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>The fact that both have the same name They don't just have the same name, they are the same thing. A Schwarzschild black hole has both: a removable singulari
by sigmoid10 1mo ago
>The fact that both have the same name
They don't just have the same name, they are the same thing.
A Schwarzschild black hole has both: a removable singularity at the event horizon that is just an artefact of a particular choice of coordinates and a true non-removable mathematical singularity at r=0 where curvature really does go to infinity. It also wouldn't be much of an issue in classical physics, because this singularity is always hidden from outside observers, so the mathematical weirdness there can't screw with your normal predictions in space outside the black hole. The problems start once you consider quantum mechanics, because any such singularity will break unitarity (a fancy way of saying that probabilities must add up to 1), which means your theory as a whole can no longer make predictions. This has opened a whole can of worms with a bunch of solution attempts, which are all sadly untestable for the foreseeable future.
- NooneAtAll3 1mo agoOSM - slight generalization of Schwarzshild BH, where you take evolving spherically-symmetric mass distribution instead of point mass - shows that point singularity in the middle can be naked (aka observable), so it's not just QM that causes worms... https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model
- sigmoid10 1mo agoIt is not difficult to construct geometries with naked singularities. Reissner, Nordström, Weyl and others individually came up with one long before Oppenheimer. You can also construct geometries where faster than light travel is possible. But all of these suffer from fundamentally unphysical energy conditions. Quantum mechanics may change the picture because it allows weirder energy states than normal physics because everything fluctuates. But it is unknown if and how this actually affects gravity.
- Ajoha 1mo agoThanks, I enjoyed your answers !
- kadoban 1mo ago> The problems start once you consider quantum mechanics, because any such singularity will break unitarity (a fancy way of saying that probabilities must add up to 1), which means your theory as a whole can no longer make predictions. How is this any different than classical? Isn't it still just an ~impossibility hidden behind an event horizon in either model?
- flavenstein 1mo agoWith our current understanding, baryon and lepton number are not conserved as a black hole radiates. I think this is a better demonstration of the incompatibility with classical and quantum mechanics.
- sigmoid10 1mo agoBaryon and lepton number conservation are what's called "accidental symmetries" in the standard models, meaning there is no real underlying symmetry that would conserve them. In fact many extensions of the Standard Model don't, while still retaining unitarity. The problems already start once you try to calculate any time evolution of anything, because the Hilbert operator not being unitary means that everything breaks. Even the total energy in a closed system might vanish or blow up to infinity. You can't calculate anything under these conditions.
- inigyou 1mo agoWe don't actually know if a black hole has an inside. Some theories/hypotheses say spacetime just stops at the event horizon.
- catlifeonmars 1mo agoThis is like saying the Riemann zeta function is only defined for real numbers. You can always extend the singularity mathematically by incorporating new axioms. My point is, it’s not super meaningful to argue whether a black hole has an inside.
- inigyou 1mo agoI don't mean the coordinate singularity, I mean there is no more spacetime after that.
- skirmish 1mo agoWhy would that happen? In a different coordinate system (Kruskal–Szekeres coordinates) nothing special happens at the event horizon at all.
- inigyou 1mo agoWhy wouldn't it happen? From the perspective of anyone outside the black hole, nothing can ever enter it. What if that's just true?
- catlifeonmars 1mo agoHave you considered that the apparent event horizon is not uniform for all observers? So does spacetime exist in some frames of reference but not others because those frames disagree on the radius of the apparent event horizon? Also note that in general an event horizon doesn’t require a singularity.
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- eru 1mo agoEven without quantum mechanics, black holes are trouble: Approximately everything in nature rotates. Including black holes. Schwarzschild blockholes do not rotate. Rotating black holes are much more complicated and don't necessarily shield their singularity behind an event horizon.
- sigmoid10 1mo agoRotating black holes are described by Kerr geometries and have more than one event horizon, but still have their singularities hidden from anyone outside behind their inner horizon.
- saidnooneever 1mo agopardon a maybe stupid quesiton but a few comments say this and i wonder. why does it matter that it is not 'visible' for anyone?
- gizmo686 1mo ago"Visible" in this case means "able to influence in any way". If a problem is not able to influence anything, even in theory, then by definition, it cannot possible influence any testable predictions we have.