Scientists may have solved Stephen Hawking's black hole paradox | Hacker News Reader

Scientists may have solved Stephen Hawking's black hole paradox

phys.org

103 pointsBrajeshwar82 comments

Sabine’s post on this topic might be relevant here:

http://backreaction.blogspot.com/2020/11/the-black-hole-info...

> The Black Hole information loss problem is unsolved. Because it’s unsolvable.

> The black hole information loss problem is not a math problem. It’s not like trying to prove the Riemann hypothesis. You cannot solve the black hole information loss problem with math alone. You need data, there is no data, and there won’t be any data. Which is why the black hole information loss problem is for all practical purposes unsolvable.

> "You need data, there is no data, and there won’t be any data."

Why won't there be any data? Is it just physically impossible to collect or is it an engineering problem?

Because data comes from experiments.

Unless we figure out how to create a black hole in the lab (spoiler, you wouldn't want to) or could get close to a natural black hole (spoiler, you wouldn't want to) you're not going to get any data for this.

>create a black hole in the lab (spoiler, you wouldn't want to)

Current understanding is that there is nothing dangerous about creating black hole in the lab: it’s going to be so small that it will evaporate quickly, and event horizon is so small that it will be very unlikely to capture new matter.

> Current understanding

I.e., if a tiny black hole had formed at LLC and swallowed the Earth, that would have been data.

Recall all the arguments quoting rates of Hawking Radiation from small black holes as if it were proven fact.

>if a tiny black hole had formed at LLC and swallowed the Earth

The point is that tiny black hole, man-made or from the outer space, can't swallow Earth in any reasonable amount of time: when it's gravitationally captured and orbits through Earth internals "forever", it's event horizon is so small, that it's very rarely interacts and captures new matter (after all, matter is not "dense" on the atomic level), so it grows very very slowly, even if it doesn't evaporate at all.

Any light black hole has really minuscule size. For comparison: black hole of mass of Earth is couple centimeters in diameter. Now, if such black hole comes from outer space and hits Earth, almost nothing will be left.

You're right, unless there is unknown physics that happens near a black hole.

It's still important to note that black holes at the atomic level, and gravitational effects of any kind at that level in fact, are entirely speculative science. QM doesn't allow for any kind of gravitational attraction, and GR doesn't account for any of the effects observed at atomic level, so we are left with only speculation.

Microscopic black holes are an entire extra level of speculation over any other gravitational effects. Even in GR, black holes are not exactly consistent with the theory - they are singularities, indicating that (even if we knew nothing of QM) GR can't be a complete theory of nature.

So, discussing what microscopic black holes can or can't do is not much more than discussing what effect photon torpedoes would have on a Holtzman shield (Star Trek x Dune fanfic).

I was thinking of a black hole that would be large enough to do any sort of science with. I personally wouldn't be concerned with it consuming the world, I would be more concerned with it evaporating!

Because the smaller the blackhole is the faster it evaporates, below a certain energy the blackhole would turn all the energy/mass that was put into it into energy close to instantly, i.e. it basically turns into a bomb.

All of the black holes we know for sure exist (those produced by supernovae) are predicted to dissolve so slowly that it would take thousands of years or perhaps even millions of years of observation to produce any data on Hawking radiation.

If small black holes can actually form (which is currently just a theory) there may be some chance to observe and measure them dissolving. However, I believe this requires energies far higher than any particle collider we can expect to build in the next hundred years, so this avenue is also closed.

So, perhaps "never" is a bit pessimistic, but not far off in human terms.

>All of the black holes we know for sure exist (those produced by supernovae) are predicted to dissolve so slowly that it would take thousands of years or perhaps even millions of years of observation to produce any data on Hawking radiation.

Isn't there also an issue that the background temperature of space is still too high compared to the temperature of Hawking Radiation and thus blackholes are still growing, even in 'empty space', because the temperature is greater than what they would lose from Hawking radiation? And this temperature difference will continue for the foreseeable future, even on galactic time scales.

> Is it just physically impossible to collect or is it an engineering problem?

Engineering as in you’d need to create and measure a shielded black hole. (Shielded because you need to isolate it from the CMB.)

Well she seems to disagree with almost anything any other physicist says.

Not at all true, it's just that most of the time her videos get posted or gain attention is when she has a disagreement with some new "groundbreaking" discovery, and often times her disagreements are well warranted. Most of her videos have nothing to do with disagreeing with others.

No, she is just pointing out something that most other physicists fail to point out, particularly when talking to reporters about things that end up in articles like this one: that when physicists talk about "showing" or "demonstrating" or "discovering" something about black holes, they mean "we built a mathematical model that says X", not "we got actual data that confirms our mathematical model that says X". (There is a similar issue when physicists talk about string theory.)

A physical model is way more than a mathematical model. It is based on theories which acquired their epistemic weight elsewhere to lend to the problem being studied. Conclusions made about black holes are made on the basis of QFT and general relativity, unlikely to be overturned so long as the researcher is working far enough from the singularity for the idea of doing QFT on a fixed background spacetime to still apply.

> A physical model is way more than a mathematical model.

Sure, there also have to be rules about how the mathematical quantities in the model correspond to physical quantities that are measured in experiments.

> It is based on theories which acquired their epistemic weight elsewhere to lend to the problem being studied.

Translated into plain language: you can take a physical model whose predictions have been confirmed over some range, and extrapolate it outside that range. Sure, physicists do this all the time.

But that does not mean that the extrapolated predictions are automatically correct. You still have to test them.

> Conclusions made about black holes are made on the basis of QFT and general relativity, unlikely to be overturned so long as the researcher is working far enough from the singularity for the idea of doing QFT on a fixed background spacetime to still apply.

I understand this is a common belief among physicists. That doesn't change the fact that, unless and until we have actual experimental data, these conclusions have not been confirmed.

Also, what is involved in black hole formation and evaporation is not "QFT on a fixed background spacetime". Quantum fields make non-negligible contributions to the gravitational source in these models. That means that, for a fully consistent model, you need a theory of quantum gravity, which we don't have. The actual framework being used basically assumes that a spacetime geometry sourced by the expectation value of the stress-energy tensor of the quantum fields is a sufficiently good approximation. Which is still an assumption, no matter how many physicists believe it, unless and until we have actual data to test it against.

> Quantum fields make non-negligible contributions to the gravitational source in these models. That means that, for a fully consistent model, you need a theory of quantum gravity, which we don't have.

Importantly, I don't think this is true. The gravitational field at the event horizon of pretty much any black hole people care to model, is actually quite "weak", when compared to the Planck scale where quantum gravitational effects are expected to become important. While quantum gravity would be needed to model phenomena deep inside a black hole (near what is referred to as the singularity), phenomena at the event horizon, such as Hawking radiation (and presumably, the phenomena these researchers claim to predict?) can be modeled quite adequately just using QFT and classical gravity.

> The gravitational field at the event horizon of pretty much any black hole people care to model, is actually quite "weak"

More precisely, the spacetime curvature at the horizon of any astronomically significant black hole (i.e., one of stellar mass or larger) is quite small--many, many orders of magnitude smaller than the Planck scale.

While this is true, it's not what I was talking about. Black holes get formed by gravitational collapse of massive objects. The models of that collapse process that were current in the 1970s, when Hawking published his original paper on black hole evaporation, were purely classical. Since then, particularly in the last decade or two, there has been a lot of theoretical work on non-negligible quantum corrections to the collapse process. Not having to do with quantum gravity, but just ordinary quantum fields (like those in the Standard Model) providing corrections that were not known when Hawking's original paper was published, or for another two decades or so afterwards.

Also, even in vacuum, quantum fields (again, not quantum gravity, just ordinary quantum fields like those in the Standard Model) can provide non-negligible corrections. The most obvious one is a nonzero cosmological constant, aka a nonzero vacuum expectation value for the energy density of the "ground state" of the quantum fields. The accelerated expansion of the universe indicates that the cosmological constant is indeed nonzero, but the value implied by those observations is about 120 orders of magnitude smaller than what our best current understanding of quantum field theory gives us. So obviously there is something important missing in our understanding of vacuum quantum fields.

Finally, to get to the main issue I was referring to in my earlier post: the problem with having quantum fields as a source of gravity has nothing to do with the magnitude of the spacetime curvature, it has to do with having superpositions of different quantum field configurations, which means superpositions of different stress-energy tensors. You can't handle that with a fixed background spacetime; there would need to be a superposition of different spacetime geometries. Which requires a theory of quantum gravity.

