Here’s an odd little story. Two physicists confined two ions in an electromagnetic trap and made them turn laps for weeks on end to compare their masses with exquisite precision. From that measurement and one borrowed from another team, they then derived the most precise estimate yet of the proton’s mass: 1.007276466574 atomic mass units (amu), plus or minus 10 trillionths of an amu. That small number may be a big deal, because knowing the proton’s mass precisely can help scientists search for new forces.
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