The Hidden Structure of the Universe
November 11, 2020 8:42 AM   Subscribe

An End to the Most Famous Paradox in Physics - "In a landmark series of calculations, physicists have proven that black holes can shed information."
Physicists had always figured that a quantum theory of gravity came into play only in situations so extreme that they sound silly, such as a star collapsing to the radius of a proton. Now Page was telling them that quantum gravity mattered under conditions that, in some cases, are comparable to those in your kitchen...

By showing that the entanglement entropy tracked the Page curve, the team was able to confirm that black holes release information. It dribbles out in a highly encrypted form made possible by quantum entanglement. In fact, it is so encrypted that it doesn’t look as if the black hole has given up anything. But eventually the black hole passes a tipping point where the information can be decrypted. The research, posted in May 2019, showed all this using new theoretical tools that quantify entanglement in a geometric way...

And that led to a remarkable twist in the story. Because the radiation is highly entangled with the black hole it came from, the quantum computer, too, becomes highly entangled with the hole. Within the simulation, the entanglement translates into a geometric link between the simulated black hole and the original. Put simply, the two are connected by a wormhole. “There’s the physical black hole and then there’s the simulated one in the quantum computer, and there can be a replica wormhole connecting those,” said Douglas Stanford, a theoretical physicist at Stanford and a member of the West Coast team. This idea is an example of a proposal by Maldacena and Leonard Susskind of Stanford in 2013 that quantum entanglement can be thought of as a wormhole. The wormhole, in turn, provides a secret tunnel through which information can escape the interior...

All this reinforces many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal. To many, that was the main lesson of the AdS/CFT duality. The new calculations say much the same thing, but without committing to the duality or to string theory. Wormholes crop up because they are the only language the path integral can use to convey that space is breaking down. They are geometry’s way of saying the universe is ultimately nongeometric.
No, Stephen Hawking's Black Hole Information Paradox Hasn't Been Solved - "We still don't know how the information encoded onto it gets out."
The space outside of a black hole is enormously complicated, even if we treat it as an idealized, rather than a physically realistic, system. While most of us think of space similarly to how Newton did — as an imaginary three-dimensional grid, perhaps with an additional layer of Einsteinian curvature to it — it’s perhaps more accurate to think of the space around a black hole as being like a moving walkway or a river: something that moves all on its own. You can walk or swim with, against, or perpendicular to the current, but the important fact is that space behaves as a non-static, in-motion entity all on its own.

In addition to that, we’re assuming that the laws of General Relativity are still perfectly accurate for describing the dynamics of space at a quantum level: we are assuming that the quantum effects that create Hawking radiation are important, but that any quantum effects that arise because treating space as a classical and continuous background can be ignored. Researchers who work on this call this approach a “semi-classical approximation,” and the suspicion is that something about it must break down.
also btw...
-A new way to visualize General Relativity
-How we know that Einstein's General Relativity can't be quite right
-Quantum Field Theory visualized
-The five most promising ways to quantize gravity
-Can we craft a theory in which space and time aren't assumed to exist?

earlier: more in the series:
posted by kliuless (36 comments total) 74 users marked this as a favorite
 
now where did I put that spare brain?
posted by thelonius at 8:49 AM on November 11, 2020 [17 favorites]


Susskind is one of the few minds who can not only grasp this stuff, but also explain it
posted by OHenryPacey at 9:02 AM on November 11, 2020 [4 favorites]


@kliuless - this is a lot for one post! I loved the General Relativity visualization. The bowling-ball-on-a-trampoline metaphor always bugs me because it ignored the temporal part of space-time curvature - that problem is solved beautifully here. This is a very good use of the video medium, and the execution was great too - the shimmery quality of the grid-lines made them look more like real objects & gives the eye some details to latch on to. Thanks for posting this!
posted by crazy_yeti at 9:17 AM on November 11, 2020 [1 favorite]


Does all this confirm, undercut, or sidestep Susskind's Holographic Principle?
posted by jwhite1979 at 9:35 AM on November 11, 2020 [1 favorite]


paging physicsmatt physicsmatt to the explanthisshit-o-phone
posted by lalochezia at 9:53 AM on November 11, 2020 [8 favorites]


Netta Engelhart was also on Sean Carroll's Mindscape podcast recently.
posted by pw201 at 9:54 AM on November 11, 2020 [2 favorites]


Wormholes crop up because they are the only language the path integral can use to convey that space is breaking down. They are geometry’s way of saying the universe is ultimately nongeometric.

