Muon G-2 Experiment at Fermilab Finds Hint of New Particles
quantamagazine.org
quantamagazine.org
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.
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.
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....).
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.
Definitely worth checking out the rest of the videos on both those channels.
While we’re at it, check out 60 Symbols’ channel for great physics discussions that hardly ever (thanks Brady!) include equations https://youtube.com/user/sixtysymbols
Compare it to this comment about General Relativity:
"Widely acknowledged as a theory of extraordinary beauty, general relativity has often been described as the most beautiful of all existing physical theories." (via wikipedia - Landau & Lifshitz 1975, p. 228).
(ignoring the cosmological constant, of course).
Compared to most, or all, other physical theories, the Standard Model feels like there's something much simpler set of equations underneath waiting to be discovered.
GR likewise becomes a brutal computational mess with these many body problems.
What will be interesting in these new experimental results is whether we’re reaching the limits where determining the correctness of our approximations becomes impractical.
For every theory, you can generate a more complicated theory with more moving parts that generates the exact same predictions. For instance, my theory of gravity is the same as general relativity, except that at 3 PM April 9th, 2021, all gravity will be reversed.
You have to appeal to some principle to sort out such theories from good descriptions of how the world works.
> For every theory, you can generate a more complicated theory with more moving parts that generates the exact same predictions.
I agree with the second point, but disagree with the first.
Yes, by Occam's razor, a simpler theory with fewer moving parts should be preferred over a complex theory, if both produce correct answers. But the most accurate theory might not be "beautiful".
What matters most is if the theory correctly predicts reality. If it has a weird arbitrary format, but correctly predicts everything, it's better than a theory that's beautiful but incorrect.
Most of science has been using "beauty" as a way to help guide it to the correct answers. For example, there's been a lot of work on string theory because it has a kind of mathematical beauty. But it may turn out that a search for "beauty" in this sense was misguided. Beauty is not the goal, truth is.
Welcome to Biology! :-)
(Except DNA -> RNA -> Protein, but it's still sloppy along the way)
Yes: testable predictions followed by experimental evidence.
Beyond that I don't see how what you're talking about is anything other than an appeal to aesthetics.
But there are an infinite number of theories matching the current evidence. Some predictions of the theories might not be testable, what do you do then?
And that's not so clearly bad there because they are obiously wrong, but then you get to complicated topics where everyone has hard to resolve disagreements about which theory should be the default from a big pool of theories that seem to agree whith the evidence.
A bold assertion.
G-2 result is seeing the next hand hold (hoping its not a mirage).
Higgs discovery and its subsequent precision measurement is resting your toe into a hold and flexing your calf upwards.
The Higgs particle was expected, and so finding it within expected ranges was confirmation of the standard model.
The G-2 results are showing deviations from the expected ranges from the standard model.
In the former case, Higgs, the standard was further, strongly confirmed. In the latter case, G-2, there's a strong indication that the standard model is fundamentally missing something.
I think that's pretty exciting!
Particle mystery: physicists confirm the muon is more magnetic than predicted - https://news.ycombinator.com/item?id=26726269 - April 2021 (282 comments)
1. One should be rather specific about what 'the complete foundation' might comprise. The depth of questions one can ask is infinite, but some questions are more important than others. The g-2 experiment is asking a specific question: "Does the anomalous magnetic moment of the muon open a door to discoveries that are inaccessible with modern colliders?"
It is tempting to regard fundamental physics as a study of "Why are we here?", but it is really a study of "How are we here?" The difference is subtle but substantial. If "Why?" is what you're seeking, you are more likely to find that answer within, rather than within the study of physics.
2. Physicists can do a lot more in these directions with greater institutional support. With great sadness, I recently departed a lab, where colleagues next-door were working on this very experiment, precisely because a collision between funding rules and administrative priorities made further progress within my specialty unsustainable. I was the third postdoc in a row to do the same; not for lack of scientific promise nor ability to attract grants.
I don't want to know why, I just want to know the complete set of rules at the lowest level.
To do so would be to have a convincing grasp of Planck-scale physics -- a detailed understanding of physics at and above energy-densities reached in the Big Bang. Some precision experiments and astrophysical measurements do constrain certain Planck-scale models, but a true test of whether or not we understand early-universe physics is likely to necessitate creating a few Big Bangs ourselves. Doing so, with known technology, seems impossible. If it were possible, doing so would seem fraught with actual risk.
(N.B. for the non-expert reader: there are cosmic rays that strike the Sun at least once every five minutes (and Earth, at least once a month) with energies 10,000,000 times greater than anything humans have ever achieved. Those collisions, over the last 4,500,000,000 years, have not yet resulted in the disruption of spacetime or the formation of a black hole. You are very, very, very safe from physicists breaking the universe as you know it. :).)
Edit to add: You, mseepgood, aren't alone in your desire for knowledge. One Stephen Hawking once stated, “My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.”
I can't pretend to be an expert here, but the quantum-fluctuations in a vacuum that we all experience at all times do, over longer times ("long" here is generally unfathomably short), need to satisfy energy-conservation. To do otherwise requires paying an overwhelming probabilistic cost (a much bigger nigh-infinity than your garden-variety nigh-infinity). If you're worried about unexpected instantaneous death from the universe, I would argue that one should be far more concerned with, say, a gamma-ray burst nearby from within the galaxy (or, far more likely yet, a nuclear war) than being overrun by a Big-Bang-like event.
For that same reason (energy conservation), perhaps the central question one might ask of the Big Bang theory is, "How did it come to pass that all of that energy wound up in such a tiny volume?" There are plenty of theories out there, the best of which attempt to confront inflation at the same time, but I am unaware of a clear leader.
I don't mean to entirely disregard the importance of quantum fluctuations in cosmology. Indeed, the large-scale structure of the cosmic-microwave background is very consistent with pure quantum fluctuation, as if the fluctuations of a tiny volume of space had been magnified to cosmic scale. It is simply this experimentalist's opinion that there is probably substantially more to the story than the universe having won the lottery of all lotteries in order to exist at all.
(I'm purposely being a bit provocative here because I haven't heard an explanation for fine-tuning that doesn't rely on multiverses or divine intervention.)
If you elect to believe in the anthropic principle, one might continue to delve into fundamental research in order to ask the question, "What configuration of all possible universes is required in order to win the lottery of all lotteries?" That is a pretty awesome question, too. It is the lottery of all lotteries, after all.
In either case, deeper research is likely to turn up new and deeply-interesting surprises. Those surprises are likely to be resolved with improved theories with greater predictive power regarding the universe in which we live.
It's hard to express how much progress we've made in such a small amount of time relative to the rest of history. It's a god damn miracle.
I feel like it’s incredibly elegant. All these low level rules are able to create such complex structures with beautiful order to it.
It’s so orderly, we can play things backwards billions of years. We can look back to how things were just a fraction of a second after this version of the universe began.
It’s astonishing.
I, personally, am partial to the idea that the anthropic principle has a pretty big role to play, at least in terms of the fundamental constants.
You should not be downvoted. Spare the world a little cruelty where we can.
Perhaps that is not how this person actually feels. If so, a little humility and empathy in their communication would go a long way to rectify that.
"I must know. Why do the mathematicians take so long to find out?"