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Constant speed of light

  1. Sep 2, 2009 #1
    I apologize if this has been covered but I did a quick search and didn't see anything.

    Alright, my question concerns the speed of light and why it's a constant. I was trying to explain special relativity to my friend and after I told him that the speed of light is always c no matter what reference frame you are in he seemed troubled by the idea. He gave me an example of how this could not be, here is what he said: "alright say light and I start at the same point and I'm going .8c along side it, you're meaning to tell me that light is still going c even though it should be going .2c?" I simply said yes, he asked why. I tried to say that the speed of light has to be constant so no reference frame was special and every reference frame in uniform motion is good for doing physics. He still went back to his example though, it's actually a pretty solid argument. I'm new to relativity so I'm having trouble explaining to him. Can someone help me explain this?
  2. jcsd
  3. Sep 2, 2009 #2


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  4. Sep 2, 2009 #3
    Thanks for that link, its very useful. Though his problem lies in the concept. The problem is that it is hard conceptually to come to terms with the fact that if you and light both start at the same time and you are moving at half the speed of light that light is still moving at point c. I truly do not know how to answer his question. Is what I already told him the correct answer? Is there a better way to put it?
  5. Sep 2, 2009 #4


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    Well, it really doesn't make much sense to say you're traveling "along side" a beam of light.

    If you and light truly start at the same point, I'm assuming you emit a photon from, say a flash light you're holding in your hand at a given time. Now, you may be moving at .8c with respect to a nearby planet or something (some outside reference frame) but surely your friend will agree that in YOUR frame, where you are motionless, the photon travels away at c. If he's concerned about the other frames, see jtbell's post.
  6. Sep 2, 2009 #5
    There is really no "why" to the constancy of c; it is an observed phenomenon and axiomatic (an assumed fact) in Relativity. All this other stuff is the set of consequences of that fact. Because c is constant, different observers' clocks and meter sticks end up measuring completely different things (different observers disagree on what interval a "second" of time or a "meter" of distance represent), and the differences in these clocks and meter sticks conspire to keep c constant.
  7. Sep 3, 2009 #6
    I thought I was explaining it right. Thank you guys for all of your help. He's having trouble because hes still assuming something has a "real" motion. He thinks he is "really" moving at speed .8c. Ill keep trying to explain more of the theory and make him understand.
  8. Sep 3, 2009 #7


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    I like to blame Maxwell. This all started in 1867 when Clerk Maxwell cast the basic relationships of electromagnetism in the form of a wave equation. In the place of the velocity of the wave was the expression:

    [tex] \frac 1 { \sqrt { \epsilon_0 \mu_0 }} [/tex]

    Where [itex] \epsilon_0 [/itex] is the permittivity of free space and [itex] \mu_0 [/itex] is the permeability of free space. When Maxwell computed the value of this constant he was amazed to find a number equal to the measured value of the speed of light. This was the start of the "schism" in physics, which lasted until Einsteins work in 1905.

    Many felt that Maxwell had errored and assumed that the error would be found. Michelson and Morly did their famous experiment shearching for a variation in the speed of light corresponding to the variation in the earths velocity over the year. They were unable to find any
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