Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is this idea on addition of velocities in SR wrong?

  1. Dec 15, 2011 #1
    I like to try and derive little things by myself if I think it is manageable. One such thing is the addition of velocities. I gave it a stab, got an answer, and it turned out to be wrong. So tell me where my logic messes up.

    There's a spaceship moving at velocity u1. A man in the spaceship throws a ball, which from his POV is thrown at velocity u2 in the same direction of the ship. Let's look at the man in the spaceship's POV:
    In 1 unit of time, the ball will move u2 units of distance.

    Now, from a man outside of the spaceship, any of distance d0 in the spaceship is observed to have a distance of d0√(1-u12/c2)

    Also, any event that takes a time of t0 in the spaceship is observed take the time of t0/√(1-u12/c2)

    So, in the space ship the ball travels d0/t0

    But outside the spaceship, I would think that it would take d0/t0 * (1-u12/c2)

    But that's not it... What's wrong with my reasoning?
    Last edited: Dec 15, 2011
  2. jcsd
  3. Dec 15, 2011 #2


    User Avatar

    Staff: Mentor

    I suspect it's because you're not taking into account the third member of the Holy Trinity of relativity (besides length contraction and time dilation): relativity of simultaneity.

    I haven't worked out this particular example, but in my experience, almost all apparent paradoxes and inconsistencies that newcomers to SR come up with, turn out to boil down to this.
  4. Dec 15, 2011 #3
    You might be right, but looking at the example, I don't see how that would be it. There's only one event occurring in one place.
  5. Dec 15, 2011 #4


    User Avatar
    Science Advisor
    Gold Member

    Many things.

    1) You define u1 and u2, but use v in formulas, without definition.

    2) You try to scale the relative speed of the ball and spaceship, to the outside observer. But this (if it worked as described ... it doesn't) scaled relative speed doesn't factor in u1. Doesn't it seem like its needed? Conceptually closer to the mark (but still wrong) would by u1 + u2 * (scale factor from u1).

    3) You don't consider relativity of simultaneity. When you make a statement: at t0, the ball is d0 from the passenger (according to the passenger), you are also implying the statement that passenger's clock reading t0 is simultaneous with the event of ball being d0 away from passenger (all according to passenger). However you ignore that these events are not simultaneous according to the outside observer. Conversely, a pair of simultaneous events for the ball and passenger according to the outside observer, will be non-simultaneous event for the passenger.

    [edit: you corrected (1) since I wrote this. Good. The other two comments still apply.]
  6. Dec 15, 2011 #5
    Hmm.... That's a good point. This seems to be trickier than I thought.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook