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Flash Light at the Speed of Light Question

  1. Feb 10, 2009 #1
    Some teens asked me a "what if" question here it is

    Lets say a person is travel at the speed of light of holding a flashlight behind him could he see the light coming from the flashlight?

    My logical answer was no because the person would be in front of light

    Where as my friend who knows a little more about astrophysics than I disagrees with me saying something about relativity
  2. jcsd
  3. Feb 10, 2009 #2


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  4. Feb 10, 2009 #3


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    These are the kinds of thought experiments which led Einstein to develop special relativity, which includes the fact that no one can go at the speed of light.
  5. Feb 10, 2009 #4
    This is more of a special relativity question than a quantum physics question. Since you can't really move at the speed of light, let's just say you're moving at almost the speed of light -- relative to the earth you're moving somewhere between 99% and 100% of the speed of light. Now, if you turn that flashlight on, the beam of light will catch up to you and pass you just as fast as it would if you were not moving at all (relative to the earth). No matter how fast you move, the light will always appear to pass you at the same speed. If you think about this a little bit you'll probably start thinking that there must be some contractions, but it all turns out to be self consistent once you include some other effects called "time dilation" and "length contraction." Look for the aforementioned words on wikipedia for more information.
  6. Feb 10, 2009 #5


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    We are traveling at very nearly the speed of light with respect to distant parts of the universe. Turn on a flash light what do you see? That is exactly what you will always see no matter what you use as a reference point for your speed.
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