[HN Gopher] Physicist discovered an escape from Hawking's black ...
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Physicist discovered an escape from Hawking's black hole paradox
Author : theafh
Score : 172 points
Date : 2021-08-23 15:10 UTC (7 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| hondo77 wrote:
| > 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.
| handrous wrote:
| Picking very difficult, specific topics like this to be
| interested in as a kid is pretty common.
|
| Sticking with it--that's unusual.
| _Microft wrote:
| What about: _" I'm glad she did run not across the Men in Black
| in the middle of the night..."_
| felipemnoa wrote:
| https://www.youtube.com/watch?v=K3hAVT2sDqQ&ab_channel=Oscar.
| ..
| andreareina wrote:
| Netta Engelhardt on Sean Carrol's Mindscape podcast:
| https://www.youtube.com/watch?v=m-6mcLX_v2I
| syncsynchalt wrote:
| > 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 wrote:
| 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.
| pvarangot wrote:
| 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.
| FatalLogic wrote:
| 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...
| PartiallyTyped wrote:
| 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.
| fallingknife wrote:
| > 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?
| pcj-github wrote:
| 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)
| TheCowboy wrote:
| 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.
| drran wrote:
| 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
| DontGiveTwoFlux wrote:
| 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
| sicromoft wrote:
| Seconded. Here's their video specifically about the black
| hole information paradox:
|
| https://www.youtube.com/watch?v=9XkHBmE-N34
| stronglikedan wrote:
| 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.
| not_kurt_godel wrote:
| Thirding PBS Space Time, and seconding taking time to
| really focus and treat watching them like studying. I
| became really interested in physics about 2 years ago.
| Initially the content was really challenging but I forced
| myself to rewatch many of the vids several times and it was
| ultimately very rewarding. I have a good enough grasp on
| the core concepts that I'm able to explain them to friends
| in-depth and it makes for great conversations, especially
| when people are in a state of mind to pontificate about the
| nature of the universe hehe.
| squeaky-clean wrote:
| 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.
| ardit33 wrote:
| 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
| zbendefy wrote:
| Not a physicist but I find the Science Asylum also pretty
| good:
|
| https://youtu.be/Q2OlsMblugo
| mrfox321 wrote:
| 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 wrote:
| 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.
| enkid wrote:
| Special relativity may not require a lot of math, but it's
| results are still very unintuitive.
| celticninja wrote:
| Not given up but I would like to find a physicist on twitter
| who can layman's terms some of thes developmemts.
| devoutsalsa wrote:
| I like Sabine Hossenfelder:
|
| - https://twitter.com/skdh?s=20
|
| - https://www.youtube.com/c/SabineHossenfelder
| T-A wrote:
| Well, then:
|
| http://backreaction.blogspot.com/2020/11/the-black-hole-
| info...
| celticninja wrote:
| Thank you
| anigbrowl wrote:
| 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.
| marcyb5st wrote:
| 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.
| lordnacho wrote:
| 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.
| WindyLakeReturn wrote:
| 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.
| ben_w wrote:
| 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 wrote:
| > 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.
| abeppu wrote:
| > 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.
| TheCowboy wrote:
| 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.)
| fho wrote:
| Mandatory link to Ian's Shoelace Site:
| https://www.fieggen.com/shoelace/
| jrace wrote:
| This site helped me to realize I had been tying one of my
| shoes wrong my entire life.
| gumby wrote:
| > 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.
| moonchrome wrote:
| >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").
|
| Even the extreme case has to be somewhat realistic to be
| worth considering. Physics has gone way past that point
| once they started building accelerators larger than cities
| to detect subatomic reactions. Not saying it's not worth
| investigating but it's not at all comparable to any
| practical domain, at this point it's l'art pour l'art.
| svachalek wrote:
| It's realistic, they are learning about the particles
| that make up everything on Earth. But not necessarily
| pragmatic, in that there aren't likely new uses for that
| knowledge in the foreseeable future. Still, imagine what
| we can do once we've achieved a model of how it all
| really works.
| moonchrome wrote:
| I don't really know how the model enables much if it
| requires that kind of effort to test the predictions. If
| the predictions implied something more practical you
| could test that. I think the time where this kind of
| physics breakthroughs lead to anything useful is gone.
|
| At the same time I think there's stuff that's much more
| practical with still a lot of discoveries to be made -
| like superconductivity.
| gumby wrote:
| The same was said about quantum mechanics (of which these
| accelerators are part of the investigation) in the 1900s,
| yet without that work there would have been no transistor
| .. or superconductors.
| moonchrome wrote:
| Umm how could the same be said about quantum mechanics ?
| Double slit experiment can be setup in a classroom.
|
| If the predictions of your hypothesis require a country
| sized accelerator to test what are the real world
| applications ?
| Rexxar wrote:
| General relativity is maybe a better example: It's
| necessary for GPS but it was initially very difficult to
| validate experimentally.
| gumby wrote:
| There's a lot more to QM than the double slit experiment
| (and a lot more experimentation before predicting
| superconductors).
|
| But if you consider basic research worthless, physics is
| hardly the only "offender".
| zopa wrote:
| 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 wrote:
| 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
| tobmlt wrote:
| >>> 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 ;)
| whatshisface wrote:
| > _Sand piles are of course the bridge between the two ;)_
|
| Bridges are an engineering topic, most coastal areas lie on
| sedimentary rocks, and wormholes, a theoretical physics
| topic, are also called "Einstein-Rosen bridges."
|
| In other words, while bridges are bridges between sand
| piles and sand piles, sand piles are bridges between
| bridges and bridges.
| hellbannedguy wrote:
| Are these videos working for you guys?
| pyb wrote:
| 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.
| immmmmm wrote:
| 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.
| PartiallyTyped wrote:
| 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.
| immmmmm wrote:
| i do partially agree. while it was once a very good
| educational channel, it has often been very opinionated,
| sometimes in good, but also in somewhat narrow ways.
|
| for instance repeating that modified gravity is great and
| that strings/supersymmetry/etc is bad is a bit weak
| especially on a science education channel.
|
| i have worked with strings and some of her criticism if
| founded, repeating over the years that people that work in
| those domains are intellectual fraudsters (i'm barely
| exaggerating) is wrong and especially damaging on an
| educational channel. consequently, there a whole mob of
| youtube commenters that repeat this (with no context) to
| whoever wants to hear that.
|
| the same happened with Smolin and his book, following
| Green's book. TBH LQG is not yet there (despite recent
| interesting progress) and Calabi Yau compaction don't yield
| the universe we observe. Modified gravity doesn't seem to
| work too well too..
|
| so yes, if you remain critical of what she says :)
| supercheetah wrote:
| > for instance repeating that modified gravity is great
| and that strings/supersymmetry/etc is bad is a bit weak
| especially on a science education channel.
|
| I'm not a physicist, but she does point out that she
| thinks dark matter is a combination of modified gravity,
| and some new particles, and not just one or the other.
|
| Also, she does usually make it clear when she has an
| opinion and bias towards less supported hypotheses, but
| it's always on things that don't already have any
| evidentiary basis, like dark matter.
| SubiculumCode wrote:
| My favorites are the articles about a new math discovery...no
| clue...but I like cheering for them!
| omgJustTest wrote:
| Startswithabang and other science educators, particularly
| Veritasium, do excellent jobs.
|
| It's complicated and worth the time to understand what they
| present.
| phendrenad2 wrote:
| 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).
| sharikous wrote:
| 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
| DantesKite wrote:
| 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.
| yawaworht1978 wrote:
| 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?
| wrycoder wrote:
| That's great - there is a massive amount of overly simplified
| physics popular journalism.
| paulpauper wrote:
| 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.
| ChrisArchitect wrote:
| Recent discussion about confirming Hawking's Black Hole theorem:
|
| https://news.ycombinator.com/item?id=27696774
| prof-dr-ir wrote:
| 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.
| f154hfds wrote:
| 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
| phn wrote:
| 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 wrote:
| > _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.
| defrex wrote:
| 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 wrote:
| 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.
| simonh wrote:
| I suspect it may be just necessarily true that information
| is preserved in a consistent universe. I don't know though,
| maybe someone could come up with a model for a consistent
| universe with information loss, but it seems to me that
| would lead to physically possible states that are not
| derivable from consistent laws of physics.
| causasui wrote:
| Physics layman, but I agree as a computer scientist. It
| also sometimes feels like there are "optimizations", e.g.,
| delayed-choice quantum erasure
| (https://en.wikipedia.org/wiki/Delayed-
| choice_quantum_eraser)
|
| I'm open to the idea that it's just me projecting what I
| understand onto what I don't.
| macrolocal wrote:
| Nb. wave function collapse messes with this, and a computer
| would use something like lazy evaluation to avoid
| generating the Everettian multiverse.
| marcyb5st wrote:
| 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.
| kstrauser wrote:
| 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?
| drdeca wrote:
| I believe the idea is that in quantum mechanics, time
| evolution is described by a unitary operator, and because
| it is unitary, it must have an inverse, and, therefore, the
| state after must determine the state before.
|
| Which, of course, reduces the question to "why does time
| evolution have to be unitary?"
|
| And, one definition of what it means for an operator U to
| be unitary, is that it preserves inner products, and is
| surjective.
|
| Why should it preserve inner products?
|
| Well, a state should have norm 1, i.e. the inner product of
| it with itself should be 1, and the state in the future
| should also have norm 1. (this 1 can be thought of
| representing the probability that "something/anything
| happens", which should always be 1.) And also, the time
| evolution should be linear (that things are done with
| linear operators is nearly the core assumption of QM ime ),
| so therefore it should also preserve the norm of other
| vectors. And, the polarization identity allows one to
| recover the inner product operation from a norm which came
| from an inner product,
|
| In what follows, "<" and ">" are angle brackets, not less
| than or greater than signs. also, by ||x||^2 I mean the
| norm squared of x, i.e. the inner product of x with x, i.e.
| <x | x> . The polarization identity (a theorem of math, not
| specific to physics) states that
|
| <x | y> = (1/4)( ||x + y||^2 - ||x - y||^2 - i||x + i y||^2
| + i||x - iy||^2)
|
| So, in particular, for some linear operator A,
|
| <A x | A y> = (1/4)( ||A x + A y||^2 - ||A x - A y||^2 -
| i||A x + A i y||^2 + i||A x - A iy||^2) = <A x | A y> =
| (1/4)( ||A (x + y)||^2 - ||A (x - y)||^2 - i||A(x + i
| y)||^2 + i||A(x - iy)||^2)
|
| And, if A preserves norms, i.e. if for all x, ||A x|| =
| ||x|| , then therefore
|
| <A x | A y> = (1/4)( ||A (x + y)||^2 - ||A (x - y)||^2 -
| i||A(x + i y)||^2 + i||A(x - iy)||^2) = (1/4)( ||x + y||^2
| - ||x - y||^2 - i||x + i y||^2 + i||x - iy||^2) = <x | y> .
|
| So, by the polarization identity, if a linear operator
| preserves norms, it preserves inner products.
|
| So, if you accept the "time evolution is linear, and the
| state should always be a unit vector in a Hilbert space",
| then it follows that time evolution should preserve inner
| products.
|
| The only thing remaining is, I guess, the assumption that
| time evolution is surjective. I.e. for any state, there is
| some state which should be able to lead to it.
|
| I suppose one could question this assumption?
|
| But I don't think giving this up would result in allowing
| the loss of information, because these requirements still
| imply that time evolution should be injective. If two
| states x and y were both sent by time evolution to the same
| state z, then, if x and y are not equal to each-other, then
| x-y is not zero, and it can be re-scaled to have norm 1,
| (specifically, giving us (x-y)/||x-y|| ) and be a valid
| state,
|
| and the time evolution would send (x-y)/||x-y|| to
| (z-z)/||x-y|| = 0. Which would mean, it would send a valid
| state to, uh, nothing. This contradicts our assumption that
| it preserves a norm of 1. To interpret this a bit, if it
| did fail to be injective in this way, sending both x and y
| to z, then if the current state were (x-y)/||x-y|| , then
| in the future, after applying the time evolution operator,
| the probability that "anything" would be 0, which is absurd
| (and also contradicts our assumption of preserving the
| norm).
