Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
That's because most physicists expect that new physics, probably some form of quantum gravity, will come into play before the singularity is reached, so that there won't actually be one. Which is a perfectly reasonable expectation.
> The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
Kerr misrepresents the actual opinion of most physicists in that paper. The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Practically nobody believes that there are actual curvature singularities; they believe that GR is not correct in that regime, and some kind of new physics is involved, as I noted above. So in the sense Kerr uses the term, there are practically no "singularity proponents" at all. He is attacking a straw man.
Maybe it was my misfortune, but the university professor who taught me GR thought that singularities are "real", so this anecdote (sadly) disproves your absolutist opinion. And there are heaps of real papers from real scientists which seriously discuss potential consequences of having "real" singularities. And if we'll take a look at the pop-sci coverage, it's orders of magnitude worse... Just see the Veritasium video linked in the other comment.
Did he teach it from Wald's classic textbook? Wald, Chapter 9, explicitly disclaims any belief that singularities are real and explicitly says the classical GR prediction of singularities indicates a breakdown in the theory in that regime. Other classic textbooks like Misner, Thorne, & Wheeler say similar things. So do many peer-reviewed papers published in the field.
Sadly, I am not surprised, though I am disappointed, that many university professors do not know the subject they are teaching well enough to be aware of what I have just said. But the fact remains that the statements I have described are the ones Kerr (who certainly cannot claim ignorance on the part of whoever taught him GR as an excuse) should have been looking at to gauge the prevailing opinion of actual researchers in the field. But he didn't.
> if we'll take a look at the pop-sci coverage
Nobody should be relying on pop science to learn actual science. Nor should anyone claim that pop science coverage is an accurate gauge of the opinions of actual researchers in the field.
But the Nobel prize was awarded for the Singularity Theorem(?). Maybe Kerr is attacking something in between a straw-man and a steel-man? Clay-man?
Sure, because those theorems played a very important role in clarifying exactly what classical GR predicts in these regimes, and therefore in focusing the efforts of physicists in fruitful directions. That remains the case even if it turns out that in our actual universe, at least some of the assumptions of those theorems are violated so that there aren't any actual singularities. One of the key contributions of the singularity theorems was in making those assumptions clear, so that physicists who did not believe singularities were physically realistic knew exactly where to look for other possibilities.
I.e. if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
No. All we have shown is that there are compact regions in our universe that look like event horizons--but could also be just apparent horizons. To know that they are actual, true event horizons, we would have to know the entire future of the universe, since an event horizon is a globally defined surface in spacetime and its presence and location cannot be known unless you know the entire spacetime, i.e., the entire future. If you don't know that--and of course we don't--you can't tell an actual event horizon from an apparent horizon.
> isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc?
None of these are actual features of actual event horizons (or apparent horizons, for that matter). The first is a missatement--the gravity of the object is not "sealed off" and can still be detected outside. The second is an illusion caused by an unfortunate choice of coordinates. The third is pure speculation, and speculation which does not appear to have gained much traction.
> if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
Most physicists do not believe that either of these (if I take "super heavy pokemon" to mean "some kind of ordinary compact object like a neutron star") is what actually happens.
We haven’t observed event horizons except as fuzzy highly processed images. They look a lot like what you’d expect from a black hole, but we have hardly ruled out all possibilities. And there are theoretical effects near an event horizon for charged and spinning black holes, that as far as I know we’ve never been able to observe.
It doesn’t seem out of the question that a black hole without a singularity might exhibit behaviour near the event horizon that is measurably different from the theories we have now.
No. Your wrong claim about the slowing of time flow has nothing to do with those.
> Have you read the Kerr paper?
Yes. It doesn't say anything like what you were saying. I have commented on it elsewhere in this discussion.
Any GR textbook. I referenced two classic ones elsewhere in this discussion.
Or, for that matter, you could read the classic paper by Oppenheimer and Snyder which was the first published model of actual gravitational collapse to form a black hole. And was published in...1939. What I'm saying is not news.
> Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
That's because "science communication" sucks at accurately portraying the actual science.
Completely agree.
There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon.
The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible.
Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
If so, then a followup: why can’t we imagine an object of arbitrary density sufficient that the massive object is also small enough? Why does it need to be a mathematical discontinuity or otherwise goofy universal exception-to-the-rule? We obviously can’t observe it, so isn’t it simply that the mathematics expresses the fact that we cannot observe it?
No, it isn't. It is impossible for a stable compact object made of ordinary matter that is smaller than the event horizon size for its mass to exist. In fact it is impossible for a stable compact object made of ordinary matter that is smaller than 9/8 of its event horizon size to exist. This result is known as Buchdahl's Theorem.
The one thing I'm not 100% sure on is whether an event horizon can form. I have never been able to shake the feeling that with Hawking radiation giving each black hole a large but finite lifetime and with matter falling in beyond the infinite future (to outside observers) you'd have the incoming matter encounter the death of the black hole before it can fall in. This would make a black hole a kind of explosion powered by Hawking radiation that looks frozen in time to outside observers.
Lacking a complete theory that can adequately explain both Hawking radiation and general relativity it's quite hard to know if this makes any sense. Though I can't rule out an answer already exists but is simply not known to me.