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Weird thought.

  1. Jul 30, 2010 #1
    Hello, I just had a weird thought and would like some clarification. Keep in mind I'm only 15 years old, and I have had NO courses in physics. I'm VERY ignorant and would like an explanation.

    If light is constant, then how do we know the true speed of light? If Earth moves at x speed this way ----> and someone uses a flashlight this way <----, won't it seem that light is traveling faster than it really is? The same could be said about our solar system/galaxy/universe if they do in fact travel. This led me to believe that light does NOT have constant velocity to certain variables. I believe the variable is gravity, or something that is affected by gravity which affects light.

    Is this info old? I would just like some answers please :confused:
  2. jcsd
  3. Jul 30, 2010 #2


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    Actually no. The measured speed of light is independent of how fast the light source is moving relative to the measuring device. I suggest you take a moment to think about how counterintuitive this is. If a train is moving at 40 m/s this way → and someone on the train is walking 1 m/s this way ←, the speed of the walking person relative to the ground should be 39 m/s, right? But if there's a light source on the train, the light it emits is always moving at 299792458 m/s relative to the ground, no matter how fast the train is going.

    This seemingly bizarre claim has been verified by experiments. Special relativity is the theory of space, time and motion that incorporates this result in a natural way. According to SR, if the train is moving with velocity u relative to the ground, and something on the train is moving with velocity v relative to the train, the velocity of that "thing" relative to the ground is not u+v, but (u+v)/(1+uv/c2).

    I suggest you try playing around with that formula a bit. What do you get when you plug in the numbers from my example? What do you get when v=c?

    By the way, "invariant" is a more appropriate term than "constant". The speed of light is constant too, but that only means that it doesn't change with time. What's relevant here is that it's invariant in the sense that it has the same value in all inertial frames (i.e. the coordinate systems we associate with physical observers that do not accelerate).
    Last edited: Jul 30, 2010
  4. Jul 30, 2010 #3
    I didn't mean relative to the ground, I meant relative to the train.

    One person walks on a train. Relative to the train, the speed of the train will not affect the speed of the person. But because light is invariant, the faster the train goes the faster light will seem to go relative to the train if the light is shining in the opposite direction of the train.

    Thank you for your help :).
  5. Jul 30, 2010 #4
    Amazingly, both the person on the ground and the person on the train will measure the speed of light to be the same.
  6. Jul 30, 2010 #5
    Wow... Thank you for these answers everyone. I appreciate them all and I am satisfied.
  7. Jul 31, 2010 #6
    I sincerely hope that you are not ar all satisfied . It was this very question and the unendurable paradox and logical riddle that it presents that led me to the study of SR in the first place.
    If you want to understand it and undertake that study, this is just the first of many weird thoughts to be encountered. All requiring the suspension of intuitive assumptions and the effort to learn the principles that resolve them.
    Personally I found the effort well rewarded and think that this would apply to anyone who wants to understand the real world and how it works.
    Good luck
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