|
| So, if [the state is represented by a vector in a Hilbert
| space, and the Born rule applies for probabilities, and
| time evolution is linear], then time evolution has to
| preserve the inner product and therefore also be injective.
|
| This I think basically justifies the "information is
| preserved" idea.
|
| You might ask "ok, how would you modify quantum mechanics
| in a way that did allow time evolution to not be
| injective?" and, I'm not sure what the appropriate way to
| do that would be.
|
| Hm, I suppose maybe you could like, use states which are
| technically different, but not in ways that any observable
| could ever (even theoretically) distinguish between?
|
| (are selection sectors relevant to that? I'm not sure.)
| amluto wrote:
| Because quantum mechanics _really_ does not like destroying
| information. Mangling information beyond recognition is
| just fine, but the laws of quantum mechanics are very
| insistent that, if you have a complete description of the
| state of the universe, you can solve the equations
| backwards and figure out what happened in the last. When
| you throw in a black hole following Hawking's rules, or any
| other device that irretrievably chews things up and spits
| them out in a way that can't, even in theory, be undone,
| quantum mechanics breaks.
| hollerith wrote:
| The problem is that black-hole evaporation is a high-level
| description of many "fundamental" events, and at the level
| of fundamental physics, there is no known process that
| destroys information.
|
| Or so popular-science articles tell me.
| macrolocal wrote:
| 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. -\\_(tsu)_/-
|
| 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 wrote:
| 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."
| thriftwy wrote:
| 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.
| raattgift wrote:
| 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).
| thriftwy wrote:
| 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.
| isk517 wrote:
| 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.
| thriftwy wrote:
| 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.
| HellzStormer wrote:
| My take on the difference (not a physicist):
|
| Take that galaxy that just crossed our "observable universe
| horizon" so that we can't see it. If there is a
| civilization halfway between Earth and that galaxy, they
| can still see that galaxy. The galaxy can see that second
| civilization and so can we. There isn't a single fixed
| "observable universe" boundary in space, it's just relative
| to the observer.
|
| With a black hole, it is different. There is no point that
| can see both sides while being seen from both sides,. If
| you are outside the black hole, you see nothing from
| within. If you are inside, then you can see the inside
| (this is speculation) and you also see the outside. It's a
| very clear boundary.
| thriftwy wrote:
| This is known to be not correct. When you are near the
| event horizon, you can still observe most of our ordinary
| observable universe, as well as subset of black hole's
| interior.
|
| This is the direct reason/consequence of not being able
| to observe the event horizon when near it, or notice when
| you cross it.
|
| The boundary is not clear, it depends on the observer.
| duxup wrote:
| > 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?
| uoaei wrote:
| This is basically stating an assumption of determinism with
| respect to physical dynamics.
| cryptonector wrote:
| Determinism.
|
| The laws of physics are mostly fully deterministic, even
| quantum laws.
| jwally wrote:
| 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 wrote:
| 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?
| lalaithion wrote:
| The evolution of the wave function is deterministic.
| However, the observables of the wave function are not
| deterministic. So if I tell you the state of a photon
| moving toward your eye, you can determine the probability
| distribution of what color you will see, but not the actual
| color, because there's randomness during collapse.
| Filligree wrote:
| It implies that the universe is deterministic, with the
| caveat that what's deterministic is the universe as a
| whole, which includes umpteenillion extra "timelines" which
| we can't see, in addition to our own.
|
| There's remains indexical uncertainty, as we can't predict
| which timelines we'll see. The answer is of course all of
| them.
| ABeeSea wrote:
| 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 wrote:
| It sounds like just the Schrodinger Equation, Quantum Mechanics
| 101
| abdullahkhalids wrote:
| 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.
| gigatexal wrote:
| 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.
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