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Swiss-cheese-like gas cloud holds clues to starquakes

By spinning ultracold sodium gas in a laboratory, scientists at MIT have created a gas cloud that resembles round Swiss cheese and is riddled with tiny whirlpools, like those that cause "starquakes" in space.

This research may teach scientists more about the history of the universe and the stars, and may eventually lead to vast improvements in highly precise atomic clocks.

The laboratory demonstration is related to puzzling glitches observed by astronomers in the otherwise smooth, rapid rotation of pulsars. A pulsar is a type of neutron star, a remnant of a dying star and one of the densest objects in the universe. Glitches in pulsar rotation are called "starquakes" and may occur when whirlpools, or vortices, form or decay.

"This was a breathtaking experience when we saw these vortices," said Professor of Physics Wolfgang Ketterle, who led the research team. "We took this ultracold, fragile gas and we were amazed that even though we put hundreds of whirlpools into it, the gas cloud remained stable and happy."

Professor Ketterle and his colleagues cooled the sodium gas to less than one millionth of a degree above absolute zero (-273��� C. or -460��� F.). At such extreme cold, the gas cloud converts to a peculiar form of matter called Bose-Einstein condensate, as predicted 75 years ago by Albert Einstein.

No physical container can hold such ultracold matter, so Professor Ketterle's team used magnets to keep the cloud in place. They then used a laser beam to make the gas cloud spin, a process he compared to "stroking a ping-pong ball with a feather until it starts spinning."

The spinning sodium gas cloud, whose volume was one-millionth of a cubic centimeter, developed a regular pattern of more than 100 whirlpools.

Previously, scientists in a laboratory had seen only one or a few quantum whirlpools in a superfluid; this was the first direct observation of many whirlpools. Both the sodium gas cloud and pulsars are superfluids, which allow matter to flow without friction. Scientists know that superfluids form quantum whirlpools as they rotate; quantum whirlpools reflect the smallest possible increase in rotation for the cloud or the pulsar. One might expect different behavior from the two systems, because the gas cloud is 100,000 times thinner than air, while a pulsar is about ten thousand trillion times denser than air.

"This was an example of a designer quantum system, where we make something happen in the laboratory that doesn't occur naturally on Earth," said Dr. Mark Lee, fundamental physics discipline scientist for the Office of Biological and Physical Research at NASA headquarters. "Astronomers had observed these phenomena on pulsars but had no opportunity to manipulate them until now."

The scientists were also challenged with how to photograph the quantum whirlpools, which were too small to be seen except with special magnification. They switched off the magnets containing the gas cloud, allowing it to expand to 20 times its original size, which made the whirlpools large enough to be photographed. As the cloud expanded, gravity made it fall, and the team had to take the picture quickly. These gravitational limitations would be absent in the near- weightless environment that will soon be available to researchers on the International Space Station.

Professor Ketterle co-authored the quantum experiment paper, which is scheduled to appear in the April 20 issue of Science, with physics graduate student Jamil Abo-Shaeer and Research Laboratory of Electronics postdoctoral associates Chandra Raman and Johannes Vogels. The research was funded by NASA, the National Science Foundation, the Office of Naval Research, the Army Research Office and the David and Lucile Packard Foundation. JPL (a division of Caltech) manages the Fundamental Physics in Microgravity Research Program for NASA's Office of Biological and Physical Research.

A version of this article appeared in MIT Tech Talk on April 11, 2001.

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