[achille]

> "At the beginning of time and the center of every black hole lies a point of infinite density called a singularity"

my understanding was that this was d̶i̶s̶p̶r̶o̶v̶e̶n̶ mathematically incorrect:

- https://news.ycombinator.com/item?id=38636225

- sabine's take: https://www.youtube.com/watch?v=nz55jONtFAU

edit: disproven -> mathematically incorrect

[ziddoap]

PBS Space Time's take on Kerr's paper:

https://www.pbs.org/video/what-if-singularities-do-not-exist...

Echoing JumpCrisscross' sentiment, though. "Disproven" is way too strong of a word.

[wigster]

"kerr black holes" should surely have been "black-kerr holes" ;-)

[mr_mitm]

They are singularities in the framework of general relativity, i.e. while ignoring quantum mechanics. I think most people expect the right version of quantum gravity to make the singularities go away, but studying classical GR is worth it on its own, so it's often ignored like in this statement you quoted.

[goatlover]

What if gravity is non-linear and thus collapses the wave function? I think Penrose has suggested gravity as an objective collapse interpretation. The measurement problem still hasn't been resolved, but we observe a classical world around us, despite the fact that decoherence simply spreads the superposition of interacting quantum systems to the world. Gravity could be what prevents the linearity of quantum systems from putting the entire universe into superposition.

[mr_mitm]

Gravity is non-linear (as in: the Einstein field equations are non-linear differential equations).

That has nothing to do with the measurement problem. Also, the measurement problem is only a problem of the Copenhagen interpretation. It doesn't exist in the many worlds interpretation.

[cma]

> Also, the measurement problem is only a problem of the Copenhagen interpretation. It doesn't exist in the many worlds interpretation.

Doesn't many worlds require branching into numbers of branches that would in some cases be irrational numbers? And you have to have some kind of index on the branch to make some of them physically distinguishable enough to still maintain probability. If equivalent branches are in there it's hard to explain how a 75%/25% branch would be distinguishable as a probability to an observer without some kind of extra index like information that has them land in the 75% more often. ( https://en.wikipedia.org/wiki/Many-worlds_interpretation#Pro...)

> That has nothing to do with the measurement problem.

He refered to I think the Diósi-Penrose model, where it would:

https://en.wikipedia.org/wiki/Di%C3%B3si%E2%80%93Penrose_mod...

[GoblinSlayer]

Born rule doesn't need index, it needs statistics of a sequence of measurements, then statistics is rationalized as a result of a stochastic process.

[cma]

In that link that says that was Everett's initial attempt to solve but it has been debated and extended. I only have a podcast understanding of it, and have heard the popular proponents of many worlds like Sean Carrol say that is the biggest problem that needs more development, he has his own self-locating thing but there are many other approaches.

But on the other point, how can there be an irrational number of branches to sample these statistics from? I just can't visualize the type of structure that would have that but I'm sure it is more subtle. I've heard the branches aren't branches under MWI but instead are something more continuous and I guess I don't understand it at that point.

[goatlover]

Gravity as in the actual force or field, not the current equations. You mentioned quantizing gravity, a few physicists like Penrose doubt it can be done. MWI is not a consensus, and there are more than two interpretations. Objective collapse theories have also been proposed, at least one involves gravity as the mechanism.

[raattgift]

The nonlinearity of gravity is obvious even in the solar system - it was discovered because Mercury's orbit, when described with the linear theory of Newton's universal gravitation, behaved as if there were an additional hidden mass between its orbit's periastron point and the sun. Or, if you prefer, summing all the sources of gravitation in the solar system is insufficient for describing all the observed orbits in the solar system. (Indeed, even if you summed all the sources of gravity in the galaxy, you wouldn't get the evolution of the relatively eccentric elliptical orbit of Mercury right.)

Precision measurements by satellites around Earth and spacecraft scattered around the solar system reveal the nonlinearity of gravitation, as do precision measurements of systems like Hulse-Taylor and PSR J2222-0137.

A linear superposition law for gravitation is consequently unavailable - again, this can be seen in the solar system, where a linear superposition model helped find a real mass (Neptune, where ultimately the telescope targetting was driven by Urbain Le Varrier's detailed study of the orbit of Uranus in the 1840s, assuming the validity of Newtonian gravitation), but misled astronomers into decades of futile searches for "Vulcan", a hypothetical body inside Mercury's orbit.

