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+--- Thread: How traveling back in time is permitted by Einstein’s physics (/thread-16062.html)
How traveling back in time is permitted by Einstein’s physics - C C - Jun 21, 2024
https://bigthink.com/starts-with-a-bang/travel-back-time-physics/
KEY POINTS: From Back to the Future to Harry Potter to Groundhog Day, time travel has been a part of our science-fiction imaginings for as long as we’ve been telling stories. While the idea of going back in time to “right past wrongs” is a strong philosophical temptation, there are important physical roots in Einstein’s general relativity that dictate this as a real physical possibility. Although an enormous amount of energy is required and certain ideas (like traveling back in time to kill your own grandfather before your parents were conceived) are forbidden, time travel cannot be ruled out.
EXCERPT: . . . How is this possible? You have to remember that in relativity, time itself is relative. This teaches us something profound: that our notion of “a year” isn’t the same for everyone, particularly if they’re moving through time and space at different rates. If we consider a hypothetical journey from Earth to the TRAPPIST-1 system, while taking one end of this wormhole with us, we’d find that the “in motion” end of the wormhole would have only aged by the amount of time we spent on our journey: 6 months. Even though, back on Earth, 40.5 years have passed, the connection between the two ends of the wormhole is still instantaneous, and if you traveled back “the short way” through the wormhole instead of “the long way” through space, you’ll find that only six months have passed when you come out of the original end.
Now imagine that this wasn’t a scenario that we were going to set up today and devise an application for it in the future. No; instead, imagine that 40 years ago, back in June of 1984, someone had created such a pair of entangled wormholes and sent one end of the wormhole off on this journey already, and that you went with it. If you were able to step into one end of that wormhole today, in 2024, you’d wind up back in time at the mouth of the other end of the wormhole… back in December of 1984. The only restriction is that you yourself couldn’t have been sticking around and stationary, here on Earth, for the past 40 years. In order for this scenario to work, you needed to be with the other end of the wormhole, or traveling through space to catch up with it, for all of that time.
One of the remarkable aspects of this physically allowable form of time travel is this: we discover that this also forbids the grandfather paradox from occurring! Even if you imagine that this wormhole was created before your parents were conceived, and that all four of your grandparents were aboard the ship, and the ship traveled near the speed of light while your parents were conceived and then you were conceived, born, and grew up, there’s no way for you to return to the original, unmoving end of the wormhole at a moment prior to your grandparents boarding that ship. There’s no way to return to an early enough moment that you could prevent your grandfather from boarding that ship, or to prevent your own parents from conceiving you.
The closest you’d be able to get would be to have put your newborn father and mother on a ship, when they were very young, to catch the other end of the wormhole, have them live, age, conceive you, and then send yourself back through the wormhole. You’d be able to meet your grandfather back on Earth, perhaps when he was still quite young — perhaps even younger than you are now — but it will still, by necessity, occur after your parents were born, and so there would be no way to prevent your own existence. It’s fascinating and true that a great many unusual things become possible in the Universe if negative mass/energy is real, abundant, and controllable, and traveling backward in time might be, perhaps, the wildest consequence we’ve ever imagined. Within the framework of both special and general relativity, traveling back in time may be difficult to achieve in any practical sort of fashion, but there’s absolutely nothing forbidding it from occurring! (MORE - missing details)