LK-99 in retrospect

post by bhauth · 2024-07-07T02:06:27.660Z · LW · GW · 21 comments

This is a link post for

About a year ago, there was a lot of public interest in a supposed room-temperature superconductor called LK-99. What I publicly said at the time was, basically:

  1. We should remember the possibility that apparent levitation is from ferromagnetism or paramagnetism. Iron filings can stand up on a magnet, and pyrolytic graphite can float over a strong magnet.

  2. If we consider some known high-temperature superconductors:

Superconductivity comes from flow of Cooper pairs, and the electron-phonon interaction must be stronger than random thermal movement. LK-99 doesn't seem to have any reason to have exceptionally strong such interactions. (Yes, I'm simplifying, you have to consider phonon bandgaps, but the point is at least directionally correct.)

  1. The focus on "room-temperature" superconductivity is a bit silly. Even with systems using liquid nitrogen cooling, the superconducting wires are much more expensive than the cooling. What's really needed for superconductors to be practical is cheaper superconducting wires, not higher-temperature ones.

At the time, I found the unusual amount of public interest a bit bemusing. There have been various claims of near-room-temp superconductivity, but none of them attracted as much public attention as LK-99. A few months earlier, Ranga Dias published a paper claiming room-temperature superconductivity; he's now up to 5 retractions.

What was different about LK-99?

There were also a few social conditions that I think were important:

  1. It had been a while since that last major excitement about fake science news. After some big story that turns out to be wrong, people are more skeptical of science stories in every field for a while, and then things gradually go back to a baseline. (That's how things were after eg the "arsenic in DNA" story, which didn't make sense either: arsenate esters aren't stable enough for DNA.) I understand the heuristic that people applied but the way it's applied here doesn't really make sense.

  2. Misleading short videos + social media is a combination that hadn't really been applied to bad science stories before.

  3. I think the atmosphere at the time had a lot of demand for ammunition in a wider techno-optimist vs techno-pessimist conflict. ("Room-temperature superconductors and Boom Technology making practical supersonic aircraft! We're so back!")

I think those overall conditions caused the LK-99 story to be self-amplifying, because:

In many cases, whether the social status of a scientific theory is amplified or diminished over time seems to depend more on the social environment than on whether it's true. For example, the amyloid theory of Alzheimer's is still going, and real money is being paid for drugs based on it that don't help people. The social environment created a demand for evidence, and so fake evidence was produced by people including the former president of Stanford.

For my part, a couple of the grad students I talked with seeing data falsification going on in their lab was a big reason for my skepticism of the university system when I was in high school. Later on, an acquaintance tried to make an issue out of apparent bad data and ended up being bullied by the professor to the point of suicide. (PIs have a lot of power over their grad students' life and career prospects.) But I wanted to warn people not to consider such things enough of a justification to avoid getting an undergraduate degree, with how things currently are. It's quite important to spend 16 years studying in school to get a certification that will get an HR person you'll never meet who spends one minute looking at your resume to not throw it out, and it does sound like a joke when I put it like that, but it isn't.

Anyway, if there's a moral of this story, I suppose it's that, if you're smart, you should learn enough technical details to be able to find experts to trust on your own instead of relying on societal consensus. Or maybe it's that you should understand the incentives of the people who determine which stories get spread and considered credible? Actually, maybe it's that people have biases towards believing or not believing in stories that often outweigh the evidence? Or maybe the moral is, real events don't have a single clear moral to them, but that's OK because you can read about as many as you want and average out the incidental details.


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comment by JenniferRM · 2024-07-07T09:47:16.038Z · LW(p) · GW(p)

My reaction to the physics here was roughly: "phonon whatsa whatsa?"

It could be that there is solid reasoning happening in this essay, but maybe there is not enough physics pedagogy in the essay for me to be able to tell that solid reasoning is here, because superconductors aren't an area of expertise (yet! (growth mindset)).

To double check that this essay ITSELF wasn't bullshit I dropped [the electron-phonon interaction must be stronger than random thermal movement] into Google and... it seems to be a real thing! <3

The top hit was this very blog post... and the second hit was to "Effect of Electron-Phonon Coupling on Thermal Transport across Metal-Nonmetal Interface - A Second Look" with this abstract:

The effect of electron-phonon (e-ph) coupling on thermal transport across metal-nonmetal interfaces is yet to be completely understood. In this paper, we use a series of molecular dynamics (MD) simulations with e-ph coupling effect included by Langevin dynamics to calculate the thermal conductance at a model metal-nonmetal interface. It is found that while e-ph coupling can present additional thermal resistance on top of the phonon-phonon thermal resistance, it can also make the phonon-phonon thermal conductance larger than the pure phonon transport case. This is because the e-ph interaction can disturb the phonon subsystem and enhance the energy communication between different phonon modes inside the metal. This facilitates redistributing phonon energy into modes that can more easily transfer energy across the interfaces. Compared to the pure phonon thermal conduction, the total thermal conductance with e-ph coupling effect can become either smaller or larger depending on the coupling factor. This result helps clarify the role of e-ph coupling in thermal transport across metal-nonmetal interface.

An interesting thing here is that, based just on skimming and from background knowledge I can't tell if this is about superconductivity or not

The substring "superconduct" does not appear in that paper.

Searching more broadly, it looks like a lot of these papers actually are about electronic and conductive properties in general, often semi-conductors, (though some hits for this search query ARE about superconductivity) and so searching like this helped me learn a little bit more about "why anything conducts or resists electric current at all", which is kinda cool!

I liked "Electron-Phonon Coupling as the Source of 1/f Noise in Carbon Soot" for seeming to go "even more in the direction of extremely general reasoning about extremely general condensed matter physics"...

...which leads naturally to the question "What the hell is 1/f noise?" <3

I tried getting an answer from youtube (this video was helpful and worked for me at 1.75X speed) which helped me start to imagine that "diagrams about electrons going through stuff" was nearby, and also to learn that a synonym for this is Pink Noise, which is a foundational concept I remember from undergrad math.

I'm not saying I understand this yet, but I am getting to be pretty confident that "a stack of knowledge exists here that is not fake, and which I could learn, one bite at a time, and that you might be applying correctly" :-)

Replies from: bhauth
comment by bhauth · 2024-07-07T10:52:02.137Z · LW(p) · GW(p)

The main point of this post wasn't to explain superconductors, but to consider some sociology. What I thought people in your position would do is click on the "Cooper pairs" link and see the references to phonons in that Wikipedia article. Phonons are definitely a real thing, broadly applicable enough that neither of the articles you mentioned are related to superconductors. I previously wrote about phonons here.

"Pink noise" refers to a noise spectrum, but "1/f noise" often refers to a source of noise also called "flicker noise".

Anyway, I'm glad you got something out of my post, and your reply might be helpful to readers here.

Replies from: JenniferRM
comment by JenniferRM · 2024-07-07T18:08:23.494Z · LW(p) · GW(p)

I previously wrote [an "introduction to thermal conductivity and noise management" here].

This is amazingly good! The writing is laconic, modular, model-based, and relies strongly on the reader's visualization skills!

Each paragraph was an idea, and I had to read it more like a math text than like "human writing" to track latent conceptual structure despite it being purely in language and no equations occuring in the text.

(It is similar to Munenori's "The Life Giving Sword" and Zizioulas's "Being As Communion" but not quite as hard as those because those require emotional and/or moral and/or "remembering times you learned or applied a skill" and/or "cogito ergo sum" fit checks instead of pauses to "visualize complex physical systems in motion".)

The "big picture fit check on concepts" at the end of your conceptual explanation (just before application to examples began) was epiphanic (in context):

...Because of phonon scattering, thermal conductivity can decrease with temperature, but it can also increase with temperature, because at higher temperature, more vibrational modes are possible. So, crystals have some temperature at which their thermal conductivity peaks.

With this understanding, we'd expect amorphous materials to have low thermal conductivity, even if they have a 3d network of strong covalent bonds. And indeed, typical window glass has a relatively low thermal conductivity, ~1/30th that of aluminum oxide, and only ~2x that of HDPE plastic.

I had vaguely known that thermal and electric conductivity were related, but I had never seen them connected together such that "light transparency and heat insulation often go together" could be a natural and low cost sentence.

I had not internalized before that matter might have fundamental limits on "how much frequency" (different frequencies + wavelengths + directions of many wave, all passing through the same material) might be operating on every scale and wave type simultaneously!

Now I have a hunch: if Drexlerian nanotech ever gets built, some of those objects might have REALLY WEIRD macroscropic properties... like being transparent from certain angles or accidentally a "superconductor" of certain audio frequencies? Unless maybe every type and scale of wave propagation is analyzed and the design purposefully suppresses all such weird stray macroscopic properties???

The main point of this post wasn't to explain superconductors, but to consider some sociology.

I think a huge part of why these kinds of things often occur is that they are MUCH more likely in fields where the object level considerations have become pragmatically impossible for normal people to track, and they've been "taking it on faith" for a long time.

Normal humans can then often become REALLY interested when "a community that has gotten high trust" suddenly might be revealed to be running on "Naked Emperor Syndrome" instead of simply doing "that which they are trusted to do" in an honest and clean way.

((Like, at this point, if a physics PhD has "string theory" on their resume after about 2005, I just kinda assume they are a high-iq scammer with no integrity. I know this isn't fully justified, but that field has for so long: (1) failed to generate any cool tech AND (2) failed to be intelligible to outsiders AND (3) been getting "grant funding that was 'peer reviewed' only by more string theorists" that I assume that intellectual parasites invaded it and I wouldn't be able to tell.))

Covid caused a lot of normies to learn that a lot of elites (public health officials, hospital administrators, most of the US government, most of the Chinese government, drug regulators, drug makers, microbiologists capable of gain-of-function but not epidemiology, epidemiologists with no bioengineering skills, etc) were not competently discharging their public duties to Know Their Shit And Keep Their Shit Honest And Good.

LK-99 happening in the aftermath of covid, proximate to accusations of bad faith by the research team who had helped explore new materials in a new way, was consistent with the new "trust nothing from elites, because trust will be abused by elites, by default" zeitgeist... and "the material science of conductivity" is a vast, demanding, and complex topic that can mostly only be discussed coherently by elite material scientists.

In many cases, whether the social status of a scientific theory is amplified or diminished over time seems to depend more on the social environment than on whether it's true.

I think that different "scientific fields" will experience this to different amounts depending on how many of their concepts can be reduced to things that smart autodidacts can double click on, repeatedly, until they ground in things that connect broadly to bedrock concepts in the rest of math and science.

This is related to very early material on lesswrong, in my opinion, like That Magical Click [LW · GW] and Outside The Laboratory [LW · GW] and Taking Ideas Seriously [LW · GW] that hit a very specific layer of "how to be a real intellectual in the real world" where broad abstractions and subjectively accessible updates are addressed simultaneously, and kept in communication with each other, without either of them falling out of the "theory about how to be a real intellectual in the real world".

Replies from: Mitchell_Porter
comment by Mitchell_Porter · 2024-07-08T13:00:27.451Z · LW(p) · GW(p)

at this point, if a physics PhD has "string theory" on their resume after about 2005, I just kinda assume they are a high-iq scammer with no integrity. I know this isn't fully justified, but that field has for so long: (1) failed to generate any cool tech AND (2) failed to be intelligible to outsiders AND (3) been getting "grant funding that was 'peer reviewed' only by more string theorists" that I assume that intellectual parasites invaded it and I wouldn't be able to tell.

I am very remote from the institutions where string theory research actually gets done, so I cannot testify to anything about how they work, but I have studied string theory, as well as various alternatives both famous and obscure, and I can say it has no serious competition as a candidate for the next stage in physics. It is also profoundly connected to the physics that already works (quantum field theory and general relativity); it's really a generalization of quantum field theory, that turns out to contain everything we need in a theory of everything, as well as being connected to vast tracts of higher mathematics. If any of the theories which present themselves as rivals to string theory, actually succeeds, I would expect it to do so by being implemented within string theory. 

You say it hasn't generated "cool tech", but that is not the primary purpose of fundamental physics; the purpose is to understand nature. I don't think electroweak theory has generated any tech - there have been spinoffs from the experiments needed to test it - but I can't think of any technologies that actually use W, Z, or Higgs bosons. 

String theory's real problem as a science is the lack of clear predictions. It's hard to calculate anything empirical, it has a googol different ground states with different empirical consequences, and the Large Hadron Collider didn't give people the guidance they expected. Particle physicists expected, with good reason, that the Higgs boson is kept light by a new symmetry that would manifest as new particles. There was going to be a new golden age of empirically driven model building. Instead we have an austere situation in which the standard model still describes everything, and the only empirical guidance we have are its unexplained parameters, and whatever clues we can eke out from the "dark sector" of cosmology. 

Replies from: mishka
comment by mishka · 2024-07-08T16:43:20.615Z · LW(p) · GW(p)

I have studied string theory, as well as various alternatives both famous and obscure, and I can say it has no serious competition as a candidate for the next stage in physics.

I've done the same, and my impression is different. Of course, all theories of quantum gravity are struggling to produce easily testable predictions (although we see more efforts in that direction recently).

But when one ponders quantum gravity, one conceptual question one really likes to be addressed is this: what is the right formalism to quantize space-time, its curvature, and so on, what is the way to talk about superposition of different states of space-time, especially if curvatures differ, and so on.

The "pure string theory" sidesteps those questions, it succeeds at making relevant series converge, but it does not seem to even try to shed any particular light on space-time quantization. So I am not too excited about string theory, as it does not seem to even try to answer the questions which interest me the most in quantum gravity.

Whereas many of the other approaches to quantum gravity (including the currently very prominent loop quantum gravity, and a number of less known alternatives) are trying to address space-time quantization heads-on. Which is why I am more excited about rather impressive progress in various flavors of loop quantum gravity (I have even been trying to do a bit of research in that field at some point, with moderate success).

And I agree that overfunding of string theory to the detriment of other directions in theory has been a bad counterproductive thing. I specifically agree with Lee Smolin's take,

Smolin suggests both that there appear to be serious deficiencies in string theory and that string theory has an unhealthy near-monopoly on fundamental physics in the United States, and that a diversity of approaches is needed. He argues that more attention should instead be paid to background independent theories of quantum gravity.

Of course, when Smolin has written that book, it has been a radical dissident take, and these days it is close to consensus, and the funding and hiring practices have shifted accordingly.

But we do see emergence of hybrid approaches, where motifs from string theory are combined with motifs from theories addressing space-time quantization, such as loop quantum gravity, and I do think that those hybrid approaches are promising. String theory might still play very interesting roles within those combinations.

Replies from: Mitchell_Porter, bhauth
comment by Mitchell_Porter · 2024-07-09T04:49:03.632Z · LW(p) · GW(p)

I can tell you my critique of loop quantum gravity, but maybe I should first ask about the successful research you say you've done in that area? 

Replies from: mishka
comment by mishka · 2024-07-09T18:16:45.101Z · LW(p) · GW(p)

I certainly would be interested in hearing your critique of loop quantum gravity. (I would not say that I am happy with the state of loop quantum gravity myself, although I am glad that that approach is at least trying to address questions I care about.)

I've helped a friend of mine with some research he has published in that and related areas over the years, and we had published one of those papers jointly in Annales Henri Poincaré in 2017 (with that one I've actually done enough to be a co-author). I had composed an informal write-up explaining parts of the motivation we have not risked to include into the paper itself (for reasons which are fairly obvious when one considers the dynamics of paper reviewing when there is an ideological struggle in the field).

Here is a link to my write-up,, and it contains the links to the paper itself. (The last page of the paper says, "Communicated by Carlo Rovelli", so, presumably, he has looked at it and decided that it is good enough for Annales Poincaré. I am quite happy about that, as one of my desires had specifically been for Rovelli to be aware of that result. I think there is enough ideological affinity there with some of his thoughts. So, perhaps, he would be able to use those considerations at some point.)

So, yes, among other things I'd like to see more developments with imaginary Barbero-Immirzi parameter (mostly likely just +/-i) and with non-unitary physics (I believe that in that particular case Penrose has the right intuition, both about the need for non-unitarity in quantum gravity, and about the imaginary values of Immirzi being preferable). And, yes, I'd like to see those explorations not only in loop quantum gravity, but also in other approaches, to the extent that these considerations are at all transferable to those approaches.

Anyway, I am looking forward to your critical thoughts on loop quantum gravity (or to any other feedback).

Replies from: Mitchell_Porter
comment by Mitchell_Porter · 2024-07-10T08:17:56.274Z · LW(p) · GW(p)

OK, thanks.... Here's the story of loop quantum gravity in a nutshell, as told by me. There have been two periods in the history of the subject, the canonical period and the spin foam period. During the canonical period, they tried to develop the quantum theory "directly", but used eccentric quantization methods that fatally broke the connection with classical geometry and with the rest of physics. The spin foam period is more promising because at least there's a connection to topological field theory, but they keep getting degenerate geometries rather than a robust 4d semiclassical limit. 

So it's not devoid of interest, but it suffers in comparisons with strings, for which there are two major paradigms that work really well (perturbative S-matrix in flat space, AdS/CFT duality in negatively curved space), and demonstrated consistency with "naive" quantum gravity in various ways. 

I actually think Ashtekar's variables (as you know, one of the ingredients that launched loop quantum gravity) are a valid window on gravity, it's just the eccentric approach to quantization taken in loop quantum gravity's canonical period that is misguided. I think there's also a chance that there will be a kind of spin foam representation of M theory (in which higher gauge theory has a role too), via the work of Sati and Schreiber on "Hypothesis H". 

Replies from: mishka
comment by mishka · 2024-07-10T14:01:00.877Z · LW(p) · GW(p)

Thanks for the write-up.

The paper I've co-authored is, of course, within the spin foam paradigm (because the EPRL itself is within that paradigm).

I feel we are not close to true understanding of quantum gravity. We are seeing a variety of important glimpses from various angles, and that's important.

For example, if one assumes, the experimental data seem to indicate that Immirzi is actually +/-i, or, at least, is very close to that, and this seems to be an important tidbit

Rovelli's hints that time might be the gradient of entropy in the 4D space is another important tidbit, together with their more formal

Etc, etc...

But I don't have feeling that we are getting close to integrating all those tidbits into a unified view and to figuring out what space-time really is.

Replies from: bhauth
comment by bhauth · 2024-07-10T15:02:50.546Z · LW(p) · GW(p)

It's not certain that there's a good reason to try to quantize gravity in the first place. The Standard Model says other forces have carrier particles, but the whole reason that's the dominant view is because W/Z masses were successfully predicted, and I don't think it can be definitively said that the forces exist because of the particles rather than particles of those masses being (briefly) stable because of those forces.

Replies from: mishka
comment by mishka · 2024-07-10T15:27:00.239Z · LW(p) · GW(p)

That's an interesting idea :-)

If we ponder that, what should we think about

Should we then think that we don't believe that result, or should we think that it is not indicative of the quantum nature of gravity despite the tight link between entropy of black holes and the number of Plank areas covering the event horizon?

Replies from: bhauth
comment by bhauth · 2024-07-10T16:30:57.110Z · LW(p) · GW(p)

The Bekenstein bound? That doesn't make any testable predictions, it's just a calculation of some theoretical implications of a theoretical model of black holes. I don't see why I should count that as evidence of anything in particular.

Replies from: mishka
comment by mishka · 2024-07-10T16:47:37.855Z · LW(p) · GW(p)

You should decide if you believe it or not.

If you don't believe that this result is likely to be true in reality, that's fine, it is one possible position, it's quite self-consistent.

But if one does believe that this result is likely to be true in reality, then that position would be difficult to reconcile with gravity not being fundamentally quantum.

Replies from: bhauth
comment by bhauth · 2024-07-10T17:30:16.199Z · LW(p) · GW(p)

You should decide if you believe it or not.

No, I don't think people should start by deciding if they think that, eg, black holes have internal structure or not. That's backwards.

I don't consider that bound a "result", just a "part of a hypothesis" or "implication of a speculation". The word "result" means, to me, something that follows from the data of experiments.

Replies from: mishka
comment by mishka · 2024-07-10T17:59:49.635Z · LW(p) · GW(p)

If we want to discuss that, then we need to step back.

How much do we believe that black holes exist at all? Are we certain, are we not quite certain? Everyone is talking as if it is certain, but how much should we believe that?

If we believe that black holes do exist to a sufficiently large degree of certainty, when did it become reasonable to believe that, after what events?

Replies from: bhauth
comment by bhauth · 2024-07-10T18:56:51.450Z · LW(p) · GW(p)

I suppose I'd say that without astronomical observations showing accretion disks and gravitational lensing without emission within an event horizon, the existence of black holes would be theoretically justified by general relativity but we wouldn't be able to make strong statements about GR holding in such extreme conditions.

Replies from: mishka
comment by mishka · 2024-07-10T19:40:52.068Z · LW(p) · GW(p)

Yeah, and if one wants to be really sure, one needs to look at raw data a bit oneself (every time I do that, I am usually taken aback by how noisy those data are, and how it must be difficult to interpret them conclusively, and how I have a very powerful built-in bias to trust the reports on experimental data and on what those data mean, and that I should try to keep updating towards higher uncertainty in order to counter my built-in bias to trust the reports).

comment by bhauth · 2024-07-08T19:46:41.570Z · LW(p) · GW(p)

There are some other proposed approaches as well.

Replies from: mishka
comment by mishka · 2024-07-08T19:55:53.329Z · LW(p) · GW(p)

Yes, indeed. This one is actually new to me (thanks!).

There are actually plenty of them:

I am particularly fond of for this interesting "emergent dimensionality" feature (the crazy 4D vs 2D aspect):

There is evidence that, at large scales, CDT approximates the familiar 4-dimensional spacetime but shows spacetime to be 2-dimensional near the Planck scale, and reveals a fractal structure on slices of constant time.

comment by Charlie Steiner · 2024-07-07T11:46:48.716Z · LW(p) · GW(p)

For anyone else confused, I can confirm that this post is normal physics.

An asterisk: superconductivity can be mediated by other excitations, like spin density wave excitations (as in iron-based superconductors), not just phonons. So the phonon spectrum of LK-99 is informative but not vital for superconductivity.

Bringing up the role of social media in why this blew up is definitely interesting. I'd mostly been shrugging my shoulders and saying "who knows why anything goes viral?"

I think a moral of the story for me is: when it comes to science reporting, just be aware a decent fraction is bullshit, and exercise caution.

Sorry for your acquaintance. Publishing bullshit is common, falsifying data is also regrettably something to be aware of especially when it seems like a "white lie," but bullying your grad student in such an egregious way is not common. I guess there's always a bad apple.

comment by Lorxus · 2024-07-08T21:25:34.113Z · LW(p) · GW(p)

...I wanted to warn people not to consider such things enough of a justification to avoid getting an undergraduate degree, with how things currently are. It's quite important to spend 16 years studying in school to get a certification that will get an HR person you'll never meet who spends one minute looking at your resume to not throw it out, and it does sound like a joke when I put it like that, but it isn't.

Sad but true. There's an undergrad at UChicago of my acquaintance already well hooked into AI alignment research circles and strongly considering dropping out. Even though I agree with them that college is likely not the best environment for them to learn and do work in, this is pretty much exactly why I'd still advise them to get an undergrad degree.