I know that there's no evidence at this point for "white holes" however would it even be mathematically possible for a black hole to be connected to a white hole (total opposite so everything would be expelled, after some really extreme physical conditions)? Maybe with a wormhole connecting them? If this was even possible (if the black hole or connection could actually be created and be stable enough), would that matter be expelled into a different universe, etc? Maybe even a different region of spacetime? Just curious, as it would be a cool idea.
[Physics] Is it mathematically possible for a black hole to be connected to a white hole or a worm hole
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This is a water flow analogy with a black hole. Take a fish tank filled with guppies and put a large drain or siphon in the tank. This opening has to be big enough to set up a decent water flow. When the water starts flowing the fish will generally swim against the current. Assume all the guppies are the same size and generally swim at the same speed.
Then the rate they flow is a vector sum of the water flow and the swimming velocity in the absence of water flow. This will provide a visual model for how photons move in spacetime. The flow of water is a model for the flow of spacetime points. The guppies represent photons (modulo the fact they change direction at will), and the guppies within some approximate radius will tend to be taken up by siphon.
Under General Relativity, Lorentzian wormholes (the kind that are traversable) require exotic matter (a kind of unobtainium which is not known to exist).
This is not true. A maximally extended Kerr black hole solution for instance has a traversable wormhole to a (different) external universe and doesn't require exotic matter. However you do have to traverse a region with a visible singularity and with visible regions where time travel is possible (where closed time like curves exist). It is unreasonable because it is an eternal solution so the black hole never formed from infalling matter.
On the other hand, we know black holes exists and these form from the collapse of large stars.
External observers, by definition, never see them form.
One of the differences that we usually associate with wormholes is that they do not have an event horizon, and are disallowed by topological censorship theorems.
Most topological censorship is based on changes in topology. And there are traversable wormholes with event horizons, but that is usually interpreted meaning they go to different universes and the alternative seems to be closed time like curves that go through the wormhole. And the latter happens anyway, just if they are different universes the closed curves loop around only in the inside regions and never in the outside regions.
But what about black wormholes? is this concept a meaningful one? what I'm thinking is a Lorentzian wormhole that is unidirectional, basically you fall in an event horizon, but instead of finding a singularity, you enter a throat and exit on the other side.
This seems lime a novel (possibly nonexistent) solution if you are trying to avoid using exotic matter.
From the exit mouth, you can 'see' the other side, you can even traverse the wormhole back to the entrance side, but you are unable to send anything to null infinity because of the event horizon.
This is very confusing. If you can cross a surface going in both directions it doesn't sound like an event horizon.
Would such exit side be essentially the same thing as a white hole?
White holes can have there own type of event horizon, ones where you can cross from the inside to the outside but not vice versa. This is the usual type of horizon to cross into an external universe, such as in the maximal Kerr solution.
Best Answer
Dear Kahtrijn, white holes are microscopically the same objects as black holes, and it's guaranteed - by the second law of thermodynamics - that all macroscopic processes occur in the way as they do in black holes and not white holes (the latter are time-reversed of the former).
However, if you don't care about the second law of thermodynamics that prevents black hole size from shrinking by emitting large objects (which objects?), then you may write down any configuration in general relativity you want. In particular, it's easy to connect white holes to black holes. After all, an "eternal black hole" is a solution that is doing nothing else.
The connection between a white hole and a black hole goes in a different direction than you seem to expect, however. It's a connection between the past and the future. It's because the (neutral) white hole singularity is a spacelike singularity in the past, and the (neutral) black hole singularity is a spacelike singularity in the future. To connect those singularities, you need timelike (rather than spacelike) trajectories.
One doesn't need any wormhole to write down an eternal black hole. It may sound cool to combine so many different holes but it is totally unnecessary: the wormholes are independent objects from the black holes and white holes.