Last week, OpenAI announced they tackled a longstanding problem in mathematical physics: the existence of quantum Yang-Mills theory. Since one of this blog’s first posts was an explainer about what Yang-Mills theory is, I figured I should probably comment.
In 1954, Yang and Mills wrote down a type of theory that would turn out to describe three of the four fundamental force of nature. Electromagnetism uses the simplest version of the theory, while the two nuclear forces are more complicated. Gravity is the only force that isn’t described by Yang-Mills theory.
But while Yang and Mills wrote down a theory that any physicist would be happy with, that’s not enough for mathematicians. Physicists take shortcuts and make assumptions in order to get a theory they can readily use, and for quantum theories those shortcuts and assumptions can paper over a lot of subtleties. Normally, that doesn’t matter much: when physicists want to use Yang-Mills theories to understand the real world, they use well-tested approximations and simulation techniques. But in a controversial-enough corner case, where you might want ironclad mathematical proof to convince skeptics, physicists wouldn’t have that option. The true theory, without any approximation tricks, wasn’t defined in terms of axioms that were proven to make sense, not with a mathematician’s carefulness. In short, despite its successes, Yang-Mills wasn’t proven to exist.
Over the years, people tried to fix this. Not many people, mind you: most physicists didn’t know enough math, most mathematicians didn’t know enough physics. But mathematical physicists had managed to find axioms and proved they worked for other theories, simpler than Yang-Mills. And a few people made progress towards more complicated theories. It’s a problem that has a lot of prestige associated with it: it’s the first step in one of the million-dollar Millennium Prize Problems in mathematics (though to be clear, just the first step: put a pin in that). But that prestige didn’t draw many people to work on it: it was viewed as too difficult and not particularly urgent, not the kind of productive direction people could build careers on.
It definitely wasn’t a type of research I understood. Hearing about it occasionally from the outside, it seemed not only mysterious but messy. It felt like there was a zoo of different assumptions and objections to those assumptions, theorems and loopholes to those theorems. It felt like the kind of research area where someone out there would be convinced that they had already solved the problem, while someone else was equally convinced that that solution was totally unsatisfactory. So even after OpenAI announced they had tackled a different Millennium problem, I didn’t expect Yang-Mills to be next on the list. Not because it was hard: it was clear to me by then that AI could solve hard problems. Rather, because it seemed like not just a mathematical problem, but a social one, of convincing people that some specific kind of approach actually did the job.
So, what happened?
I’m probably wrong about that, though we don’t quite know yet.
AI has made prestigious problems a lot more tempting. If you don’t need to dedicate your career to becoming the right kind of expert, then why not give it a shot?
Not every prestigious problem can be solved, of course. And trying to solve one you can’t solve yet is more likely to just make the AI “cheat”, like how an impossible hacking challenge got an AI to hack the website HuggingFace.
But at this point, some people with the right AI tools and the right expertise had been working on this particular problem. Some of them were Ethan Sussman and Levent Alpöge (the Anthropic employee who was involved in the controversy around OpenAI’s solution of the Navier-Stokes Millennium problem). Others, apparently, were working at OpenAI.
Much like with Navier-Stokes, OpenAI got there first. They haven’t said much about how, besides a rough timeline, which puts their result before Sussman and Alpöge posted their in-progress work online.
Instead of posting their own results online, OpenAI reached out to David Gross. David Gross isn’t known for this kind of mathematical work, but he is a Nobel prizewinner who made an important discovery about Yang-Mills. He’s also one of the founding figures of string theory.
Reaching out to David Gross was pretty clearly a way for OpenAI to try to get approval from someone considered a pillar of the theoretical physics community (at least, by the people who don’t think of him as an arrogant jerk). David Gross in turn put together a team of people to check the result. Of them, Arthur Jaffe is a major figure in making these types of theories mathematically consistent, while Michael Douglas is someone who has been trying to get people more involved in these kinds of prestigious or mathematically deep problems, and has been keeping track of progress on this problem specifically for the last twenty years. The other two, Igor Klebanov and Nikita Nekrasov, are more generally known as smart, mathematically savvy physicists and don’t have a direct connection to the topic that I’m aware of.
The team has decided not to release the result until they’ve checked it and written a decent explanation. This has annoyed a lot of people, including the Advisory Group on Mathematics and Artificial Intelligence, who think the result should be released immediately, so everyone has a fair chance to try to understand and use the result. I can appreciate some of the logic behind waiting: the AI-written papers OpenAI has released before have been pretty poorly written, the kind of thing that would be heavily edited before being published if they were written by a human. And even if the math works, the important question is whether it actually represents Yang-Mills theory, which is something you need physics expertise to answer. Still, it’s hard to argue against more transparency. My goofy recommendation is that they don’t release the paper, but do release the version of the result in the mathematical programming language Lean, and people can work to understand that.
Until they do make something public, or release a decision, we shouldn’t count on the result actually doing what it was supposed to, and proving Yang-Mills exists. It still probably does, these AI systems are increasingly reliable. But there could well be a loophole that wasn’t considered.
What is it good for?
I’ve seen physicists shrug at this result. But I think some of the intuitions there are outdated.
We already know Yang-Mills is fine. Not in a mathematically rigorous way, but through experiments and tests and more or less consistent procedures. Nobody was doubting that Yang-Mills existed in the everyday sense of the word.
Now, mathematical rigor can still help. With a nice functional set of axioms, it can become possible to prove other things. The rest of the Yang-Mills Milennium problem is proving one of those things, namely that the massless gluon particles in Yang-Mills theories group up into massive glueballs. That’s still not really in doubt, it’s been checked in computer simulations pretty extensively. But the idea is, if you can prove that, maybe you can prove other interesting things.
This hasn’t been high priority, because math is slow. Often, by the time mathematicians have carefully nailed down a result, physicists have raced ahead and done something reckless. So while some physicists will pay lip service to the idea of getting mathematicians interested, most are pessimistic about it helping.
But I’ve seen a few people put two and two together here, and notice that math is no longer slow.
AI is good at math, better than it is at most other things. That isn’t because it’s some ultra-logical beep boop sci-fi robot. It’s because the best way to get an AI good at something is to train it on a huge number of examples of that thing, and you can generate a lot more examples automatically than you can by hand. So an AI trained on math can use the Lean programming language during training to check it got the answer right, rather than waiting for human graders. That makes it a lot easier to train AI to be good at math.
So when a part of physics becomes expressible in Lean code, it gets a lot easier for AI to get good at it.
What happens when an AI gets good at Yang-Mills, or similar physics theories?
First, the rest of the Millennium problem. If it can’t do that, it probably can’t do much else.
But after that?
Physics is an experimental science. You can’t prove a theory is correct with math alone, you have to test it.
But many of the most controversial questions in physics aren’t actually questions about experiments. They’re questions about theory.
You can’t use mathematics alone to prove which theory of quantum gravity is correct, you eventually need experiments. But as it happens, every theory of quantum gravity is controversial for mathematical reasons as well. String theorists believe there is one unique theory that unites the different types of string theory, others think they’re full of hot air. Loop quantum gravity researchers think their theory can describe a world like ours, others think the theory forces space and time into a crumpled up wad of nonsense. Asymptotic safety researchers assemble evidence that gravity has a special property that keeps it safe from the problems that make quantum gravity seem hard, others think the evidence doesn’t go far enough.
If any of those theories had axioms, maybe someone could write a proof to settle one of these controversies. If AI gets good enough at this, it might be able to.
Now, all of this still might not matter very much. Right now, AI contributions to math and physics seem to be a mix of public relations and advertising, benchmarks (where AI companies test how powerful their setup is by seeing how many math problems it can solve) and individual employees playing around with the system in their free time. That mix makes it hard to predict what will be tackled next. If making a system able to resolve big longstanding physics questions is an expensive detour and doesn’t look like it will generate good PR, then it might not make sense for AI companies to pursue it, and if the technology stays expensive it may be out of reach of users. But if it’s cheap, or for some reason they think it’s a high priority, then it might happen.
As always these days, who knows?



