# Could Kerr Metric Black Holes hypothetically be used for Interstellar Information Transfer?

Kerr Metrics should (extremely hypothetically) allow travel into "mirror" universes. However, I have heard that some theories estimate the time-swapped black hole on the other side of the ringularity may be within the same universe, simply at a displaced time.

I am aware that they may be very unstable under perturbation, however. If, for example, a civilisation were to create a rotating black hole then feed it to keep it alive, then somehow entangled the two sides then assigned opposite electric charges to the two ends. Then made sure the formation of the black hole was almost perfect, and that the black hole underwent minimum perturbations, could an extremely large black hole (many times the size of the sun) possibly allow matter, or at least information travel through.

I should preface this by saying my knowledge of general relativity is very rusty, and I'm going based off courses in my physics degree from several years ago

If you want your world to be based around the current understanding of physics, then no.

The Kerr black hole 'wormhole' is effectively a mathematical quirk that arises because of how the black hole is treated.

The general gist is that the 'toy wormhole' is a quirk of the ideal Kerr solution; once you cross the event horizon the black hole is no longer described by this solution

While it is mathematically valid within the Kerr black hole model, it is accepted that this is not a physical property, and therefore the idea of firing some particle in to a black hole and it reappearing somewhere else in the universe does not hold up. As such, it would not be possible to use black holes for interstellar information transfer.

# Not within known physics

A black hole can be defined in a number of ways. My favorite one is this: a body whose escape velocity is greater than $$c$$.

Therefore, even for topographies where a black hole connects to another universe, or to another place in the same universe, once you enter one you cannot get out. That is why traversable wormholes need negative mass.

There is one exception. If you have watched Interstellar, you'll have seen the characters escaping a black hole. That is possible if a black hole is rotating (here's how and why. But with full realism, anything going in would be spaghettified, and as in the movie, you get out in the same universe and vicinity that you came in.

As for information, this is where stuff gets weird. The black hole information paradox is my favorite paradox in physics (the twin paradox isn't really one, and time-travel related ones can only go so far). From the wiki:

The black hole information paradox is a puzzle resulting from the combination of quantum mechanics and general relativity. Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state. This is controversial because it violates a core precept of modern physics—that in principle the value of a wave function of a physical system at one point in time should determine its value at any other time.

There are some interesting solutions in that article. All hypothetical though, and none which would allow for a practical way to send information through a black hole.

• This is a very good answer! However... if you entered the alternate-universe black hole, it would appear to the original-universe observer like a white hole, propelling the person out. I was thinking that if the observer sees on exiting a white hole as the object he's within, then as he exits it effectively transitions into a black hole. You could make an traversable 2-way wormhole system with the ringularity. Commented Apr 20, 2020 at 11:16
• @MrKred I don't think so. The idea of a white hole as a necessary region of a black hole only works for eternal black holes, which are not realistic. For non-eternal black holes, a white hole region does not make sense. Without eternal holes, the only way to have a white hole is through the "death" of a black hole. Commented Apr 20, 2020 at 12:42
• that in principle the value of a wave function of a physical system at one point in time should determine its value at any other time. I tend to disagree with this formulation. It's missing *iff* there's no interaction with any other system , no matter how small. It means: throw just a neutrino in the blackhole, neutrino that originated from another system, and the wave function of the original system collapses. So either you consider the wave function of the entire Universe (and nothing was lost) or don't expect the wave function of an isolated system to conserve interacting with a BH. Commented Apr 20, 2020 at 13:52