I agree with you, if the broader point is that she always finds the contrarian position, even if there's no evidence for it.

For example, she makes a point here that you need data to gain useful insight into the black hole radiation problem.

That is not true and has no compelling evidence that there will be true, because she cannot predict fundamental new insights about the future nor is it clear this is a problem strictly dependent on data, since we can build models of our world with computational structures.

> she cannot predict fundamental new insights about the future nor is it clear this is a problem strictly dependent on data, since we can build models of our world with computational structures.

I think she is saying that these models cannot solve anything because multiple contradictory models can each be valid within our current mathematical / theoretical framework. We need actual data to rule some of them out to find a real solution of what is physically happening.

This is a wonderful response in that it points clearly to the modern disconnect between physics theory and the experimentation which led to its greatest advances. Simplifying assumptions can be very useful for didactic purposes, but nature isn't simple. There are many sciences which put observations that threaten the status quo into a (physical or dogmatic) closet.

"the problem comes from combining a certain set of assumptions, doing a calculation, and arriving at a contradiction.... without data, the question is not which solution to the problem is correct, but which one you like best."

To the same extent the problem is unsolvable, it also doesn't exist in the first place. No one has observed Hawking radiation. From an empirical standpoint, the whole thing started as a math problem, so it's appropriate to solve it as a math problem.

Yes, and as a math problem, it's been solved a thousand times already. This is not like Fermat's last theorem where no one could find a solution that worked, or prove that none existed.

This is a situation where we know for sure no solution exists with the axioms of GR or QM. So, you need to add new axioms, and it's easy to add new axioms to solve it, as has been done already in various ways.

The remaining problem is to see if any of these axioms are in any way physical, which is a problem of physics, one that can only be solved with data.

Science requires good hypotheses to contruct good experiments to learn new things.

Math is absract, but it can have implications that are testable in the real world.

> Math is absract, but it can have implications that are testable in the real world

The complaint is that there are possibly infinite mathematically-valid solutions to the problem. Without more data we can’t weigh them against each other. The work is interesting. But it should be pitched as a series of thought experiments or philosophical entreaties, not a solution.

ELI5: Matter sucked into a black hole leaves a gravitational trace, meaning all black holes are gravitational data centres logging the neighbourhoods historical activities.

Wow that’s pretty awesome. Has this been proven?

I'm not sure this Could Ever Be proven. Outwith some unknown hyperscience, at least.

We'll have plenty of time to develop hyperscience before we can watch any known black hole start shrinking.

No. What the article is describing is a proposed theoretical model. There is no prospect of gathering any data to test it now or in the foreseeable future.

Mostly just faint hope that Planet 9 both exists and is a primordial black hole.

So the thought here is that particle spins/charges/mass/etc. are all encoded in the gravitational field of the black hole?

While the previous assumption was that black holes are completely characterized by only three parameters - mass, electric charge, and angular momentum - which is called the no-hair theorem [1]. This is at odds with the time-reversibility of physical laws as there are many ways to make a black hole with a certain mass, electrical charge, and angular momentum but given such a black hole it is not possible to go backwards in time and reconstruct the formation process of the black hole as you have only three numbers and therefore not enough information to figure out when, where and what was thrown into the black hole.

[1] https://en.wikipedia.org/wiki/No-hair_theorem

So space archaeology is dead before it got started?

Not sure if understand what you mean, but if you mean that black holes could eat up the traces of the past and make history inaccessible, then no, at least theoretically. I would assume that the current consensus is that black holes can not do this - which implies that the no-hair theorem is at best approximately true - but we only have unproven ideas which mechanism prevents black holes from destroying information. In practice recovering information from black holes is probably no easier than trying to recover the text of a book by inspecting the smoke of the fire you threw the book into.

It was a little joke about rooting around inside a blackhole discovering things from the past. ;-)

"Dead" barely even begins to describe the horror of passing the event horizon of a black hole - essentially every particle of your body that gets closer to the center becomes unable to ever, in any possible future, communicate back to the parts that are farther away (according to the GR understanding, which to be fair is not consistent with what we know of QM).

I find this a better explanation:

> But the team's calculations showed there were differences in the gravitational field around the black hole. Specifically, information about the composition of the black hole was stored in gravitons, a hypothetical elementary particle that mediates gravitational forces in quantum gravity. "We found that quantum gravity enables us to find the difference in the gravitational field," Calmet said. "There's a memory in the gravitational field of what went into the black hole."

https://www.livescience.com/black-hole-quantum-hair

I’m confused how you can find that something is stored in a hypothetical particle. Is it only hypothetically stored?

The particle is hypothetical in the sense that scientists hypothesize it exists, but it hasn't been proven to exist. If it helps, you might think of it similar to the Higgs boson before the Large Hadron Collider.

They do calculations using a theory where gravitons are assumed to exist, and then their results are valid in that theory.

Wouldn't that just prove the theory is internally consistent, without any further indication it can explain real-world observations?

I think the way to think about it is, they "invented" a particle that has all the right properties to explain real world data. Then they can say, if this is right, we can do such and such experiments to observe it.

Note that the design of the experiment needs to be able to disprove theory. That is to say if the results don’t match calculations then their theory is falsified and we make progress.

Interesting to think that a successful experiment is one that disproves a theory, or, in other words, one where the numbers aren't the ones one expected.

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

A successful experiment can also support a prediction. I.e., "using this model, we predict that a bagel with blueberry cream cheese will remain on the counter for 94+/-4 seconds before disappearing." The team will then release the data from their experiment, which either supports or disproves a model.

This kind of work is critical to refining theories and highlighting/fill gaps in knowledge.

Having a hypothesis that is consistent with both relativity and quantum mechanics, as they are currently conceived, is no small matter. According to one of the paper's authors:

"It was generally assumed within the scientific community that resolving this paradox would require a huge paradigm shift in physics, forcing the potential reformulation of either quantum mechanics or general relativity.

"What we found—and I think is particularly exciting—is that this isn't necessary."

I'm a noob here,and maybe this is not a meaningful question, but why is it a paradox that matter falling into a black hole causes information to be lost?

Are there not many other physical processes where this happens? For example, if a sound is made, does the universe permanently keep a record of it? I would imagine the waves would die down, until at a certain quantum level, they will stop entirely.

Or, if two particles annihilate each other, is it possible from the resulting particles/energy produced to know which particles were involved?

Yes, the known laws of physics are time-reversible - two distinct states A and B evolve over time but they stay distinct, i.e. they will never evolve into the same state C. If the the universe is also deterministic, then there is also also only one possible way a state can evolve over time. In the examples you mentioned, like sound waves dissipating, the energy just gets spread out over countless particles as heat. If you could measure all the particles in the air and environment precisely enough, you could in theory reconstruct the sound.

Is that true? Here's a chamber with two equal halves, with a wall between them. I put air in the left half or the right half, and a vacuum in the other half. Then I remove the wall. Once things reach equilibrium, how are you going to determine which half had the air in the beginning?

So it looks to me like, even apart from black holes, no, you can't reverse things. Am I missing something here?

Equilibrium is local in your example. Globally, the rush of air from one side to the other would result in detectable oscillations whose released energy could be used to determine which chamber was initially empty, assuming one can find and isolate the information.

I see. All right, I put oxygen at some pressure on one side, and nitrogen at the same pressure on the other side, and then remove the barrier. Now there are no oscillations, just mixing.

This doesn't change the situation. Shortly after removing the barrier, some molecules will start moving into the other half. Just reverse all velocities and see all the molecules move back into the half they started in. If you go further forward in time, you will eventually see the first collision. But that doesn't really change the situation, reverse the velocities, see the molecules collide in reverse and then move back into their starting halfs.

And this never really changes, particles move and collide and this can be reversed, you just have to do this for very, very many particles and collisions. And this is probably chaotic, the tiniest error in any of the positions or velocities will get massively amplified and give you the wrong initial state. But in principle it could be done. And it has to be done on the microscopic level, macroscopic things like concentrations, temperatures, or pressures by definition do not reflect the microscopic states.

If you know the information about every single molecule of each, you could calculate backwards the probability of where they came from. Eventually you would reach a state where the most likely probability would show all the oxygen on one side and all the nitrogen on the other.

This is a purely theoretical calculation, far outside the scope of our ability to calculate. It is one of those, if we turned all known matter into the possible fastest computer, could that even calculate it before the heat death of the universe sorts of questions.

Right but they leave a forensic trace, I.e. if you had perfect measurement you would be able to tell which sound was made.

Like a slight difference in the pattern of sand or where the grass is, but at a much lower level.

A huge part of Quantum Mechanics is Conservation of Information, or better maybe to think of it as a conservation of entropy?

Your example of a soundwave is too local in scope, sure the wave will die down, but while the soundwave was propagating it's bumping into other particles and those particles affect other particles, which affect other particles, and on and on and on.

If you look locally, sure, eventually you wouldn't be able to tell the wave ever existed, but if you pull back far enough and take the state of the system and rewind it you will eventually reconstitute the wave.

Your example with annihilating particles is basically how CERN works, though we do know what we started with in some places.

I had started to describe the whole of how Hawking Radiation works, I'm sure you can find that easily elsewhere.

> if a sound is made, does the universe permanently keep a record of it? I would imagine the waves would die down, until at a certain quantum level, they will stop entirely.

> Or, if two particles annihilate each other, is it possible from the resulting particles/energy produced to know which particles were involved?

These are really great questions! The facile answer is that all known physical processes are reversible in principle, but the actual truth is more complicated than that. For starters, quantum measurement may or may not be reversible depending on which interpretation of QM you subscribe to. On the currently-most-popular account, the Copenhagen interpretation, measurement is by assumption irreversible. There are many reasons to believe that this is not the case, that measurements are in fact reversible (in principle, not in practice) but no one really knows, and no one can know because the process of reversing a measurement is indistinguishable form the normal state of affairs. It might actually be happening all the time. There is no way to know.

For more info see:

http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

(And its prequel: http://blog.rongarret.info/2014/09/are-parallel-universes-re...)

If you want a deeper understanding of all this stuff I recommend:

https://www.amazon.com/Philosophy-Physics-Princeton-Foundati...

https://www.amazon.com/Quantum-Mechanics-Experience-David-Al...

You can go meta and think about what is knowledge and what is scientific method.

Knowledge is inherently axiomatic. In other words there are certain unprovable truths (“axioms”) you agree on as granted and then build your theories from based on certain principles (and this is “scientific method”).

In this case “conservation of information” is one of the axioms quantum mechanics is build on. Now, thinkers operating on the fringe of our understanding and advancing our theories have a choice to make: (a) assume it’s true on the black hole event horizon - thus paradox and theories on how to resolve it, (b) assume it’s not true on event horizon. So we don’t know if it’s a paradox at all, we choose to think it’s a paradox because we want our theories to work everywhere. But it’s not necessary correct assumption. When these theories are discussed this is implicitly implied.

Is there a link to the paper or a different article that contains information, instead of… not?

Links to papers are at the end of the article

There are two papers linked at the bottom of the not-so-fine article, both leading to "green lock" unpaywall https://research.lib.buffalo.edu/articleaccess/unpaywall https://unpwaywall.org/ destinations.

The articles correspond to the following arxiv preprints:

https://arxiv.org/abs/2112.05171v2 [Quantum Hair and Black Hole Information]

which is an overview work published in Elsevier's Phys.Lett.B on an open access basis; and

https://arxiv.org/abs/2110.09386 [Quantum Hair from Gravity]

which is the main work (published to APS's Phys.Rev.Lett) that the not-so-fine article attempts to summarize.

Can't you say that the blackhole as a whole maintains a quantum state that accounts for every particle in it? Then, the paradox does not arise in the first place, compared to considering a blackhole as a gravitational singularity only determined by its mass.

If I am not mistaken, I believe the paradox arises not when matter enters the black hole, but rather in the anonymity of its expression as Hawking radiation later in the black hole's lifespan.

I was also under the impression that a solution to the paradox had already been discussed, just not conclusively or in detail.

The linked article is pretty atrocious.

But if the black hole then evaporates by emitting Hawking radiation, where did the state go when it has disappeared?

> Can't you say that the blackhole as a whole maintains a quantum state that accounts for every particle in it?

No - in current theories of black holes (GR), the strength of gravity (the curvature of space time) at the center of the black hole is such that there is no room for a single particle to exist, everything is condensed into a single mathematical point (well, it's actually a disk I believe, since it must be able to rotate). As such, a black hole is characterized by only three things: mass, charge, and angular momentum. If it has more degrees of freedom, it's not consistent with GR.

Now, the clear assumption is that mathematical solutions that involve singularities indicate limits of a theory - that is, that GR breaks down inside a black hole, requiring some other theory of gravity to explain what goes on in there. But QM is not that theory, and there is no theory which encompasses both GR and QM - though there are a few incomplete candidates.

AFAIU this is pretty much what they are saying. The gravitational field of the black hole is not as simple as we thought it would be (i.e. it cannot be fully described by just a few numbers like mass, angular momentum and charge) and it carries information about the original matter that formed the BH. This information then manages to "escape" along with the Hawking radition as the gravitational field influences the emission of said radiation.

It hasn’t been a ‘paradox’ since at least 2001 when people found fuzzball solutions. Today we get a headline that somebody resolved this ‘paradox’ every month or so and each time they make it sound like this is something new.

There are many ways to describe a quantum black hole, none of them lose information because QM is unitary.

If you think classical mechanics is real than yeah there is a paradox, but that’s like asking why electrons don’t spiral into atoms radiating UV radiation —- no paradox there, that’s what they do in classical mechanics, but we live in a quantum world.

There is a ELIPhD student version here[1].

Personally, though I’ve not read the papers, I doubt that the description of a quantum eigenstate of QED with charge Q can just be copied over mutatis mutandis to describe a quantum eigenstate of quantum gravity with energy E, the theories are just too different.

[1] https://youtu.be/Jn0hsZg_3sE

Other than to denote a hypothesis, why do these articles use the term “may.” I think it implies “have” to the layman and implies hasn’t to anyone with some math or science based education. If nothing has been proven yet what’s the story here? It seems to me, and I’m happy to be wrong, that science journalism is indulging in clickbait.

When i read „may have solved…“, i instantly think: „…but they didn‘t“.

Newton "may" have solved what makes objects to fall, until Einstein gave a more complete explanation, and we still don't fully understand gravity. So it's always a "may" until you get a better theory.

This would be a measure of may I’m okay with, but the article isn’t denoting that since it is discussing theories about gravitational waves that have not been verified. If we roll with your analogy we’re at the Galilean period where we don’t yet even have methods to prove that reality reflects the maths we contrived.

This is why I take issue with it. When I go to walk my dog in an hour I may solve the pieces to a theory of unified gravity. If I did it would be okay for me to make a claim like that, if I thought about how cool circles looked in space and solved some random equations for replicating those circles I don’t think anyone would appreciate me saying I may have solved an actual theory.

Obviously I’m walking over the part where they actually do research, but only to drive the point home of it being clickbait (I see these titles more and more in r/physics)

This dichotomy - either it's proven or it's nothing - has been alluded to in several comments here, and shows up whenever similarly far-reaching hypotheses are raised on HN. The thing is, though, you need a hypothesis before you can get to a proof [1], and a hypothesis that is consistent with everything we think we know is a significant advance, especially when the absence of such a hypothesis has ben gnawing at the foundations of physics for several decades.

The history of science provides plenty of examples. Take the atomic theory for example, where Einstein's Brownian paper theory is generally regarded as the step that silenced the positivist opposition. Does that mean that none of the prior work by Boltzmann and in chemistry was newsworthy? On the contrary, it paved the way to Einstein's paper, and was an essential part of the history.

[1] Not that scientific theories are ever formally proven, of course, but that line of argument would be a diversion here.

Can someone provide an ELI5 of this quantum hair theory?

There has been no experimental evidence for Hawking radiation, so does it matter all that much?

I'm not a physicist, so my explanation is most likely off the mark, simply see [1].

There was something recently about whirlpools that showed that Hawking's math worked out [1]. So yea, no experimental evidence, but at least the math works out for whirlpool-style experiments.

[1] https://www.youtube.com/watch?v=HuIxSkD6wsA&ab_channel=Astru...

Very nice, thank you for sharing.

Sure if it is being debated why not?

Yeah I guess... so to me that's the analog of conjectures conditionally true on the Riemann hypothesis being correct. There is indeed value in it, though I'm not gonna hold my breath.

I have always suspected that there is no interior.

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