4 ms·

Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fr

Popular scientists have been saying it for decades and always use the flexible rubber sheet as the learning aid. Serious physicists tended to stay out of the fray, preferring to just "shut up and compute", as they do for quantum mechanics. It's a relatively recent development that physicists have entered the fray and say maybe curved spacetime isn't a model, maybe it's reality. I urge caution to all as we have no evidence that the model of curved spacetime is indeed reality. I think the caution is warranted considering that GR contains singularities, which is a sign the theory has issues.

Filligree 2y ago
GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole. At larger scales, however, fluid mechanics is extremely reliable.
api 2y ago
What if singularities do in fact exist? What issues does that create? They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.
- Filligree 2y ago
  We’d have no way to predict what they do, since the math breaks down. This is fine if you treat the math as only a model, and you’re willing to accept that it can’t predict the behaviour of a thing that can’t affect you anyway, but given your wording — “do in fact exist” — I assume that’s not what you mean. A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. At a minimum it would need additional postulates about computational order that allows the inside of the black hole to be NaN without that causing external reality to seize up.
  - taylodl 2y ago
    > “and now the universe bluescreens”. I love it!
  - toast0 2y ago
    > A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. A theory that provides useful predictions in some conditions and doesn't provide useful predictions in other conditions is a useful theory if you can determine which conditions are which. My lay understanding is general relativity provides useful predictions as long as mass isn't too dense (black hole) and as long as nothing breaks the speed limit; which is pretty general for everyday use, even if it doesn't cover the whole universe.
- layer8 2y ago
  Singularities mean that the math stops working, so we don’t know what’s really happening there, the mathematical model is failing. We’d like to have a working model.
  - api 2y ago
    It would be fascinating if the universe actually has a “NaN” phenomenon that conveniently can never be observed or directly interacted with because it’s always inside an event horizon.
    - taylodl 2y ago
      Event horizons aren't physical objects, they're mathematical constructs.
atombender 2y ago
A fun fact about black holes that isn't often mentioned is that a black hole has two singularities. One is in the center — the one people always talk about — and the other one is at the event horizon. My impression is that the second one is eliminated by choosing to represent the calculations with a coordinate system to eliminates it at the geometrical level, but it's unclear if this is a "hack" or not. But I find it interesting (as a layman) that one can be handwaved away as irrelevant, while the other cannot.

Filligree 2y ago

GR is a theory of the bulk. It’s like fluid mechanics, which treats fluids as infinitely divisible and doesn’t acknowledge the existence of molecules: There’s no way it’s accurate at small enough scales that molecules matter, which is what would be happening inside a black hole. At larger scales, however, fluid mechanics is extremely reliable.

api 2y ago

What if singularities do in fact exist? What issues does that create? They might not be observable if superextremal black holes don't exist and thus naked singularities do not exist.

Filligree 2y ago
We’d have no way to predict what they do, since the math breaks down. This is fine if you treat the math as only a model, and you’re willing to accept that it can’t predict the behaviour of a thing that can’t affect you anyway, but given your wording — “do in fact exist” — I assume that’s not what you mean. A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. At a minimum it would need additional postulates about computational order that allows the inside of the black hole to be NaN without that causing external reality to seize up.
- taylodl 2y ago
  > “and now the universe bluescreens”. I love it!
- toast0 2y ago
  > A theory that purports to describe how the universe actually functions, can’t have places where the theory says “and now the universe bluescreens”. A theory that provides useful predictions in some conditions and doesn't provide useful predictions in other conditions is a useful theory if you can determine which conditions are which. My lay understanding is general relativity provides useful predictions as long as mass isn't too dense (black hole) and as long as nothing breaks the speed limit; which is pretty general for everyday use, even if it doesn't cover the whole universe.
layer8 2y ago
Singularities mean that the math stops working, so we don’t know what’s really happening there, the mathematical model is failing. We’d like to have a working model.
- api 2y ago
  It would be fascinating if the universe actually has a “NaN” phenomenon that conveniently can never be observed or directly interacted with because it’s always inside an event horizon.
  - taylodl 2y ago
    Event horizons aren't physical objects, they're mathematical constructs.

atombender 2y ago

A fun fact about black holes that isn't often mentioned is that a black hole has two singularities. One is in the center — the one people always talk about — and the other one is at the event horizon. My impression is that the second one is eliminated by choosing to represent the calculations with a coordinate system to eliminates it at the geometrical level, but it's unclear if this is a "hack" or not. But I find it interesting (as a layman) that one can be handwaved away as irrelevant, while the other cannot.