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A quick question

  1. Dec 6, 2003 #1

    Poy

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    I think this is the right place to put this question
    Special reletivity says light always seems to move at c. But wouldn't it (relative to you) be moving faster than c if its moving away from you and you are moving away from it? I might misundestdn something here, but if someone could clearify for me, thanks.
     
  2. jcsd
  3. Dec 6, 2003 #2

    Doc Al

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    No. Light moves at speed c in any reference frame. No matter how fast you move, you'll measure any light passing by to have speed c. It's a feature of the "spacetime" structure of the universe.
     
  4. Dec 6, 2003 #3

    Poy

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    I know how that ussually works, but I wasn't sure in this one condition... thanks.
     
  5. Dec 7, 2003 #4

    turin

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    You may want to reconsider your conceptualization. SR says that light literally moves at c for every observer, not just appearing to do so.
     
  6. Dec 7, 2003 #5

    Integral

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    If you want to be nit picky it is Maxwell's Equations which state that the speed of light is constant to all observers. Einstein was able to state a constant c as a postulate, because it was derived as such by Maxwell in 1867. The Constancy of c was a major dilemma for Physicists in the last half of the 1800s.
     
  7. Dec 7, 2003 #6

    turin

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    I wasn't trying to be "nit picky." There is a lot of misconception about what SR says concerning the nature of space-time, i.e., "time dilation is a consequence of the delay of light due to its finite speed," being such a misconception. From the wording, I thought that Poy might have such a misconception. Sorry for being presumptuous.




    I'll be nit picky only to point out that a statement by a set of equations does not exclude a statement by SR.
     
  8. Dec 7, 2003 #7

    jcsd

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    There really should be a sticky thread about this as variations of this one seems to be the most oft-asked question:

    In special relativity velocities ([itex]u[/itex] and [itex]v[/itex])are summed:

    [tex]w = \frac{u + v}{1 + \frac{uv}{c^2}}[/tex]

    take the case where [itex]u = c[/itex]

    [tex]w = \frac{c + v}{1 + \frac{v}{c}} = \frac{c(c + v)}{c + v} = c[/tex]

    So the sum of the two velocites when one is equal to [itex]c[/itex] is always equal to [itex]c[/itex].
     
  9. Dec 7, 2003 #8

    Integral

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    There is a big difference between a derivation from basics and a postulate. Maxwell dervives, Einstein postulates. He was able to do that based on Maxwell's derivation.

    Special Relativity does not contain a reason for the constancy of c, it is the starting point. From there SR explores the results of a constant c.
     
  10. Dec 8, 2003 #9

    turin

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    So what? I didn't claim a derivation of any sort. SR says that the speed of light is literally the same in every inertial frame of reference. Do you disagree with this?

    I thought that Poy misunderstood this statement, based on his wording. I didn't want to make an issue of whether or not SR is true, and why.
     
    Last edited: Dec 8, 2003
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