Reality only splits in two in the sense that the observer-dependent reality of the person falling into the black hole and the observer-dependent reality of the person remaining outside of it can no longer exchange information anywhere except inside of the black hole's event horizon.

The half of your light cone that extends into the future remains entirely within that event horizon. The outside observer's cone can remain outside of it only as long as she avoids crossing the event horizon.

Even if the outside observer decide to jump in just seconds after you, it is possible that the physics inside the event horizon would prevent her from ever catching up with you to compare notes, or even sending you a message. You could send a message with modulated x-rays, and she might pick them up with a VLF antenna. She might reply with a deep infrared laser and accidentally cook your brains with cosmic rays. Assuming a steep spacetime gradient as you approach the singularity, everything sent inward would be severely blueshifted, and everything sent outward severely redshifted.

There might even be a sort of Zeno's Paradox inside, where once you cross the event horizon, there's another event horizon beyond that, such that anything that much closer to the singularity than you are can never communicate with you, in the same way that you can never communicate with the universe outside the first event horizon.

> There might even be a sort of Zeno's Paradox inside, where once you cross the event horizon, there's another event horizon beyond that, such that anything that much closer to the singularity than you are can never communicate with you, in the same way that you can never communicate with the universe outside the first event horizon.

Isn't every step in from the event horizon effectively an event horizon? If all the mass of a black hole is effectively at the singularity in the center, and escape velocity is the speed of light at the event horizon, won't there be no point within the event horizon where escape velocity is less than the speed of light, such that at any point within the event horizon, no point farther from the event horizon is within the future light cone of that point.

I don't think that's quite what they meant by that.

In silly "outside universe coordinates", inside the black hole you can define a "minimum inwards speed" as a function of distance from the singularity. This is how fast a photon will go towards the singularity if fired directly away from it. It's zero at the event horizon, and it increases as you go in.

Now, what (I think) logfromblammo was saying was, "If I'm at the event horizon, can there be someone close enough to the singularity that I can never catch up?"

Communication isn't much of a problem for two people falling into a black hole one after another if they're close enough. The messages from the person lower down don't "go up", they just go down more slowly than the person higher up. If they're too far apart, though, this might not work.

The distance between fallers determines the energy requirements for communication. The lower person will also experience slower time than the higher.

The analogy is as follows:

Two BASE jumpers leap from the top of an infinitely tall radio mast in a vacuum, one after the other. They are in free-fall, so do not directly experience the effects of gravity, but the increase in the gravitational field as the distance to the singularity decreases will be detectable as a pseudoforce in that reference frame.

The first jumper writes a note on a baseball and throws it at the second. The second also writes a note on a baseball and throws it at the first.

The concern for the first jumper is not whether the baseball can be thrown back to the top of the radio mast, but whether it can be thrown hard enough and accurately enough for the second to ever reach it. The concern for the second is whether the first can catch his ball without exploding like a paper sack full of wet spaghetti.

The other thing to consider is that both fallers will "see" multiple images of the other, and of themselves, because there is a direct light path, the light path that twists around once, the light path that twists around twice, and so on. Images further away will be lower, slower, and older. If you aimed correctly, you could send messages to your older self, but I'm not entirely certain if your older self could reply.

> The concern for the first jumper is ... whether it can be thrown hard enough and accurately enough for the second to ever reach it. The concern for the second is whether the first can catch his ball without exploding like a paper sack full of wet spaghetti.

This is just a practical difficulty. There might actually be stronger constraints in play -- the two jumpers might be causally disconnected from one another entirely (that is, their forward light cones might intersect only at the singularity.) I have no idea whether that's possible, though.

> If you aimed correctly, you could send messages to your older self, but I'm not entirely certain if your older self could reply.

Any message has to be received "lower" than where it was sent, so all messages go from younger selves to older selves.

Which would also imply that you cannot communicate with a person who jumped in right after you did. I think your model does not account for the jumper having mass and his communication photons being massless.

The reason the photons cannot escape is not because they are affected by the gravity, but because the space they must traverse is stretched and twisted to such an extent that to an observer inside the event horizon, the entire outside universe presents an infinitesimally small target that is receding rapidly. Anything you might manage to shoot out would be indistinguishable from the Hawking radiation coming from the event horizon.

On a closer scale, you could probably communicate uphill, but the conversation would be like an ent talking to a chipmunk. At some point, you simply can't target your uphill counterparty accurately enough, or with enough bandwidth to hold their interest.

> Which would also imply that you cannot communicate with a person who jumped in right after you did

That doesn't follow at all. It's conceivable we could drop three things into a black hole one after another, and at some point the first two could send a message to each other, and so could the last two, but the first and last could not (in a situation similar to the cosmological horizon.)

> the entire outside universe presents an infinitesimally small target

No. There is literally no direction you could fire a photon that could take it out of the black hole. You can talk to someone falling behind you only because they can "fall onto" your transmissions. Light cannot go uphill at all.

If you can communicate with someone who jumped in one second after you, you could communicate with your younger self via a gravity-warped path that has the same distance as the direct distance between you and the second jumper.

There are no one-way trapdoors with massless particles. If a photon can get in, an anti-photon (aka a photon) following the reverse trajectory (including moving backward through time) can get out. It seems almost tautological, but the photon doesn't really care which way the time arrow points, and can't tell whether it is coming or going.

The warping of spacetime is such that no photon crossing the event horizon from the outside can return to the outside. All straight light paths between any arbitrary point on the event horizon and any arbitrary point inside it do not intersect any other point on the event horizon. If your point of origin is inside the event horizon, you can fire a photon out of it. But you literally need a perfectly accurate and perfectly predictive model of all mass in and around the black hole to make the shot. A single neutron unaccounted for could bend the spacetime that the photon traverses in such a way that it misses the event horizon. So for all practical purposes, you cannot ever be certain if your photon made it out or not, especially after your first second beyond the event horizon.

There's also the little matter that your photon, if it does make it out, may have done so billions of years later, and possibly with a wavelength longer than the diameter of the event horizon, which might make it appear as though it were radiating from the event horizon itself, rather than somewhere inside. It would convey no information across. It would probably look exactly like the Hawking radiation.

> But you literally need a perfectly accurate and perfectly predictive model of all mass in and around the black hole to make the shot

I think where you're going wrong is assuming that gravity acts by "distorting angles". Like, near a massive object, maybe more than 180 degrees point downwards, and less than 180 point upwards. In a black hole, then, maybe 360 degrees (minus epsilon) point downwards, and there's a peephole pointing back up. You might litter breadcumbs along that trail, or unspool some twine to find your way back.

Alas, that's not the picture we have. It's closer to the truth to say that gravity gives space a "base velocity" (downwards) equal to the escape velocity at that point. Inside the Schwartzchild radius that velocity is higher than the speed of light, and because you can't go faster than light you can't get out.

>> The instant you entered the black hole, reality would split in two.

Ah, I'd assumed this meant that "you" would split in two, both copies being "in the same universe". I don't know enough about the physics to know which is meant.