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Homework Help: Help on special relativity

  1. Aug 7, 2007 #1
    Ok, let's imagine an observer on the ground, he witnesses a large spacecraft flying pass him relatively at 3 x 10^5m/s.

    Now let's imagine he saw through a large window, an alien shining a torch across the window in the spacecraft.

    By Einstein's second postulate on special relativity, we expect that the observer will see the light moving across the spacecraft at c.

    By considering the lorentz's factor, the spacecraft now appears to be travelling at 299999.7m/s to my observer.

    With respect to the observer, the alien still sees his light beam travelling at c. But to the observer, although the spacecraft appears to be travelling at a slower velocity than its actual velocity, it still doesn't account for the discrepancy between the 2 time frames! Someone please help me on understanding this concept!
  2. jcsd
  3. Aug 7, 2007 #2
    How, exactly? You just said the spacecraft's velocity with respect to the observer was 300,000 m/s. And relative velocities don't change like that unless acceleration is involved.

    Both observer and alien (in spacecraft) see the light travelling at c.
  4. Aug 7, 2007 #3
    well, i factored in the length contraction and the time dilation, is there something wrong with it?
  5. Aug 7, 2007 #4
    What exactly is the question? Relative velocity doesn't change between inertial frames. (Ground observer sees spacecraft move with speed v to the right, alien sees ground observer move to its right at speed v)

    I'm not sure where/why you applied length contraction and time-dilation. If it's a text book question, post it as it is in the book. If it's your own, then please describe it more clearly.
  6. Aug 7, 2007 #5
    dont time dilation and length contraction apply to moving objects?
  7. Aug 7, 2007 #6
    Yes, they do. But I don't see how they would affect relative velocity.
  8. Aug 7, 2007 #7
    oh... so if time dilation and length contraction apply to moving objects, is there no frame of reference that would be able to observe this phenomenon?
  9. Aug 7, 2007 #8


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    Before we can go anywhere with this, you need to properly define the event(s) in question. If this is from a book, please quote the book verbatim. You are making too many vague, unspecific statements to have a meaningful discussion.
  10. Aug 7, 2007 #9
    i thought up this question because i simply dont understand special relativity...
    Ive read texts but they dont seem to provide a satisfactory understanding of the theory to me.

    I was dreaming about how an observer would see that the light travelling in a spacecraft would be a constant as that outside the spacecraft when the spacecraft is moving at a high speed for the effects of time dilation to be of any significance. Even yet, he would still see the spacecraft moving, so is it not true that the spacecraft should be totally still so as to view the speed of light as a constant? What is wrong with my way of thought!! I seriously need some help over here sorry if i sound confusing.
  11. Aug 9, 2007 #10
    any help please?
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