Are you referring to the idea of time travel using a transversable wormhole, singularities and that kind of thing? That stuff is really interesting.
They are indeed interesting, but no: I was referring to simultaneity in Relativity.
Consider this hypothetical: you have two people, O1 and O2, standing on a train and a train platform, respectively. On top of the train in the middle is a device that emits two laser beams, one towards the front of the train (i.e., in the direction of movement), and one towards the back. At either end are laser detectors, and inside the train is enough C4 to make Guy Fawkes salivate. When both detectors are
simultaneously hit with the lasers, the C4 will go off. A delay will not set it off.
Now, the lasers are turned on, the beams emitted. The question is, does the C4 go off?
From the point of view of O1, standing on the train and moving with it, yes: the laser hit the detectors simultaneously, and boom goes the cyclotrimethylene trinitramine.
From the point of view of O2, standing on the platform and watching the whole rig trundle past at (let's say) 0.9
c, no: the aft laser hits first, then the fore laser. Thus, the C4 doesn't explode.
So, according to General Relativity, the two observed events are not simultaneous: two events that occur at the same time in one inertial frame, might well occur at different times in another. That is, simultaneity is not invariant.
(Obviously the C4 either
will go off or
won't go off, regardless of inertial frame; the solution to this apparent paradox is that the trigger mechanism is itself experiencing a delay: a signal has to be sent from both detectors to the 'fuse', and only if
that is hit simultaneously (from its own point of view) will the C4 go off - and, indeed, it will. So, O2, who sees the laser hit non-simultaneously, will also see the burning 'fuse' travel at various speeds due to length contraction and time dilation - and the C4 explode)
(It's far too early in the morning for this...)
