Superfluids and Neutron Stars

Yet superfluids can rotate. And when they do, weird things happen. "Superfluids cannot turn as a rigid body - in order to rotate, they must swirl," explains Ketterle. Among physicists he would say that "the curl of the velocity field must be zero." This basic physics holds for BECs and neutron stars alike.

In 2001, following similar experiments in 1999 by researchers in Colorado and France, Ketterle and his colleagues at MIT decided to spin a BEC and see what would happen. Ketterle says he didn't have neutron stars in mind when he did the experiment: "BECs are a new form of matter, and we wanted to learn more about them. By rotating BECs, we force them to reveal their properties." Simulating the inside of a weird star was to be an unintended spin-off.

Ketterle's team shone a rotating laser beam on the condensate, which was held in place by magnets. He compares the process to "stroking a ping-pong ball with a feather until it starts spinning." Suddenly, a regular array of whirlpools appeared.

"It was a breathtaking experience when we saw those vortices," recalls Ketterle. Researchers had seen such whirlpools before (in liquid helium and in BECs) but never so many at once. The array of quantum tornadoes was just the sort of storm astronomers had long-suspected might swirl within neutron stars.

No one has ever seen superfluid vortices inside a neutron star, but we have good reason to believe they exist: Many neutron stars are pulsars - that is, they emit a beam of radiation as they spin. The effect is much like a light house: we see a flash of light or radio waves each time the beam sweeps by. The pulses arrive at intervals so impressively regular that they rival atomic clocks. In fact, when Jocelyn Bell Burnell and Tony Hewish discovered pulsars in 1967, they wondered if they were receiving intelligent signals from aliens! Sometimes, though, pulsars "glitch" like a cheap wristwatch that suddenly begins to run too fast. The glitches are likely due to superfluid vortices forming or decaying within the star, or perhaps vortices brushing against the star's crust.