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

A couple of simple questions I'm boggled by

  1. May 5, 2013 #1
    A couple of simple questions I'm boggled by....

    Suppose two people, each in his own craft, set apart by significant distance and stationary to each other. Both crafts accelerate to 1/2 light speed from their origins directly towards each other. This results in a relative speed between the two craft of light speed. Now lets say both crafts have a light, one orange and the other blue so they can discern between each, and once they have reached 1/2 they both turn on their lights, both pointing towards the other craft.
    It is obvious that from both crafts the light the light must travel forward away at speed c towards the other craft. But from the other craft's perspective the light would be moving towards him at the same speed as the craft emitting the light resulting in neither seeing each other's light at all. From each craft's perspective though, their light will travel forward at speed c towards the other craft.

    Questions are, does the light from each craft reach the other before the two crafts collide head on to each other? How is it possible that each craft's pilot can see their light projecting forward through space when turned on but at the same time the other craft's pilot cannot see the light until the point of collision?

    Another hypothetical: Lets put Jim into a particle accelerator. He is wearing a nice watch. You accelerate him to a whisper of light speed in this accelerator and each time he makes the 186,000 mile loop you see how much time has passed on his watch. On your watch he's whizzing by once a second but his watch ticks none. How is it possible that he is traversing distance at such speed but yet no time ticks by on his watch?

    Something has to give here but I can't figure out how to resolve these questions.
  2. jcsd
  3. May 5, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    No. Each will see the other as moving at 0.8c, not at light speed. Look up the relativistic addition of velocity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook