Probing the mysteries of neutron stars with an amazing terrestrial analogue Ars Technica

Since neutron stars were discovered, researchers have used their unusual properties to probe our universe. The superdense remnants of stellar explosions, neutron stars pack more mass than the Sun into a ball about the size of San Francisco. A single cup of this star matter would weigh as much as Mount Everest.

These strange celestial bodies could alert us to distant disturbances in the fabric of spacetime, teach us about the formation of elements, and unlock the secrets of how gravity and particle physics work in some of the most extreme conditions in the universe.

They are at the heart of many open questions in astronomy and astrophysics, says astrophysicist Vanessa Graber of the Barcelona Institute of Space Sciences.

But to accurately interpret some of the signals from neutron stars, researchers must first understand what’s going on inside them. They have their own insights, but experimenting directly on a neutron star is out of the question. So scientists need another way to test their theories. The behavior of matter in such a superdense object is so complicated that not even computer simulations are up to the task. But the researchers think they’ve found a solution: a land analog.

Although young neutron stars may have temperatures of millions of degrees inside them, for an important energy measure neutrons are considered cold. Physicists think it’s a feature they can exploit to study the inner workings of neutron stars. Instead of looking at the sky, researchers are peering into clouds of ultracold atoms created in laboratories here on Earth. And that might help them finally answer some long-standing questions about these enigmatic objects.

Space weirdness

The existence of neutron stars was first proposed in 1934, two years after the discovery of the neutron itself, when astronomers Walter Baade and Fritz Zwicky wondered whether a celestial body made entirely of neutrons could remain after the explosion of a supernova. While they didn’t get all the details right, their general idea is now widely accepted.

Stars feed by fusing the nuclei of lighter atoms into those of heavier atoms. But when the stars run out of those lighter atoms, nuclear fusion stops and there is no longer an external pressure to fight the internal gravity. The core collapses and the star’s outer layer rushes inward. When this layer hits the dense core, it bounces and explodes outward, producing a supernova. The dense core that remains after that is a neutron star.

It was not until the 1960s that the hypothetical neutron stars of Zwicky and Baade were finally detected. Radio astronomer Jocelyn Bell Burnell noticed a strange signal of regularly pulsed radio waves from space while working as a graduate student at Cambridge University. He was detecting something he’d never seen before: a special kind of neutron star called a pulsar, which emits beams of radiation at regular intervals as it rotates, like a lighthouse. (His advisor, along with the director of the observatory, but not Bell Burnell, later received the Nobel Prize for the discovery.)

Since then, thousands of neutron stars have been detected. As some of the densest and highest pressure objects in the universe, neutron stars could help us understand what happens to matter at extremely high densities. Understanding their structure and the behavior of the neutron matter that composes them is of fundamental importance for physicists.

Scientists already know that the neutrons, protons and other subatomic particles that make up a neutron star arrange themselves differently depending on where they are in the star. In some sections, they pack as rigidly as water molecules in a block of ice. In others they flow and swirl like a frictionless fluid. But exactly where the transition takes place and how different phases of matter behave, physicists aren’t sure.

A superdense star born from a nuclear fireball appears, at first blush, to have very little in common with a diluted cloud of ultracold particles. But they can share at least one useful characteristic: they are both below a threshold known as the Fermi temperature, on which it depends and is calculated on the basis of the matter of which each system is composed. A system that is well above this temperature will behave largely according to the laws of classical physics; if it is well below, its behavior will be governed by quantum mechanics. Certain ultracold gases and neutron star material may both be well below their Fermi temperatures and as a result may behave in similar ways, says Christopher Pethick, a theoretical physicist at the Niels Bohr Institute in Copenhagen and co-author of an early overview of the neutron stars in the 1975 Annual Review of Nuclear SciencecAnd.

Matter that is below its Fermi temperature can obey remarkably universal laws. This universality means that while we don’t have easy access to neutron star matter of several million degrees, we could learn some of its behavior by experimenting with ultracold gases that can be created and manipulated in laboratory vacuum chambers on Earth, says the theoretical astrophysicist. James Lattimer of Stony Brook University in New York, author of a summary of nuclear matter science in the 2012 Annual Review of Nuclear and Particle Science.

