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Superfluids and Neutron Stars

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

by Dr Tony Phillips

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.


Neutron stars are formed in supernova explosions. This supernova remnant (known as the Crab Nebula) harbors one that spins 30 times every second.

Perhaps the best-known example of a superfluid is helium-4 cooled to temperatures less than 2.2o K (-271o 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.

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.


Arrays of vortices that form in spinning BECs resemble the
insides of 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.

The swirling vortices in BECs offer scientists an opportunity to study such processes first-hand - without burrowing into a distant star.

The possibilities don't end there: "If the condensed atoms [in a BEC were to] attract each other, then the whole condensate can collapse," Ketterle adds. "People have actually predicated that the physics is the same as that of a collapsing neutron star. So it's one way, maybe, to realize a tiny neutron star in a small vacuum chamber."

Small, confined and tame - a pet neutron star? It sounds far-fetched, yet researchers are learning to make BECs collapse in real-life experiments.

Credit: NASA

An artist's concept of a magnetized neutron star in space.

BECs are formed with the aid of magnetic traps. Carl Wieman and colleagues at NIST have discovered that atoms inside a BEC can be made to attract or repel one another by "tuning" the magnetic field to which the condensate is exposed. Last year, they tried both: First, they made a self-repelling BEC. It expanded gently, as expected. Then, they made a mildly self-attracting BEC. It began to shrink - again as expected - but then it did something wholly unexpected.

It exploded!

Many of the atoms in the BEC flew outward, some in spherical shells, others in narrow jets. Some of the ejecta completely disappeared - a lingering mystery. Some remained as a smaller core at the position of the original condensate.

To an astrophysicist, this sounds remarkably like a supernova explosion. Indeed, Wieman et al dubbed it a "Bosenova" (pronounced "bose-a-nova"). In fact, the explosion liberated only enough energy to raise the temperature of the condensate 200 billionths of a degree. A real supernova would have been 1075 times more powerful. But you have to start somewhere.

If researchers eventually do craft miniature neutron stars, they might learn to make white dwarfs and black holes as well. Such micro-stars pose no danger to Earthlings. They are simply too small and their gravity too feeble to gobble objects around them. But such pets would no doubt be popular among physicists and astronomers.

Personally, I think I'll stick to dogs.


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First Science 2014