What would time travel make possible?
Spacetime Paradoxes: Are Time Travel Possible?
The possible existence of such curves leads to paradoxical scenarios. How can you change the past when it has already happened? Those who believe in causality should be reassured that astronomers have found no evidence of a rotating universe. Godel himself has apparently unsuccessfully scoured galaxy catalogs for evidence that would support his theory. Although he did not develop a realistic model of the universe, he did show that closed time-like curves are completely in accordance with the equations of general relativity. So the formulas do not prohibit travel into the past. Lately, time travel enthusiasts have also been devising ways of using local curvatures of space-time. According to general relativity, planets, stars, galaxies and other massive bodies distort space-time. This in turn affects the movement of these objects. In extreme cases, spacetime could bend so much that it creates a path from the present to the past.
Theorists have suggested some exotic mechanisms for creating such paths through time. In a paper published in 1991, the US astrophysicist J. Richard Gott from Princeton University showed how cosmic strings - infinitely long structures, thinner than an atom, which could have formed in the young universe - make closed, time-like curves possible at their intersections. And in 1983, Kip Thorne from the California Institute of Technology began investigating so-called wormholes, which are a kind of tunnel between different regions of space-time and could thus allow journeys into the past. "If you connect two different regions of space in general relativity, you connect two different regions of time at the same time," explains Sean Caroll, a colleague of Thorne.
The entrance to a wormhole would be spherical - a three-dimensional entrance into a four-dimensional tunnel. As with all closed time-like curves, the journey through a wormhole would also be unspectacular, says Caroll. "You don't dissolve, nor are you put back together again at another point in time. Such visions from science fiction are not possible in any recognized theory." While physicists can formulate equations that describe wormholes and other closed time-like curves, all models have serious pitfalls. To create a wormhole at all, you need negative energy. Without it, the spherical entrance and the four-dimensional tunnel would implode instantly. Such a structure would hardly be stable in practice or perhaps even fundamentally impossible, says Caroll: "Negative energy causes a lot of problems in physics."
But even if a wormhole could be kept open, the next trouble would come. Particles moving through would loop back through this path. In doing so, their energy grows across all boundaries. And since energy, like mass, deforms spacetime, the wormhole collapses into a black hole in this scenario. "While we're not 100 percent sure that this will happen," admits Caroll, "it seems a reasonable assumption that the universe is preventing a time machine from being built by turning it into a black hole."
In contrast to black holes, which are a natural consequence of general relativity, wormholes and closed time-like curves are artificial constructs that are intended to explore the limits of the theory. Even if wormholes are not physically plausible, the important thing is that they do not contradict general relativity. "It's strange that we're so close to eliminating the possibility of time travel and still not really making it. I find that annoying," says Caroll, unnerved that a theory as elegant as Einstein's should make something so obvious Incredible allowed. But thinking about it also leads physicists to a better understanding of our natural laws. It is possible that our universe could not have come into being if a path backwards in time were fundamentally forbidden.
The general theory of relativity describes the universe on its largest scales. Quantum mechanics, on the other hand, is a kind of manual for atomic orders of magnitude and offers a playground for particularly surprising phenomena with closed time-like curves. The appearances can even concern the origin of the cosmos. For very small lengths of around 10-30 Centimeters, researchers like John Friedman from the University of Wisconsin-Milwaukee expect the topology of spacetime to boil. And such random fluctuations could also lead to closed time-like curves if nothing fundamental prevents it. Can such fluctuations be increased to build a time machine? "There is no formal evidence that macroscopic, closed time-like curves are completely impossible," replied Friedman. "But most researchers who have dealt with these general questions would probably bet against it."
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