Force of Nature

Haiyan Gao, chair of Duke’s physics department, probes inside atoms to study the structure and spin of neutrons. A native of Shanghai, Gao was inspired to pursue physics by her father’s stories about female Chinese physicist Chien-Shiung Wu, who came to the U.S. in the 1930s and helped scientists unravel the chain of reactions needed to create the atomic bomb. Now Gao is designing experiments that test for the existence of a new, fifth, fundamental force of nature.

Photo above by Les Todd

What forces do we already know about? There are four fundamental forces we already know about. Some of them we are more familiar with than others—for example, gravitational force and electromagnetic force. The third one is strong force, which is the force responsible for binding neutrons and protons into the nucleus of an atom. The fourth force is the so-called nuclear weak force, which is responsible for the fact that a neutron decays into a proton, electron, and anti-electron neutrino. The name can be misleading, because you would think weak force is weak, but the weakest force we know is actually gravity. And because gravity is so out of range with all the other three forces, we have not been successful in unifying all four forces. So far, the theory we have is the standard model of particle physics, which will allow you to unify electromagnetic, strong interaction, and the weak interaction.

If you can’t unify all the other forces, is that why you need another force?

To some extent. The reason we need, or we think we need, new forces is because we do not really understand everything in nature. This Standard Model of particle physics is successful in describing a lot, but it cannot describe, for example, why 74 percent of energy in the universe is actually dark energy and why visible matter is only 4 percent, or why the universe is predominantly matter, rather than matter and anti-matter. What physicists are trying to do is extend standard models by introducing new physics.

In terms of looking for this new force, you can’t necessarily use just a neutron, because it decays pretty quickly, right?

Fortunately and unfortunately, a neutron decays in around fifteen minutes. That makes my job very difficult. For the same reason that a neutron decays, however, you can actually use that to look for this new kind of physics. To do my work, I want to use something very similar to neutrons—but stable—because I want to play with the polarization, or alignment, of the neutron spin. And then you know, you can ask your graduate students to work twenty-four hours a day, seven days a week. I’m joking, but the point is you have a lot of time to work on your experiment and don’t have to worry about finishing everything in fifteen minutes.

Spin zone: Haiyan Gao uses tabletop equipment to control the spin of subatomic particles. [Megan Morr]

I thought everything in particle physics now was about giant, expensive particle-smashers.

The way to look at new physics is to go to extremely high-energy regions, big colliders where you’re at the high-energy frontier. But one can also go to the lower-energy regions and perform high-precision measurements, using weak interaction. You want to do experiments from different ways to get a complete picture. It’s like looking at an elephant. You have to look at one from different angles; otherwise you just get a long nose or long leg.

What are the essential points about the atom that we should be teaching in high-school physics?

You can learn so much about the basics of modern physics that are important for chemistry, for biology, and materials science. I cannot emphasize enough the importance of studying atoms. And it’s very important to give students the impression that even though the atom is part of our everyday lives, we still don’t know everything about it. We don’t know why a neutron is neutral, why a proton has a charge of 1, why the masses of the particles are so close but not quite equal, why the proton is stable but the neutron decays. We need more youngsters to be interested and inspired and motivated to carry on the adventure of understanding more about the subatomic world. 

Adapted from a video interview conducted by Ashley Yeager, science writer for the Office of News and Communications.

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