Physicist discovered an escape from Hawking’s black hole paradox

https://www.quantamagazine.org/this-physicist-discovered-an-escape-from-hawkings-black-hole-paradox-20210823/

by theafh • 5 years ago

255 163 5 years ago

quantamagazine.org

andreareina• 5 years ago

Netta Engelhardt on Sean Carrol's Mindscape podcast: https://www.youtube.com/watch?v=m-6mcLX_v2I

ChrisArchitect• 5 years ago

Recent discussion about confirming Hawking's Black Hole theorem:

https://news.ycombinator.com/item?id=27696774

prof-dr-ir• 5 years ago

That is an unrelated result, although the confusion is understandable.

You refer to a theorem, named after Hawking, that states that in classical general relativity the area of black hole horizons must always increase. The experimental confirmation of this theorem refers to the fact that the total area indeed appears to have increased for the black hole merger event observed by LIGO (which should indeed fall well within the realm of classical general relativity).

However, once quantum gravitational effects are taken into account this black hole theorem no longer holds. Indeed, since the real world should be quantum, it is expected that the area of black holes eventually does decrease: they evaporate by emitting Hawking radiation. This is however purely a theoretical expectation, since these evaporation effects are a loooong way from being observable; evaporation probably only becomes a significant effect on time scales far beyond the current age of the universe.

In quantum gravity there are nevertheless plenty of theoretical paradoxes and open questions, and the above article describes some recent progress by the theoreticians in this area.

hondo77• 5 years ago

Engelhardt set her sights on quantum gravity when she was 9 years old.

I tried to come up with a witty comment about that but words fail me. Wow.

_Microft• 5 years ago

What about: "I'm glad she did run not across the Men in Black in the middle of the night..."

felipemnoa• 5 years ago

https://www.youtube.com/watch?v=K3hAVT2sDqQ&ab_channel=Oscar...

handrous• 5 years ago

Picking very difficult, specific topics like this to be interested in as a kid is pretty common.

Sticking with it—that's unusual.

duxup• 5 years ago

information about particles’ past states gets carried forward as they evolve

So I was curious and googled and found:

the value of a wave function of a physical system at one point in time should determine its value at any other time

Is there a layman's explanation about what this ... is?

jwally• 5 years ago

I'm a layman, so I'm not 100% certain on this, but I think this is a quantum version of Laplace's Demon; where if you know all the information about the state of a system, you can calculate what the system will look like at any point in the past, present, or future.

ctlachance• 5 years ago

Does this imply that if we knew the total state of the universe, we could calculate its future state?

Basically, does this imply the universe is deterministic, or that we're living in a simulation?

ABeeSea• 5 years ago

Mathematically, thee solutions to the differential equation have a “time evolution operator” that allow the quantum states to be pushed forward or backward in time.

martincmartin• 5 years ago

It sounds like just the Schrodinger Equation, Quantum Mechanics 101

uoaei• 5 years ago

This is basically stating an assumption of determinism with respect to physical dynamics.

abdullahkhalids• 5 years ago

This is a vague statement that could mean two different things.

1. That quantum mechanics is deterministic (as far as wavefunctions go) and time-reversible. Knowing the state of a system at any given point, you can use the differential equation (Schrodinger's equation) that determines the evolution of the system, to find the state any other time.

2. The should in the statement is referring to the philosphical idea that we expect that the true laws of physics will always be deterministic and time-reversible.

cryptonector• 5 years ago

Determinism.

The laws of physics are mostly fully deterministic, even quantum laws.

nerdponx2• 5 years ago

This is where my layperson's intuition totally breaks down.

Matter goes into black hole. Black hole destroys everything because too much gravity and spacetime itself might or might not literally have a break in it. Eventually the remnants of said matter radiates out as random particles.

That seems totally logical and not paradoxical at all to me. My uneducated mental models of black holes and information theory are so crude that I can't even see what the problem with this is. All I can figure is that the conclusions of a theorem in Research Area A turn out to violate an axiom that is used and dependend-on in Research Area B.

I did however find this very interesting document that seems to cover some of what I'm missing: https://plato.stanford.edu/entries/spacetime-singularities/. I'll have to make my way through it over the next few days, maybe it will benefit other people, too.

syncsynchalt• 5 years ago

In the past two years, a network of quantum gravity theorists, mostly millennials,

There you have it: millennials are killing the Hawking Information Paradox industry.

(It's always been said that mathematicians do their best work when they're young, it seems weird to mix generational cohorts into it)

coliveira• 5 years ago

It is a favorite pastime of journalists to attribute events to a particular "generation", as if there was any kind of explanatory power in doing this.

FatalLogic• 5 years ago

Maybe it's a subtle reference to Planck's Principle [0], which is often summarized as "Science advances one funeral at a time"?

In this case, suggesting that younger scientists might invest their time into new ideas that older scientists would dismiss.

[0] https://www.chemistryworld.com/news/science-really-does-adva...

PartiallyTyped2• 5 years ago

There is also the statement that when an old scientist tells something is possible, then it is certainly possible, but if they express it is impossible, then it is possible.

pvarangot• 5 years ago

I think the blurry line for millennials put the oldest of us at something like 34 to 38 years old now? So yeah most people that got into academia after their bachelors and just finished their post-doc should be a millennial.

hanniabu• 5 years ago

Classic millennials and their cancel culture /s

TheCowboy2• 5 years ago

Has anyone else (without a background in physics) kind of given up trying to understand developments in physics? It seems like the reporting is usually completely inaccurate or a metaphor at best. And that to have some grasp of it requires having a solid foundation in undergrad physics at a minimum.

I'm just wondering if I'm being lazy by not trying hard enough or efficient. It's definitely not for lack of curiosity, but I also don't like to fool myself into thinking I understand something that I don't.

DontGiveTwoFlux• 5 years ago

I worried about this too. But I think my attitude has changed after watching lots of the PBS Space Time youtube channel. They do a great job of breaking down these concepts at a level where highly interested non-physicists can get what feel like the real details without dumbing it down too much. They have good videos on many physics topics, and regularly explain new discoveries.

https://youtu.be/QLSIZg0npuA

stronglikedan• 5 years ago

PBS Space Time seconded. I recommend taking the rabbit-hole approach with them - i.e., blocking out a considerable more amount of time than the length of the video you're about to watch. They always reference past videos that expand on the building blocks of whatever the topic is in the video you're watching, and it helps to go watch those if it's a new topic to you, before continuing with the current video. I absolutely love that channel.

zbendefy• 5 years ago

Not a physicist but I find the Science Asylum also pretty good:

https://youtu.be/Q2OlsMblugo

squeaky-clean• 5 years ago

The Fermilab channel is also quite good for short-form content. ScienceClic English has absolutely wonderful visualizations. All of them do make some subtle inaccuracies or skip things for the sake of brevity though. I think Sabine Hossenfelder's channel has the most accurate videos in that 10-15 minute range, but they're still only about 15 minutes long.

I don't have a proper education in Physics, but have been trying to self-teach and I think that none of the ~15 minute video channels really cover things to a very detailed degree. You really do need textbooks/lectures/real papers to actually understand it. The channel "Physics Explained" is pretty good for more in depth breakdowns of things, but it is quite dry compared to those other channels and still not really a substitute for a textbook or class.

And I don't even mean learning things well enough to get a job as a particle physicist or anything. Just some things, like say particle spin, just can't be explained in under a few hours and without the math behind them. They don't have a proper intuitive analog to our macro-level world.

sicromoft• 5 years ago

Seconded. Here's their video specifically about the black hole information paradox:

https://www.youtube.com/watch?v=9XkHBmE-N34

ardit33• 5 years ago

I like Sabine's channel even better. She is great at simplifying things, and explaining the raw concepts and what some equations / findings really mean

Sabine Hossenfelder https://www.youtube.com/channel/UC1yNl2E66ZzKApQdRuTQ4tw

mrfox321• 5 years ago

The main issue is that quantum mechanics and {special | general} relativity are non-intuitive. Any classical analogy is a leaky abstraction at best.

There are communicators who can pierce through this effectively. However, they tend to be researchers who do not have the time to spend writing pop-sci articles.

marbu• 5 years ago

You are right about quantum mechanics and general relativity, which requires more advanced math, and it's hard to create good and useful analogies without it.

That said, if you can handle basic high school math, you don't need any leaky abstractions for special relativity: explain experiment with speed of light measurement and it's consequences, then explain thought experiment about the light clock and mathematically derive time dilatation out of it.

lanstin• 5 years ago

And sadly quantum mechanics is the simple and obvious version of quantum field theory, which is what the standard model is. Electron energies around a proton in the atom are one thing, but particles as fields and renormalization kill. Tho Feynman wrote a non maths book on QED.

celticninja2• 5 years ago

Not given up but I would like to find a physicist on twitter who can layman's terms some of thes developmemts.

devoutsalsa• 5 years ago

I like Sabine Hossenfelder:

- https://twitter.com/skdh?s=20

- https://www.youtube.com/c/SabineHossenfelder

immmmmm2• 5 years ago

PBS space time is the best ressources out there. i have a phd in this kind of things and still learn a lot. but you're right, while nature is fundamentally weird to our brains (and often involve very complex maths), physicists (including myself) are typically very bad at explaining things in a simpler language. one of the reason i stopped was the very little (between 10 and 100) people around the world i could talk of my work.

PartiallyTyped2• 5 years ago

I second PBS space time, and would also like to mention the notorious Sabine Hossenfelder, she has a great educational youtube channel and does a good job at explaining things to the lay person.

sharikous• 5 years ago

It is actually a very famous controversy in physics between Hawking and Leonard Susskind.

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

And if you care about understanding physics you absolutely have to check Susskind's "the theoretical minimum" videos. He explains advanced concepts with remarkable clarity. You really can grok string theory if you watch some of his series

abeppu2• 5 years ago

I'm just wondering if I'm being lazy by not trying hard enough or efficient.

Leonard Susskind at Stanford has a series of courses specifically aimed at helping people get up to speed in understanding modern physics. Every several months I work up the enthusiasm to watch several lectures, and then I get distracted and forget it all. Clearly, it's because I'm not trying hard enough because there are so many other things to also be curious about. https://theoreticalminimum.com/

But I also wish that physics were less focused on understanding extremes, and more interested in understanding physical systems closer at hand. There are interesting recent-ish results found in the behavior of crumpling paper, collapsing piles of sand, or the 'legs' of wine in a glass. I suspect that we could have much richer conceptual tools for thinking about the physical world actually around us if only more resources went into looking at it, rather than into exploring how laws do or don't break down in extreme conditions that don't naturally exist near earth.

TheCowboy2• 5 years ago

I can relate to this. It might seem stupid but understanding this made me change the way I tied my shoes, and my laces come undone way less frequently now.

https://news.berkeley.edu/2017/04/11/shoe-string-theory-scie...

(Also thanks to everyone for their responses, a lot of good stuff to check out.)

gumby3• 5 years ago

But I also wish that physics were less focused on understanding extremes

To defend this approach: Extrema are a good way to get a handle on a problem, which can then be extended.

In programming we almost always have to handle the null case ("what if the graph is unpopulated?") and often an extreme case as well ("What if we end up with 10 million users? How to we quickly respond to XXX").

In physics, like so many other domains, the poles are often the most enlightening region.

However, as you say, work on those extrema rarely translates into more common situations until a breakthrough happens.

tobmlt2• 5 years ago

>> I suspect that we could have much richer conceptual tools for thinking about the physical world actually around us if only more resources went into looking at it

What I hear in this statement is more towards the domains of research engineering as opposed to theoretical physics. Sand piles are of course the bridge between the two ;)

hellbannedguy• 5 years ago

Are these videos working for you guys?

zopa• 5 years ago

Quantum gravity gets the headlines, but only a pretty tiny minority of physicists actually work on stuff like that. Most people are researching problems much closer to hand. Basically the utopia you're looking for is already here.

sharikone• 5 years ago

Let's be real. There are more physicists than fitting research problems. That's why your average physics PhD graduate is typically employed outside of the field they learned and earns less than an average React programmer.

That said, theoretical physics has the goal of understanding reality from a reductionist point of view. At the scales that goes from the nucleus to the Solar system there are few questions left. We know the Standard Model and GR and they fit the data perfectly. Of course some questions remain for how things interact when there are many of them (e.g. warm superconductivity) but there are few questions about fundamental laws

You can think about it like bootstrapping an open source system (it was in the homepage today). There are still many technical hurdles downstream but we are interested in reducing the binary blob from which all starts to its perfect minimal form. And the only places we still have not figured out well are things at the limit of our instrument capacity. Black holes (GR and relativity, we still cannot figure out that and proving black hole seems experimentally challenging), exotic particles (what are quarks composed of?), dark matter (why far away galaxies seem to rotate so quickly?), dark energy, inflation, that stuff.

I think physics should be smaller, it has too many graduates. But actually these problems should be researched. They are the fundamental questions that remain and there is a reason that the layman considers this stuff to be "real physics" and not origami folding

pas4• 5 years ago

wine legs on applied science: https://www.youtube.com/watch?v=s6w0tSg-msk !

And let me randomly throw in metallic water too: https://www.youtube.com/watch?v=Vdz18ibX7rE

pyb• 5 years ago

This is the frontier of knowledge. By definition, things that theoretical physicists themselves barely understand.

For instance, this physicist reportedly "Discovered an Escape From Hawking’s Black Hole Paradox". If true (I presume it is), this implies that other physicists before her didn't understand the black hole paradox all that well!

It also implies that you'll never get a crystal clear understanding of it from reading popular science.

SubiculumCode• 5 years ago

My favorites are the articles about a new math discovery...no clue...but I like cheering for them!

lordnacho2• 5 years ago

Isn't it like this with any serious subject? You need to have read quite a few books to get it?

One positive thing is that if you just pick up the university books/papers, there's no exam. You can just read them for understanding the concepts instead of for passing tests.

I picked up a book about relativity (both) years after graduating, and it was an interesting read. I won't claim to understand how Ricci tensors work, but it made sense at the time.

crazygringo• 5 years ago

Isn't it like this with any serious subject? You need to have read quite a few books to get it?

No, I'd say physics/math is uniquely hard in this regard.

A lot of stuff in the humanities, you can still get the gist of a paper even if you're not precisely familiar with the field, authors, etc. Then read a couple of textbooks, a handful of survey articles, all of which is a few days' reading, and you'll understand nearly of all it.

But physics? A well-educated layperson can read a paper and not have the slightest idea what any of it means, no clue whatsoever. And a few days' reading isn't going to help much -- it's quite likely to need a couple years' of study at a minimum to understand the context of the paper.

Remember, math/physics isn't just its own subject, it's its own language. For most people, the average math/physics paper might as well be written in Chinese. While a history, sociology, or political science paper is incredibly more accessible.

phendrenad22• 5 years ago

Yeah. I wish magazines would do more work bridging the gap between scientist and layperson. Most articles are either "completely inaccurate or a metaphor at best" as you say, or they're just an unabashed, untranslated interview with a key scientist (like this article).

ben_w2• 5 years ago

The latest developments (with a few exceptions) generally really are impenetrable without a solid foundation in undergrad physics, which meant that many of my trips to Physics Stack Exchange have given me a direct personal experience of being on the dumb side of the Dunning-Kruger effect.

The good news is, as others have already suggested, this level of education is now very accessible by e.g. YouTube channels:

https://youtube.com/c/pbsspacetime

https://youtube.com/c/SabineHossenfelder

https://youtube.com/playlist?list=PL701CD168D02FF56F (Susskind, The Theoretical Minimum: Quantum Mechanics)

I would add the channels:

https://youtube.com/user/EugeneKhutoryansky

https://youtube.com/user/minutephysics

and the MOOC Brilliant.org

That said, I’m still definitely in the “undrergrad” level despite all this; I can recognise the equations of GR and QM, but not use them, and there’s plenty which I know I must be misunderstanding.

queuebert• 5 years ago

The good news is, as others have already suggested, this level of education is now very accessible by e.g. YouTube channels:

With the caveat that you must work on on every problem set you are confronted with.

An essential part of a physics education is struggling over extremely hard problem sets. (I'm assuming/hoping these channels offer decently hard problem sets.) This more than lectures teaches you how to flail about in unknown areas of physics and to gauge your own understanding. This I think is a physicist's superpower.

drran• 5 years ago

If you cannot understand it, then probably it doesn't matter for you, because you have no problem to solve. Most of the time, physicists are trying to invent a mathematical formula or construct to elegantly describe a physical process, to make an accurate prediction.

Just imagine, that we have a game, where we want to accurately predict next frames of a video. It's an interesting game on its own, because you need to understand deeply what happens on the video to be able to accurately predict behavior of all objects, animals, and persons in the video.

Such game requires a lot of skill, to accurately guess and predict, but most of the time it's not important for us, mere mortals. For example, we put a lot of effort into OpenGL, PBR, physic engines, etc. to make realistic games. Do you feel obligated to study all of that when you are interested in a realistic fly simulation? Do you feel obligated to study construction of AK when you like to play a 3D shooter?

If you really want to understand physics, then I suggest to perform experiments, or play with a physical simulation, or, even better, to implement your own physical simulation. Look, for example, at this beautiful simulation of black hole done in OpenGL shader:

https://ebruneton.github.io/black_hole_shader/demo/demo.html https://www.youtube.com/watch?v=_hhOd7GDboM https://github.com/ebruneton/black_hole_shader https://ebruneton.github.io/black_hole_shader/paper.pdf

marcyb5st2• 5 years ago

I am one of these with a background in physics, but have been out of the circle for the last 10y. I highly recommend the videos from PBS Space Time on YT. They have playlists on various topics and, if you are willing to put the mental effort, can give a solid foundation and allow to understand to some extent papers/theories like the one reported here.

WindyLakeReturn• 5 years ago

I would suggest starting with YouTube. Start consuming a variety of videos from different sources and you'll begin picking things up and gaining enough knowledge to start identifying bad info. You can start with lectures of complex topics by physicists meant for the general public. Their is a lot of handwaving to remove the math and oftentimes they go into personal beliefs concerning theoretical physics, but they are very good at identifying each time they do so.

PBS spacetime is one that seems informative while giving enough clarifications. Including looking into a few pop sci theories, explaining what they would mean, and ending with why science don't consider them as serious explanations.

You can also find some more math heavy explanations and slowly build your math skills to be good enough to understand the physics that uses it.

DantesKite• 5 years ago

That's why I like Quanta Magazine. They do a great job of laying down enough material to get a sense of what's going on.

stevenhuang2• 5 years ago

Especially in this article, the explanation of the proposed breakthrough was pretty clear:

Old: Errant entropy curve (Hawking's entropy calculation) due to wrong black hole surface area used

New: Resolved entropy curve (Page curve) via quantum-corrected surface area (quantum extremal surface)

anigbrowl• 5 years ago

Kind of, as you say it is time-consuming to keep up. A bigger problem is that it's not obvious what the impact of many discoveries would be on daily life; physics sometimes seems to have been stuck in an era of diminishing returns after important breakthroughs that led to a short era of rapid innovation - sort of how nuclear fusion has been 20 years away my whole life.

I've often felt part of the problem here is the relative decrease of manned missions in space, which are not the best bang for the buck in scientific terms, but at one time created a widespread of view of 'humanity's future in space' that provided the motivation for and wide public interest in many scientific endeavors.

Nowadays, there's a widespread feeling that planet earth has got very crowded and there aren't as many opportunities for big new discoveries (the sort that can be appreciated by anyone without specialist education/training), that the long-term viability of our habitat is Not Great, and that the prospect of space exploration is too remote and costly to have any impact on the lives of ordinary people, but is limited to a microscopic scientific or financial elite.

Of course, this is somewhat irrational; as a society we've chosen to have ubiquitous worldwide real-time communications devices that would have once seemed limited to star Trek. Computing has made big science small enough to fit in our pocket and allowed anyone who is really interested to be a software maven. But it's not as spectacular as the future anticipated a few decades ago.

omgJustTest• 5 years ago

Startswithabang and other science educators, particularly Veritasium, do excellent jobs.

It's complicated and worth the time to understand what they present.

wrycoder2• 5 years ago

Natalie Wolchover is one of the finest science journalists around, especially for physics. You can rely on her.

hinkley3• 5 years ago

Isn’t it kinda true that A Brief History of Time was so important in part because it successfully dumbed down a bunch of esoteric physics? Maybe we are just reverting to the mean?

I recall years ago mentioning offhand to someone that I read SIGPLAN proceedings and their eyes bugged out and they asked if I could understand those. I said “only about 3/4s” but I knew exactly what he meant. Learn to human.

toast0• 5 years ago

I'm just wondering if I'm being lazy by not trying hard enough or efficient.

I don't think anyone else specifically said this, so allow me to be one of the few who say you're being efficient. Sure, you could spend time to understand this stuff, and yes it's important knowledge for the world/society, but knowing about black holes really isn't going to change anything you do, unless you're a physicist or working on something involving deep space (and likely not really then either).

Sometimes esoteric knowledge is useful in other areas, and sometimes learning esoteric knowledge is fun or helpful to build learning skills, but sometimes it's just more junk to fill your brain sponge with. If it doesn't tickle your interest, leave it be and it's fine. If it becomes useful, chances are over time more ways to explain it will have been made and some might be more comprehendable by you.

codethief• 5 years ago

I'm just wondering if I'm being lazy by not trying hard enough or efficient. It's definitely not for lack of curiosity, but I also don't like to fool myself into thinking I understand something that I don't.

I don't think you're lazy or not trying hard enough. The truth is just that… it's a hard and long road. Even more so when you're studying by yourself.

I took about the straightest path you can take to learning Quantum Mechanics and General Relativity (and all the math you need for it), attending classes and sitting down on my butt every day for 3 to 4 semesters. There are ways to shorten it but I'm not sure how rewarding that would be.

It also depends on what you mean by "understanding". If you really want to understand things at the deepest level possible, there's no way around a university-level education. I'm saying "university-level", not "university", because one could certainly learn these things on one's own. But to be honest with you I think chances of pulling this off are very small, mostly because physics outsiders don't have access to the same resources or social networks as enrolled students, which makes studying even more frustrating.

Anyway, FWIW here's a roadmap for learning General Relativity at the deepest-possible level along with the minimum timeframe needed (IMO) and the most important topics / keywords you really need to understand well:

    Linear Algebra (1-2 semesters): vector spaces, linear maps, dual maps, matrices, symmetric bilinear forms – These things lay the foundation for pretty much anything in math and are needed to understand differentiation in several variables (see below) as well as pretty much anything in Differential Geometry and Relativity.

    Real Analysis (1-2 semesters): Mostly differentiation of functions of one to several variables + a bit of integration theory – needed for pretty much anything in Differential Geometry and Relativity (especially coordinate changes and manifolds) but also in mechanics.

    Differential Geometry aka (Semi-)Riemannian Geometry (1 semester): manifolds, tensors, metric, connections, geodesics, curvature – These concepts are at the heart of General Relativity

    Mechanics (1-2 semesters): Newtonian Mechanics, Lagrangian Mechanics, Special Relativity (all with a focus on both theoretical and experimental physics) – without these there's no hope of understanding (or appreciating) the physics content of GR

    General Relativity (1 semester)

(Side note: Don't let anyone tell you that you don't really need Differential Geometry and all the other math to understand Relativity. They're lying and probably don't really understand Relativity, either.)

For quantum mechanics it's a bit shorter because the math is not as involved (at least to understand the basic concepts:

    Linear Algebra: (1-2 semesters): (finite-dimensional) vector spaces, linear maps, dual maps, matrices, symmetric bilinear forms – No way around this. 99% of quantum mechanics is encoding fuzzy concepts in linear algebra.

    Mechanics (1 semester): Newtonian Mechanics, Lagrangian Mechanics

    Quantum Mechanics

    Bonus: Hilbert space theory, basics in functional analysis

CephalopodMD• 5 years ago

Yes. This is nonsense to me.

That said, check out scienceclic. They do good run downs of BOTH the metaphorical stuff AND the math that underpins it. PBS spacetime is a fun place to check out too. Between the two of them, you should have enough of an idea of these things to wax scientific at a cocktail party.

thriftwy6• 5 years ago

Our visible universe has event horizon around it, which has thousands of galaxies falling away from it and becoming unobservable every day, due to cosmological redshift.

For some reason, physicists are not concerned with information loss via this one. I would be glad if somebody explained the difference.

isk517• 5 years ago

Black holes are thought to be information destruction not loss. Things moving super far away are lost but should still be out there, things falling into a black hole disappear and then once the black hole evaporates are gone forever. Or at least that use to be the theory.

thriftwy6• 5 years ago

It sounds a mistake to me, thinking that black holes' contents are still "out there". They are no longer observable, just like the galaxies that are no longer observable due to expanding universe.

In both cases it is due to curvature of space, so I think these are essentially the same case.

I would definitely like to hear something verifiable on why there's a difference and why only one of these susceptible to information paradox.

raattgift4• 5 years ago

That's a good question.

The tl;dr is that if information hides on the other side of an event horizon and doesn't come back, we can pretend unitarity (and all the rest of the physics we've discovered) continues where we can't see it.

A non-evaporating black hole forever holds within it the information about what fell into it.

A forever-expanding universe causes information to exit observability forever.

Partitioning away -- hiding forever -- information is not the same as losing track of it when it comes out of hiding.

There are some differences between these two types of horizon because they are generated by different metrics : one for an expanding spacetime and one for a collapsing one.

We can see the differences by adapting these theoretical (as opposed to astronomically observed) objects.

If an expanding universe's expansion slows and reverses, then eventually all the galaxies that exited from one observer's view return into its view (having evolved with stars forming, aging, dying, galaxies merging, and so forth). If we are talking timescales of a few billion years, then if an us-like observer has detailed information about a galaxy now leaving its view, it can in principle predict what it will look like in billions of years when the galaxy returns back into view. The gentle assumption here is that stellar physics does not stop when the most distant galaxies go out of view.

If the timescale is pushed out to trillions and zillions of years, these us-like observers could still maintain the idea that the galaxies which exited from visibility continue to evolve like the closer galaxies which continue to be seen. A star which ends up on the other side of the cosmological horizon continues being that same star, evolving as normal.

A black hole is different, precisely because we should expect unknown extremely high energy physics to occur as e.g. protons fall in. What happens as you crush some quarks and gluons together at energies enormously higher than that we get from the LHC, or even from supernovae? We don't know. In fact, when we try to answer that, we lose track because our calculations tend to become singular : https://en.wikipedia.org/wiki/Singularity_(mathematics) We don't know what should pop out of a black hole late in evaporation, but we do know when a star crashes through a black hole event horizon, it will stop behaving like a star very quickly.

Indeed, even just on "our" side of the two horizons we can see differences near them. The furthest galaxies, at the edge of what we can see of the cosmos, are filled with normally behaved (young) stars. The shapes of those galaxies are not distorted by proximity to any horizon. We expect that to continue as we see galaxies deeper and deeper into our sky. By comparison we can see gas clouds falling into the black hole in the centre of our galaxy, stars orbiting it, and distortions to these caused by these close approaches to the central black hole. We have even found evidence of stars ripped apart by more distant extragalactic black holes. Crossing a black hole horizon does violence to the bit of the star that has not yet crossed; crossing the cosmological horizon would not change the star's basic behaviour.

If the universe were to collapse in the future, we would expect to see disappeared stars returning into view. Those stars stayed in locally gently curved spacetime, just like our local star did. If a black hole were to shrink in the future, we would be surprised if it spat out intact stars, or space probes, or whatever fell in emerging unscathed. Those objects did not stay in locally gently curved spacetime, and indeed would have encountered the locally enormously curved spacetime inside the black hole. That strong curvature spaghettifies things, at the very least.

These are just the consequences of our best theories of gravitation and matter applied to situations we have no reason to expect to be able to observe. As far as we know our universe is not accelerating towards a recollapse, it is accelerating towards faster expansion. And as far as we know no astrophysical black hole in our universe is presently shrinking. It's fairly safe to bet that if there is ever to be a reversal of the expanding cosmological metric or the collapsing black hole metric it's not going to be soon, so humanity and its descendants have lots of time to think about evaporating black holes (including those that evaporate in a contracting "anti-de Sitter" universe with a big crunch, which is the setting (sometimes including extra spatial dimensions than the three we're used to) for many approaches like the one in the fine article in Quanta Magazine linked at the top).

Now, a more direct answer to your question: in an almost-completely-flat-space universe if we have all the data (position, momentum, particle species, etc) at every point in a time-indexed spatial slice of our universe, we can calculate the entire data in neighbouring slices, and the data in those slices' neighbours, and so forth, into the infinite future and the infinite past. An expanding universe doesn't break this, it just means that we can't choose any arbitrary slice and march forwards and backwards from there, we have to take initial data from the hottest densest earliest part of the universe. From complete initial data and appropriate dynamical laws we can (in principle) describe anything in the future, even if the parts we describe are so separated from one another (in that future) that they can't exchange light with one another. The formation of non-evaporating black holes doesn't change the picture much: we know that things fall into a black hole and stay there in some unknown state, unable to exchange light with things outside the black hole.

However, once we introduce black hole evaporation we have the problem that we don't know how the stuff inside the horizon evolved inside the horizon, so we have no idea what should pop out through the last stages of evaporation.

In our standard cosmology, we can expect black holes to have evolved from stuff that was close to us in the hot big bang era but which is now almost certainly forever outside our cosmological horizon. A general solution to the black hole information paradox should not create craziness in those so-distant-we-will-never-see-them black holes, much less in the earliest visible quasars. That tends to get forgotten until someone asks what the interviewer asked:

    Most of the justification for the quantum extremal surface formula comes from studying black holes in “Anti-de Sitter” (AdS) space — saddle-shaped space with an outer boundary. Whereas our universe has approximately flat space, and no boundary. Why should we think that these calculations apply to our universe?

That's an excellent question, and it was not answered by the interviewee. (I'd love to be persuaded that it has ever been reasonably answered by anyone).

thriftwy6• 5 years ago

Thank you for the detailed considerations.

First I want to say that black hole does not imply extreme conditions. You will not notice when falling into a really large black hole. They are violent only when small. Large black holes are almost as benigh as outer event horizon, shredding-and-tearing-wise. We can't observe singularity, so whatever matter state it is on has no bearing on information paradox.

With regards of reappearing from black hole. When the universe is close to big crunch, a lot of very heavy black holes begin to merge. When we are virtually inside a black hole, it may merge with more black holes, and if they are sufficiently large, we will be able to interact with objects (such as stars, even) inside the black holes in which they disappeared from our sight previously. Moreover, we will see that they have evolved during their absense in line with how objects outside of observable universe evolved in absense of observation.

This is when talking about very large black holes, the size of our galaxy. These are easier than it sounds due to very fast black hole volume growth.

About evaporation, I can't say too much. But I also don't see how it

UPD: ...I don't see why it needs introduction of new physics, given that it is a virtual phenomenon - nothing interesting really happens near the event horizon, it only becomes interesting at a distance.

nerdponx2• 5 years ago

My intuition is failing here.

If I put a log through a wood chipper, I can't un-chip it! Why should we expect the same for stars getting torn apart by intense gravitational fields?

Does it matter that the gravitational field is literally "spacetime itself" changing shape, versus two pieces of matter interacting with each other as in the wood chipper case?

Here's my guess of what's happening based on what you read, please tell me where I'm going wrong:

Physics generally assumes that it can be theoretically un-chipped; if somehow I could run time backwards, that I would end up with exactly the same log that I started with. But the results of interacting with with the mathematical singularity in the black hole cannot be "undone" in this way. And so far, all attempts to avoid trying to model "matter that has passed through a singularity in spacetime" with some kind of firewall, etc. have failed.

fallingknife• 5 years ago

In the past two years, a network of quantum gravity theorists, mostly millennials, has made enormous progress on Hawking’s paradox.

Wonder why they feel the need to mention that they are mostly millennials?

phn• 5 years ago

So, layman's question about black holes, almost 100% sci-fi derived.

Starting from the time mechanics shown in, e.g., Interstellar. If when you're near a massive black hole time passes differently (more time passes away form the hole, so to speak), couldn't it be said that the regions near and far the black hole are drifting apart in the "time dimension"?

If we take the black hole to be an extreme case of that, isn't the black hole a region that is drifting away so "fast" that light isn't fast enough to reach "us" on the outside?

In that case, there would be no paradox, right? Whatever is inside the black hole is still there, but with no way to communicate.

philipov• 5 years ago

isn't the black hole a region that is drifting away so "fast" that light isn't fast enough to reach "us" on the outside?

I've seen some models of black holes that are similar to this. Specifically, what is happening in those models is that the space inside the event horizon is growing faster than the speed of light, so more space is created than light can traverse.

This is the inverse of how cosmological horizons work. The reason we can only observe a limited portion of the universe is because objects are uniformly moving away from all other non-gravitationally bound objects. Space is being created between them. The farther you look, the faster galaxies are moving away from us because space is being created at every point in between. If you try to look far enough, the speed that objects are moving away from us becomes faster than the speed of light: space is being created faster than light can traverse it.

This sort of faster-than-light travel doesn't break the relativistic speed limit because these objects aren't inertially accelerating inside their frame of reference, the frame of reference itself is expanding.

hinkley3• 5 years ago

space is being created faster than light can traverse it.

Which is precisely the sort of behavior one might expect if they lived inside of a black hole.

defrex• 5 years ago

This is a reasonable conceptualization, IMO. However, the problem isn't that we can't access the information in a black hole (there are other places in the universe where information becomes inaccessible).

The problem is that black holes evaporate. If the particles released via evaporation don't contain the information about the particles that entered, information is lost when the black hole is completely gone.

The proposed solution is that the information is encoded onto the surface of the black hole and thus into the hawking radiation being released from that surface.

eloff• 5 years ago

This idea in physics that information is conserved, neither created nor destroyed, just transformed seems awfully similar to a computer to me. A classical computer is not the right metaphor really when you think of the universe as a possible computational process, but the parallels are striking to me.

pygy_• 5 years ago

I'm tying loose ends in my head that probably have long been tied in other ones...

Reading this next to the comment making a parallel between the black hole event horizon and the cosmological horizon...

Wouldn't this give credence to holographic model of the universe?

Could the so-called heat death of the universe and black hole evaporation be identical phenomena seen from either the inside or the outside of the boundary?

macrolocal3• 5 years ago

Well, objects falling into a black hole can reach the singularity in a finite amount of time. So you're going to have to enrich these singular spacetime points with a lot of extra structure if you want whatever passes through them to still exist. ¯\_(ツ)_/¯

To wit, you can imagine classical black holes as pulling whatever's near them into the future. The effect is so severe when you pass the event horizon that escaping the black hole amounts to traveling backwards in time. The effect is so severe when you reach the singularity that the entire timeline of the universe is in your past. So the singularity itself is more like an infinitely distant future than a point in space, with the caveat that the black hole slings you toward it with enough acceleration that you either actually reach it or something about this classical picture breaks down.

saalweachter• 5 years ago

I feel like there's something so scary about falling into a black hole, literally unable to escape, that we just really, really want it to be "survivable", somehow.

Which is kind of funny when you consider that no one would expect to survive falling into a star, but we don't grasp at straws the same way to say, "Oh, you wouldn't actually be immolated, the solar wind would blow you back into space first."

marcyb5st2• 5 years ago

The problem is that black hole evaporates due to Hawking radiation, which is a special case of Unruh radiation. This radiation is independent of what falls into the black hole, but just its size (and hence mass). This is the paradox. Two black holes of the same mass can be created with completely different matter and they would radiate exactly the same way and in so doing destroying the quantum information of matter they are made of.

Your point of view/theory would hold if black holes were eternal, but they probably are not if our understanding of physics is correct. In fact, if black holes "die" then the quantum information has to be released back into the universe somehow. This paper proposes a mechanism for that to happen.

kstrauser3• 5 years ago

Why does that information have to be released? To my naive layman's thinking, if you told me that a black hole permanently destroys that information, I'd think "sure, it destroys lots of other things, so that makes sense". What problem does it cause if we believe that the information is gone forever?

f154hfds• 5 years ago

For those of you like me having trouble with the abstract nature of modern physics, I have to recommend the relevant PBS Space Time on the subject: https://www.youtube.com/watch?v=HF-9Dy6iB_4

gigatexal• 5 years ago

Amid the pandemic and the increasingly scary bits of international politics and increasing nationalism I’m just so happy that very exotic science continues to happen.

yawaworht1978• 5 years ago

While I find pbs space time very good and entertaining, I am wondering if I am the only one who thinks the content is a bit towards the complex side?

wrycoder2• 5 years ago

That's great - there is a massive amount of overly simplified physics popular journalism.

dvsfish• 5 years ago

This is hardly a bad thing. Not all content should necessarily be easy to digest.

pcj-github2• 5 years ago

I recently heard somewhere that the cosmic background radiation (CMR, 2.7 kelvin or so) is so much hotter than the "temperature" generated by Hawking radiation of black holes (apparently on the order of a billionth of a kelvin) such that they effectively do not radiate, and are not expected to do so for a loooong time (until expansion of the universe drops the background temp to an incredibly cold temperature).

What I have not been able to determine is when they might be expected to occur? How far in the future will black holes start evaporating? (I believe the answer depends on the size of the black hole as well)

cdumler• 5 years ago

According to this, on the order of million trillion trillion trillion trillion trillion years [1].

[1] https://www.youtube.com/watch?v=uD4izuDMUQA

mr_toad• 5 years ago

Some of the larger black holes could take 10^100 years to evaporate even after the universe grows cold. I don’t know how long that would take, but the whole process will take an unimaginably long time.

T-A3• 5 years ago

You can easily find out yourself. First use a black hole temperature calculator like

https://www.omnicalculator.com/physics/black-hole-temperatur...

https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati...

to find the Hawking temperature T_BH of your black hole. Then use the fact that CMB temperature T_CMB is inversely proportional to the cosmological scale factor a(t), where t is time:

https://physics.stackexchange.com/questions/76241/cmbr-tempe...

Set a(t_now) = 1 for convenience; then

T_CMB(t) = T_CMB(t_now) / a(t)

and you want to find the time t when T_BH = T_CMB(t).

That depends on how the scale factor will change over time:

https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

We don't know that, but if we assume for simplicity that dark energy will keep dominating, a(t) will grow exponentially, i.e.

a(t) = a(t) / a(t_now) ~ exp(H_0 * (t - t_now))

where H_0 is Hubble's constant. So

T_CMB(t) = T_CMB(t_now) / a(t) ~ T_CMB(t_now) / exp(H_0 * (t - t_now))

Numbers:

T_CMB(t_now) ~ 2.7 K

H_0 ~ 71 km/s/Mpc ~ 2.3e-18 s^-1

t_now ~ 14 Gyr ~ 3.2e16 s

so, with t expressed in seconds,

T_CMB(t) ~ 2.7 / exp(2.3e-18 * (t - 3.2e16)) K

Setting this equal to T_BH and solving for t, I get

t = 3.2e16 + ln(2.7 / T_BH) / 2.3e-18 s

So let's say you have a solar mass black hole. Then

T_BH ~ 6.16871e-8 K

and so

t ~ 7.7e18 s ~ 2.4e11 years

i.e. 240 billion years (17 times the current age of the universe).

That's actually not a humongous number, thanks to the exponential expansion of a(t). It would take a lot longer with the expansion rate of a matter-dominated universe (exercise for the reader!)

pcj-github2• 5 years ago

Thanks, all great responses!

FeepingCreature3• 5 years ago

Does that mean you can build a Dyson sphere around a black hole, then use the black hole as a thermal sink?

T-A3• 5 years ago

http://www.weidai.com/black-holes.txt

paulpauper• 5 years ago

Reading quanta magazine always makes me feel like a failure inside . All these ppl doing cutting edge research about things that matter. Thank God for crypto investments , stock investments or else i'd have nothing going for me.

bryan0• 5 years ago

How are these extremal surfaces related to the event horizon? How are these surfaces related to the "firewall" solution to the blackhole information paradox that everyone was talking about a few years ago?

https://en.wikipedia.org/wiki/Firewall_(physics)

zachf• 5 years ago

The question about firewalls is such a good one. In short, physicists identified this problem involving three parts of an evaporating black hole, and all three seemed to be entangled according to the usual understanding, but that was bad because “entanglement is monogamous”, meaning that a system A can’t be “married” (maximally entangled) to system B and system C at the same time. So it was proposed that maybe spacetime just ends, just inside the event horizon, and so the metaphorical third wheel, system C, is killed off or actually just never existed. It’s not like anyone’s been inside a black hole and then came out to tell us if spacetime ends there. So that’s a firewall, and it was a hypothesis for how to resolve the non-monogamy issue. But it does require a bizarre new phenomenon where spacetime ends suddenly in a region of low curvature where nothing was supposed to happen. So many people did not like this idea.

However, there was an alternative idea, which was that C was the “same person” as B, in which case its perfectly legal to be married to B and C at the same time. But B and C are really far apart, so that seemed weird. It would have to be that spacetime was joining these distant systems together via their entanglement. This was called ER=EPR, or roughly speaking the idea that the connectivity of space is defined by entanglement, so that these distantly separated systems B and C were actually connected via a very quantum sort of wormhole and thus were the same thing.

This new work on the evaporation paradox, amazingly, makes it clear that at least in some cases, ER=EPR is actually happening in an unexpected but concrete way—in these models, there is the emergence of a spacetime bridge of the distant entangled systems. And that’s what’s so awesome. We always suspected the firewall paradox would prove something about what spacetime is, because it was a very sharp paradox. And now, in this unexpected and un-firewally way, it has!

GoblinSlayer2• 5 years ago

The black hole model used here is the elephant in the room, namely the no-hair conjecture. It's derived for the classic black hole, which has a problem that it's assumed into existence and has no means of formation (collapsars are only asymptotically close, but not exactly classic black holes, they are furry and the no-hair conjecture doesn't hold).

phendrenad22• 5 years ago

So in layman's terms, we just redefine the surface of the black hole until the equations work? (I'm sure I'm wrong, but maybe this'll help kick off the discussion)

Physicist discovered an escape from Hawking’s black hole paradox

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