 28, 2009 #1
    I've had a debate with someone recently about whether or not light is constant in GR. I think that it is. Is there any debate on this point?
     
  2. jcsd
  3. Apr 28, 2009 #2
    I assume you are asking if the speed of light is constant. Yes it is.

    But it also can be viewed as accelerating: that is, it changes direction in a gravitational field, but locally it's speed remains "c".

    Also, the frequency/wavelength of light varies: as light climbs out of a gravitational potential, say from a star towards earth, it loses energy and is consequently red shifted....
     
  4. Apr 28, 2009 #3
    How can it be constant and viewed as variant?
     
  5. Apr 28, 2009 #4

    DaveC426913

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    Speed (scalar) is constant.
    Velocity (vector) is variable.
     
  6. Apr 28, 2009 #5
    No, actually it all depends on how the speed of light is measured.
     
  7. Apr 28, 2009 #6
    What's the difference between speed and velocity?
     
  8. Apr 28, 2009 #7

    DaveC426913

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    http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/1DKin/U1L1d.html" [Broken] describes it pretty well.
     
    Last edited by a moderator: May 4, 2017
  9. Apr 28, 2009 #8

    HallsofIvy

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    Just what DaveC426913 said:
    "Velocity" is a vector. "Speed" is the norm of the velocity vector.

    A car driving east at 50 mph and a car driving north at 60 mph have different velocities but the same speed.
     
  10. Apr 28, 2009 #9
    The speed of light being constant to all observers is one of the fundamental aspects of special relativity, so it can't be different in general relativity, as it arose from special relativity.
     
  11. Apr 28, 2009 #10

    O.K.

    Then it is absolutely correct to say that the speed of light is constant in GR?
     
    Last edited by a moderator: May 4, 2017
  12. Apr 28, 2009 #11

    djeitnstine

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    Yes the speed of light for all observers is constant.
     
  13. Apr 28, 2009 #12

    DrGreg

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    Actually the whole truth is not quite so simple as that. It depends how you measure speed.

    If you are falling freely and you use your own clock and ruler to measure the speed of some light that is near you, then yes you will always get the same answer, no matter where you are or how quickly you are falling. But if you are not falling freely (i.e. you are undergoing proper acceleration) or if you try to measure the speed of some light that is some distance away from you, you might get a different answer.
     
  14. Apr 28, 2009 #13

    But I thought that if light is not constant, then relativity must be wrong.
     
  15. Apr 28, 2009 #14

    DrGreg

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    Even in Special Relativity, it is only inertial observers who measure a constant speed of light. Accelerating observers do not.

    In General Relativity, gravitational tidal effects mean that someone who is an inertial observer of nearby events cannot also be an inertial observer of distant events.
     
  16. Apr 28, 2009 #15
    Would you like to try that again? :confused:
     
  17. Apr 28, 2009 #16

    DaveC426913

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    :biggrin:
     
  18. Apr 28, 2009 #17
  19. Apr 28, 2009 #18

    HallsofIvy

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    Oh, blast! Always a typo to mess things up! I meant to say that a car moving east at 50 mph and a car moving north at 50 mph have the same speed but different velocities!
     
  20. Apr 28, 2009 #19
    I figured as much but I just couldn't stand to let it go :biggrin:
     
  21. Apr 28, 2009 #20

    atyy

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    In special relativity, spacetime is flat and the speed of light is constant.

    In general relativity, spacetime is globally curved, but local regions of spacetime are approximately flat - just like the earth is round, but a local region of the earth like Kansas is approximately flat. Within every local, approximately flat region of globally curved spacetime, the speed of light is constant. If one measures the speed of light over globally curved spacetime, then its speed will not be constant (actually there isn't even a standard way to measure the speed of light globally over curved spacetime, so one has to define that first, whereas to measure the speed of light in local approximately flat bits of spacetime, one just takes over the definitions from special relativity.)
     
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