11 ms·

There have been a bunch of articles near the top of HN on this result, but I found this one from Quanta magazine to be one of the best non-technical explanation

There have been a bunch of articles near the top of HN on this result, but I found this one from Quanta magazine to be one of the best non-technical explanations. Particularly interesting was the focus on an alternative theoretical calculation for the muon magnetic moment: the “BMW” approach, which uses a numerical lattice model and produces a number much closer to the Fermilab experiment. For an outsider, one of the surprising things here must be just how hard the theoretical calculation actually is, especially as you get deeper into the decimal places and have to include the vacuum effects on the muon of basically all particles in the standard model, including hadrons which means trying to calculate the Strong Force. Back when I studied quantum field theory in college we never got anywhere near actually producing numbers for strong force effects; the equations weren’t soluble with the usual perturbation theory techniques. But it’s interesting to read about how it is possible to get useful numbers with these different cutting-edge theoretical/computational methods - but with some uncertainty about whether they’re actually right! Curious to hear any practicing physicists on HN weigh in on the BMW calculation versus the orthodox theoretical approach that everyone’s comparing the Fermilab result against.

tobmlt 6y ago
Can’t upvote you enough. This is driving me crazy-curious. It must be eating at the HEP folks... unless it isn’t, hah. If this becomes a quagmire nested with the muon quagmire within the new physics quagmire. I will surely have to find a way to renormalize my self energy.
whatshisface 6y ago
The class of integrals that QCD solutions belong to is known to be NP-hard. It is not known specifically that QCD itself is NP-hard but it is suspected to be by many.
- penteract 6y ago
  Would this mean that either the strong Church-Turing thesis(any model of computation based on physical reality gives the same class of problems solvable in polynomial time as a Turing machine) is wrong or P=NP?
  - whatshisface 6y ago
    If what is suspected is true, then yes, strong Church-Turing is false. If there's a symmetry in QCD that makes simulation possible in polynomial time, i.e. a helpful property that is true of QCD specifically but absent from the more general class of integrals its solutions belong to, then no. If you're looking for something to raise your hair even farther, some suspect that gravity is outright uncomputable[0] due to the unclassifiability of 4-manifolds and the expectation that quantum gravity will require summing over possible spacetimes. Since all of our computers are built with QED, it should come as no surprise that everything else made out of QED-obeying-matter, like Turing machines are imagined to be, is equally difficult to compute and equally powerful at computing. I don't see why you'd expect the other field theories to fall in to the same computational class. [0]https://arxiv.org/pdf/gr-qc/0506019.pdf https://arxiv.org/pdf/gr-qc/0506019.pdf
podiki 6y ago
Someone linked to this Twitter thread in the other discussion, from a theorist (one I happen to know a very little, from back in the day): https://mobile.twitter.com/GordanKrnjaic/status/1379834308844781569 https://mobile.twitter.com/GordanKrnjaic/status/137983430884... Not a lattice theorist, but I can tell you lattice stuff is very hard, and also very cool to be calculating such bare bones nature on a computer. Lattice results can be hard to parse for the outsiders, and I know there is often debate over how trustworthy the results are when it is so hard to check (you don't just build and perform another experiment). That's not to say the people doing it aren't very smart and capable, but it is an unbelievably difficult problem to do, and just like experiments have problems and errors, so to does lattice calculations. Treating them like almost another form of experiment is perhaps best (how does it fit in with other experiments? with theory expectations? how can we know if it is correct or not and correlate with other results? etc.). In other words, theorists and model makers (like work I used to do), may or may not include lattice results based on how they feel about some result (or what it does to their own model....).
deleted 6y ago
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tobmlt 6y ago

Can’t upvote you enough. This is driving me crazy-curious. It must be eating at the HEP folks... unless it isn’t, hah. If this becomes a quagmire nested with the muon quagmire within the new physics quagmire. I will surely have to find a way to renormalize my self energy.

whatshisface 6y ago

The class of integrals that QCD solutions belong to is known to be NP-hard. It is not known specifically that QCD itself is NP-hard but it is suspected to be by many.

penteract 6y ago
Would this mean that either the strong Church-Turing thesis(any model of computation based on physical reality gives the same class of problems solvable in polynomial time as a Turing machine) is wrong or P=NP?
- whatshisface 6y ago
  If what is suspected is true, then yes, strong Church-Turing is false. If there's a symmetry in QCD that makes simulation possible in polynomial time, i.e. a helpful property that is true of QCD specifically but absent from the more general class of integrals its solutions belong to, then no. If you're looking for something to raise your hair even farther, some suspect that gravity is outright uncomputable[0] due to the unclassifiability of 4-manifolds and the expectation that quantum gravity will require summing over possible spacetimes. Since all of our computers are built with QED, it should come as no surprise that everything else made out of QED-obeying-matter, like Turing machines are imagined to be, is equally difficult to compute and equally powerful at computing. I don't see why you'd expect the other field theories to fall in to the same computational class. [0]https://arxiv.org/pdf/gr-qc/0506019.pdf https://arxiv.org/pdf/gr-qc/0506019.pdf

podiki 6y ago

Someone linked to this Twitter thread in the other discussion, from a theorist (one I happen to know a very little, from back in the day): https://mobile.twitter.com/GordanKrnjaic/status/1379834308844781569 https://mobile.twitter.com/GordanKrnjaic/status/137983430884... Not a lattice theorist, but I can tell you lattice stuff is very hard, and also very cool to be calculating such bare bones nature on a computer. Lattice results can be hard to parse for the outsiders, and I know there is often debate over how trustworthy the results are when it is so hard to check (you don't just build and perform another experiment). That's not to say the people doing it aren't very smart and capable, but it is an unbelievably difficult problem to do, and just like experiments have problems and errors, so to does lattice calculations. Treating them like almost another form of experiment is perhaps best (how does it fit in with other experiments? with theory expectations? how can we know if it is correct or not and correlate with other results? etc.). In other words, theorists and model makers (like work I used to do), may or may not include lattice results based on how they feel about some result (or what it does to their own model....).

deleted 6y ago

[deleted]