This is an astonishing pair of sentences.
posted by mhoye at 9:56 AM on November 11, 2020 [11 favorites]


That said, I'm really just here to make this joke:

Now Page was telling them that quantum gravity mattered under conditions that, in some cases, are comparable to those in your kitchen...

Old news: Molecular gastronomy.
The new thing: Quantum gravity kitchens.
posted by mhoye at 10:00 AM on November 11, 2020 [5 favorites]


Do not understand, but am looking forward to the Greg Egan novel.
posted by clew at 10:15 AM on November 11, 2020 [14 favorites]


Highly nontrivial calculations indeed. To a layperson, it sounds more like physics as we conceive of it breaking down into pure information theory. If this holds up, you get the sense of a revolution in understanding coming.
posted by blue shadows at 10:24 AM on November 11, 2020 [3 favorites]


OHenryPacey, maybe that's a good explanation if you have a solid grounding in quantum theory already. I hope maybe Dr. Katie Mack will get around to helping the rest of us comprehend it.
posted by rikschell at 10:30 AM on November 11, 2020 [2 favorites]


Paging physicsmatt ...
posted by seanmpuckett at 10:31 AM on November 11, 2020


many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal

This notion from Stephen "Stephen Wolfram" Wolfram appears to address the mechanism. A self-instantiating algorithm from which emerges the big simulation...or something like that?
posted by The Half Language Plant at 11:18 AM on November 11, 2020 [5 favorites]


Stephen "Stephen Wolfram" Wolfram

this got an involuntary guffaw out of me
posted by BungaDunga at 11:33 AM on November 11, 2020 [9 favorites]


I have to say, I think of Wolfram as kind of a blowhard, and titling one's book "A New Kind of Science" just screams self-important. But I will admit that I've found his latest excursions into the nature of spacetime to be very compelling, both to read and think about. Pretty much completely un-falsifiable though, so not anything I would call Physics, per se.
posted by dbx at 11:53 AM on November 11, 2020 [1 favorite]


many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal

I understand this at about 37% capacity and my mind already can't handle it. Imagining that something like consciousness is a by product of a by product just doesn't sound... real. Are we the root to something bigger? What are those byproducts?
posted by FirstMateKate at 1:03 PM on November 11, 2020


Re: The holographic principle, I don't think it would have much of a bearing on it, but of course IANAP.

My understanding is that the point of the holographic principle is that the amount of space required to describe the state(s) of a given region of space time is exactly the same as the surface area of that spacetime.

Obviously, a sphere is the simple/obvious example. Basically any point within the volume of the sphere can have a given state and the information regarding that state can be written as a point on the surface. It's called holographic, in the same way a 2D surface can project a 3D image of a fully 3d object. The 2 surface contains the image in 3 dimensional space. likewise, the surface of an N-Dimensional space can be fully describe N-1 surface area bounding that space (regardless of its shape - I think).

Is this wrong? Anyways, if that's the conjecture/point of it, I can't imagine much about this new result would tell us anything except that perhaps, as you disentangle/evaporate the data inside, the surface would also shrink in order to compensate. It's just a natural result? Black Hole evaporation, that is - it wouldn't have any effect on the principle, as it's apples and oranges?
posted by symbioid at 1:03 PM on November 11, 2020 [1 favorite]


I would like a definition of the word “information”. Is it a description of the current state of all the subatomic particles in a thing? If I knew all the properties of all the particles I could recreate the thing? In one of the articles they use a book as the thing. Throw a book into a black hole and you lose all the information of the book and/or in the book. It’s not clear. If I throw a book into a raging fireplace, I do lose the information in the book, but do I lose the information of the book? One of my problems with physics, is that they use common words in a deeply technical way and they do not try to clarify their use versus the common understanding. They know what they are talking about, but we poor laypeople have to thrash about trying to apply referents to terms we thing we know and understand but when we do apply a referent it still doesn’t make any sense. All of this sounds interesting but not being a certified mason of physics, I don’t know the secret passwords nor the secret handshakes. Some serious clarification would be nice.
posted by njohnson23 at 1:45 PM on November 11, 2020 [5 favorites]


many physicists’ hunch that space-time is not the root level of nature, but instead emerges from some underlying mechanism that is not spatial or temporal.

