How to Build a Neutron Star

Scientists can probe the conditions just after the Big Bang. They can detonate a supernova in a test tube. They can even study black holes. What about neutron stars?

We've heard of ‘black holes' being built in the lab. We keep hearing physicists referring to ‘Big Bangs' inside the LHC. Most recently, those crazy scientists have even recreated a ‘supernova' in a test tube! It might therefore seem like an easy task to create a ‘neutron star' at home. Neutron stars are, after all, a little less extreme than black holes, Big Bangs and supernovae...

You might have noticed quite a few ‘scare quotes' in that paragraph, but it doesn't mean I'm unsure about the what the words inside the quotes mean. We associate black holes, Big Bangs and supernovae with extreme astrophysics and mind-boggling destructive energies, so obviously scientists can't really build the genuine article in a lab.

It is impossible (at our current state of technological prowess) to blow up a star, say, but we can build an analog by recreating some of the conditions of a supernova. This is when the fascinating science of very big cosmic phenomena can be done with a test tube filled with glycerol and water. It might sound weird, but glycerol and water make great analogs for the exploding cloud of supernova matter!

That's pretty much what a team of Duke University physicists have done when probing the nature of the bizarre neutron star - although it's taking something way more sophisticated than glycerol to do it.

But they didn't have to scoop a chunk of neutron degenerate matter from the surface of a real neutron star either.

Armed with lasers and a near-absolute-zero-chilled sample of gaseous lithium-6 atoms, the researchers have bypassed the need for a real neutron star sample (of which a pinhead-sized sample of material would weigh the same as a battleship), and built a scale model analog of neutron star material.

The physics is fiendishly complex, but the conclusions from this experiment are fascinating.

Cool It, Trap It, Poke It

The first step in creating this exotic form of material is to trap the ultra-cooled atoms of lithium-6 with lasers. The laser trap can keep a tiny spherical sample - measuring only a millimeter across - levitated so experiments can be done.

When the Duke scientists say "ultra-cool" they mean uber-cooled to around 150 micro-Kelvin - that's only a few billionths of a degree Kelvin above absolute zero. At these temperatures, the gas takes on a very exotic form; it becomes a Fermi gas.

The Fermi gas is governed by quantum dynamics and held in place by the laser trap, allowing the group to simulate the conditions of the tightly packed neutrons inside a neutron star.

Due to quantum constraints, the atoms sit at a fixed distance from one another (they can't get any closer as the Pauli exclusion principle forbids it). There is therefore an outward quantum pressure forcing against the constraining laser trap. This pressure is known as a "degeneracy pressure." It's a kind of "we-can't-squeeze-this-matter-any-more pressure."

This is where the experiment becomes analogous to the material of a neutron star. A neutron star is under a constant pressure battle: massive gravitational pressure squeezing the neutrons together, but a countering neutron degeneracy pressure pushing out. If the gravitational pressure overwhelmed the neutron degeneracy pressure, the neutron star would be crushed into a singularity, producing a black hole.

In the Duke experiment, the lithium-6 atoms are cooled so much, thermal motion of the individual atoms in the gas are at a bare minimum. When constrained within the laser trap, the atoms are bunched close together, mimicking the neutron degenerate pressure of a neutron star. The lithium-6 Fermi gas may not be as dense as the neutron degenerate matter of a neutron star, but the effects are simulated (albeit on a larger scale).

This is when the Duke team had some fun with their laser-trapped neutron star analog.

When the laser trap was turned off and back on again, the ultra-cooled gas oscillated like a jelly sphere. Critically, they found that when the gas was heated a tiny fraction (only by millionths of a degree Kelvin), the oscillation slowed, signifying an increase in viscosity.

The big finding here is that the more the gas was cooled, the more it acted like a perfect fluid (i.e., a frictionless fluid), not dissimilar to how the universal quark-gluon plasma acted fractions of a second after the Big Bang.

Although fascinating in understanding the real nature of neutron stars, this research also has far-reaching implications in the field of condensed physics and high-temperature superconductors. It may even help to indirectly test some components of superstring theory.

So, scientists can add neutron stars to their list of cosmic things they can test in the lab. But the best thing is that they don't have to actually create neutron degenerate matter itself, just cool some lithium-6 gas to nearly absolute zero, grab it inside a laser trap and make it wobble. Easy!

Image credit: NASA/CXC/M.Weiss