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Speed of light is set to be 299,792,458 m/s

  1. Oct 8, 2009 #1
    I have a question...
    The speed of light is set to be 299,792,458 m/s
    but if time slows as you approach the speed of light,
    how can we tell how fast Light itself is actually traveling?
  2. jcsd
  3. Oct 8, 2009 #2


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    Gold Member

    Re: Light

    As you approach c relative to some external reference frame, you do not experience a time dilation, you experience time passing normally. But the observers in that external reference frame experience your time as increasingly attenuated.

    All observers - on the spaceship and off it regardless of their speed with respect to each other - will measure the speed of light as 299,792,458 m/s.

    There are many searchable posts on this forum that go into it in more detail.
  4. Oct 8, 2009 #3
    Re: Light

    Length also contracts when approaching the speed of light. Relativistically speaking, the speed of light is constant in all reference frames; measurement of it shouldn't change from one frame to another.
  5. Oct 8, 2009 #4
    Re: Light

    yeah, but the are all external references...
    if time starts to slow down around you, then a second becomes elongated...
    to people around you the second might remain a second, but if to you the second becomes, lets say an hour... how fast are you moving per second.

    does this make sense?
  6. Oct 8, 2009 #5


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    Staff: Mentor

    Re: Light


    The first postulate of Special Relativity is that the laws of the universe are the same for any inertial (non accelerating) observer. What this means here is that there is no experiment you can perform to distinguish an absolute motion. You may be stationary with respect to one outside observer, moving at 1,000 mph with respect to another and moving at 10,000 mph with respect to another. And all 3 of these observers (and you) can consider themself stationary and measure the speed of light to be C in their frame.

    Don't say "if time starts to slow down around you" - that's nonsensical. You never see any change in your clock, but your clock runs at one rate for you, another rate with respect to the clock of that guy moving at 1,000 mph and yet another rate wrt the guy moving at 10,000 mph. Time isn't variable, it is relative.
  7. Oct 8, 2009 #6
    Re: Light

    Briliant, thanks all.
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