I'd also appreciate someone taking a crack at explaining this. Hunch based upon what? Does anyone have an inkling about the nature of the underlying mechanism?
posted by leotrotsky at 1:56 PM on November 11, 2020 [2 favorites]


There’s the physical black hole and then there’s the simulated one in the quantum computer, and there can be a replica wormhole connecting those

So in other words, the proposal that we are all just living in a simulation and the proposal that we are actually real are both right simultaneously, and possibly there's no actual difference between those two concepts in an absolute frame of reference, if those frames of reference become entangled. Also, omelets.
posted by It's Raining Florence Henderson at 1:57 PM on November 11, 2020 [6 favorites]


njohnson23 I think the concept is that if you "ran the film backwards" of the book burning, applying known physical laws, the book would re-assemble. All the information is theoretically there to reverse the process from smoke to poetry.
posted by Mei's lost sandal at 3:35 PM on November 11, 2020 [4 favorites]


This is how you get TRON, isn't it? Stick someone in a pod, entangle the pod with a quantum computer, drop the pod in a small, handy black hole, reassemble them in a videogame on the quantum computer, Bob's your animated uncle.
posted by It's Raining Florence Henderson at 4:00 PM on November 11, 2020 [3 favorites]


Mei’s Lost Sandal,

But then again they use the example of a dropped tea cup or fried eggs not being reversible in their entropy examples. Since frying eggs increases the entropy of the universe, unfrying them would decrease the entropy of the universe, which is a no-no. Playing the film backwards appears to be an abstract action and not an actual action. A film is a recording of an existing event. Are they implying that there is a “recording”? Probably not. A clump of ashes does not tell us what it was. Or what was written on its pages. My main complaint is that these terms, these real world models all seem to have issues. I can’t either understand them or critique them except from an ontological/epistemic point of view. The formulas are gibberish to me. I’m honestly poorly trained in math.
posted by njohnson23 at 4:07 PM on November 11, 2020


IANAP at allllllllllll but I do love reading pop physics about this shit. I'm just now settling in to read all the links in this post for the rest of this evening with pen and notebook in hand; I have no idea what any of the OP means. But I can contribute something to this thread, I think: background info that may be relevant.

In quantum mechanics there is a concept known as a "wave function", which is a mathematical way of describing all the parameters - think of it as metadata - of an UNOBSERVED independent quantum system. (I should note here that I'll be using entirely laypeople's language; for instance the word "parameters" may mean something wildly different in the context of quantum physics but whelp IANAQP at alllllllllllllll so idk.)

Anyway, if, like, a breadbox was your assigned independent quantum system for which you want a wave function, then you'd be like:

Step 1: DO NOT FUCKING LOOK AT THE FUCKING BREADBOX, because observation = collapsed wave function i.e. game over before you begin

Step 2: come up with a set of measurables that would describe it, e.g. dimensions, shape, material composition, mass, velocity, positions, its probability of flickering out of existence right before your very eyes due to its quantum-entangled twin anti-breadbox getting caught up in black hole shenanigans, etc.

(If the quantum system in question is not a breadbox but rather an elementary particle, a wave function would describe its spin, isospin, a matrix assigning probability values for its variables at each possible point in all of space (!!) that the particle may occupy, how much to worry about its evil twin, etc.)

Once you decide how to represent any given quantum system, you can derive its wave function by plugging in whatever information you have about its quantum state (information obtained in some nefarious way other than observation). Schrodinger's equations govern how the wave function will behave over time - boom, now you have a fully deterministic model for the behavior of your quantum system. You can add or multiply or fourier transform two different wave functions - boom, now you're studying interactions between two quantum systems under various conditions. So wave functions are super useful. Also wave functions are one way to mathematically arrive at wave-particle duality, so this is not just a nifty trick to represent quantum fields in the form of manipulatable numbers; it's meaningful in a real sense, with scientific validity and predictive power.

Now, as mentioned above, Schrodinger's equations governing how wave functions of quantum systems evolve means that if I know its wave function now, then I should be able to determine its values at any other time - past or future. Wave functions can only do two things: either collapse (observation) or evolve (according to Schrodinger's equation). No third possibility. This principle is considered non-negotiable, at least according to the most popular interpretations of quantum mechanics.

However, when black holes show up, this non-negotiable principle appears to break down. If your breadbox falls into a black hole, then the metadata i.e. the descriptive, predictive, deterministic information contained in the wave function of the breadbox is just ~gone~. General relativity decrees it. The wave function neither collapses via observation nor does it continue to evolve according to Schrodinger's equation inside the black hole. Instead, all that information is *lost*, gone, poof. That's not supposed to happen. This is known as the black hole information paradox. It is one of the ways in which quantum mechanics and general relativity do not agree with each other.

BUT WAIT DON'T PANIC, because there's a way it all works out with no need to violate any rules.

Black holes must have entropy, because we (normal folks who exist well away from any black hole) would be able to violate our second law of thermodynamics: for example, we might decrease the average entropy of our universe just by chucking a bunch of very hot gases into a black hole. Obviously that is bonkers. Throwing high entropy things into a black hole MUST be increasing the black hole's entropy at least as much as it decreases ours. That entropy can't disappear, poof, *lost*, ~gone~ just because it's in a black hole now. If a black hole has entropy, can it be measured or described by equations? Yes, it can. Some guy came up with statistical modeling of the upper and lower bounds of entropy for black holes. (My memory is weak here, this last sentence is .. approximate. He might have come up with upper and lower bounds of entropy for only the event horizon, or something like that? I don't remember.)

Anyway, his equation was VERY interesting because he showed that the upper and lowe bounds for entropy of a black hole were related to the radius of the black hole *squared* (area), not the radius *cubed* (volume) as might be expected. So from this he concluded that the entropy information for whatever is chucked into a black hole remains on the surface of the black hole on the event horizon. Entropy is not affected by the volume and does not affect the volume because entropy never gets past the surface. Any "information" i.e. metadata about the breadbox that has fallen into a black hole has not actually fallen into a black hole (and disappeared), but rather it is forever preserved on the surface of the black hole at the event horizon. Forever, because from the observer's perspective, time stops at the event horizon.

This is the solution to the Black Hole Information Paradox. We are back to the original two possibilities. The wave function has not collapsed due to observation, and it cannot evolve according to Schrodinger's equation because it is stuck in time. Quantum systems can't evolve unless time moves. Everybody's happy. The information isn't lost, it is preserved.

At this point my memory is failing me once again but IIRC Stephen Hawking , while trying to prove that nothing could possibly leak out of a black hole, ended up with a bunch of equations that showed the opposite. He presented that as "omg there is random radiation coming out of the black hole every time it absorbs something nonrandom." I'm oversimplifying here because I have terrible recall, but the problem with his equations and his claim was that it contradicted a different principle of quantum mechanics known as superposition. Superposition required that the leaked radiation should have some correlation with whatever was absorbed.

That's when we get the Holographic Principle which Leonard Susskind was developing independently on his own; he was coming at it from a different angle but also ended up with the same conclusions about the Black Hole Information Paradox, viz., information is preserved at the event horizon. AND Susskind used a very specific version of string theory to describe the mathematics of how whatever falls into the blackhole modifies what leaks out of the black hole. This was HUUUUUUUUUUUGE because it was the first time that string theory had been used to describe quantum gravity effects. Suddenly, string theory was the one way to completely describe the "oscillation" of the black hole event horizon as a self-contained system; "oscillation" here meaning absorbing and emitting, with each affecting the other - what is absorbed contains information about what is emitted, what is emiitted contains information about what is absorbed, all parts of this system affect all parts of the system, and string theory is what ties it up in a neat bow. (Pardon the pun.)

(BTW I have no idea why it's called the Holographic Principle. If anyone knows, please enlighten me! Are there holograms involved? It can't be *that* cool, can it?!)

IDK if knowing this will help me understand what's going on in the OP, but this is my best guess as to what background info might be relevant to it. Off to read!
posted by MiraK at 4:48 PM on November 11, 2020 [9 favorites]


It's called the holographic principle because it's the best analog that people are familiar with of a lower dimensional surface projecting a higher dimensional volume and vice versa. ADS-CfT "proves" the math, but we don't live in anti-DeSitter space.

There is at least one lecture by Juan Maldacena himself on YouTube that is reasonably accessible to normal folks. I believe Sean Carroll also discussed the holographic principle in one of his "Biggest Ideas in the Universe" videos earlier this year. There are a couple of Lenny Susskind's lectures on it on YT also, and he has discussed it in more informal terms in a couple of Closer to Truth videos.
posted by wierdo at 5:11 PM on November 11, 2020 [5 favorites]


>I would like a definition of the word “information”. Is it a description of the current state of all the subatomic particles in a thing?

I've studied physics and -do not- understand what the term is supposed to mean. WPedia has an article called "physical information" that is slightly penetrable. It says:
Information can also be understood as a measure of probability as follows: a physical state with a low initial probability of observation contains a relatively high quantity of physical information.... [and vice versa]
This 2018 comment ... somewhat more informative ... explains that we're not alone. It concludes:
Let’s summarize. In physics, the word information is closely related to microstates and probabilities. In some limited circumstances information is equal to entropy, but in most cases not. Information should never be confused with knowledge despite what natural language and the dictionary say. And never ever confused with the knowledge of intelligent beings.
(I for one would welcome more ... knowledge on the topic.)
posted by Twang at 5:31 PM on November 11, 2020 [4 favorites]


AFAIK information refers to the wave function of a quantum system. Observation causes wave functions to collapse, which is why "low initial probability of observation contains a relatively high quantity of physical information". The information we have pertaining to the probability of Schrodinger's cat being dead vs. the probability that the cat is alive is at its peak when we have not opened Schrodinger's box. Opening the box collapses the probabilities, lowering the physical 'information'.
posted by MiraK at 5:41 PM on November 11, 2020 [1 favorite]


A good way I've seen information explained is as a number that tells you how surprised you should be when you see something unexpected or out-of-the-ordinary.

In human beings, for instance, you can read out their genetic sequence as a long series of letters (A, C, T, and G). Most of the time, generally speaking, you should expect to find a letter to be "X" at position Y, in some fraction Z of a set of people. (The values of X, Y, and Z can change over time, but that doesn't matter because generally everyone agrees on these sets for doing research.)

A number — a measure of information — can be assigned to that position Y to suggest how "surprised" you should be when you don't see X where you're looking for it. This number can be measured in various units: bits, nats, whatever. Mainly, when you see something other than X at Y, something that you didn't expect to see, that could be (biologically) interesting — like something that might contribute to causing a genetic disease, say. Maybe you could even use that information to find a cure for said disease.

In physics, it's the same idea, but writ larger: information is an arrangement of stuff in some ordered fashion in the universe around us. Most of the universe is randomly distributed, like a shuffled deck of cards — small parts of it are ordered.

Usually we don't care too much about randomness, as it is boring. A fair coin is boring. If you flip a fair coin, it is 50/50 odds you will get a heads or tails. You don't have certainty about the outcome. It's the ordered stuff where we are looking that we care most about: moons, planets, stars, galaxies. Or a biased coin — or a loaded die or stacked deck of cards at the casino — where you can measure how certain you expect to see a side of the coin, die, or turn of the card. Those are interesting. How things are ordered, and the odds that we see that order, can be translated into a numerical measurement called information.
posted by They sucked his brains out! at 6:25 PM on November 11, 2020 [3 favorites]


"...example, we might decrease the average entropy of our universe just by chucking a bunch of very hot gases into a black hole. Obviously that is bonkers. Throwing high entropy things into a black hole MUST be increasing the black hole's entropy at least as much as it decreases ours"


"The discovery leaves open the question of when the Milky Way was last active. A minimum age can be calculated by dividing the jet's 27,000-light-year length by its approximate speed. However, it may have persisted for much longer.

