Superfluids and Neutron Stars

Using a new form of matter researchers are bringing astrophysics from deep space right into their laboratories.

Neutron stars are weird.

They're about the same size as Manhattan Island yet more massive than the Sun. A teaspoonful of one would weigh about a billion tons. On the outside, neutron stars are brittle. They are covered by an iron-rich crust. On the inside, they are fluid. Each one harbours a sea of neutrons - the debris from atoms crushed by a supernova explosion. The whole ensemble rotates hundreds of times each second, and so spawns powerful quantum tornadoes within the star.

You probably wouldn't want one on your desktop.... That is, unless you're an experimental physicist.

Neutron stars and their cousins, white dwarfs and black holes, are extreme forms of matter that many scientists would love to tinker with - if only they could get one in their lab. But how? Researchers experimenting with a new form of matter called Bose-Einstein condensates may have found a way.

Bose-Einstein condensates (BECs) are matter waves formed when very cold atoms merge to become a single "quantum mechanical blob." They contain about ten million atoms in a droplet 0.1 mm across. Physicists Eric Cornell (NIST), Carl Wieman (University of Colorado) and Wolfgang Ketterle (MIT) - who shared the 2001 Nobel Prize in Physics - created the first ones from vaporous gases in 1995.

In most ways, BECs and neutron stars are dissimilar. BECs are 100,000 times less dense than air, and they are colder than interstellar space. Neutron stars, on the other hand, weigh about 100 million tons per cubic centimetre, and their insides are 100 times hotter than the core of the Sun. So what do they have in common? Both are superfluids - that is, liquids that flow without friction or viscosity.

Perhaps the best-known example of a superfluid is helium-4 cooled to temperatures less than 2.2^o K (-271^o C). If you held in your hand a well-insulated cup of such helium and slowly rotated the cup, the slippery helium inside wouldn't rotate with it.