80 years? I would date its birth as 1968-9 (Veneziano), it’s hard for me to imagine calling prior work than that as “string theory”. But never mind that—the bigger problem with this (quite common) argument is that everything about quantum gravity, not just string theory, has avoided testability because our other theories are too good, and because we’re limited to doing experiments on Earth with equipment built on human scales with human budgets, and that’s just not where quantum gravity would naturally make itself known. So really this argument just suggests we shouldn’t study quantum gravity at all. Maybe that’s your actual opinion—it’s a waste of time if we can’t access the Planck scale, we should table it all and sit on our hands until we can. But string theory really is quite interesting to study, stuff like AdS/CFT is just really surprising and magical when you get what it’s about, and it would be a real pity to not pay the meager salaries of theoretical physics just because of pessimism. String theory is so far from fully understood! It’s actually…really hard!
BtW I think you got this 80 years number from looking at the earliest date on the Wikipedia page. You might want to read it more carefully. Not everything leading up to string theory is string theory.
Fair enough - 50 to almost 60, not counting s-field precursor work.
I’m not saying string theory isn’t potentially interesting from a mathematics perspective, I’m just saying treating it like physics (which is, explicitly about testable/falsifiable theories) is BS.
If we were honest about it, it would be a maths speciality eh?
At least until there are more clear attempts at making testable hypotheses.
But that would cause other issues with funding I imagine.
If quantum loop gravity comes up with a testable hypotheses, then hey, maybe I’m wrong. But so far, not so much yeah? And I’m not talking ‘we’d need to spend a lot of money to test it’, I mean an actual testable/falsifiable hypotheses at all.
The kind of supersymmetry you’re referring to (global spacetime supersymmetry) is not required by string theory; this is a common misconception. Looking for super partners in a collider is actually only telling you about global supersymmetry, which unlike local supersymmetry is not a universal feature of string theory at low energy, in fact the opposite, it is probably non-generic. It so happens that a class of appealingly simple vacua do have this property, which led to some inappropriate optimism among string theorists that has entirely abated with more experiments. Unfortunately this has been widely misunderstood to rule out the whole enterprise of string theory, which is unreasonable for the reason stated above, it is much more likely to not see SUSY below the Planck scale. [0] (Unless you just like to mock string theorists for hoping that the universe would be kind to them.)
Also global supersymmetry has not been experimentally disproved (how would you do this, even?) but it is true that current or even near-term experiments are not nearly sensitive enough to get close enough to answering this definitively, which is obviously upsetting.
And that's why the US is such a terrible place right now; more than 150 million people voted in the 2020 presidential election, a stark decline from the days of 1776 when the entire country had about 2.5 million people in total, many of whom were ineligible to vote, as God intended.
You're asking the wrong question. Conflicts arise not just from differing views on general questions about governance, but from competing groups each fighting for their own self-interest. "Agreeing with views", for one fleeting moment in time, means little for the long-term cohesion of a country, or the prosperity and sovereignty of a people.
See for example Kashmir [1] - whether the coming immigrants agree with the natives on e.g. the tax rate, term limits for politicians, or environmental laws, don't even come up as concerns. Nor did Czechoslovakia split over gay marriage, or Yugoslavia over differing views on abortion.
This focus on "views" to the exclusion of all else is a purely American phenomenon, and a recent one at that - only six decades old.
America has a long history of successful integration of racial groups. Once upon a time, the Irish were extremely unwelcome in Boston, simply for being Catholics. Dozens of examples like this abound in American history. America is stronger than the countries you’ve listed because of the melting pot, not in spite of it. That’s because the selfishness you alluded to is contrary to a core American value: that a person is not defined by their ancestors’ virtues or sins, but by the content of their character. All the immigrants I know are fierce defenders of this principle.
It’s a pity some people can’t think of politics as possibly more than zero sum. A good policy should actually lift all boats. Middle America simply will not survive without more people and more economic activity, period. If native born children choose to move to the coasts, and no immigrants fill in the gaps, you’re not going to have the long-term prosperity you’re imagining.
By 'America', do you mean the landmass controlled by some administrative entity that calls itself the US government, or do you mean some group of people?
> It’s a pity some people can’t think of politics as possibly more than zero sum. A good policy should actually lift all boats.
That there are such policies doesn't mean one can ignore the zero-sum ones, or the downright hostile ones, as the Uyghurs could tell you.
I mean the people and land area governed by the United States Constitution, a document that provides explicit protections against the kind of ethnostate policies you are so worried about.
No. Look at who backs Trump, who is himself of recent immigrant ancestry. His base is heavily composed of Germans in the Midwest, European Catholics, and Latinos. He reversed the trend of Vietnamese and Cuban voters away from the GOP and has been making huge inroads with recent immigrants from Latin America.
The death of the GOP and the rise of MAGA has more than a little to do with immigrants.
Is your preferred remedy that quantum gravity be entirely defunded, or instead that more funding be redirected to any of the other programs to study quantum gravity? If the latter, which ones in your opinion are more likely to be productive than string theory?
Your suggestion implicitly asserts that string theory was productive, which is exactly the claim that seems to be in contention.
I don't think it's too wild to suggest that, without the constraints of string theory imposed by advisors, lots of novel approaches would have been tried. We have no idea what could have been produced.
As for quantum gravity specifically, arguably not much progress will be made without more data, and we now have some proposed experiments that can be conducted here on Earth to test them.
There are in fact exceptionally strong incentives to discover alternatives to quantum gravity which could be tested in experiments. These are the same incentives that always drive the scientific process, and new theories cost next to zero to produce. The reason string theory is popular is not because string theorists somehow prevent funding of other directions. It is because string theory has given us tools like AdS/CFT that are useful in other contexts to understand real physics—-and the alternatives have not (yet). There are many physicists who spend their lives studying alternatives to string theory with 100% of their time. I hope for their sake that there is a similar pot of gold at the end of their rainbows. It has not yet materialized.
Oh Ads, you mean that space that emphatically does NOT describe our universe? Ads/CFT is overblown. It's just an interesting mathematical result that hasn't borne much meaningful fruit for actual physics.
I'm sorry, but string theorists absolutely do prevent funding other research because funding is finite, grad students have to research something their advisors think is worthy, and their advisors have their heads full of "beautiful math" so that's what they tell their students to work on if they want their PhDs, and that's what they hire their post grads to work on if they want a job.
Only now as the strong theory haze has started dissipating are we starting to see novel approaches, like Oppenheim's post quantum gravity theory.
No, this is a shallow understanding of AdS/CFT. If you want to study quantum gravity when it is weakly coupled to matter, you can use AdS/CFT regardless of whether the background space is asymptotically AdS by embedding a brane near the boundary and working in a perturbative expansion. If you want to study the physics of quantum de Sitter space with a field theory dual, you can study any of the recent work on TTbar deformations. And anyway, surely you aren’t arguing that conformal field theories are irrelevant for physics? Because that would obviously be an untenable position, and the whole point is that quantum gravity AdS (basically) is CFT (it’s an equality! It goes both ways), just in different variables. You can actually study non-gravitational physics with it, using a gravitational language. That’s awesome stuff! Please don’t dismiss this fascinating field too quickly.
By the way, I know Oppenheim personally. He gets funding from string grants. Nobody is angry about that. Anybody can do this. I don’t think his theory is going to pass any experimental validation (it requires a really severe violation of a physical principle we have tested over and over) but the entire community has always supported and listened. He gives talks at major universities. He’s not an outcast or renegade or something.
> Because that would obviously be an untenable position, and the whole point is that quantum gravity AdS (basically) is CFT (it’s an equality! It goes both ways), just in different variables. You can actually study non-gravitational physics with it, using a gravitational language. That’s awesome stuff!
Which makes it an interesting mathematical construct, but in what way does that actually help physics? I included a link to one critique of Ads/CFT in another post, and others have critiqued its applications to QCD and other alleged "successes" because the important properties to do meaningful work in those domains just aren't there.
The versions of this correspondence that are easy to work with also depend on supersymmetry, for which every experiment has failed to find any evidence in the expected regimes. In the old days we'd call this "refuted", but these days it just means reworking it (adding a new epicycle?) to get "new bounds".
Ads/CFT is a mildly interesting mathematical derivation, but its actual utility for physics is questionable.
> He gets funding from string grants. Nobody is angry about that. Anybody can do this.
Maybe anybody can do this now, and I think that's because, as I said, string theory's stranglehold has weakened because of well-motivated criticisms over the past 15 years or so. The evidence of string theory's former dominance is right in what you said: string theory grants.
> but the entire community has always supported and listened.
I think some physicists are open minded, and some are not. You need only look at how physicists who work MOND are treated to see how not open minded some physicists are. MOND is not a final theory, but it and the people who work on it are scorned despite it's unreasonably good predictive success over the last 40 years.
Okay, I’ll tell you about my own research. From studying the way that geometric surfaces work in AdS, we conjectured a relationship between the stress tensor of QFT and entanglement entropy. This is because those quantities translate into geometrical analogs in the quantum gravity theory. We then proved this same relationship holds in some simple field theories and then other physicists proved it in the general case. So we learned something about non gravitational physics from gravitational physics. We study a specific, tractable case (AdS, mapping onto CFT) and then use it to learn about the general case (every QFT). That’s how physics works! You study the spherical cows. Eventually you learn something universal. All this is because I started with an open mind, and pursued the full consequences of AdS/CFT.
Your complaint about supersymmetry is like saying that Newtonian physics can’t work because objects are not rigid, continuous solid bodies. And yeah, that’s true, there are none of those in nature. Does that mean Newtonian physics is not useful? NO! It’s a model that’s useful. Is it wrong? Kinda. And the models that have unbroken SUSY are “wrong” too, in the same way. But the point is—-it’s obviously useful!
Please try to be open minded about string theory, especially if you wish to lecture about small-mindedness around MOND. Diminishing the real accomplishments of physicists doesn’t make other fields more likely to get funded—it makes it more likely that bureaucrats defund everyone. That’s the lesson of the SSC.
This is a preposterously uncharitable characterization of something that again, was I think a triumph of string theory, the likes of which cannot be claimed by any competing theory. It is a framework for understanding black hole information loss, and it even has specific applications in condensed matter physics for modeling high temperature superconductors.
Like I said, Ads/CFT's alleged "successes" are overblown.
As for it being a framework for understanding black hole information loss, it's merely one idea that has questionable application to our universe. We'll see if anything actually useful comes from it.
There are exactly zero free parameters in string theory [0]. The details of why string phenomenology is hard is a difficult subject, but the characterization you've given of it is not correct. If you have a proof that string theory is not self-consistent, you should publish it, because there is no such proof in the scientific literature today. (Source: my PhD in physics.)
Unfortunately, there is a ton of misinformation about this topic on the web. For example, people love to say that string theory predicts anything and everything. But it predicts (and rejects) a lot; it’s just that all of the known predictions happen to fall into the categories of (1) predicting things that are very hard for humans to measure (behavior of black holes at long time scales, graviton scattering, etc) or (2) retrodicting things we already know are true (e.g. gravity, Lorentz invariance, etc.). This state of things isn’t by design of nefarious string theorists designing their theory to be untestable, it’s just cruel fate of what comes out of the math. Hopefully someday we can find some other type of prediction, but string theory isn’t easy.
I thought lots of variants of string theory do predict things inside human means. But they've all failed, leaving only variants that predict things outside of it.
You would probably learn more by listening to Cumrun Vafa [0] than anything I could say. It’s hard to say much about string theory without space time supersymmetry not because it doesn’t exist (we know it does) but because it’s so hard to calculate anything…physicists are very reliant on a few tools, supersymmetry is a big one, and without it it’s really hard to say anything concrete, yet.
To whom? To other branches of physics? Look up AdS/CFT. To the general public? Dunno, I guess the pursuit of understanding the universe is its own reward for some.
You mean, if string theory does not turn out to describe the universe, how could it be useful? Well, by giving extraordinarily powerful tools for understanding things that are well established to be useful, like quantum field theory. AdS/CFT gives us the only tool we have to analytically understand quantum field theories in the so-called “strong coupling regime”. This is useful for discovering new properties of quantum matter in systems where you would otherwise need simulations. You can think of it intuitively as string theory providing a glue between two descriptions of the quantum matter, like a “type cast” in programming where you start with one kind of object but reinterpret it as another. The thing that is incomprehensible in one representation is simple in the other. This was discovered by studying limits of string theory in interesting geometries.
Most quantum computer experts think that this is not the case, that it is very unlikely that quantum computers can solve NP-complete problems like travelling salesman more efficiently than classical computers. (In particular, the popular intuition that quantum computers “try a lot of solutions in parallel” is basically totally wrong, at least from the perspective of making a useful heuristic for what quantum computers are good for.) This is an older article, but its conclusions are still about right: https://www.cs.virginia.edu/~robins/The_Limits_of_Quantum_Co...
This is one of those things that laymen often misunderstand, actually. (I don’t blame you, it’s often not well explained.) General relativity is easy to make into a QFT. There’s an action and you can easily derive amplitudes from it. And it works well, as long as you don’t ask questions of what happens at arbitrarily high energies. That’s what “nonrenormalizable” actually means: it only answers questions up to some energy scale, after which it stops being predictive. But believe it or not, physics has been here before, so we’re not totally clueless what to do! Fermi’s theory of beta decay [0] had the exact same problem. And the moral there was that there was something new that had been missed in Fermi’s theory that was only important at high enough energy. The theory was correctly predicting its own limitations. In fact we figured out what was missing. Fermi didn’t know about the W boson, and at the energy where W boson exchange becomes important, that’s where the old theory breaks down!
But Fermi’s theory works well up to that scale. It’s like how Einstein’s relativity didn’t mean that everything in Newtonian physics was totally wrong, but just that you need it if you want to think about really fast moving objects. Newton’s theory is still an outstandingly good approximation.
So if history is any guide, we don’t need to change the low energy theory of GR (at least not much), that works just fine. We just have to find out what’s the high energy phenomenon that we’re missing. That’s stuff that comes into play with black holes and very high energy collisions. Anyway we have some guesses here, too. In string theory, that’s the stringyness of the strings, their 2D nature, that fixes the problems automatically. (This was not by design, it was completely unexpected! You start playing with dimensions and it just falls out.) In LQG it’s about the fuzziness or discretization of space (which sounds appealing, although it’s really tough in practice to make this work).
If anyone is looking to learn some actual physics instead of media crit, it would be worthwhile to sit down with the wikipedia pages for the AdS/CFT correspondence and the black hole information paradox. There’s a lot of things you can learn about the nature of quantum gravity. Some of it is built on assumptions about nature, of course, which leads a lot of people to assume that the enterprise is built on a house of cards, but even this is worth really digging in to. You may find that in fact there are only a few assumptions about reality that need to be made, and that removing any one of them is harder than you might think.
And AdS/CFT is genuinely fascinating. It’s a type of theoretical construct that’s quite unique in the history of physics. So it’s very hard to talk about it in English sometimes! That’s partly what happened in this Quanta article. AdS/CFT asserts an equality between two very different systems (here, the quantum system is identified with the wormhole) in an extremely complex and nonlinear way. Does this mean that the quantum system is a wormhole? It’s a harder question than it appears on the surface.
A tangle in the interior of a cylinder loses information when viewed on the boundary because you lose a dimension (ie, the shadow cast by string around a light bulb as you see it on the lamp shade). You then have to allocate probabilities to pseudoknot resolutions (ie, guesses about crossings). So you end up with something that looks like continuous geometry on the inside and quantized statistics about interactions on the boundary.
