Scientific Method —

Scientists re-weigh the electron, get more precise mass

Value is an input into many key calculations in physics.

A trap used to hold ions. Bottle cap presumably for scale.

Ernest Rutherford, pioneer in studying the world inside atoms, famously remarked that all science is either physics or stamp collecting. But sometimes physics itself involves dutifully collecting the stats on the world, in the same way that a naturalist might capture and examine butterflies.

A precise value for the mass of the electron is one example of the sort of statistic that physicists are eager to collect. Last Wednesday in Nature, a team of German physicists reported a new electron-mass measurement that offers a precision to parts per trillion. It is a “remarkable 13-fold increase in precision,” according to Florida State University physicist Edmund G. Myers, who published an accompanying perspective on the research paper.

Scientists have been on a quest for a better and better value of the tiny particle's size for decades. The goal with each new measurement is to get closer and closer to the true value of me, which sharpens our understanding of the way that atoms form molecules and is key to a variety of important calculations.

How do you measure something so small? Bind an electron to a reference ion—the team used the “hydrogen-like” carbon nucleus, stripped down to a single electron. The nucleus of that ion has a known, precise mass. You then pop it into an apparatus called a Penning trap, which has been in vogue since the 1980s. A magnetic field whips the ion around a circular path, while an electric field keeps it secured in this motion. Measure the frequency of the whole nucleus-electron system, then the frequency of just the electron. The mass of the electron can be calculated using this ratio, the mass of the ion, the ratio of the electron’s charge to that of the ion, and one other factor: the “g-factor.”

Most recent advances in understanding the electron's mass have been thanks to better and better predictions of the g-factor. (Two decades ago, scientists last published an electron mass measurement based on a direct cyclotron measurement described above.) The g-factor is a dimensionless number that is crucial in calculating the frequency of the electron spinning around in the Penningtrap. A “state of the art QED calculation” was used to pin down the g-factor of an electron tethered to a carbon nucleus.

(You can view the updated g-factor value as another “stamp” in the collection, one that allowed a more precise measurement of another stamp, the electron’s mass. Which, in turn, could improve measurements of most everything else.)

Electrons underlie our physical world—everything we interact with is, in part, made of them—as well as our mathematical interpretations of it. The value of their mass is a crucial parameter in the Standard Model of particle physics, which explains electromagnetism, as well as the weak and strong nuclear interactions. The mass of the electron contributes to other key values, such as the Rydberg constant and the fine-structure constant.

Physicist know that the Standard Model—great for explaining the world of the very small, but useless when it comes to gravity—either unravels somewhere or must be woven into something else. “Any difference” between a theoretical calculation and an experimental one, Myers explains, “could indicate physics beyond the Standard Model of particle physics.” Both kinds of calculations require the mass of the electron as input, so a better and better measurement of electron mass will lead to better and better calculations of all sorts—and thereby help us identify any discrepancies between them.

Nature, 2014. DOI: 10.1038/nature13026  (About DOIs).

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