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White holes appear as part of a solution to the Einstein field equations known as the maximally extended version of the Schwarzschild metric describing an eternal black hole with no charge and no rotation. Here, "maximally extended" refers to the idea that the spacetime should not have any "edges": for any possible trajectory of a free-falling particle (following a geodesic) in the spacetime, it should be possible to continue this path arbitrarily far into the particle's future, unless the trajectory hits a gravitational singularity like the one at the center of the black hole's interior. In order to satisfy this requirement, it turns out that in addition to the black hole interior region which particles enter when they fall through the event horizon from the outside, there must be a separate white hole interior region which allows us to extrapolate the trajectories of particles which an outside observer sees rising up away from the event horizon. For an observer outside using Schwarzschild coordinates, infalling particles take an infinite time to reach the black hole horizon infinitely far in the future, while outgoing particles which pass the observer have been traveling outward for an infinite time since crossing the white hole horizon infinitely far in the past (however, the particles or other objects experience only a finite proper time between crossing the horizon and passing the outside observer). The black hole/white hole appears "eternal" from the perspective of an outside observer, in the sense that particles traveling outward from the white hole interior region can pass the observer at any time, and particles traveling inward which will eventually reach the black hole interior region can also pass the observer at any time.

Just as there are two separate interior regions of the maximally extended spacetime, there are also two separate exterior regions, sometimes called two different "universes", with the second universe allowing us to extrapolate some possible particle trajectories in the two interior regions. This means that the interior black-hole region can contain a mix of particles that fell in from either universe (and thus an observer who fell in from one universe might be able to see light that fell in from the other one), and likewise particles from the interior white-hole region can escape into either universe. All four regions can be seen in a spacetime diagram which uses Kruskal–Szekeres coordinates.

In this spacetime, it is possible to come up with coordinate systems such that if you pick a hypersurface of constant time (a set of points that all have the same time coordinate, such that every point on the surface has a space-like separation, giving what is called a 'space-like surface') and draw an "embedding diagram" depicting the curvature of space at that time, the embedding diagram will look like a tube connecting the two exterior regions, known as an "Einstein-Rosen bridge" or Schwarzschild wormhole.[4] Depending on where the space-like hypersurface is chosen, the Einstein-Rosen bridge can either connect two black hole event horizons in each universe (with points in the interior of the bridge being part of the black hole region of the spacetime), or two white hole event horizons in each universe (with points in the interior of the bridge being part of the white hole region). It is impossible to use the bridge to cross from one universe to the other, however, because it is impossible to enter a white hole event horizon from the outside, and anyone entering a black hole horizon from either universe will inevitably hit the black hole singularity.

Note that the maximally extended Schwarzschild metric describes an idealized black hole/white hole that exists eternally from the perspective of external observers; a more realistic black hole that forms at some particular time from a collapsing star would require a different metric. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole interior region.[5] But because the equations of general relativity are time-reversible (they exhibit T-symmetry), general relativity must also allow the time-reverse of this type of "realistic" black hole that forms from collapsing matter. The time-reversed case would be a white hole that has existed since the beginning of the universe, and which emits matter until it finally "explodes" and disappears.[6] Despite the fact that such objects are permitted theoretically, they are not taken as seriously as black holes by physicists, since there would be no processes that would naturally lead to their formation, they could only exist if they were built into the initial conditions of the Big Bang.[6] Additionally, it is predicted that such a white hole would be highly "unstable" in the sense that if any small amount of matter fell towards the horizon from the outside, this would prevent the white hole's explosion as seen by distant observers, with the matter emitted from the singularity never able to escape the white hole's gravitational radius.

Recent Speculations

There are theories suggesting that white holes create new universes from matter originating in another universe's black hole.[8]

A more recently proposed view of black holes might be interpreted as shedding some light on the nature of classical white holes. Some researchers have proposed that when a black hole forms, a big bang occurs at the core, which creates a new universe that expands outside of the parent universe.[9][10] See also Fecund universes.

The initial feeding of matter from the parent universe's black hole and the expansion that follows in the new universe might be thought of as a cosmological type of white hole. Unlike traditional white holes, this type of white hole would not be localized in space in the new universe, and its horizon would have to be identified with the cosmological horizon.

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Originally posted by dreamtrove:
Whatever. It is clear to me that if anyone disagrees with you it must be because they are stupid, and that once they have disagreed with you one one thing, like politics, then you must always assume that they are a moron, and this must be true of all things. I have no interest in communicating with someone with this perspective.

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True. But in a black hole, at the singularity, these forces are no longer separate. Thay can't act against each other. Different rules apply.

