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Light speed doubts

  1. Mar 28, 2012 #1
    Hi all

    One things that puzzles me is the speed of light. We measure it always the same speed independently if we are moving or still. But here is where i get really confused: if we are moving against a light ray (imagine that I will "bump" with the light ray), the speed that we measure is the same if we are moving in an oposite direction (light moves one direction while I move the oposite direction but moving away)?

    Second doubt: Is the slowing of satelites clocks in relation to the Earth clocks the proof (or at least on of the proofs) of the constant of the light speed? I don´t get it. Its to confusing to me.

    Thank you
  2. jcsd
  3. Mar 28, 2012 #2
    There is a huge, huge amount of experimental data supporting special and general relativity (which in this context can be used interchangably with the constancy of the speed of light). I could list some of them, but the FAQ in the Relativity Forum has this link, which details them all: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

    I would start there.
  4. Mar 28, 2012 #3
    Thank you

    I will read it very attentively
  5. Mar 28, 2012 #4
    Is there a special name for the suposed paradox that I proposed in this post: convergence vs divergence between observer moving and light moving (like two cars that are about to cross each other vs two cars that are moving away in the same straight line road)? Is this doubt pertinent? Am I being clear?

    Im asking in order to get the answer to this question faster.

    Thank you
  6. Mar 28, 2012 #5
    Could you be more specific, I don't really understand your question.
  7. Mar 29, 2012 #6


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    If two cars are moving away from you, one East with speed u, the other West with speed v, then each driver will observe the other moving with relative speed
    [tex]\frac{u+ v}{1+ \frac{uv}{c^2}}[/tex]

    Since the speeds of cars is very small compared to c, [itex]uv/c^2[/itex] is essentially 0 and so the speed is indistinguishable from u+ v. But it, instead of two cars, we are dealing with light beams going in opposite directions [itex]uv/c^2= (c)(c)/c^2= 1[/itex] so [itex]1+ uv/c^2= 2[/itex] and the speed of each is [itex](c+ c)/2= c[/itex].
  8. Mar 29, 2012 #7
    Try this overview:

    The general method for deriving special relativistic effects (as in the link above) is to assume that the speed of light is constant (which experiments have proven over and over again), and everything else comes into place. The key to understanding the, seeming paradoxical, results is that the speed of light is intrinsically connected to how we measure/observe both the passage of time, and the length of physical objects.

    Lets say you are (at rest) watching someone travelling at a large velocity in the same direction as a light-beam. You can measure the difference in velocities between the person and the light-beam, and the person can also try to measure the velocity of the light-beam relative to themselves. At low speeds (and using classical, newtonian physics) your results will would agree, but in special relativity (and in actuality) your results disagree. You can make sense of this, because the way the moving person measures time and distance are different from yours.
  9. Mar 29, 2012 #8
    There is one catch in the GPS time dilatation. The GPS clocks apply 38.6μs of time correction in the satellites clocks per day because of GR and SR time dilatation. The catch is that the correction is constant for all satellites, which move in different directions relative to earth rotation.
    But the 1971 Hafele & Keating plane experiment showed that the amount of time change depends greatly on the direction of movement relative to earth rotation.

    Secondly the GPS position accuracy depends mostly on clock synchronization between the satellites, not on synchronization between the earth time and satellite time. Most GPS receivers don't have built-in atomic clocks and achieve accuracy of few meters. They relay on time stamp information sent from the satellites.

    But the two way constancy of the speed of light is a proved thing (see Michelson-Morley experiment). The SR uses this fact, but beyond that the SR predicts some effects (like the twins paradox) that are contentious and cause many heated discussions.
  10. Mar 29, 2012 #9
    I don't know about being the "key" to understanding the "paradox" but it is one of the best explinations for layman like me regarding the invariance of c.
  11. Mar 29, 2012 #10


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    The twin paradox may cause some heated discussion by folks who do not yet understand it but it is not contentious at all once you understand it and certainly causes no consternation for physicists since it is a simple outcome of SR (or maybe it's GR ... I can never remember)
  12. Mar 30, 2012 #11
    The problem always seems to be believing that light is somehow like anything else in the universe when according to relativity it doesn't work the same as the rest of reality in most any way...
  13. Mar 30, 2012 #12
    Does it help if you consider the observer as always being in the present and moving at light speed and yet the observation or measurement as always being in the observers past
  14. Mar 30, 2012 #13
    The way I understand it - The speed of light does not change because the distances change, as you approach speed of light, as an effect of your clock rate changing.

    Imagine this scenario. Aplha Centauri is ~4 ly away from you when you are standing still. It may become only 1 ly away if you start traveling at some small fraction of the speed of light (Edit:) towards it. The space contracts in front of you, and it affects the measurement of light speed.

    If this was not the case, you could instantly speed up to near speed of light, shoot a bullet near speed of light that would reach to Alpha Centauri in less that 4 years. That is of course not not the case. You see alpha centaury come closer to you as you accelerate and you normally shoot the bullet at it, but an observer sees you shoot a bullet in slow motion and the bullet still reaches the star in more than 4 years.
    Last edited: Mar 30, 2012
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