Reading The world is upside down - when time travel comes true. Wormholes could get us into our own future or past - that's what physicists read from their formulas. But time travel would have paradoxical consequences: they would confuse cause and effect. The general theory of relativity makes clear statements about the time flow at both openings of such a wormhole. Due to the time dilation described by Einstein, the clocks at a moving aperture are slower than the clocks at a stationary one. At least that's the case when viewed from the outside.

Seen from the wormhole, the time flow is the same at both gates. "An infinitely advanced civilization could construct a time machine out of a wormhole, " speculates Kip Thorne of the California Institute of Technology in Pasadena.

For example, an astronaut has found a small wormhole in space and flies away with one end of the hose at near the speed of light. When he turns and returns to the starting point, decades may have passed for his twin brother, who had stayed at the other end, but the spaceman himself has barely aged.

This thought experiment is based on Einstein's twin paradox, according to which the time for an astronaut in a fast spaceship is slower than for his twin brother on earth. This twin paradox can be picked up by the wormhole. The backward brother just has to slip through the wormhole's moving opening as soon as his brother brings it back, and so gets into his own past - back to his younger self, which was just left by the twin brother. display

The only restriction is that it is impossible to travel back in time to a time past when the wormhole was first used as a time machine. Conversely, the rejuvenated twin brother can catapult itself through the dormant opening of the wormhole into the future.

David Deutsch and Michael Lockwood of the University of Oxford have been looking for a different way for time travel without paradoxes. But your hypothesis has a staggering price. The two physicists rely on a provocative interpretation of quantum mechanics proposed by American physicist Hugh Everett in 1957. He suggested that whenever nature has a choice of two or more states, the universe splits into two or more identical parallel universes. Then there is a universe in which this sentence ends with a dot, but also one in which this is not the case. And both have their own story from now on.

So the son can actually travel to the past and murder his father. He enters a universe that was exactly identical to his own until the moment of his arrival, but now takes a different course. In this universe, the time traveler will never see the light of day. But he did not come from this universe, but from another in which he was born, because his father did not die prematurely there.

The last word has not been spoken yet. "The question of time travel remains open, " admits astrophysicist Stephen Hawking. "I will not bet on that, though. The other one could have the unfair advantage of knowing the future. "

=== Rüdiger Vaas

© science.de

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