5 ms·

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

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.

logfromblammo 11y ago
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.
- repsilat 11y ago
  > 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.
  - logfromblammo 11y ago
    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.
    - repsilat 11y ago
      > 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.

logfromblammo 11y ago

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.

repsilat 11y ago
> 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.
- logfromblammo 11y ago
  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.
  - repsilat 11y ago
    > 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.