Entanglement looks like taking two filaments in a plasma globe and moving one in a circle around the other. Their ribbons are now tangled. (As a 2+1-D analog.)
Respectfully, your metaphor doesn’t bear any similarity to the way AdS/CFT works. In AdS/CFT there are dual quantum systems with no information loss. A bit more precisely, the correspondence relates the partition functions of quantum gravity in AdS with the partition functions of conformal fields. This implies the existence of a map between the two, which in this case happens to be extremely complex and nonlocal.
Another way of saying it is that the two descriptions are different representations of the same object. There is no projection or information lost in switching between descriptions.
Yes — that non-locality on the surface is because the braiding structure is non-local when projected.
You are correct and I misspoke:
You don’t lose the information, it becomes non-local on the surface — and so if you’re building a model of the interior from a local sampling of the surface, you get a statistical model built on pseudoknots (again, like a shadow from a lamp).
I think it proves my point that AdS/CFT is hard to talk about :) I’ve still got mixed feelings about the lamp/shadow analogy but thanks for the clarification.
The thread-lamp model isn’t arbitrary; ultimately, we’re looking for some kind of tangle model that rescues geons. So we’re going to need something that looks like continuous tangles on the inside and Feynman diagrams on the outside.
Newtonian space == a universe with nothing in it, which is then populated with fictitious continuous rigid bodies. Yet we use it to build bridges.
Physics is about models. Some are useful. AdS/CFT is useful. I know this because I spent the time to study it in grad school, rather than dismiss it out of hand.
This is a very cool and important result. Here's a rough analogy to what's happening. You take two drums, and you hit one of them. Then you wait for a while. After waiting, you hook up a special kind of wire between the drums, and then quickly unhook it. Then, you wait the same amount of time again. And bizarrely, you hear the original sound coming from the other drum!
How did this happen? It seems like no wire would be capable of moving the excitation from one to the other in a clean way. But if the system is set up correctly, it's because your "drums" obey identical mathematics to a pretty interesting gravity setup. Each drum individually represents a region of space, and the two spaces are initially unconnected. The wire is a way of establishing a traversable wormhole between them. These wormholes are very unstable, so to actually send something through it is a delicate affair. But with the right kind of entanglement, you can send the excitation through, to emerge on the other side after a propagation delay. (Nothing is happening "faster than the speed of light", though, even in the gravity setup. It takes time to pass through the wormhole. It's tricky to show there's no problems with causality, but the "wire" is already messing with the ordinary story about causality, so it does all work out.)
This is a lovely example of how string theory, via AdS/CFT, can produce fascinating insights into the nature of spacetime. It's an extremely nontrivial bit of stringy math that leads to this connection between quantum mechanics and gravity. Of course, if one's prior is "string theory can never be useful for anything", it makes sense to downplay the importance of the result. I think that's a shame--AdS/CFT is an extraordinarily fascinating and powerful tool.
(I didn't know this result was coming out today, but I did talk excitedly about the possibility of this experiment earlier this week on this very website! https://news.ycombinator.com/item?id=33735328 )
Since this comment was more controversial than I expected, let me add a few notes.
1. Everything done on the quantum computer here can be (and was) also simulated on a classical computer. The most interesting part of the story here is the algorithm, which is derived from a string theory argument.
2. Even so, since entanglement plays a key role, a quantum computer is way more natural to implement this on than a classical computer.
3. There are some profound philosophical consequences of AdS/CFT. A lot of the discussion about simulation vs reality here would be better served by taking some time to deeply understand the topic in detail, because it is extremely counterintuitive and interesting. The argument that “everything done on a computer is by definition a simulation so it is not interesting” is far from the end of the story.
BtW I think you got this 80 years number from looking at the earliest date on the Wikipedia page. You might want to read it more carefully. Not everything leading up to string theory is string theory.