3 ms·

Personally, I’m not convinced that there is an “inside”. From the outside there doesn’t appear to be one — you never see anything cross the horizon, including t

Personally, I’m not convinced that there is an “inside”. From the outside there doesn’t appear to be one — you never see anything cross the horizon, including the substance that originally formed the black hole! Conversely, observers see the black hole evaporating from just above the surface, with all infalling matter recovered in this way. World lines approach the horizon, hug it closely, then leave. They don’t fall “in”. In my mind scientists filled in a blank with their imaginations, but the blank they are filling in may not even exist as a location. It’s like complex (imaginary) numbers, you can talk meaningfully about solutions to equations: but ordinary +, -, *, / arithmetic “can’t get you there”. A black hole could be like a pinhole stretched out into a larger hole in a stretchy sheet of fabric. From the point of view of an ant walking on the surface the hole is a boundary that can be approached, but not entered, and around it the fabric is highly distorted. There have been some good papers published recently on related topics. For example, Penrose diagrams as typically drawn use a simplification that black holes extend forever forward and backward in time. This allows infinitesimals to add up over infinity, which is a non-physical sleight of hand. Real black holes form over time, and worldlines don’t enter them — they just approach the horizon and then “boil off” in the distant future due to Hawking radiation. There is this obsession in modern physics of clinging to overly simplified models and then treating the edges of their capabilities as real things to discover instead of modelling failures to get past with better models. You can’t climb the mountain range represented by the fold crease in your map.

sigmoid10 2y ago
>Real black holes form over time, and worldlines don’t enter them That's not correct. It only appears that way in some forms of diagrams because of their choice of coordinate system or the way they display lines of constant time/space. But all black holes (eternal Penrose and normal ones) have worldlines entering them normally. The claim that these worldlines would suddenly stop at the horizon for no particular reason is pretty wild (but not completely new). I don't think the majority of physicists would doubt that an observer could fall inside a black hole (particularly large black holes with low tidal forces), because from their point of view there would be absolutely nothing unusual happening at the horizon.
- jiggawatts 2y ago
  From the point of view of a distant observer, objects falling into a black hole become infinitely redshifted as they approach the horizon. Nobody outside the black hole observes anything completely falling in. Penrose diagrams aren't drawn from the perspective of an in-universe observer. They're drawn from the perspective of a god, looking "down" at the universe from outside it. Most such diagrams -- irrespective of coordinate system -- are drawn with these world lines continued into the black hole, typically ignoring the change-over-time of the destination itself. If you were to "ride the world line" into the black hole in the physical universe, you will slow down so that the hole in front of you appears to speed up, evaporating faster and faster as you approach. You never observe falling into a "black" hole, it'll become brighter and brighter and explode in your face as you approach. A black hole is a supernova explosion smeared out in time.
  - sigmoid10 2y ago
    That's not true. You can check out chapter 32 in Misner's Gravitation for a detailed discussion of a free falling observer's worldline into a black hole.
    - jiggawatts 2y ago
      In that section "infinite" is used a bunch of times, which is a non-physical simplification. All of the sections using Penrose diagrams talk explicitly about "the point at infinity", but Hawking showed that black holes don't last for infinite time. You have to keep reading past the overly simplified models. E.g.: right after that section: "But from an external vantage point the star requires infinite time to reach the horizon". In other words, observers don't ever see a black hole form completely, which is an observation that infalling observers can't contradict. Either you believe that there are two different universes for non-infalling and infalling observers, or you have to accept a single physical reality that black holes never completely form, and then evaporate in a finite time.
      - sigmoid10 2y ago

sigmoid10 2y ago

>Real black holes form over time, and worldlines don’t enter them That's not correct. It only appears that way in some forms of diagrams because of their choice of coordinate system or the way they display lines of constant time/space. But all black holes (eternal Penrose and normal ones) have worldlines entering them normally. The claim that these worldlines would suddenly stop at the horizon for no particular reason is pretty wild (but not completely new). I don't think the majority of physicists would doubt that an observer could fall inside a black hole (particularly large black holes with low tidal forces), because from their point of view there would be absolutely nothing unusual happening at the horizon.

jiggawatts 2y ago
From the point of view of a distant observer, objects falling into a black hole become infinitely redshifted as they approach the horizon. Nobody outside the black hole observes anything completely falling in. Penrose diagrams aren't drawn from the perspective of an in-universe observer. They're drawn from the perspective of a god, looking "down" at the universe from outside it. Most such diagrams -- irrespective of coordinate system -- are drawn with these world lines continued into the black hole, typically ignoring the change-over-time of the destination itself. If you were to "ride the world line" into the black hole in the physical universe, you will slow down so that the hole in front of you appears to speed up, evaporating faster and faster as you approach. You never observe falling into a "black" hole, it'll become brighter and brighter and explode in your face as you approach. A black hole is a supernova explosion smeared out in time.
- sigmoid10 2y ago
  That's not true. You can check out chapter 32 in Misner's Gravitation for a detailed discussion of a free falling observer's worldline into a black hole.
  - jiggawatts 2y ago
    In that section "infinite" is used a bunch of times, which is a non-physical simplification. All of the sections using Penrose diagrams talk explicitly about "the point at infinity", but Hawking showed that black holes don't last for infinite time. You have to keep reading past the overly simplified models. E.g.: right after that section: "But from an external vantage point the star requires infinite time to reach the horizon". In other words, observers don't ever see a black hole form completely, which is an observation that infalling observers can't contradict. Either you believe that there are two different universes for non-infalling and infalling observers, or you have to accept a single physical reality that black holes never completely form, and then evaporate in a finite time.
    - sigmoid10 2y ago