What is Polchinsky Paradox?

Who doesn't love a good old-fashioned time paradox? Theoretical physicist Joseph Polchinski posed a puzzle to another physicists in a letter:


If you throw a billiard ball into one end, at the right Speed and angle, such that it comes out of the other end.


The time it comes out is exactly same as to prevent its previous self from entering the very hole.


The twist is that the pool table is a time machine and the 2 pockets of the pool table are connected through a Wormhole which allows time travel.


lt is very popular theory among Scientists and it is taken very seriously. Many research papers have been published on this topic in various journals.


General Relativity allows Time travel. In fact, one of the solutions suggests the present of Closed Time-Like Curve (CTC) which is the key to Time Travel.


CTC is the path through the space-time that ends back where it started. Meaning you can make a round trip through this curve and return back to where you started.


It is another version of grand father paradox. The difference is that here no time traveller is involved. Instead you are the observer who has created this situation.


Normally, a closed timelike curve comes out of the equations through something called frame dragging, where a massive object or intense gravitational field moves and literally "drags" spacetime along with it. Many results that allow for a closed timelike curve involve a black hole, which allows for a singularity in the normally smooth fabric of spacetime and often results in a wormhole.


One key thing about a closed timelike curve is that it is generally thought the worldline of the object following this curve doesn't change as a result of following the curve. That is to say, the worldline is closed (it loops back on itself and becomes the original timeline), but that has "always" been the case.


Should a closed timelike curve be used to get a time traveler to travel into the past, that most common interpretation of the situation is that the time traveler would have always been part of the past, and therefore there'd be no changes to the past as a result of the time traveler suddenly showing up.

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