How do white holes work?

A physicist that specializes in relativity will probably say that white holes are simply time-reversed black holes in which all geodesics must emerge but not enter. A geodesic, by the way, is the 4-dimensional world-line of a particle as it traces its path through space and time. For a black hole, all geodesics at the event horizon may enter but not leave, however, if you reversed the direction of time, the same black hole with 'positive time' becomes a white hole with 'negative time'.

What this means for you and me is pretty complicated because the particular 'geodesically-complete' solution of general relativity that gives you a rotating black hole in one part of space-time, also has attached to this same black hole, other space-times in which the black hole can appear as a white hole depending on the trajectory you take, and where the outside observers are located!

Geodesic completeness means that all world lines either start on a singularity or end upon one. The only way to avoid geodesic incompleteness in Kerr space-times is to make certain that when a world line passes from large radial distances to 'zero', that the world lines continue through 'zero' and extend to negative radial distances. The time-like components to these world lines are also 'completed' this way by making certain that 'time zero' is just an inflection point that transitions to negative times. By continuing all world lines from positive to negative quantities in space and time, you avoid all non-physical 'coordinate singularities' and are left with only the important physical ones where space-time curvature really does go to infinity. Only for rotating 'Kerr' black holes' does this lead to interesting world lines.

In the non-rotating 'Schwarschild' case, all world lines entering the event horizon absolutely must terminate on the singularity at 'R = 0'. However, 'behind' this singularity there exists another space-time for which the radial coordinate distance R is less than zero. The singularity is the 'non- traversable' doorway to this other space-time. For Kerr space- times, the singularity is avoidable, and these other space-times can be reached, at least mathematically, by a single world-line from our own space-time. This is sometimes called the Kerr worm hole solution.

Physically, traversable worm holes do not exist naturally. Black holes can only be produced by the evolution of massive rotating stars, or may exist as supermassive monsters in the hearts of galaxies. No matter what the physical origin, so far as we understand the physics of real systems, these are not 'vacuum solutions' of general relativity, but must have within them the physical object that formed them. As you enter a black hole, the object that formed it is still in front of you, filling the space-time with gravitational radiation, which makes the entire edifice of stacked, connected universes dynamically unstable. Worm holes do not get born this way because there is always star-stuff blocking the doorway!

I would not be surprised if a theoretician were to tell me that the worm hole 'solution' is only a so-called asymptotic one that only obtains after nearly an eternity of time after the formation of the black hole. You would then have a long time to wait before nature would be able to use it for anything interesting. One end of the worm hole would appear as a 'white hole' and the other a 'black hole'. There would be no trajectories that could get you inside the worm hole end, but on the black hole end, just about every trajectory would lead you inside its event horizon.

Would anything be emerging from the white hole in a constant stream? Not unless it entered the black hole end in the first place, or spontaneously was produced inside the Kerr worm hole by perhaps a quantum mechanical process of some kind.