Science & Tech

Measuring one of the universe’s building blocks

5 min read

Reducing uncertainty about electrons

Electrons are everywhere. There are trillions of them around you as you read this. They help make your computer, TV, cell phone – even the universe – work.

Every atom boasts a thin cloud of them orbiting its core, or nucleus. When they jump from one orbit to another, they create the electric and magnetic forces that power the universe. Their behaviors in the most energetic orbits determine the chemical properties of everything you can think of.

Yet only a few people think deeply about electrons. One is Gerald Gabrielse, Leverett Professor of Physics at Harvard University. In the past 20 years, he has discovered new things about them, things that even Albert Einstein never knew. And he’s trained a half-dozen young Ph.D.s in the business of how subatomic particles make the universe what it is.

Despite its role as a fundamental building block of everything, there are still some mysteries about the electron’s size and character, its motion and magnetism, its energy and natural spin.

These mysteries are difficult to solve because, as you might guess, single electrons are hard to isolate and work on. First of all, they are among the smallest of particles, about 1,000 times smaller than the protons in the atomic cores they orbit, or much less than a trillionth of an inch across.

Also, electrons are the world’s smallest magnets, and they carry a minute electric charge. This negative charge keeps them in orbit around the positively charged nuclei of atoms. You can’t keep them alone in metal containers unless you find a way to neutralize their electricity and magnetism. They move with almost the speed of light at room temperatures. Gabrielse and other physicists have to cool them down to temperatures near minus 460 degrees Fahrenheit, or near absolute zero, a temperature at which electrons and other particles hardly move.

Gabrielse has earned himself a place in the history of physics for skillfully building exotic traps that hold single electrons at very low energies so they can be studied and measured (see Nov. 14, 2002, Gazette). In one of these traps, he and a student once carried electrons, delicately suspended in magnetic and electric fields, across the United States inside a truck.

In two papers about to be published in the scientific journal Physical Review Letters, he and his colleagues describe how they used such a trap to measure the electron’s magnetism with an accuracy of more than one part in a trillion. That’s a sixfold improvement over a historic measurement of electron magnetism made in 1987, a feat that later won a Nobel Prize.

“Peeking inside a particle as fundamental to us and our universe as the electron is an exciting and difficult undertaking,” Gabrielse notes. “The puzzling electron seems to have no discernable size or substructure, yet it has intrinsic magnetism. We finally devised a way to peek inside it at a level of precision 10 times better than any other method used to date.”

Nature’s dance

Why is this so important? To start with, it shows that physicists are on the right track with their theories of how most of the universe operates, how electricity and magnetism combine with light in the way that they do.

So far, no big errors have been found in the basic theory of interaction between light and matter that physicists came up with in 1949. As one of those physicists, Freeman Dyson, commented in a congratulatory letter to Gabrielse, “I’m amazed at how precisely Nature dances to the tune we scribbled so carelessly 57 years ago. I’m also amazed that the experimenters and theorists can measure and calculate her dance to one part in a trillion.”

The Harvard experiments also place a limit on the size of electrons, a very basic number for determining other facts about the dance and dancers. Electrons can be no larger than about a thousand-trillionth of an inch. Discovering such limits is one of the best ways to build accurate models of the universe.

The electron trap effort has been going on for 20 years. Gabrielse says it’s a great way to train the physicist of the future, So far, more than six graduate students have earned their Ph.D. degrees working with the devices. Those who assisted him with his latest experiment include Ph.D. students Brian Odom, David Hanneke, and Brian D’Urso. Odom won an international prize from the American Physical Society for his contribution. Also working on Gabrielse’s team are theoretical physicists Tiochrio Kinoshita of Cornell University and Makkio Nio, a researcher in Japan.

Intimate knowledge of the electron might well lead to practical applications that we can’t even imagine now but that will boggle the minds of consumers in the 22nd century. It’s happened before. Lasers and transistors emerged from efforts to solve fundamental physics problems. The traps that Gabrielse developed to hold basic bits of matter and antimatter have already produced new methods for analyzing the ingredients of medications and a better way to shield medical imaging equipment from stray magnetic fields.