Of particular interest to Lattimer is a theoretical state called a unit gas. A gas is unitary when the sphere of influence of each of its particles becomes infinite, meaning that they would influence each other regardless of the distance between them. This is impossible to achieve in reality, but clouds of ultracold atoms can get close and so can matter inside neutron stars. It’s similar to a unit gas, Lattimer says, but it’s not a perfect unit gas.

Down to earth

For a long time, the exact relationship between a gas’s pressure and its density was simply too complex to calculate precisely. But when experimental physicists developed the ability to control clouds of cold atoms and tune them to closely approximate a unit gas, it opened up a new avenue for determining such properties of the gas: just measure it directly, instead of struggling to argue the cumbersome math on a computer.

These clouds of ultracold atoms are actually closer to being a unit gas than neutron star matter, so the analogy isn’t perfect. But it’s close enough that Lattimer has been able to take near-unit gas measurements from clouds of cool atoms and apply them to neutron matter to refine some of the theoretical models that describe the inner workings of neutron stars. And experiments with cold atoms may help scientists develop theories about what physics might be at play in some unexplained neutron star phenomena.

In particular, Graber and other scientists hope to find clues to one of the biggest mysteries, called the pulsar glitch. In general, the regular ticking of a pulsar clock is so reliable that its accuracy rivals that of atomic clocks. But not always: Sometimes, the rotational speed of pulsars increases sharply, causing a glitch. Where that extra oomph comes from isn’t clear. The answer lies in the way matter moves inside a neutron star.

Both the cold gases and the neutron matter in some parts of a neutron star are superfluid: the particles flow without any friction. When a superfluid rotates, small eddies or eddies develop. How exactly these eddies move and interact with each other and with other structures within a spinning neutron star is still an open question. It’s probably not this nice regular vortex lattice, says Michael McNeil Forbes, who studies theoretical physics at Washington State University in Pullman. It could be a tangle of eddies that is found throughout the star. We do not know.

Forbes and others suspect that the flaws they observe in the rotation of pulsars have something to do with how these eddies become attached to the star’s structures. Generally, a single vortex snakes freely around a fluid. But when the fluid contains a rigidly packed area of ​​matter that hinders the movement of the vortex, the vortex will stop and sometimes even wrap its swirling arms around the rigid object and position itself so that its center is right above it .

Vortexes tend to get stuck like this, but can sometimes disengage and migrate away from the object. When this happens, the fluid flow exerts a torque on the object. If hundreds of thousands of eddies detach from various structures in a neutron star at once, they can suddenly accelerate the rotation of the stars. Forbes explains how so many whirlpools could unclog all at once: Like dropping sand on a pile of sand, nothing happens until you get a whole avalanche.

But it’s nearly impossible for classical computers to calculate exactly all the intricacies of the dance of so many eddies at once. So Forbes plans to partner with experimental groups that can form these eddies in their clouds of cold atoms and see what happens. The idea is to use experiments with cold atoms like analog quantum computers to calculate things we can’t do any other way, she says.

Researchers are busy examining how other ultracold phenomena they regularly see in the laboratory could inspire new lines of research into the behavior of neutron stars. Recently, Graber and his colleagues outlined so many possibilities that it took them 125 pages to publish them all. In 2019, dozens of astronomers, nuclear physicists and ultracold atomic physicists from around the world gathered to discuss the surprising connections between their fields. Researchers are just starting to test some of the ideas generated by these brainstormings.

They’re also learning more from the stars themselves, Pethick says. It’s an exciting field, because there are a lot of submissions coming right now.

With better telescopes and new ways to glean the properties of a neutron star’s inscrutable interior, scientists can hope to discover how far this analogy between cold atoms and neutron stars can be carried.

Katie McCormick is a physicist turned science writer living in Sacramento, California. In his previous life, he spent a postdoctoral appointment experimenting with ultracold atoms like the ones in this story. This article originally appeared in Knowable Magazine, an independent reporting effort of Annual Reviews. Subscribe to the newsletter.