- #1
jaredmt
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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? I am 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?
but in the beginning they talk about it being a possible way of timetravel. then they say it "appears" to travel through time.
If CTCs exist, their existence would seem to imply at least the theoretical possibility of making a time machine
In extreme examples, in spacetimes with suitably high-curvature metrics, the light cone can be tilted beyond 45 degrees. That means there are potential "future" positions, from the object's frame of reference, that are spacelike separated to observers in an external rest frame. From this outside viewpoint, the object can move instantaneously through space. In these situations the object would have to move, since its present spatial location would not be in its own future light cone. Additionally, with enough of a tilt, there are event locations that lie in the "past" as seen from the outside. With a suitable movement of what appears to it its own space axis, the object appears to travel though time as seen externally.
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:
A closed timelike curve can be created if a series of such light cones are set up so as to loop back on themselves, so it would be possible for an object to move around this loop and return to the same place and time that it started. An object in such an orbit would repeatedly return to the same point in spacetime if it stays in free fall. Returning to the original spacetime location would be only one possibility; the object's future light cone would include spacetime points both forwards and backwards in time, and so it should be possible for the object to engage in time travel under these conditions.
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?