(Please note: I've seen some other questions asked like the one I myself am asking, I realize that I am indeed late to the party, but I am genuinely curious and none of the previous answers have satiated me as of yet. Note that this is a highly hypothetical question, asked by somebody who isn't even a college student studying the idea of physics at this point in time, so go easy on me here. Sorry if this isn't worded well, but I still haven't studied much on the subject, nor am I used to typing up thesis papers. Excuse me please.)

Theoretically, it is stated that if you were to travel through a Kerr Black Hole- a Black Hole with a rotating ring singularity- it would allow you to go back to an earlier point in time. Another says you can travel through time using a Wormhole and exotic matter. What I'm curious about is whether or not time travel is hypothetically feasible were one of these true, as in whether or not the sheer force would crush a human by attempting to use one of these routes.

  • $\begingroup$ space.com/… $\endgroup$ – user83548 Nov 24 '15 at 1:49
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    $\begingroup$ First you say "who isn't even a college student studying the idea of physics at this point in time", then you say "if you were to travel through a Kerr Black Hole..." Well, that escalated quickly... $\endgroup$ – raul Nov 24 '15 at 1:52
  • $\begingroup$ The perhaps unpopular answer is that time travel of this sort probably isn't possible. I'm also pretty sure that traveling through a black hole isn't possible either. The reason why gets a little complicated. (if Time travel was easy - everybody would be doing it). It makes good science fiction. But in real science, Time travel to the past is, according to articles I've read, probably not possible even in theory. Now, maybe some new science could change that at some point, maybe, but for now, nobody has a good model to do it and traveling inside a black hole isn't a good solution. $\endgroup$ – userLTK Nov 24 '15 at 2:01
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    $\begingroup$ Also, already asked: physics.stackexchange.com/q/2166 $\endgroup$ – userLTK Nov 24 '15 at 2:06
  • $\begingroup$ you need to precise what time do you want to turn backward. The one of a traveling bubble ? of its environment ? the whole time ? what is the scenario ? $\endgroup$ – user46925 Nov 24 '15 at 6:32

There are quite a large number of theoretical obstacles to the possibility of travelling back in time using general relativity :

a) A lot of these solutions are unstable, making them rather benign. If you actually try to cross the Cauchy horizon (that's the point where spacetime ceases to be causal and allows you to travel back in time), it will collapse. This is among other things the case for the Kerr black hole.

b) Rather general theorems says that you cannot form a time machine (that is, make a time machine through human means, this does not concern the cases where the universe already contains time machine-like structures) without either violating the null energy condition (that the energy must always be positive) or producing singularities (cf for instance Tipler's "Causality violation in asymptotically flat spacetimes" or Hawking "Chronology protection conjecture")

c) Another even more restrictive theorem says that the energy of the quantum vacuum will diverge on the Cauchy horizon, which means that it will probably collapse on its own before forming. (cf Hawking "Chronology protection conjecture")

d) Some theorem also hints at the possibility that, if you try to form a time machine, the laws of physics as they stand cannot let you decide if what will form will be a time machine or another type of spacetime (loss of uniqueness of the solution). If the conditions are met for a time machine to form, it is possible that either a time machine will form or a spacetime with singularities. (cf Krasnikov's " Time machines with the compactly determined Cauchy horizon")

There are a lot of other problems associated with time machines, such as the difficulties of defining quantum fields on them, loss of quantum unitarity, breakdown of the Cauchy problem and such

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  • $\begingroup$ You are oveselling it a bit. Conditions of the theorems you are referring to all fail in presence of quantum corrections:"Over the last decade or so it has become increasingly obvious that there are quantum effects that are capable of violating all the energy conditions, even the weakest of the standard energy conditions" arxiv.org/pdf/gr-qc/0205066v1.pdf And about the energy of quantum vacuum near the Cauchy horizon nothing conclusive can be said without quantum gravity. Semi-classical coupling of classical gravity to quantum fields is inconsistent, so divergencies mean little. $\endgroup$ – Conifold Nov 24 '15 at 2:35
  • $\begingroup$ Yes the energy conditions can be violated quite a bit (in the case of Casimir-like effects, all of them can be violated), but those effects are overall very small and probably nowhere near enough to produce causality violations. As for quantum gravity, pretty tough to say. I have seen some efforts to get around chronology protection but it is rather unconvincing (for instance Li's paper on using complex geometries to get around it in path integral formalisms). Not a lot of work is done on quantum gravity for CTCs unfortunately, as a lot of them have causality built directly in them. $\endgroup$ – Slereah Nov 24 '15 at 2:46
  • $\begingroup$ We won't know what QG is like for the foreseeable future, string theories alone are million, and energy levels to test them are way beyond reach. But without QG there is nothing to get around, energy conditions don't work, and chronology protection is just wishful thinking:"Semiclassical methods become unreliable at the horizon and quantum gravity effects would be important there". en.wikipedia.org/wiki/… $\endgroup$ – Conifold Nov 24 '15 at 3:04
  • $\begingroup$ It's a myth that GR permits time travel. Spacetime is a "block" universe which models all times. So there's no motion in spacetime. So you don't travel along a worldline. So you don't travel around a CTC. See this answer where you can read how Wheeler conflated a cycle with a circle. Ditto for Kip Thorne. If you were subject to a 24-hour CTC it wouldn't be Groundhog day, it would be Mayfly day. $\endgroup$ – John Duffield Nov 24 '15 at 13:42

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