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Originally posted by Bytemite:
The question about the singularity and my thinking there can't be an exact singularity based on how we define it is because some of the matter that's supposed to be in the singularity is, simply, not. A singularity requires zero volume, particles in the singularity being OUTSIDE the theoretical point despite gravity would cause the singularity to have non-zero volume.

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In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible.

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BTW, any response about degenerate states?

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It is not a condition of a singularity that ALL matter be in it.

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Originally posted by dreamtrove:
CTS,

Why was I disputing it? I just posted to you basically the same thing as what shell theorem said in wikipedia. No, I think I was saying the exact same thing, not disputing it at all.

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DT:

Alas, the dispute was more basic than that: The gravity inside the moon is less than on the surface because the moon is not a point. If you're standing, say, 400 miles from the center, then you have an 800 mile sphere of moon more pulling you in one direction vs. every other direction. Ergo, density aside, there's a 400 mile radius or 800 mile diameter sphere of moon on one side of you causing net gravity, and that is smaller than the moon. You proximity to is not enough to make that gravity higher.

A secondary question might be whether the surrounding moon gravitational force pulling you in separate directions would be enough to pull you apart, but we can calculate that it isn't.

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Originally posted by Bytemite:
I'm afraid I must quibble here. An Event Horizon is when what you get when gravitational forces are so strong that nothing - not even degenerate states - can escape.

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I'm confused... degenerate states don't escape. Did you make a typo here?

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Or you mean that degenerate states don't exist in black holes?

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The degeneracy is able to prevent collapse. Once collapse occurs, the mass becomes a singularity, a small enough point that it can have an event horizon.

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I believe that if matter has not collapsed into a singularity, the body cannot have enough density to be a black hole. It cannot have a big enough M and small enough r for the escape velocity to reach and exceed c.

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Originally posted by Bytemite:

I learn everything I know from a combination of logical induction and wikipedia (and also donate to wikipedia because I would become the stupidest person on Earth without it).

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Originally posted by Bytemite:
I learn everything I know from a combination of logical induction and wikipedia

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Originally posted by Bytemite:

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I'm confused... degenerate states don't escape. Did you make a typo here?

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No, I don't believe so. Perhaps it would have been better to say "where they don't escape," but ultimately I think can't escape and don't escape mean the same thing because they have the exact same end result in this case.

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Not at all. Based on your representation, I can buy completely that they exist in black holes.

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I think something like a singularity is probably found in most black holes (I'll have to think about if there might be black holes that DON'T have one.

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Originally posted by dreamtrove:
Well it does, but not in a uniform direction. This is where the anti-grav effect comes from. If you take the shell of the moon from 400 miles to the surface, you are also on the inside edge of that hollow sphere, which, were it a real hollow sphere, would hold you there as it has a gravitation force, that being small but directly opposed to that of the inner core. I think you can work out for yourself that this would be so.

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you have no ability to address your mistakes. I have pointed out several

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No, you have not. You haven't pointed out any.

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In a pure Newtonian world, yes, you would be correct, but that is clearly not the universe we live in.

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I have no interest in debating misinterpretations, or arguing over minutiae.

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My last contribution was "where does the space go?" Which is a valid question. It can't be a point if all of the space went into it, because then there is internal space.

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Also, on a personal note, it feels like you harbor a hostility in your posts to me that you don't hold to others. We might politically disagree, but this is a discussion of science.

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Originally posted by dreamtrove:
Yes, that would be the anti-grav effect, but as it is a shell, you're being pulled in all directions by that shell as it is all around you. It is just closer to you on the spot you are in, so it is stronger there. The net effect is a small anti-grav.

It shouldn't disregard it completely, but it will be a more minor effect than that of the inner core ISLG.

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See the calculations above - you cannot have an event horizon unless the mass has a density beyond what any degeneracy pressure can support. Hence, any body dense enough to have an event horizon will collapse totally into a singularity.

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In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole.[57] The possibility of traveling to another universe is however only theoretical, since any perturbation will destroy this possibility.[58] It also appears to be possible to follow closed timelike curves (going back to one's own past) around the Kerr singularity, which lead to problems with causality like the grandfather paradox.[59] It is expected that none of these peculiar effects would survive in a proper quantum mechanical treatment of rotating and charged black holes.[60]

The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.[61] This breakdown, however, is expected; it occurs in a situation where quantum mechanical effects should describe these actions due to the extremely high density and therefore particle interactions. To date it has not been possible to combine quantum and gravitational effects into a single theory. It is generally expected that a theory of quantum gravity will feature black holes without singularities.

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