Campus & Community

Looking for magnetic quiet

3 min read

Gerald Gabrielse needs magnetic quiet to work. The professor of physics generates magnetic and electric fields, super-empty vacuums, and supercold temperatures in his basement workshop in Jefferson Laboratory at Harvard University. He needs such exotic conditions to hold bits of antimatter steady. If antimatter touches normal matter, such as makes up the walls of ordinary containers, the antimatter annihilates itself in a flash of energy.

The slightest breath of a stray magnetic field, say from an elevator moving from floor to floor, will upset Gabrielse’s experiments. When Gabrielse moved from the University of Washington in Seattle to Harvard in 1987, he supervised the building of special instrument pits that extend 8 feet into the foundation of his laboratory. Then he put in place various devices to protect his equipment from stray electric and magnetic fields.

When Gabrielse installed a custom-made magnetic device to trap antimatter particles, however, the device would not work. “Magnetic fluctuations were confoundingly high except for three hours in the early morning,” he recalls.

There had been no such disturbance when he first checked the suitability of the basement for his experiments. Then he discovered that a subway line had been extended from Harvard Square northward in the interval between the time when he first checked the site and when he installed his instruments. The tracks passed about a city block from his laboratory.

Gabrielse called the Massachusetts Bay Transportation Authority, which operates the subway, and learned that the trains run all day except for the three morning hours when his equipment worked the way it should.

Left with no choice, Gabrielse had to find a way to build a gadget to sense stray magnetic fields and cancel them out. “We had to push technology hard, but we were able to come up with a superconducting solenoid that blocks natural and man-made magnetic fields,” he says.

Harvard received a patent on the shield, and several companies now produce it for a variety of commercial uses, including screening out natural distubances in the Earth’s magnetic field.

Hospitals demand a protected environment for magnetic resonance imaging, a technique used to make images of patients’ soft tissues such as brains and hearts. One medical building uses it, for example, to suppress magnetic changes caused by elevators moving up and down.

“This is a familiar story in science,” Gabrielse notes. “You can’t always predict the results of basic research. Both lasers and transistors were not invented to revolutionize the way we live, but to increases understanding and to solve problems in physics.”

– William J. Cromie