There are a bunch of ways one can capture the nonlinearity of observed gravitation. A nice slogan: gravitation self-gravitates. A nice theoretical framework: Newton-Cartan gravity is great for exploring the failure of linear gravity. An easier theoretical approach: the relativistic two-body effective radial potential energy <https://en.wikipedia.org/wiki/Two-body_problem_in_general_re...> or less encyclopedically <https://spaceengine.org/articles/the-anomalous-advance-of-th...> where you can see the same effective potential term written slightly differently, with more about Mercury's orbit.

Quoting the latter:

  Again I don't want to get lost in math, but it's worthwhile
  just to look briefly at what the math is saying here.
  Notice this still has the exact same two terms from the
  Newtonian effective potential: an attraction that goes as
  -1/r, and a repulsion that goes as +1/r^2.  But a new term 
  is added: another attractive term that goes as -1/r^3. This
  means that at very small radii, the -1/r^3 term dominates, 
  and gravitation becomes attractive again, dominating even 
  over the centrifugal effect of your orbital velocity.  

The increased attraction mimics an additional mass in a linear theory.

No linear theory of gravitation is viable for known planetary and astrophysics. At best one can come up with a quasi-linear theory.

This is calculationally unfortunate: solving the full nonlinear Einstein Field Equations exactly is fiendishly hard. Where one can use a linear approximation, essentially every physicist would choose to do so. Indeed, Einstein invented linearized gravity (and various other approximation techniques). Unfortunately, linear theories of gravity can only ever be approximate, as they fail to deal with multibody systems, systems where orbital velocities are even only thousands of kilometres per second, where one wants to trace out radiation (including gravitational radiation), and so forth. Convincing proof of incompatibility between any possible linear theories of gravity and numerous observed physical orbits have been known since the 1960s. Roman Sexl did some really interesting work (alone and with collaborators like Otto Nachtmann) in that area in that decade.

These days one can turn to box 7.1 (sec. H) of Misner Thorne & Wheeler as a textbook starting point.

None of this really has anything to do with quantum mechanics, except with respect to a correspondence principle (e.g. Fraunhofer-like spectral lines have their origin in quantum mechanics, and we can see how gravity rather than just motion affects them).

[GoblinSlayer]

Decoherence splits your state into superposition of several coherent states then you observe one classical result in each state in superposition, that's why it looks classical.

[nathan_compton]

As far as I know the limits on physical collapse theories are extremely strong and there are some reasonably good philosophical reasons to doubt them as well. I don't have the text in front of me at the moment but Aaronson's Quantum Computing Since Democritus has a section on this, I think. If physical collapse were true then the implications for quantum computing would beggar belief. P=NP stuff.

[empath75]

Sabine doesn't even say it's disproven, and the paper doesn't claim that it's disproven, it just claims that one of the earliest proofs that it was a singularity was incorrect. There's an important distinction there. If someone points out a flaw in a proof of the pythagorean theorem, that doesn't mean the theorem is disproved, it just means that the proof was wrong.

[JumpCrisscross]

> my understanding was that this was disproven

To the extent anything in this discussion can be absolute, it's the wrongness of your statement. Nothing about singularities has been empirically proven (or disproven).

[credit_guy]

You don’t seem to be new around here, so this quote from this forum’s guidelines is more for the benefit of others

  > When disagreeing, please reply to the argument instead of calling names. "That is idiotic; 1 + 1 is 2, not 3" can be shortened to "1 + 1 is 2, not 3."

[JumpCrisscross]

You’re right. My apologies to OP and y’all. Can’t edit, but the snark was uncalled for.

[slwvx]

I don't see any name calling. Could you eplicitly state what the problem is?

[JumpCrisscross]

The comment stands factually with just the second sentence.

[wavewrangler]

But that is not name-calling. In your words, that is snark. Are all snarky comments also name calling now, and vice-versa? One could argue that the comment stands factually with just the first sentence, too. It’s strong, yes, but very different from name calling. I think it’s a valuable tool in learning in that it [snark] reminds us to choose our words very carefully, and we all need reminders of that sometimes. Also…

[strogonoff]

There are plenty of various oblique ways of expressing similar sentiment that can hurt an educated person much more than name-calling (which, conversely, can be more amusing than anything). Digs like “the wrongness of your statement” are definitely far on relevant spectrum.

Therefore, I believe the rules against name-calling are not literal. As you noticed, attempting to restrict discussion more strictly would make it bland, but on the other hand when it comes to literal name-calling in a civilized discussion it’s way past all limits.

Tangentially, I was surprised to learn recently that merely the use of specific “you” in an argument is already considered unnecessary and perceived as somewhat confrontational. Haven’t confirmed it from multiple sources (not sure how to search for), but in hindsight it makes sense: the mood changes, and the argument can quickly devolve thereafter. I suspect it might be something from psychotherapy practice.

