The mass of an electron has been measured to 12 decimal places, 13 times more precise than previous efforts. Such precision may seem unnecessary, but it enables more exact predictions of atomic behavior, which in turn can be used to test competing models of physics.
The Nature paper announcing the results notes, “the electron mass me is prominent, being responsible for the structure and properties of atoms and molecules.”
The authors drily note, “The low mass of the electron considerably complicates its precise determination.”
Electrons weigh 5.5 x 10-4 atomic mass units (roughly the mass of a proton or neutron). However, the figure of 9.1 x 10-31 kg may give a better idea of just how mindbogglingly small the electron mass actually is. Measuring with a precision one ten trillionth of that is not for the faint-hearted.
To conduct the measurements the team, mostly based a the Max Plank Institute, used a Penning Trap made from a carbon atom. Penning Traps combine a consistent magnetic field with four-pole electric fields. The electron traces out a complex orbit (curiously similar to the Ptolemaic model of planetary orbits that was overthrown by Copernicus) within the trap with a frequency that is based on a ratio of its charge to its mass. Calculations of the mass are then limited in precision only by our knowledge of the value of the charge, and the exactness with which the frequency can be measured.
An editorial in the same Nature edition praised the work saying, “A new value for the atomic mass of the electron is a link in a chain of measurements that will enable a test of the standard model of particle physics with better than part-per-trillion precision.”
Measurement of electron mass assists with observations of the Rydberg constant used in spectroscopy, and the fine structure constant. The fine structure constant α has come under particular attention recently with evidence that it may not be the same everywhere in the universe. If true, such a finding would challenge our models of the universe so deeply that measurements of α have taken on renewed importance.