What's the exact mass of an electron?

This illustration looks nothing like an actual atom. The electrons that orbit an atomic nucleus are actually in all possible places at the same time.
Illustration by Jake Turcotte

Using a novel technique, scientists have greatly improved the precision with which they can measure the mass of an electron, a new study reports.

Scientists have made the most precise measurement yet of the electron's atomic mass.

"It is a major technical improvement," said Edmund Myers, a physicist at Florida State University, who wrote an accompanying News & Views article today (Feb. 19) in the journal Nature, where the new measurement is detailed. "They have improved the precision by a factor of 13." The new value is just the tiniest bit smaller than the previous best value, though not by a significant amount.

The new measurement could one day be used in experiments to test the Standard Model, the reigning physics theory that describes the tiny particles that make up the universe.

But before the new value can be used to test the basic physics theory, other fundamental constants need to be measured at higher precision, Myers said.


Electron mass

The electron's mass is one of a few key parameters that govern the structure and properties of atoms, yet because the electron is so tiny, precisely measuring its atomic mass has been difficult. The most precise measurement so far was one adopted by the Committee on Data for Science and Technology, in 2006.

To improve on this value, Sven Sturm, a physicist at the Max Planck Institute for Nuclear Physics in Germany, and his colleagues bound an electron to a bare carbon nucleus, which has a mass that's already known. The result was a charged carbon nucleus or ion. Next, they pinned the bound electron into place using electric and magnetic fields.

The team developed a technique to measure the ion when it was almost at rest, which limits uncertainty in the system, Sturm said in an email.

Electrons' intrinsic angular momentum, or spin, act like tiny bar magnets, which, when exposed to a magnetic field, rotate around the field's axis. By combining information on the carbon nucleus with the frequency at which the electron's spin rotates in the presence of a magnetic field, the team deduced the electron's mass more precisely than ever before, Sturm said in an email.


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