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Speed of light

  1. Apr 18, 2009 #1
    if, a particular body rotates ,with a certain velocity
    then every particle of that moves with the same velocity as that of body.if
    the earth is moving at a velocity,we also move with same velocity daily .
    for eg:the speed of light was discovered to be 3x10^8m/s wrt to earth,but is it the same everwhere,including outer space??
     
  2. jcsd
  3. Apr 18, 2009 #2
    My understanding is that the speed of light is the same everywhere and has been the same since the beginning. There is some speculation that the speed of light may have been different in the past but no observational evidence exists.
     
  4. Apr 18, 2009 #3
    well but when speed is calculated ,sitting on earth it varies,right,relative velocity!!!
    but in space it is different
     
  5. Apr 18, 2009 #4
    I accept the speed will differ in space. because the speed of the light is not same when it travels in different mediums so this may differ in space as well.
     
  6. Apr 18, 2009 #5

    Janus

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    The speed of light in a vacuum is invariant. It is measured to be the same by everyone regardless of their relative motion. For example, a beam of light traveling past the Earth will have a measured relative velocity of 3e8 m/s wrt to the Earth as measured by the Earth, but an observer flying past with a speed of 1.5e8 m/s relative to the Earth will measure that same beam of light as having a relative velocity of 3e8 m/s wrt to himself
     
  7. Apr 18, 2009 #6
    The historical measurements of the speed of light indicate that it dropped by about 100 kilometers per second from 1880 to 1980. See http://www.sigma-engineering.co.uk/light/lightindex.shtml
    This drop is nearly within experimental error, and is probably due to experimenters fudging their numbers until partial agreement with other measurements was obtained.
     
    Last edited by a moderator: Apr 24, 2017
  8. Apr 18, 2009 #7
    Light always travels at a constant speed of a bought 2.99*10[tex]^{8}[/tex] m/s.

    The only reason that it would appear to travel slower when traveling through matter is because, photons excite the adjoining particles that in turn transfer the energy to the neighbor.

    The time it takes for the light to be absorbed and then transfer out again is what would make light seem slower than in a vacuum.
     
  9. Apr 18, 2009 #8
    This has to do with special relativity. You should google on the Michelson Morley experiment. It is an experiment in around 1900 I believe, where they basically tried to measure different speeds of light in different directions.
     
  10. Apr 19, 2009 #9

    Born2bwire

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    Agreed, the Michelson-Morley experiment is proof that the speed of light is invariant to the motion of the observer.
     
  11. Apr 24, 2009 #10
    how do you say michelson's experiment is a proof,it was conducted on earth,and was found to be 3x10^8. Do we have any other experiment whose value for speed of light
    coincided with michelson's.well,consider the light rays emitted from stars,in space
    ,they won't travel with speed-3x10^8,they would vary ,due to relative velocity.
     
  12. Apr 24, 2009 #11

    Hootenanny

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    Not true, the speed of light is absolute. In other words the speed of light is always c irrespective of the relative motion of the observer and the emitter (as Janus has already said).

    In terms of experimental verification, see here: http://math.ucr.edu/home/baez/physi...ts.html#3. Tests of Einstein's two postulates
     
    Last edited: Apr 24, 2009
  13. Apr 24, 2009 #12
    The only thing that varies is the frequency of the light (Doppler effect), which is dependent on the radial velocity of the stars with respect to earth. The frequency of the light decreases for receding objects (redshift) and increases for approaching objects (blueshift).
     
  14. Apr 24, 2009 #13
    so according to what you say,anywhere in the universe,any object
    acting as a light source would emit and will emit light at the speed of
    3x10^8m/s.
     
  15. Apr 24, 2009 #14

    Hootenanny

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    Yes. And that speed will be constant and have the same value as measured by any locally inertial observer.
     
  16. Apr 24, 2009 #15
    yes - this is exactly the weird thing about reality that made special relativity so hard for people to accept at the time (100 yrs ago now).

    also, aside from the MM experiment, the invariance of light speed (constant c) can be inferred from the Maxwell equations. I think that's what Einstein said he was doing (and the basis for his statements that he didn't hear about MM experiment until later).

    If you are interested in this (as you appear to be) get yourself a book on special relativity thats written for someone with your background, and have a go at it.
     
  17. Apr 24, 2009 #16

    Born2bwire

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    Michelson-Morley is not about proving that the speed of light is c, it proves that it is invariant to the frame of motion. It addresses your assertions in the OP.
     
  18. Apr 24, 2009 #17
    *First time poster*
    I'm not too clued up on things of this nature and so thought I'd ask a question.
    If extreme gravity can bend light then can it speed it up or slow it down?
     
  19. Apr 24, 2009 #18
    No, since the speed of light is constant according to special relativity.

    Welcome to the forums, by the way!
     
  20. Apr 24, 2009 #19
    The reason I ask is that if gravity can have an effect of the physical elements within light regarding its direction, then might it be possible for it to affect its speed in a straight line?
    I understand the idea of the speed of light being constant, though I can't help but wonder.
    Thanks for the welcome, and for the speedy reply.
     
  21. Apr 24, 2009 #20
    The basic idea of general relativity is that masses create curvature in space-time. So although the path of the light looks bent to us, it's actually following a straight path (geodesic) in space-time. So masses change space time, which affects the path of the photon that travels through it, but not the speed.

    I know it's odd, but that's why I like physics :)
     
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