4 ms·

Consider this argument: if you can see the person falling in for "infinitely long", then you can actually communicate with this person falling in. For example,

Consider this argument: if you can see the person falling in for "infinitely long", then you can actually communicate with this person falling in. For example, through light or gestures. Since you can communicate, this means the image you see of the person falling in is not just an illusion or artifact of general relativity, you both must agree on reality, that the person still hasn't crossed the horizon.

corethree 3y ago
Yeah, you're right. It's not an optical illusion. I think I'm wrong about the infinite acceleration part though. We never observe the person hitting the event horizon so time never actually freezes. It just gets really really slow and approaches the point of time freezing. The person falling into the black hole will have time in the outside universe accelerate relative to him. It will get faster and faster and when he hits the event horizon we don't know what happens as that's the singularity where the math no longer works. We can speculate that time actually freezes but that leads to paradoxes.
- deleted 3y ago
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lamontcg 3y ago
Nope, your signals also slow down and never reach them, and they appear to be 'frozen' and cannot respond to you. As they approach the event horizon their "reflexes" slow down in a kind of Zeno's paradox. Your signals never reach the infalling observer, so you cannot communicate. And they have a finite amount of reaction time as they fall through the black hole, but to your perspective that gets smeared over an infinite amount of time in the future. This is only due to the fact that you both disagree over how to measure space and time at the event horizon. To the infalling observer this isn't of any consequence though since all actual physics is local and your ruler and stopwatches aren't relevant to the infalling observer.
- gnramires 3y ago
  If their light reaches you, your light could reach them, this is a reciprocity condition that I believe is valid in GR too (because effectively light follows null geodesics, and it can always go both ways, geodesics don't have a preferred direction). It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down. In fact, as they approach the Event Horizon, they would appear to experience diverging time dilation. So in effect, they become "frozen in time". But remember that you can always maintain two-way communication with light. The very fact that both the outside observer can always see the infalling person and the infalling person can always see the outside world imply there can be two-way communication all the way up to crossing the event horizon (which would take infinitely long for the outside observer). This ought to be enough to conclude the infalling person can in fact never reach the event horizon.
  - lamontcg 3y ago
    > It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down. And the infinite passage of all external time is observed in a finite time by the infalling observer. All of the communication from the external observer is going to become a high frequency chirp at the event horizon, and the last message the infalling observer could send will take nearly an infinite amount of time to reach the external observer. And in the next instant of subjective time by the infalling observer they will be past the event horizon and unable to communicate any more.
    - gnramires 3y ago
      Sure, but it seems this is unphysical if this passage occurs after essentially an infinite time for an outside observer. Before crossing the event horizon, the infalling person sees the whole history of the universe. But the problem is, if crossing the horizon takes infinitely long, how could the horizon form in the first place? That's what I've never seen addressed to a satisfactory degree.

corethree 3y ago

Yeah, you're right. It's not an optical illusion. I think I'm wrong about the infinite acceleration part though. We never observe the person hitting the event horizon so time never actually freezes. It just gets really really slow and approaches the point of time freezing. The person falling into the black hole will have time in the outside universe accelerate relative to him. It will get faster and faster and when he hits the event horizon we don't know what happens as that's the singularity where the math no longer works. We can speculate that time actually freezes but that leads to paradoxes.

deleted 3y ago
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lamontcg 3y ago

Nope, your signals also slow down and never reach them, and they appear to be 'frozen' and cannot respond to you. As they approach the event horizon their "reflexes" slow down in a kind of Zeno's paradox. Your signals never reach the infalling observer, so you cannot communicate. And they have a finite amount of reaction time as they fall through the black hole, but to your perspective that gets smeared over an infinite amount of time in the future. This is only due to the fact that you both disagree over how to measure space and time at the event horizon. To the infalling observer this isn't of any consequence though since all actual physics is local and your ruler and stopwatches aren't relevant to the infalling observer.

gnramires 3y ago
If their light reaches you, your light could reach them, this is a reciprocity condition that I believe is valid in GR too (because effectively light follows null geodesics, and it can always go both ways, geodesics don't have a preferred direction). It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down. In fact, as they approach the Event Horizon, they would appear to experience diverging time dilation. So in effect, they become "frozen in time". But remember that you can always maintain two-way communication with light. The very fact that both the outside observer can always see the infalling person and the infalling person can always see the outside world imply there can be two-way communication all the way up to crossing the event horizon (which would take infinitely long for the outside observer). This ought to be enough to conclude the infalling person can in fact never reach the event horizon.
- lamontcg 3y ago
  > It's true that infalling observers will experience outside time speeding up, and conversely outside observers will see infalling objects slow down. And the infinite passage of all external time is observed in a finite time by the infalling observer. All of the communication from the external observer is going to become a high frequency chirp at the event horizon, and the last message the infalling observer could send will take nearly an infinite amount of time to reach the external observer. And in the next instant of subjective time by the infalling observer they will be past the event horizon and unable to communicate any more.
  - gnramires 3y ago
    Sure, but it seems this is unphysical if this passage occurs after essentially an infinite time for an outside observer. Before crossing the event horizon, the infalling person sees the whole history of the universe. But the problem is, if crossing the horizon takes infinitely long, how could the horizon form in the first place? That's what I've never seen addressed to a satisfactory degree.