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Do I arrive before a light-speed train if I jump off the front?

  1. Jul 7, 2013 #1
    I'm having a lot of trouble wrapping my mind around this... It seems there are some similar questions in this forum, but I can't tell if the answers can apply to my question... I'm sorry if this is an extra post:

    I am traveling in a vacuum, on a train, at the speed of light, from planet A to planet B. I go to the front of the train, get outside, and jump forward/propel myself toward planet B. Do I arrive at planet B first?

    I understand, that I am not going faster than the speed of light relative to the train, but relative to an observer on planet B, who arrives first? If I arrive first, does that mean I traveled from planet A to planet B faster than light?

    Thanks in advance.

  2. jcsd
  3. Jul 7, 2013 #2


    User Avatar

    If the train is travelling near the speed of light, then yes, you get there first. (Relative to the planets you'd be travelling faster than the train, but still less than the speed of light no matter how hard you jumped.)

    If the train is travelling at the speed of light, then you're describing something that's fundamentally impossible and nonsensical in the context of relativity.
  4. Jul 7, 2013 #3


    Staff: Mentor

    You cannot go on a train at the speed of light.

    Let's say you were on a train at c - 1 m/s relative to earth and you jumped off the train at 10 m/s relative to the train. Then you would arrive before the train, but after light from your jump. According to the relativistic velocity addition formula you would be travelling at c - 0.99999993 m/s relative to earth.

    EDIT: like wle said.
  5. Jul 8, 2013 #4
    No you will not. Speed of light is a barrier. If you somehow increase your kinetic energy, your mass will increase instead of your velocity. So even if you jump off from the train travelling at speed of light, light will surely reach planet B before you because of your increase in moment of inertia due to increase in your mass.
  6. Jul 8, 2013 #5
    Thank you all for the quick responses! Really appreciated. I understand it's nonsensical to think that you can be in a train in space traveling at the speed of light, but wasn't really sure of a better way to explain the answer I was looking for. Thanks for looking past that and providing great answers.
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