"These jets probably flickered on and off as the supermassive black hole alternately gulped and sipped material," said Finkbeiner.

It would take a tremendous influx of matter for the galactic core to fire up again. Finkbeiner estimates that a molecular cloud weighing about 10,000 times as much as the Sun would be required.

"Shoving 10,000 suns into the black hole at once would do the trick. Black holes are messy eaters, so some of that material would spew out and power the jets," he said."
posted by clavdivs at 10:35 PM on November 11, 2020


If information is preserved on the surface of the black hole, this seems to imply that the surface is a thing, and that this thing can encode information which implies that this thing is made up of smaller things, which can hold different states. By surface do you mean the Schwarzshild (spelling) radius? I was led to believe that this was a boundary based on gravity and not a thing.
posted by njohnson23 at 7:31 AM on November 12, 2020 [1 favorite]


> If I throw a book into a raging fireplace, I do lose the information in the book, but do I lose the information of the book?

netta engelhart helps explain it in the podcast pw201 links to -- worth a listen! (oh and sabine hossenfelder does, too ;)

> I will admit that I've found his latest excursions into the nature of spacetime to be very compelling, both to read and think about.

fwiw...
-Stephen Wolfram: Fundamental Theory of Physics, Life, and the Universe
-Stephen Wolfram vs. Eric Weinstein: Mathematical Reality & Their Two New Theories of Everything (previously)
-Garrett Lisi ... with host Eric Weinstein

but note why these TOEs don't impress sabine hossenfelder!

also btw, quanta's fourth article in their 'hidden structure' series is out:
What Is a Particle? - "It has been thought of as many things: a pointlike object, an excitation of a field, a speck of pure math that has cut into reality. But never has physicists' conception of a particle changed more than it is changing now."

oh and check out :P
Meet the kaon - "Nearly 75 years after the puzzling first detection of the kaon, scientists are still looking to the particle for hints of physics beyond their current understanding."
posted by kliuless at 12:13 PM on November 12, 2020 [2 favorites]


njohnson, yes, I understood surface to mean the concept of a surface determined by the Schwarzschild (sp? indeed) radius, which would also be synonymous with the event horizon of the black hole. I have no idea how such an "encoding" might work, and from my reading of the links in OP last night, it seems like my understanding of this is in fact disputed? Or else it isn't the heart of the matter, because whether and where this information is preserved is, strictly speaking, irrelevant if the information doesn't leak out.... or if we don't get some evidence that the information at least WAS preserved while it was inaccessible. A tree might fall and be perfectly preserved on the event horizon, but if we neither have nor can have any evidence of this happening, did it make a sound? My focus on the info being preserved on the surface appears to be misplaced. The real question seems to be about whether there's any evidence for the information being preserved which we can access, even in theory.
posted by MiraK at 1:25 PM on November 12, 2020 [1 favorite]


That is almost exactly how I always pictured relativity, but I think viewing it as a “flow” works better. They do a good job of stretching the grid lines to show space warping, but the metaphor still breaks when they splice in new ones. I think a particle flow would work really well at representing this in a continuous way — something like https://earth.nullschool.net/

Damn, I wish I were as good at building shit as I am pontificating about it.
posted by bjrubble at 9:39 PM on November 12, 2020


Check out black hole complementarity to really bake your noodle.
posted by swr at 9:49 PM on November 12, 2020


Ahmed Almheiri: From the UAE to Firewalls - "'Black Holes: Complementarity or Firewalls?' by Almheiri, Donald Marolf, Joseph Polchinski, and James Sully (collectively known as AMPS, after their initials) tried to understand how information gets out of a black hole." :P
posted by kliuless at 11:42 PM on November 13, 2020


The Black Hole Information Loss Problem is Unsolved. And Unsolvable. - "Today I comment on the recent claim that the black hole information loss problem is 'near its end' and explain why this is nonsense."
posted by kliuless at 10:01 PM on November 19, 2020


« Older Bear Dogs to the Rescue   |   "The price of food keeps us from realising our... Newer »


This thread has been archived and is closed to new comments