I've seen time inside a black hole described as gaining space-like properties - essentially swapping its properties with space, which becomes time-like, meaning there is only one possible direction of movement in space left - and consequently that more directions of movement in time are opened. Which would mean traveling backwards in time becomes possible. Or sideways, whatever that means (possibly choosing to move backwards so slowly so as to freeze in time?), depending on how many dimensions of movement in time we supposedly gain.

But while those properties make sense on paper and as a result of mathematical calculations, what does it entail in practice? Could there exist a practical way to take advantage of this fact? What would it require to commence movement backwards in time in an environment that supposedly allows it? Energy, or something else? Explaining reaching the singularity inside a black hole as akin to an inevitable future point in time is fine and all, but what about the other side of the story, where there exist alternate places in time to move into?

  • $\begingroup$ Related: physics.stackexchange.com/q/336133/123208 $\endgroup$
    – PM 2Ring
    Jul 4 '21 at 18:21
  • $\begingroup$ In $(-,+,+,+)$ signature, the metric always takes the form $ds^2 = (-ve) dt^2 + (+ve) dx^2$ where $t$ is a time. The coefficient of "time" $t$ is negative and the coefficient of "space" $x$ is always positive. However, inside the black hole, the metric becomes $ds^2 = (+ve) dt^2 + (-ve)dx^2$ so inside the black hole $x$ becomes time and $t$ becomes space. $\endgroup$
    – Prahar
    Jul 4 '21 at 19:50
  • $\begingroup$ When you go North along a meridian on the Earth, after you cross the North Pole, your direction becomes to the South. Does this mean “North gains South-like properties”? No. Time nowhere “gains space-like properties”, but the same coordinate that reflects space in one region may reflect time in another. Your question is based on a false premise. $\endgroup$
    – safesphere
    Jul 6 '21 at 15:18
  • $\begingroup$ What does it even mean it's "based on a false premise"? It's precisely how many people phrase it. $\endgroup$
    – spiffles
    Jul 7 '21 at 23:55
  • 1
    $\begingroup$ Even if many people phrase it wrong, it is still wrong. However, chances are that you may be misreading what they say. A correct statement is that the $t$ coordinate becomes spacelike inside a black hole. This is not the same as "time gaining spacelike properties". $\endgroup$
    – safesphere
    Jul 8 '21 at 3:52

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