[oneshtein]

We can empirically prove that gravitation cancels out in the gravitational center of an object, if we will dig into Moon.

[mr_mitm]

What does this have to do with singularities? No one expects any kind of singularity anywhere around or in the moon.

[oneshtein]

Singularity is not possible at 0G, isn't?

[JumpCrisscross]

> Singularity is not possible at 0G, isn't?

One divided by zero is a singularity. Singularity, mathematically speaking, means your math breaks. Calculus gets around this problem with limits. But there is absolutely nothing about physics that prohibits singularities, even gravitational singularities, in a zero G space because by definition a gravitational singularity per se has an undefined G.

[oneshtein]

Singularity means that at least some barions will be at the same place in the same time, which against nature of fermions.

Moreover, it hard to imagine that Higgs bosons will act at same place and time with same effectiveness.

So, I cannot believe in a singularity unless it will be physically demonstrated.

[api]

If matter falling into a singularity never reaches it because time slows down infinitely as you approach, wouldn’t this be a physical representation of a mathematical limit from calculus? The actual literal 1d singularity never forms but it is approached infinitely close.

[pdonis]

In the GR model of black holes, the singularity is at the end of time inside the hole, not the beginning.

[Twisol]

I think the "singularity at the beginning of time" being referenced here is the one postulated before / at the instant of the Big Bang.

[pdonis]

Ah, I see, I was parsing the sentence wrong.

[uoaei]

A more diplomatic and uncontroversial way to put it is that the event horizon is the only thing we have any evidence for.

[GoblinSlayer]

We don't have evidence for event horizon. Black hole is a hypothetical object to begin with, it exists only in mathematics, what evidence.

[blovescoffee]

The photos we have of black holes?

[nathan_compton]

I am a physicist but I think he is more or less technically correct. We have photos of black hole like objects but no evidence that they conform to the object described in general relativity except in broad terms. There are the obvious issues with quantization, for example, but there are also multiple ways we can formulate GR-style theories which give different black-hole solutions which have not yet been disambiguated by experiment.

I don't think there is any harm really in calling the objects we have "taken photographs of" (these images are model dependent, so to call them photographs is a bit of a stretch) "black holes," but if we want to be totally precise a black hole is a specific concept in GR, a theory which most people think is incomplete, and we have only found some correspondences between that theoretical object and some observations in the world.

It is an interesting exercise to apply this sort of thinking to (for example) electrons. Do we know electrons exist? In an informal sense, obviously, but in a more detailed sense I would argue care must be taken. We know that QED, for example, is not renormalizable, and thus we ought to be careful to distinguish the notion of "QED electrons" from "Standard Model Electrons" from "the things that leave exposures on our detectors."

But we do know considerably more about the qualities of the physical objects we measure and call electrons than we know about the qualities of the physical objects we measure and call black holes. I don't think its unreasonable to be careful about these things.

[GoblinSlayer]

Observation of collapsars nicely corresponds to GR predictions about collapsars without event horizon, there's no real need to invoke black holes here. You might call them black holes, but I imagine people will be confused why these kinda black holes don't have event horizon, singularity, coordinate discontinuity, information paradox, cosmic censorship and all that stuff black holes are famous for. They already conclude there's evidence for event horizon, because it's a widely advertized feature of black holes and there's a photo of black hole.

[GoblinSlayer]

We only have photos of collapsars, not black holes. For mathematics the difference is big, only black holes have peculiar mathematical properties.

[braiamp]

> We only have photos of collapsars

Yes, we have photos of collapsed stars, some of which were above the Tolman–Oppenheimer–Volkoff limit and became black holes. When they are not above it, they either become white dwarf or neutron stars.

[GoblinSlayer]

Volkoff's calculation demonstrates that a star above the limit begins to collapse. It doesn't support the claim that they already became black holes, especially this can't happen globally with galilean synchronism even under slightly unrealistic assumptions. In the end, it's a mathematical calculation under assumptions; if you want to connect it to reality, you need to understand what it claims exactly and estimate what error is introduced by difference between its assumptions and reality.

[devnullbrain]

That definition requires a higher standard of proof than a Nobel prize committee.

[GoblinSlayer]

A photo of a star is cool, why not give a prize for it. Or maybe they use attention economy. You can't exactly blame them for attention economy, can you?

[oneshtein]

Two event horizons, because gravitation cancels out in the center of a black hole.

ps. Energy is sucked up from the center by second event horizon, but matter is pushed inside, forming a dense and cool crystal, a solid foundation for second order effects to play.

[uoaei]

That assumes there is gravity, or even universe, "inside" the black hole. We don't have any evidence of that.

