Ars Technica: A new measurement of the mass of the electron has significantly increased the precision of the value, to parts per trillion. The new measurement makes use of a Penning trap, in which magnetic and electric fields hold ionized particles in a regular circular path. For this specific measurement, the researchers bound an electron to a carbon atom that had been stripped of its own electrons. They then measured the frequencies of the orbits of the carbon ion, for which the mass of the nucleus is precisely known, and of the electron itself in the trap. From the ratio of the values, combined with a dimensionless number called the g-factor, the researchers were able to determine the electron's mass. The value of the g-factor itself is continually being refined through advancements in calculations based on quantum electrodynamics.
