| But this is not the same as a time machine. The article doesn't explain the difference between a machine you can use to travel back in time, and a contrived spacetime in which events run in a loop. While it's dismissive of "paradox mongers" it doesn't deal with any of the very real issues created by very obvious potential paradoxes - not least of which is that physics fundamentals like conservations of energy stop working. (If you can move anything or anyone back in time you have a universe with two of something instead of one of something - which means you can get something for nothing, over and over. If you can't do this because the CTC forms a time loop, you don't have a paradox, but you don't have much else of interest either.) A contrived spacetime with a CTC is fundamentally useless as a practical time machine. And the whole point of a Thornian wormhole is that it offers practical engineering benefits - such as being able to travel in a way that effectively bypasses the speed of light limit. In any case - relativistic speculations are metaphysical, because it's a fair bet that spacetime isn't a smooth continuous manifold at all. It just looks like one from a distance under normal conditions - normal meaning anything outside a singularity or black hole. No one has much of a clue what spacetime is made of, and that makes it hard to say much about its properties under stress. For all anyone knows CTCs may be impossible because if you stress spacetime enough it may not act like a manifold any more, and a clean description of what happens at that point needs new completely physics. |