[mr_mitm]

Black holes are a prediction of general relativity. The same theory predicts that all properties of spacetime exist up until the singularity. You cannot simultaneously believe in black holes and some sort of discontinuation of spacetime before the singularity.

[uoaei]

That is a theory that makes predictions, not evidence. As you may note in my comments above, I am speaking exclusively to evidence.

[mr_mitm]

It doesn't make sense to talk about black holes outside the context of GR. What do you even mean by black hole if you can't describe it in the language of GR?

[oneshtein]

Occam's razor says that you must present a proof that they are not existing in a black hole.

[JumpCrisscross]

> Occam's razor says that you must present a proof that they are not existing in a black hole

Occam’s razor absolutely doesn’t predict that the weird thing that breaks physics occurs twice and then precipitates a crystal.

[oneshtein]

Physics is fine, it just model, which breaks.

However, we can see that stars are eaten by black holes, and then can be partially released back years later, so it's proven that 1) «an event horizon» exists, 2) matter can pass the «event horizon» in both directions, 3) light cannot pass the «event horizon» in one direction.

I do not introduce a new physics, like a «singularity», without any evidence. Occam's razor is in my hands now.

[biimugan]

Your first link goes to a 2023 arXiv pre-print that never landed in any journals as far as I can tell (could be wrong though). And there seems to be some controversy about whether Kerr's math shows what he says it shows.

This is the danger of trying to sensationalize science and putting any special weight on science influencers, especially ones who very often seem gung-ho about any story that challenges the status quo despite the evidence.

[monocasa]

To be fair, it's written by literal Roy Kerr.

https://en.wikipedia.org/wiki/Roy_Kerr

[spwa4]

Layman opinion here: If a black hole forms, the point where it forms is an event horizon, but not a singularity. Then, while things get worse, it disappear from the universe.

So why would a singularity ever form? And what can't be formed, can't exist.

[mr_mitm]

Cosmologist here, the event horizon is not a true singularity. There is a singularity in certain coordinates, but it goes away when doing a coordinate transformation. There is nothing physically strange going on at the event horizon. The physical singularity is only at the center.

[rbanffy]

I have the (layman) impression that there is no inside - that spacetime is so stretched around the event horizon that there is no spacetime beyond it.

But, then, I've never seen anywhere that the mass of the black hole (which is very much a real thing that exists in spacetime) is distributed over the event horizon, which would be at the biggest amount of mass a given region of spacetime can hold, and is not concentrated on a point with infinite density inside it.

[nh23423fefe]

black holes have an interior, you wouldn't notice if you passed the event horizon of a large enough black hole.

[spwa4]

Wouldn't you?

1) you'd see infinite space dilation. Distance between you and anything would increase to infinity. It doesn't even matter if it's closer to the black hole from you or further away. I wonder if it even occurs with things that fall into the black hole with you.

2) You'd see time pass infinitely fast. I'd say "behind you", but not really of course: anything already fallen in you wouldn't see, and you wouldn't see anything exactly at the same position as you do, so anything you can see would see time pass infinitely fast. You'd be "transported to the end of time".

3) You'd see the whole universe compress into a single point (like in a lens I presume)

I wonder if this wouldn't present problems. The whole point of Hawking radiation is that the black hole will stretch out the wave function of light, in both ways (it "transports energy" from the virtual photon falling in to the particle escaping. The particle falling in loses energy, and keeps losing energy forever, while the particle escaping gains energy and since gravity has infinite range, it also keeps gaining energy forever, it's just that the amount decreases exponentially, never quite reaching zero. The particles never quite become real particles, however virtual particles do interact, so I'm not quite sure what makes them virtual, aside from their origin). Wouldn't it stretch out the wave function of any particle? And if you stretch out the wave function of a particle, you reduce it's energy. If you stretch it out infinitely you reduce it's energy, in the limit (but the limit is the event horizon), to zero. And the particles themselves aren't zero size: they would "see" this happen. Even the famous "point-particle" that is the electron has a wave function that occupies a portion of space (in fact, it occupies a surprisingly big volume)

So the mass of black holes could be in these frozen particles being teleported to the end of time, while losing energy.

[ndsipa_pomu]

> You'd see time pass infinitely fast

I've often wondered about that - whether you'd see to the end of the universe. However, I've seen various descriptions using Penrose diagrams that show that events from far enough in the future would never be able to reach the intrepid explorer before they hit the singularity at the middle of the black hole (assuming that one exists).

[GoblinSlayer]

You won't. Infinitely fast time means infinitely strong Hawking radiation that will just burn you instantly before you reach event horizon, but you will first catch a glimpse of heat death of the universe.