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

FTL travel in a light cone?

  1. Jan 10, 2005 #1
    FTL travel in a light cone???

    I read in several places there there is one condition in GR, that will allow you to travel faster than light, and allow time travel to the past. Can someone explain this further, without getting too mathamatical (Ok, maybe a little math).

    Thanks in advance :smile:
    Last edited: Jan 10, 2005
  2. jcsd
  3. Jan 10, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    I think you are referring to one of two outcomes of GR. First it is not possible for a massive body to reach light speed measured locally, i.e. relative to local frames of reference.

    Note: Depending on how you measure distance and time at events far from home it is possible to both have and observe distant bodies that are receding faster than light because of cosmological expansion, but this is not what you are talking about.

    However there are at least two occasions when GR suggests time travel into the past is possible.

    The first is when solutions are found with closed world-lines. i.e. like in "Groundhog Day" you keep revisiting the same place and time that you were at before. These solutions generally require some pathological combination of multiple spinning black holes and a spaceship doing a figure of eight around them or so - I wouldn't like to try it - apart from the inherent danger I would not want to be stuck in a closed time loop!

    The second involves an Einstein-Rosen Bridge, a worm-hole or tube, connecting the annular singularities at the centres of two spinning black holes. You go in one and pop out the other - at least that’s the idea! You may well pop out within your own past light cone and therefore able to revisit yourself before you went in! Again I wouldn't like to try it - there's no guarantee the theory, having been extrapolated to extreme and untested regimes is correct – and you could pop out anywhere even in a different universe!

    I hope this helps.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook