In classical physics, a vacuum is a total void—a true manifestation of nothingness. But quantum physics says that empty space isn’t really empty. Instead, it’s buzzing with “virtual” particles blipping in and out of existence too quickly to be detected. Scientists know that these virtual particles are there because they measurably tweak the qualities of regular particles.
One key property these effervescent particles change is the miniscule magnetic field generated by a single electron, known as its magnetic moment. In theory, if scientists could account for all the types of virtual particles that exist, they could run the math and figure out exactly how skewed the electron’s magnetic moment should be from swimming in this virtual particle pool. With precise enough instruments, they could check their work against reality. Determining this value as accurately as possible would help physicists nail down exactly which virtual particles are toying with the electron’s magnetic moment—some of which might belong to a veiled sector of our universe, where, for example, the ever-elusive dark matter resides.
In February, four researchers at Northwestern University announced they had done just that. Their results, published in Physical Review Letters, report the electron magnetic moment with staggering precision: 14 digits past the decimal point, and more than twice as exact as the previous measurement in 2008.
That might seem like going overboard. But there’s much more than mathematical accuracy at stake. By measuring the magnetic moment, scientists are testing the theoretical linchpin of particle physics: the standard model. Like a physics version of the periodic table, it’s laid out as a chart of all the particles known in nature: the subatomic ones making up matter, like quarks and electrons, and those that carry or mediate forces, like gluons and photons. The model also comes with a set of rules for how these particles behave.
But physicists know the standard model is incomplete—it’s likely to be missing some elements. Predictions based on the model often don’t line up with observations of the real universe. It can’t explain key conundrums like how the universe inflated to its current size after the Big Bang, or even how it can exist at all—full of matter, and mostly absent of the antimatter that should have canceled it out. Nor does the model say anything about the dark matter gluing galaxies together, or the dark energy spurring cosmic expansion. Perhaps its most flagrant flaw is the inability to account for gravity. Incredibly precise measurements of known particles are therefore key to figuring out what’s missing because they help physicists zero in on gaps in the standard model.
“The standard model is our best description of physical reality,” says Gerald Gabrielse, a physicist at Northwestern University who coauthored the new study, as well as the 2008 result. “It’s a highly successful theory in that it can predict essentially everything we can measure and test on Earth—but it gets the universe wrong.”
In fact, the most precise prediction the standard model makes is the value of the electron’s magnetic moment. If the predicted magnetic moment doesn’t match up with what’s seen in experiments, the discrepancy could be a clue that there are undiscovered virtual particles at play. “I always say that nature tells you what equations are correct,” says Xing Fan, a physicist at Northwestern University who spearheaded the study as a Harvard University graduate student. “And the only way you can test it is if you compare your theory to the real world.”
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