Apr 13, 2021,02:00am EDT|1 views
Starts With A Bang
Ethan Siegel Senior Contributor
In early April, 2021, the experimental physics community announced an enormous victory: they had measured the muon’s magnetic moment to unprecedented precision. With the extraordinary precision achieved by the experimental Muon g-2 collaboration, they were able to measure the spin magnetic moment of the muon not only wasn't 2, as originally predicted by Dirac, but was more precisely 2.00116592040. There's an uncertainty in the final two digits of ±54, but not larger.
Therefore, if the theoretical prediction differs by this measured amount by too much, there must be new physics at play: a tantalizing possibility that has justifiably excited a great many physicists.
The best theoretical prediction that we have, in fact, is more like 2.0011659182, which is significantly below the experimental measurement. Given that the experimental result strongly confirms a much earlier measurement of the same “g-2” quantity for the muon by the Brookhaven E821 experiment, there’s every reason to believe that the experimental result will hold up with better data and reduced errors. But the theoretical result is very much in doubt, for reasons everyone should appreciate. Let’s help everyone — physicists and non-physicists alike — understand why.
The Universe, as we know it, is fundamentally quantum in nature. Quantum, as we understand it, means that things can be broken down into fundamental components that obey probabilistic, rather than deterministic, rules. Deterministic is what happens for classical objects: macroscopic particles such as rocks. If you had two closely-spaced slits and threw a small rock at it, you could take one of two approaches, both of which would be valid.
More:
https://www.forbes.com/sites/startswithabang/2021/04/13/the-big-theoretical-physics-problem-at-the-center-of-the-muon-g-2-puzzle/?sh=7fbb4017422c
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