Apr 3, 2002

My Pet Neutron Star




Using a new form of matter called Bose-Einstein condensates, researchers are bringing astrophysics from deep space right into their laboratories.




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April 3, 2002: 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 harbors 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.

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




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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 centimeter, 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.


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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.

Above: Superfluids circulate around quantized vortex lines. Here the vortex lines are yellow pillars; the circulating flow around them is indicated by arrows. [more]

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 shined 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.

Below: Arrays of vortices that form in spinning BECs resemble the insides of neutron stars. [more]


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"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.


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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."

Above: An artist's concept of a magnetized neutron star in space. Credit: NASA.

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

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!

Below: A Bosenova -- dubbed by experimenters "the puniest explosion ever." [more]


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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.



Editor's Note: The author is a driver of sled-dogs, hence the dog-friendly ending to this story. In fact, desktop neutron stars will probably be less troublesome than either cats or dogs. Ketterle's ongoing research is supported in part by NASA.

more information


A New Form of Matter -- (Science@NASA) Scientists have created a new kind of matter: It comes in waves and bridges the gap between the everyday world of humans and the micro-domain of quantum physics. (A good introduction to Bose-Einstein condensates....)

This work was supported in part by NASA's Office of Biological and Physical Research. Learn more about ongoing research at NASA's Fundamental Physics in Space web site.

More reading:

  • "Bose-Einstein Condensation of Atomic Gases" by James Anglin and Wolfgang Ketterle, 2002, Nature, v. 416, 211.
  • "Why Trapped Atoms are Attractive" by James Anglin, 2001, Nature, v. 406, p. 29.
  • "Tabletop Astrophysics" by Philip Ball, 2001, Nature, v. 411, 628.

Bosenovas: Implosion and explosion of a Bose-Einstein condensate (NIST); Researchers have the bosenova blues (Nature); Supernova in a Bottle (AIP); The "Bosenova" (Carl Wieman); Researchers Can Now Vary the Atomic Interactions in a Bose-Einstein Condensate (AIP)


Electric gravity -- another way to make BECs collapse? Some researchers have argued that bathing a BEC with long-wavelength lasers could induce electric fields that decay with the square of the distance from each atom -- like gravity only orders of magnitude stronger. So far it's only an idea, and there are many engineering obstacles: preventing the lasers from destroying the BEC, for example. But the obstacles may yet be overcome. For more information see: O'Dell, D., Giovanazzi, S., Kurizki, G. & Akulin, V. M. Phys.Rev. Lett. 84, 5687 (2000).


Right: The rotating lighthouse model of pulsars.

Neutron stars: Introduction to Neutron Stars (University of Maryland); Another neutron star lecture (Ohio State University); Neutron Stars and Pulsars (NASA); Neutron Stars: X-ray Astronomy Field Guide (Harvard)

Neutron stars and superfluids: Superfluidity in the Interiors of Neutron Stars (Northwestern University); Pulsars, Glitches and Superfluids (PhysicsWeb)

Superfluids: Double first for superfluids (PhysicsWeb); Superfluidity and Quantized Vortices (; An Optical Spoon Stirs Up Vortices in a Bose­Einstein Condensate (American Institute of Physics); Liquid hydrogen turns superfluid (PhysicsWeb);

Quantum Vortices in Atomic Bose-Einstein Condensates -- this poster shows pictures of vortices in BECs and in superfluid helium. (Adobe pdf file)

Carl Wieman -- Shared the Nobel Prize for Physics in 2001 for the creation of vaporous Bose-Einstein Condensates.



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