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Closed timelike curves

  1. May 17, 2009 #1
    im trying to understand the theory of this. is a CTC supposed to actually bring an object back to the original time? or is it supposed to make it appear that way to an outside observer? im reading up on it from wikipedia: http://en.wikipedia.org/wiki/Closed_timelike_curve
    but in the beginning they talk about it being a possible way of timetravel. then they say it "appears" to travel through time.

    but then once again, the next paragraph talks about multiple light cones curving all the way around and back to the original position and time:

    the last paragraph appears to claim that an object can physically go back in time right? so then what exactly is the other paragraph talking about saying "the object appears to travel through time as seen externally"?
    somehow 1 light cone only makes it appear like time-travel but a loop of light cones makes it physically possible?
  2. jcsd
  3. May 17, 2009 #2
    Think of stepping into a wormhole that transports you to the past. And later, on your deathbed, you realise that you are your own great grandparent. That's what CTCs may depict: fully self-consistent histories (smacking of determinism) involving time travel (from the perspective of an outside observer, in this case meaning anybody that did not step into the wormhole; from your point of view, of course, time is always progressing forward: even as you fell down the wormhole, the hands of your fob watch kept on ticking forward at the normal rate).
  4. May 17, 2009 #3
    o ok. that makes sense, i was just a little confused with the way they worded it.

    my other question is with the last paragraph. it talks about "a series of light cones". if a light cone is all the different possible worldlines for a person/object then how can there be a series? wouldnt a person's worldline end somewhere within the first light cone?
  5. May 17, 2009 #4
    Light cone often just refers to a range of directions in the immediate local vicinity of a single point. The full extension (or the region it encompasses) is sometimes called the null evolution (or the causal future); you're correct that the complete causal future from every point of the CTC is identical.

    Can you make the article more clear to laypeople without sacrificing technical precision of language?
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