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Absolute speed of light conundrum

  1. May 19, 2008 #1
    I would greatly appreiciate some insight into this question to allow me to beter understand the nature of relative simultainety.

    A rocketship is traveling at v=.5c and passes a tree at the instant a lightning bolt strikes it.

    Now some time later (in referance to the rocket) say the light wave from the lightning has propegated distance L. As the rocket is traveling at speed .5c will it be at a distance .5L???
    This was my first intuitive answer but then i remembered that the speed of light is absolute regardless of which refereance frame you are in

    Now if this is true does this mean that at some time later (time t) the light wave is C times t meters infront of the spaceship and infact an external observer would not observe the rocket passing the tree and the lightning strike to be simultaneous??? In fact they would see the lightning strike the tree before the rocket got to it??

    This entire idea has got me perplexed just because i have so many different possibilities in my head and no intuitive instinct to base a true answer on.

    Any input will be greatly appreiciated.
  2. jcsd
  3. May 19, 2008 #2
    From the rocket's point of view, it is standing still, and the tree went past it at 0.5c when the lightning went off. From the rocket's frame, the tree will be at 0.5L whenever the light pulse is at L.

    From the tree's point of view, it is standing still, and the rocket went past it at 0.5c when the lightning went off. From the tree's frame, the rocket will be at 0.5L whenever the light pulse is at L.

    Your next question has to do with what I believe to be the #1 most important realization that someone learning relativity must make. Even my relativity textbook does not hammer this point well enough. Making this realization helped me personally go on to learning the basics of relativity. The only place I have found this important piece of information was actually on the wikibooks.org site:


    Put aside the effects of what you as an observer sees as a result of light taking a while to show you events. When we are calculating events in time and space we are calculating as they actually happen having already accounted for the effects of light's latency in showing us these events.

    As the page above says, crazy stuff is really happening, even after you take into account the fact that you see events after they happen as a result of light's limited speed.
    Last edited: May 19, 2008
  4. May 19, 2008 #3
    Thank you i didn't really get into the thought of the stuff moving past the rocket that helps alot. But do you mean that the tree will be .5L behind the rocket and the Light a whole L infront of the rocket or the rocket will still be half way between the tree and the front of the light wave at all times??
  5. May 19, 2008 #4
    Well to the rocket, the light will travel in all directions (forming a sphere) and the tree will always be at half the radius of the sphere being formed by the light emanating from the rocket (to the rocket, the rocket is standing still and light is emanating from it at the same speed in all directions).

    Also please note I was only talking about magnitude of distance in the preceding post, I assume you understand that the directions are of course opposite between the rocket and the tree.
    Last edited: May 19, 2008
  6. May 19, 2008 #5
    Thankyou for the help makes alot more sense now
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