Maybe you’ve recently seen a headline like this:
Actually, I’m more worried that you saw that headline before it was edited, when it looked like this:
If you’ve seen either headline, and haven’t read anything else about it, then please at least read this:
Physicists have not created an actual wormhole. They have simulated a wormhole on a quantum computer.
If you’re willing to read more, then read the rest of this post. There’s a more subtle story going on here, both about physics and about how we communicate it. And for the experts, hold on, because when I say the wormhole was a simulation I’m not making the same argument everyone else is.
[And for the mega-experts, there’s an edit later in the post where I soften that claim a bit.]
The headlines at the top of this post come from an article in Quanta Magazine. Quanta is a web-based magazine covering many fields of science. They’re read by the general public, but they aim for a higher standard than many science journalists, with stricter fact-checking and a goal of covering more challenging and obscure topics. Scientists in turn have tended to be quite happy with them: often, they cover things we feel are important but that the ordinary media isn’t able to cover. (I even wrote something for them recently.)
Last week, Quanta published an article about an experiment with Google’s Sycamore quantum computer. By arranging the quantum bits (qubits) in a particular way, they were able to observe behaviors one would expect out of a wormhole, a kind of tunnel linking different points in space and time. They published it with the second headline above, claiming that physicists had created a wormhole with a quantum computer and explaining how, using a theoretical picture called holography.
This pissed off a lot of physicists. After push-back, Quanta’s twitter account published this statement, and they added the word “Holographic” to the title.
Why were physicists pissed off?
It wasn’t because the Quanta article was wrong, per se. As far as I’m aware, all the technical claims they made are correct. Instead, it was about two things. One was the title, and the implication that physicists “really made a wormhole”. The other was the tone, the excited “breaking news” framing complete with a video comparing the experiment with the discovery of the Higgs boson. I’ll discuss each in turn:
The Title
Did physicists really create a wormhole, or did they simulate one? And why would that be at all confusing?
The story rests on a concept from the study of quantum gravity, called holography. Holography is the idea that in quantum gravity, certain gravitational systems like black holes are fully determined by what happens on a “boundary” of the system, like the event horizon of a black hole. It’s supposed to be a hologram in analogy to 3d images encoded in 2d surfaces, rather than like the hard-light constructions of science fiction.
The best-studied version of holography is something called AdS/CFT duality. AdS/CFT duality is a relationship between two different theories. One of them is a CFT, or “conformal field theory”, a type of particle physics theory with no gravity and no mass. (The first example of the duality used my favorite toy theory, N=4 super Yang-Mills.) The other one is a version of string theory in an AdS, or anti-de Sitter space, a version of space-time curved so that objects shrink as they move outward, approaching a boundary. (In the first example, this space-time had five dimensions curled up in a sphere and the rest in the anti-de Sitter shape.)
These two theories are conjectured to be “dual”. That means that, for anything that happens in one theory, you can give an alternate description using the other theory. We say the two theories “capture the same physics”, even though they appear very different: they have different numbers of dimensions of space, and only one has gravity in it.
Many physicists would claim that if two theories are dual, then they are both “equally real”. Even if one description is more familiar to us, both descriptions are equally valid. Many philosophers are skeptical, but honestly I think the physicists are right about this one. Philosophers try to figure out which things are real or not real, to make a list of real things and explain everything else as made up of those in some way. I think that whole project is misguided, that it’s clarifying how we happen to talk rather than the nature of reality. In my mind, dualities are some of the clearest evidence that this project doesn’t make any sense: two descriptions can look very different, but in a quite meaningful sense be totally indistinguishable.
That’s the sense in which Quanta and Google and the string theorists they’re collaborating with claim that physicists have created a wormhole. They haven’t created a wormhole in our own space-time, one that, were it bigger and more stable, we could travel through. It isn’t progress towards some future where we actually travel the galaxy with wormholes. Rather, they created some quantum system, and that system’s dual description is a wormhole. That’s a crucial point to remember: even if they created a wormhole, it isn’t a wormhole for you.
If that were the end of the story, this post would still be full of warnings, but the title would be a bit different. It was going to be “Dual Wormholes for My Real Friends, Real Wormholes for My Dual Friends”. But there’s a list of caveats. Most of them arguably don’t matter, but the last was what got me to change the word “dual” to “simulated”.
- The real world is not described by N=4 super Yang-Mills theory. N=4 super Yang-Mills theory was never intended to describe the real world. And while the real world may well be described by string theory, those strings are not curled up around a five-dimensional sphere with the remaining dimensions in anti-de Sitter space. We can’t create either theory in a lab either.
- The Standard Model probably has a quantum gravity dual too, see this cute post by Matt Strassler. But they still wouldn’t have been able to use that to make a holographic wormhole in a lab.
- Instead, they used a version of AdS/CFT with fewer dimensions. It relates a weird form of gravity in one space and one time dimension (called JT gravity), to a weird quantum mechanics theory called SYK, with an infinite number of quantum particles or qubits. This duality is a bit more conjectural than the original one, but still reasonably well-established.
- Quantum computers don’t have an infinite number of qubits, so they had to use a version with a finite number: seven, to be specific. They trimmed the model down so that it would still show the wormhole-dual behavior they wanted. At this point, you might say that they’re definitely just simulating the SYK theory, using a small number of qubits to simulate the infinite number. But I think they could argue that this system, too, has a quantum gravity dual. The dual would have to be even weirder than JT gravity, and even more conjectural, but the signs of wormhole-like behavior they observed (mostly through simulations on an ordinary computer, which is still better at this kind of thing than a quantum computer) could be seen as evidence that this limited theory has its own gravity partner, with its own “real dual” wormhole.
- But those seven qubits don’t just have the interactions they were programmed to have, the ones with the dual. They are physical objects in the real world, so they interact with all of the forces of the real world. That includes, though very weakly, the force of gravity.
And that’s where I think things break, and you have to call the experiment a simulation. You can argue, if you really want to, that the seven-qubit SYK theory has its own gravity dual, with its own wormhole. There are people who expect duality to be broad enough to include things like that.
But you can’t argue that the seven-qubit SYK theory, plus gravity, has its own gravity dual. Theories that already have gravity are not supposed to have gravity duals. If you pushed hard enough on any of the string theorists on that team, I’m pretty sure they’d admit that.
That is what decisively makes the experiment a simulation. It approximately behaves like a system with a dual wormhole, because you can approximately ignore gravity. But if you’re making some kind of philosophical claim, that you “really made a wormhole”, then “approximately” doesn’t cut it: if you don’t exactly have a system with a dual, then you don’t “really” have a dual wormhole: you’ve just simulated one.
Edit: mitchellporter in the comments points out something I didn’t know: that there are in fact proposals for gravity theories with gravity duals. They are in some sense even more conjectural than the series of caveats above, but at minimum my claim above, that any of the string theorists on the team would agree that the system’s gravity means it can’t have a dual, is probably false.
I think at this point, I’d soften my objection to the following:
Describing the system of qubits in the experiment as a limited version of the SYK theory is in one way or another an approximation. It approximates them as not having any interactions beyond those they programmed, it approximates them as not affected by gravity, and because it’s a quantum mechanical description it even approximates the speed of light as small. Those approximations don’t guarantee that the system doesn’t have a gravity dual. But in order for them to, then our reality, overall, would have to have a gravity dual. There would have to be a dual gravity interpretation of everything, not just the inside of Google’s quantum computer, and it would have to be exact, not just an approximation. Then the approximate SYK would be dual to an approximate wormhole, but that approximate wormhole would be an approximation of some “real” wormhole in the dual space-time.
That’s not impossible, as far as I can tell. But it piles conjecture upon conjecture upon conjecture, to the point that I don’t think anyone has explicitly committed to the whole tower of claims. If you want to believe that this experiment literally created a wormhole, you thus can, but keep in mind the largest asterisk known to mankind.
End edit.
If it weren’t for that caveat, then I would be happy to say that the physicists really created a wormhole. It would annoy some philosophers, but that’s a bonus.
But even if that were true, I wouldn’t say that in the title of the article.
The Title, Again
These days, people get news in two main ways.
Sometimes, people read full news articles. Reading that Quanta article is a good way to understand the background of the experiment, what was done and why people care about it. As I mentioned earlier, I don’t think anything said there was wrong, and they cover essentially all of the caveats you’d care about (except for that last one 😉 ).
Sometimes, though, people just see headlines. They get forwarded on social media, observed at a glance passed between friends. If you’re popular enough, then many more people will see your headline than will actually read the article. For many people, their whole understanding of certain scientific fields is formed by these glancing impressions.
Because of that, if you’re popular and news-y enough, you have to be especially careful with what you put in your headlines, especially when it implies a cool science fiction story. People will almost inevitably see them out of context, and it will impact their view of where science is headed. In this case, the headline may have given many people the impression that we’re actually making progress towards travel via wormholes.
Some of my readers might think this is ridiculous, that no-one would believe something like that. But as a kid, I did. I remember reading popular articles about wormholes, describing how you’d need energy moving in a circle, and other articles about optical physicists finding ways to bend light and make it stand still. Putting two and two together, I assumed these ideas would one day merge, allowing us to travel to distant galaxies faster than light.
If I had seen Quanta’s headline at that age, I would have taken it as confirmation. I would have believed we were well on the way to making wormholes, step by step. Even the New York Times headline, “the Smallest, Crummiest Wormhole You Can Imagine”, wouldn’t have fazed me.
(I’m not sure even the extra word “holographic” would have. People don’t know what “holographic” means in this context, and while some of them would assume it meant “fake”, others would think about the many works of science fiction, like Star Trek, where holograms can interact physically with human beings.)
Quanta has a high-brow audience, many of whom wouldn’t make this mistake. Nevertheless, I think Quanta is popular enough, and respectable enough, that they should have done better here.
At minimum, they could have used the word “simulated”. Even if they go on to argue in the article that the wormhole is real, and not just a simulation, the word in the title does no real harm. It would be a lie, but a beneficial “lie to children”, the basic stock-in-trade of science communication. I think they could have defended it to the string theorists they interviewed on those grounds.
The Tone
Honestly, I don’t think people would have been nearly so pissed off were it not for the tone of the article. There are a lot of physics bloggers who view themselves as serious-minded people, opposed to hype and publicity stunts. They view the research program aimed at simulating quantum gravity on a quantum computer as just an attempt to link a dying and un-rigorous research topic to an over-hyped and over-funded one, pompous storytelling aimed at promoting the careers of people who are already extremely successful.
These people tend to view Quanta favorably, because it covers serious-minded topics in a thorough way. And so many of them likely felt betrayed, seeing this Quanta article as a massive failure of that serious-minded-ness, falling for or even endorsing the hypiest of hype.
To those people, I’d like to politely suggest you get over yourselves.
Quanta’s goal is to cover things accurately, to represent all the facts in a way people can understand. But “how exciting something is” is not a fact.
Excitement is subjective. Just because most of the things Quanta finds exciting you also find exciting, does not mean that Quanta will find the things you find unexciting unexciting. Quanta is not on “your side” in some war against your personal notion of unexciting science, and you should never have expected it to be.
In fact, Quanta tends to find things exciting, in general. They were more excited than I was about the amplituhedron, and I’m an amplitudeologist. Part of what makes them consistently excited about the serious-minded things you appreciate them for is that they listen to scientists and get excited about the things they’re excited about. That is going to include, inevitably, things those scientists are excited about for what you think are dumb groupthinky hype reasons.
I think the way Quanta titled the piece was unfortunate, and probably did real damage. I think the philosophical claim behind the title is wrong, though for subtle and weird enough reasons that I don’t really fault anybody for ignoring them. But I don’t think the tone they took was a failure of journalistic integrity or research or anything like that. It was a matter of taste. It’s not my taste, it’s probably not yours, but we shouldn’t have expected Quanta to share our tastes in absolutely everything. That’s just not how taste works.
4 gravitons 
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“ They view the research program aimed at simulating quantum gravity on a quantum computer as just an attempt to link a dying and un-rigorous research topic to an over-hyped and over-funded one, pompous storytelling aimed at promoting the careers of people who are already extremely successful.”
I shouldn’t assume that this “They” refers to anyone in particular now should I… ? :-). But I agree that the anti-hype seems at least as overblown as the hype (maybe there’s a hype conjugation symmetry at play ?)
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Enjoyed the tone section of your analysis. It is quite a hard life lesson to not feel personally hurt when someone/something stops agreeing with one’s personal views/opinions even momentarily! 🙂
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People are excited ( or annoyed) by different things, that’s for sure, and that’s true for almost any human activity ( arts, music, sports …) not only our subject.
But the point here is that a serious science magazine adopts a blatantly populist approach that realizes once again the very old strategy that says:
” Say whatever you want, no matter how outrageous or even ridiculous, and you can be certain that you’ll find a big audience for that”.
This advertising tactic is not new; it has been practiced many times in various cases in the past with sometimes seriously damaging results.
See, for example, the comments section of that already infamous video, the enthusiastic reaction of the vast majority of viewers.
Sooner or later, they will find out that no actual spacetime wormhole was “created”. Their previous enthusiasm will be inverted to something else:
Scoff, mistrust, rejection of science news as “fake” all over the place…
How much nonsense can be be accepted by “the public” without consequences?
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Good post. Just one minor (but I think important) clarification. A duality is something that can exist between different theories. Reality is just reality and there is no duality involved there. We use theories to model reality. So when different theories are related by a duality then it may be that they describe different aspects of reality, but that does not mean that the existence of one of these real physical objects described by one of the theories imply the existence of the other object described by the other theory. Does that make sense?
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I think your mistake is the following:
“So when different theories are related by a duality then it may be that they describe different aspects of reality”
Maybe this is just how you phrased it, but when different theories are related by a duality, then they describe the same reality in different ways. They don’t describe different “aspects”, at least in the sense of different physical objects or the like, they’re two alternative descriptions for the same “physical objects”.
So when you have an aspect of reality that is accurately modeled by a theory with a dual (if it is completely accurately modeled by that theory anyway), then it is equally well modeled by the dual theory. That means that whatever reality is, it has to be something that can be described in either way, no one description is preferred.
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You can take an eggshell and make its stereographic projection over the whole plane. You can study properties of the eggshell via looking at the projected plane, and you may find something interesting that way. It does not turn the eggshell into a planar structure though. And calling the eggshell a plane covering, because you made such a mathematical construct, is wrong.
Any apologetics that it is just about a tone of the statement that it is a plane covering, that because no description is preferred, is terribly wrong; especially when it is made by a scientist.
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In this case, though, the stereographic picture is incomplete, because it distorts distances. So while you can study some properties of the eggshell by mapping it to a plane, if you assume that an ant traveling on the eggshell travels at a constant speed on your projected plane then you will get its motion wrong.
For theories that are actually dual, this doesn’t happen. Any calculation in one theory has an analogue in the other, which gives exactly the same result. It’s really a different situation, and I suspect that makes it rather counterintuitive.
A better analogy might be electric charges. Imagine if you swapped every negative charge in the universe for an equal positive charge, and vice versa. (Ignore the weak force for a moment, just think about classical E&M.) The universe would be completely indistinguishable. It’s because of this that we think of what we label positive and negative charge as a convention, not a fact of nature: if things had been discovered in a different order, we might say that electrons had positive charge and protons had negative charge.
Essentially, duality suggests that things like dimensionality are like this too: conventions based on how we happened to discover things.
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Um, no. Relabeling gauges is exactly what gauge groups are for. No one would complain on that. Contrary to that, both you and me can distinguish the number of dimensions we are moving at. Those AdS/CFT dualities break that, and thus we can tell whether we are within bulk or on its surface. Even a theoretical physicist that pretends that reality does not exist (I saw theoreticians damaged that way) can find it out.
Regarding the stereographic projection, it distorts distances alike distances are distorted when you claim that we are at a surface of the bulk which we are within. By that, this example is better pertaining to the discussed stuff. And why the hack should the laws on the eggshell and on the projection be the same? The discussed dualities have different laws at the bulk and at the surface too.
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If you’re allowing different metrics on the eggshell and the projection then you’re back to the usual diffeomorphism invariance of GR, and they are indeed equivalent, that’s what diffeomorphism invariance means. The only things that are physical in GR are diffeomorphism invariant quantities, much like the only things that are invariant in gauge theory are gauge-invariant quantities. “Whether or not you’re on an eggshell” is not diffeomorphism invariant, unless you define it in terms of a statement about curvature (and thus don’t get to arbitrarily vary the metric).
From the other direction, suppose you happened to have a red color charge. From your perspective, it would be obvious which things had red color charges and which had blue and green ones. But that’s because you’re a red color-charged object, and you’ve chosen to describe yourself in that way. Similarly, if you start out by describing yourself as 5-6 feet tall etc, then of course there is a preferred description in terms of our familiar three large dimensions of space. But you’ve smuggled that in by how you’ve chosen to describe yourself, it’s a preferred description relative to you.
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Moment, do you mean that it’s just my feeling that I can move in three dimensions? Look, it is something different than doing diffeomorphisms in GR or labeling colors in QCD. Apropos, I don’t question your expertise. You do not need to repeat an exposition of invariances to show me that you know physics 🙂
Regarding my example, I wanted to show two different theories that are coupled by a duality. By that I use the same metrics on both the eggshell and the plane. And why I wanted to show it? To make a more mundane example that still shows the absurdity of the claim that dual (here projected) phenomena are real.
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Not knowing your background either, I was just trying to be clear. 😉
And yeah, if it turns out the physics that actually describes our world has a dual theory (very much not guaranteed!) then it would indeed be just your feeling that you can move in three dimensions! Put another way, it would just be that the degrees of freedom that make you up are easier to approximately describe in three dimensions, and thus that was the way evolution ended up presenting them to you. That doesn’t seem all that much stranger than that it’s “just your feeling” that there is a fixed order of distant events, or that solid objects are not mostly empty space. Physics has a tendency to reveal that certain features of our everyday experience are, in some sense, “just a feeling”.
For the eggshell, again, if you give them the same metric the example doesn’t work, for the reason I described up-thread. Dual theories actually give identical observables, an eggshell and a plane with the same metric do not.
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Your argument on the feeling part is a good one with a caveat (that you stated) that the 2D dual theory would need to exist. And AFAIK that is not the case: AdS/CFT works for AdS, because AdS is scale invariant alike CFT (while our universe is not).
I understand that people make conjectures when they do not have proofs, but the conjectures should be supported by something to make sense at all. And while AdS and CFT have alike properties, our universe is different. By that the support is lacking here.
Along with that, you can make dS space that has voluminous number of microstates simply by taking it with matter and expanding. The well known blackholish 2D limit is for static (parts of) universes only, as even our universe when it was young had much denser matter than it would be for a black hole. The ending part of the linked article has an example of it too (not taking the speculations there though).
https://www.quantamagazine.org/why-this-universe-new-calculation-suggests-our-cosmos-is-typical-20221117/
Regarding hep-th/0407125 (that you got as a link for dS holography), I’ve just looked at it, and it seems to not contain matter in the dS space (at least I did not spot it). Vacuum without any matter around can be quite simple, described with a lesser count of degrees of freedom than the naive count, for sure. The Quanta article linked above has it alike. BTW I found a statement in the dS/dS article that is misleading if not simply wrong:
“For instance, in general relativity, from the point of view of a static, outside observer, it takes forever for a probe to reach a horizon.”
The thing is that when a probe gets close enough to the horizon of a black hole, the overall mass of that black hole plus of the probe gets big enough for a black hole that has radius equal to the original radius plus the distance to the probe. That is, the black hole increases towards the probe. The result is a larger black hole containing the probe. Here the probe does not need to go exactly up to the original horizon, and it is all finite. I’ve thought that this point is well known.
Regarding the eggshell and the plane, they share observables, e.g. points moving over them. And two mutually dual theories can have quite different laws. It seems that what you mean is that those two theories are not two different gauges of a single theory. I agree with that and adding: duality is something different than gauge invariance.
Notice that dual theories can have the corresponding states looking even more wildly different than it is at the eggshell example (and AdS/CFT admits such wild duals). Translating between states of dual theories can need an involved dictionary, see e.g. the link below.
https://scottaaronson.blog/?p=6599
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Yeah to be clear, I am certainly not saying that I expect that our spacetime has a gravity dual, merely that if it did, it would be as “real” as the original description.
I get the impression the conjectures here are on a bit firmer ground than you’re making them out to be (in particular, I think there are duals that don’t involve CFTs, where the bulk theory captures the RG flow). But still there’s plenty that’s shaky enough to motivate skepticism here, I agree.
Regarding black holes expanding (and thus an external observer actually seeing things fall in), I’m a bit hazy on the proper resolution of these things myself actually, and I’ve certainly seen the “outside observers just see the infalling matter get infinitely redshifted” point made in many places before. Dimitris, who commented elsewhere on the page, is much more of a relativist than I am, maybe he can clarify.
Regarding dual theories, I think what you’re complaining about is that some of the dynamics can be hidden in the dictionary, rather than in either theory? While the dictionary can be very involved in some ways, it shouldn’t be time-dependent, so there’s a limit to what you can hide there. Certainly you can make one side or the other arbitrarily inconvenient to work with, to the extent that you would never actually choose to use it in a practical situation: but that doesn’t mean it doesn’t describe the same physics!
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It did not allow me to reply to your last comment, thus putting it here.
It grew to several topics here.
1) Black holes: You know about mergers of massive bodies, like a black hole with a neutron star. The thing is that even a tiny probe has some equivalent mass, and by that it undergoes a merger when it reaches a black hole. A neutron star is a quantitatively but not qualitatively larger probe than a particle (from the point of view of GR).
When you use e.g. Schwarzschild metrics for the overall BH/probe system, you make an approximation that is pretty good for probes that are neither too huge nor too close to the BH horizon. It is important to understand that simplifications are done, so that we do not get led to wrong conclusions that are based on the simplifications put to regions where they are not valid.
A link below notices the BH growth, even though I tend to disagree with the text, as I consider the differences to be qualitative. E.g. we can detect signals from the very merger.
https://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html
Trying to make a particle-wise analogy: taking anomalous magnetic moment as 0, alpha/(2*pi), up to 5-loop diagrams is alike taking the probe without effects on metrics, increasing the BH horizon when it gets close enough, computing mergers at LIGO/Virgo respectively.
This stuff has actually generally big consequences w.r.t. simplifications. It is common to particle/duality persons to approximate GR by gravitons moving on a fixed background. It is a far cry from GR. The point that matter alters the space which it is within means that spaces with matter have much greater counts of configurations, are less symmetric, and for some of them, their degrees of freedom cannot be (easily or at all, here we possibly disagree) decreased.
2) Regarding dualities, let’s take it as holographic dualities (i.e. they need not be with CFT) to specify what we argue about; and that for the description of this universe as it is. My view is that the presence of matter (when it is taken correctly, i.e. with its effects on metrics) disallows 2D duals.
All that I’ve seen for this global-wise approach is that first, it grew out of the AdS/CFT that works for the share properties there, second, people tried to put it to dS when AdS got ruled out (courtesy of dark energy) just for the sake of keeping the already used approach, and third, the attempts that have no matter inside are based on the above described simplifications. By that I consider them broken beyond repair.
If you mean that a part of energy range of the strong interaction can be approximated as a CFT, and by that having approximate gravitational dual, I have nothing against that. It is an auxiliary tool alike amounts of other auxiliary tools.
Let’s take the dualities even more generally, as say, mappings between observables of some two theories, with the mappings commuting with evolution operators on the observables within the two considered theories.
I feel several points here. First, the theories can be more differing than with just differing gauges (like the eggshell example). Second, some theories fit better to reality as they do not bring unnecessary clutter. Third, using famous (e.g. gravitational) notions within an analogous theory and presenting it to the public as if it were about the famous stuff is evil.
Adding two examples to show some issues with dualities.
* You can make duality about an experiment at your lab and at an imagined lab on a planet in M31. As the physics should be the same here and there, you can claim (via this duality) that you made an experiment in a different galaxy.
* The blogpost linked at the previous comment presents a next situation: Let a probe at AdS bulk enter a black hole there. As it has CFT dual, you can make respective probe at the CFT surface, and it has evolving dual to the dynamics of entering a black hole in the bulk. Since quantum theory is reversible, you can reverse the dynamics of the probe at CFT. And it means that its AdS dual in the bulk leaves the black hole there (the same way as it entered it). Thus when you do a CFT experiment fit to this example, you can claim that you entered BH and jumped out of it then.
Well, I guess that we both have more important work to do, thus I’m leaving this thread. Bye.
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Since you’re leaving the thread, I’m mostly just putting the following here so I can keep track of it, and in case others find it interesting.
The Baez page you link is quite nice. The argument he makes there, though, is precisely why I was uneasy about the claim that an external observer can see someone pass the event horizon. Even if the event horizon expands, the horizon itself is still a surface from which light cannot escape, so it seems like you should still see the infalling person slow down/dim/redshift, just slightly earlier than you’d expect if you modeled the spacetime as fixed. Would be nice if a GR-focused commenter could confirm anyway.
For the various duality stories, I’ve realized a better example than your eggshells are canonical transformations. You can map any integrable system to a harmonic oscillator, but it would be very weird to say that all integrable systems are “really” harmonic oscillators.
I think the important distinction here is the difference between the usual approaches to classical and to quantum physics. In classical physics, you have classical trajectories, and you think of the whole motion of objects in spacetime as real. In quantum physics, you have observables. The observables have labels, and the meaning of those labels comes from which different observables can coexist, the algebra of observables. AdS/CFT is, at its core, a map of observables, with every observable on one side corresponding to an observable on the other. Since the observables are the only thing that’s real in a quantum theory, in that context it doesn’t make sense to think of one description as more real than another, merely more useful.
(Regarding the example of someone reversing the dynamics of someone falling into a black hole on the CFT side to get someone climbing out of the black hole: I’m not an expert here, but usually black holes on the AdS side correspond to something thermodynamic on the CFT side. If I had to guess, I’d say that what happens when you reverse the CFT dynamics is that the AdS black hole’s Hawking radiation happens to produce an exact copy of you heading out of the black hole. This is mind-bogglingly unlikely, but not impossible, and would likely correspond to the CFT dynamics being thermodynamically extremely favored to go the one way, not the other. But a holographer can correct me here.)
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Yes, basically we agree. (The reality is the same but the different theories could be focusing on different degrees of freedom in the reality.) Then there does not exist something else that is dual to a wormhole, right? Because the wormhole is something that is assumed to exist in reality.
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4gravitons writes:
“Theories that already have gravity are not supposed to have gravity duals. If you pushed hard enough on any of the string theorists on that team, I’m pretty sure they’d admit that.”
This seems to depend on context. I have seen a number of claims to the contrary:
1) At his blog, Matt Strassler mentioned hep-th/9906182, a 500-citation paper which is explicitly about “a generalization of the AdS/CFT correspondence to boundary theories that include gravitational dynamics”.
2) JT gravity is supposed to be an AdS2 dual to SYK. But there are papers about coupling JT gravity to conformal matter so as to produce an AdS3 dual. This may be an instance of “double holography”.
3) hep-th/0407125 proposes a “dS/dS correspondence” in which dynamics in dS^n is holographically dual to CFTs in two dS^(n-1) spaces. They seem to be saying it can be iterated until you have a pure quantum mechanical system with no gravitational component, and possibly no space-time component?
There’s a lot to work through here, and conjectures upon conjectures. But it seems conceivable e.g. that even just simulating a teleportation protocol on a quantum computer, creates an AdS2 x “something” wedge, in a dual geometry equivalent to the standard model physics underlying it.
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Ah, interesting, I didn’t know about any of this! Ok, this probably merits an edit to the post.
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A maybe naive question from a mathematician –
Holography apart, when does a simulation a phenomenon start counting as creating the phenomenon itself?
Presumably this is when the physical simulation components correspond to the ones we try to simulate, otherwise the classical simulations would generate the headline by themselves.
But is it necessarily an almost-1-to-1 correspondence between quantum degrees of freedom? For example, if the same program was run on a quantum computer that uses error-correcting codes to represent its state, would a wormhole not be created? Or if a QC is used to determine molecular dynamics, but only electron degrees of freedom are considered and not nuclear ones, is the molecule really created or not? (in the sense used by the experiments and theorists quoted in the article)
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Yeah, so of course the overall “when/to what extent is a simulation real” question is a big philosophical tangle of unresolved stuff…
One division I’d like to draw though, is between reproducing a system approximately or exactly. I don’t think it makes sense to say that a system that behaves approximately like another system is making the latter “real”. So error-correcting codes wouldn’t qualify, nor would a simulation that ignored nuclei. You should have actually identical dynamics, not merely approximately.
I think if you allow approximations to count as “really creating” things, then you have to go down the full philosophical slippery slope and say that the things always exist in the Platonic realm or something, so whether you actually build the computer doesn’t actually matter. I think the only way you can get away with drawing a line in between is if you insist the dynamics has to actually be exactly the same.
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AdS spacetime has all these unique interesting properties, being non- globally hyperbolic with Cauchy horizons and instabilities that lead to the formation of black holes in the bulk, yet it also has that “reflective” timelike “boundary” with one “macroscopic” dimension less, that differentiates it from other geometries.
And the proper distance from any regular point in the interior to the boundary being infinite, yet only a finite amount of proper time is needed for light ( or gravitational waves) to be reflected back.
One thing that makes me wonder, about the AdS/ CFT correspondence, is the following: Is the duality a consequence ( merely) of the specific characteristics ( e.g. symmetries/ causal structure) that this special spacetime geometry has , or can it be generalized so as to be the basis for a really compelling theory of quantum ( or emergent) gravity..
Most experts that working on that field are optimistic and reassuring that this will be the case eventually. Others believe that a breakaway from AdS to more general spacetimes is not near, and others that, perhaps it will be unnecessary, or even irrelevant. That makes me wonder..
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About the wormhole Quanta article, you said “I don’t think anything said there was wrong”
What about their description of a qubit – “Entangling two qubits — quantum objects like particles that exist in two possible states, 0 and 1 — yields four possible states with different likelihoods (0 and 0, 0 and 1, 1 and 0, and 1 and 1). Three qubits make eight simultaneous possibilities, and so on”
I was under the impressions each qubit had 4 states.
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In the most recent Quantum Magazine, in the article “One Quantum Advantage Survives a Classical Clobbering”, a researcher says “I think the field as a whole has been reshaped” to move away from algorithm-hunting, she said. She expects that quantum computers will be most useful for learning about quantum systems themselves, not for analyzing classical data.”
Is this an example of that?
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In principle, yeah. In this case, the quantum computer wasn’t exactly useful for this either, since the classical simulation was easier. But that’s the eventual aim of these guys, to learn about quantum mechanical systems that might be tough to simulate, some of which might say something about toy models of quantum gravity.
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I’m not sure where you got the impression that each qubit has 4 states. Usually qubit refers to the quantum analogue of a classical bit, so it has two states (and can be in a superposition of them…I don’t know if you were thinking of superposition as “an extra state”, but then I’m not sure where you got the number four!)
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Yes, I thought they usually talked about two superposition states.
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Ah I see. No, there’s an infinite number of possible superposition states, you can make a combination of the original two in any proportion you like, for example. The “two states” is counting the two different possible measurement outcomes, not the different states the qubit can be in.
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Lee Smolin has said “The real ensemble theory can be falsified by making a large quantum computer that works exactly as predicted by quantum mechanics.” This seems interesting, not sure what “large” and “exactly” mean, however.
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Yeah, no idea. Depends on what the people who made up “real ensemble theory” say!
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A brief comment about duality in the real world, that I think gets very muddied when AdS/CFT is discussed. We have dualities all over in physics that, while may not be proved, are extremely useful. Perhaps the most familiar to particle physicists is in QCD. There are two dual descriptions of QCD: one in which the fundamental degrees of freedom are quarks and gluons, and the other in which the hadrons are the fundamental degrees of freedom. At high energies, quarks and gluons interact weakly and so are the good quasi-particles in that regime. At low energies, hadrons interact weakly, and so are the good quasiparticles in that regime.
Which description of QCD is correct? Is it just quarks and gluons or is it just hadrons? As always, the answer depends on the questions you ask, and no description is more right than the other, but one can be more useful for your purposes.
Applying this familiar case to this quantum simulation, I still think it’s an extreme stretch to say that a wormhole was created because I would guess that the relevant, “good” degrees of freedom of a 9 qubit quantum computer are not some gravitational dual. Just guessing.
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I don’t think those descriptions are dual in the same sense. While you can in principle describe the degrees of freedom at low energy in QCD in terms of quarks and gluons (though yes you would not want to!), you cannot describe the quarks and gluons at high energy in terms of hadrons. The RG flow only goes one way.
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4gravitons
Hello again
Regarding your discussion with another commenter ( bystander) about black holes: The simplest model is the spherically symmetric collapse that reaches asymptotically the static Schwarzschild solution. A test particle ( that has negligible backreaction on the geometry) models an infalling object and this approximation is ok when its mass is very small in comparison.
The infalling object ( as it is seen from a distant observer) gets dimmer and redshifted and the geometry, subsequently, does not deviate significantly from the static Schwarzschild.
These cases ( another one that’s frequently used is a symmetrically infalling thin shell of matter) are the usual “spherical cow” models that simplify the calculations. In these, the trapping horizon practically coincides with the event horizon afterwards, as the geometry settles down to the final static state.
Generically, though, black holes are not stationary, as they consume matter , they merge with other black holes etc. so they cannot described by simple metrics like Schwarzschild.
Other metrics ( like Vaidya, for example) are more useful when the black hole grows, and things are trickier and way more difficult for mergers ( with neutron stars or other black holes). In these latter cases usually computer simulations can do the job.
The ‘event horizon ‘ is a global property of spacetime and , generically, its ‘location’ cannot be specified by local experiments. This concerns both theorists ( for example in the case of black hole thermodynamics, holographic bounds etc) and astrophysicists , so other notions of horizons ( like ‘apparent’ or quasi-local ‘trapping’ or ‘dynamical’ ) are considered more useful.
So, in a generic ( or more realistic ) situation, the question about what a distant observer sees as another observer approaches the black hole is trickier than the simple asymptotically stationary cases: even the definition of the event horizon presupposes the knowledge of the future history of spacetime ( at least, practically, until the hole gains its maximum mass in the future, before starts to shrink due to Hawking evaporation).
‘Dynamical’ horizons ( that are actually spacelike hypersurfaces) or trapping/ apparent horizons that are quasi local, provide a way out but other difficulties apear ( slice dependence e.g.).
Exponential redshift and dimming occurs again of course, but this old “frozen star” analogy is not adequate anymore to describe time dependent growing or merging black holes.
Practically, all that means that as an infalling astronaut merges with the hole, from the distant observer point of view, the quasi-local horizon grows and afterwards it continues to grow ( from other infalling stuff / radiation etc) and the mass ( another tricky thing to define! ) of the hole grows also, but it’s not accurate anymore that it takes an infinite amount of time for the astronaut to reach the ‘horizon’ ( as seen from far away), because it is not exactly the ‘event horizon’ that the distant observer “sees”.
Actually, in the classical black hole ( neglecting evaporation) the event horizon is always in the future of an external observer, in the sense that it never intersects her past light cone.
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Thanks!
Ok, but thinking about it the statement in the article bystander was complaining about isn’t all that unreasonable. They do specify that they’re considering a probe, so they’re certainly taking a probe-particle limit, and they’re taking an equivalent limit in the field theory. I agree that it’s misleading to think that’s the whole story (and it’s good to know that’s because “the horizon is ill-defined when you leave the probe limit” and not because “an external observer definitely sees objects cross the horizon at finite time”!), but as a statement restricted to the probe limit it seems fine.
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Oops, yeah, you’re right! My comment was more relevant to the Baez GR page link ( that was of course nice , but mostly – and understandably – avoided the complications of the trickier dynamical cases).
But it was rather irrelevant to the dS / dS paper ( hep-th/0407125) that you’re referring to… Sorry about that. I don’t think that the subtleties about horizons and complicated strongly dynamical cases has really something to do with the suggested dS holography paper.
As you already know, in papers like this ( that explore new territories) they’re usually take care to incorporate probes that do not change dramatically the background. (Similar is the case with the other model in the Gao/ Jafferis/ Wall “wormhole” paper, that has to do with ‘test’ observers).
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You mention the possible phrasing “dual wormhole”. But that by definition of duality is just a wormhole. There is no observation that we can make on a “dual wormhole” that would distinguish it from a wormhole, if there was the duality would be false. “Dual” is just a word for theory, not for experiment: realizing in the lab the dual of something is exactly the same as realizing that thing. So indeed there is a lot of difference between the phrasing “simulated wormhole” and “dual wormhole”.
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I mean, it’s still “dual with respect to you”, though. It wouldn’t be a wormhole in the same picture of spacetime in which you are around two meters tall. It’s still worth putting in a qualifier, but I agree it’s quite a different qualifier than “simulated”.
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