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Cool thought experiment.

  1. Jan 3, 2010 #1
    I'm still really confused about the speed of light. I'm kinda new at this so bear with me. Does the speed of light depend on the frame of reference it is in? For example lets say you have a ball moving at 10m/s in a car moving at 20m/s. This would mean that from the earth's frame of reference the ball is moving 30m/s.

    But what if you had a car moving half the speed of light. And a man in the car fired a light gun traveling at the maximum speed of light. What would the speed of the light from the gun be from the earth's frame of reference.

    Wouldn't it be 1.5 the speed of light? Why or why not is this not possible?
  2. jcsd
  3. Jan 3, 2010 #2


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    Welcome to PF.

    It doesn't. Actually, that is one of the axioms that the whole of special relativity follows from, and it is experimentally verified to great accuracy. That is, in two "equivalent" frames (moving with constant velocity with respect to the other) the speed of light always has some fixed value, c.

    Now, when you perform your thought experiment, all kinds of weird stuff follows (like, when you use the light gun in your moving car to construct a clock, it will tick slower to a spectator along the track).
  4. Jan 3, 2010 #3
    hmmm so does this mean that the light fired from the gun will move have a slower time interval once it reaches maximum speed observed from the earth?
  5. Jan 3, 2010 #4


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    Not usre what you mean by a "slower time interval," but a person travelling at enar lightspeed will age more slowly, yes.
  6. Jan 4, 2010 #5


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    Also note it doesn't reach its maximum speed. It just always travels at that speed, light is not something that needs to be or even can be "accelerated from rest".

    May I ask how your mathematical knowledge is? E.g. what is your level of algebra? Because I have a document that allows you to figure out some of the weirdnesses, starting from basic postulates for yourself. I'm not sure where you are from, but in American terms end-of-highschool (~ 16 - 18 years of age) math definitely suffices (as long as you can substitute and simplify fractions, and use a function prescription to sketch a graph and derive basic properties from it.).
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