From Wormholes to T-Symmetry: Navigating the Complex Terrain of General Relativity and Time Travel

In certain spacetime geometries governed by general relativity, such as cosmic strings, traversable wormholes, and Alcubierre drives, there exists a theoretical possibility of traveling to the past by surpassing the speed of light. The scientific foundations of general relativity lend support to the notion of backward time travel in specific extraordinary circumstances. However, when quantum effects are taken into account, arguments from semiclassical gravity indicate that these potential avenues might be closed. Notably, these semiclassical arguments prompted Stephen Hawking to propose the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel. Nonetheless, physicists are unable to reach a definitive conclusion on this matter without a unified theory of quantum gravity that harmonizes quantum mechanics and general relativity into a comprehensive framework.

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The laws of physics, as we currently understand them, suggest that traveling backwards in time would violate fundamental principles such as causality, which states that an event must occur before its effects. Additionally, time travel would require the manipulation of space-time

There are some theories, such as the concept of closed time-like curves, that allow for the possibility of time travel, the concept of time travel

also raises numerous philosophical and ethical questions, such as the potential for altering history and the implications of meeting one’s past self.

While the idea of time travel may be intriguing, it remains a subject of speculation and imagination rather than a scientifically valid.


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Come on, my friend. If you’re going to lift an entire section from another website and paste it into a post at least put quotation marks around the text and give the author or site proper credit. For instance, your post comes from the “General Relativity” section of Wikipedia’s “Time Travel” page Time travel - Wikipedia

That’s correct. A host of physical laws would seem to be violated. Think about this situation:

You film two tennis players volleying a tennis ball back and forth. Then you ask yourself, “Does a simple physical law of time reversal (T-symmetry) exist? If I run the film of the players volleying the tennis ball backwards does it look the same as running the film forwards?” The answer is no, absolutely not.

If you run the film backwards you’ll see the ball approaching one of the players. The player’s racket is extended forward, the racket strings are stretched back away from the ball and as the ball approaches the racket it becomes deformed - flattened - until it strikes the racket. The racket, which is moving backwards away from the incoming ball, collides with the ball. The strings are moving from their stretched position back toward their flat unstretched position. The incoming ball becomes less and less distorted - it returns to being round. When the racket has moved backward to a position almost parallel with the back line the ball jumps off the racket as if it has been repelled, not propelled, by the racket, toward the other player. If you also film the event with an infrared camera you’ll see a flash of infrared light go back into the ball and racket. The ball appears to lose energy by cooling in order to be propelled toward the other player.

We don’t have to even refer to causality violations to see that simple T-Symmetry (time reversal) does not conform to the known laws of physics. Good post.

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The way time flows and its direction (or “arrow,” as it’s sometimes called) is an interesting part of our world that makes time travel difficult in many ways. T-symmetry, also called time reversal symmetry, is a topic that comes up a lot in talks about this, and @Darby’s tennis example is a great explanation.

General relativity does allow for some solutions in which time-like loops (or closed time-like curves) are possible, which in theory would make it possible to move through time. But these answers often involve strange things, like negative energy densities or structures that go on forever and cover the whole universe, which are not very useful.

Also, as you pointed out, even if such scenarios were possible, the reversal of events wouldn’t happen the way we think it would. Time has a clear ‘direction’ because of the conservation of energy, the second law of thermodynamics, and many other scientific concepts.

Quantum effects are also something to think about. As far as we know, quantum physics doesn’t like the idea of traveling through time very much. Here we run into what Hawking called the “chronology protection conjecture.” This says that the laws of physics work together to keep time travel errors from happening.

But what’s really interesting is that we still don’t have a clear “no” to the question of time travel, even though there are a lot of problems. It’s still not clear, and there are still a lot of things to figure out. It shows how beautiful and mysterious our world is and reminds us to keep our minds open to the unknown.