The alerts started in the early morning of Aug. 17. Gravitational waves produced by the wreck of two neutron stars — dense cores of dead stars — had washed over Earth. The thousand-plus physicists of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) rushed to decode the space-time vibrations that rolled across the detectors like a drawn-out peal of thunder. Thousands of astronomers scrambled to witness the afterglow. But officially, all this activity was kept secret. The data had to be collected and analyzed, the papers written. The outside world wouldn’t know for two more months.

The strict ban put Jocelyn Read and Katerina Chatziioannou, two members of the LIGO collaboration, in a bit of an awkward situation. In the afternoon on the 17th, the two were scheduled to lead a panel at a conference dedicated to the question of what happens under the almost unfathomable conditions in a neutron star’s interior. Their panel’s topic? What a neutron-star merger would look like. “We sort of went off at the coffee break and sat around just staring at each other,” said Read, a professor at California State University, Fullerton. “OK, how are we going to do this?”

Physicists have spent decades debating whether or not neutron stars contain new forms of matter, created when the stars break down the familiar world of protons and neutrons into new interactions between quarks or other exotic particles. Answering this question would also illuminate astronomical mysteries surrounding supernovas and the production of the universe’s heavy elements, such as gold.

In addition to watching for collisions using LIGO, astrophysicists have been busy developing creative ways to probe neutron stars from the outside. The challenge is then to infer something about the hidden layers within. But this LIGO signal and those like it — emitted as two neutron stars pirouette around their center of mass, pull on each other like taffy, and finally smash together — offers a whole new handle on the problem.

Strange Matter

A neutron star is the compressed core of a massive star — the super dense cinders left over after a supernova. It has the mass of the sun, but squeezed into a space the width of a city. As such, neutron stars are the densest reservoirs of matter in the universe — the “last stuff on the line before a black hole,” said Mark Alford, a physicist at Washington University in St. Louis.

To drill into one would bring us to the edge of modern physics. A centimeter or two of normal atoms — iron and silicon, mostly — encrusts the surface like the shiny red veneer on the universe’s densest Gobstopper. Then the atoms squeeze so close together that they lose their electrons, which fall into a shared sea. Deeper, the protons inside nuclei start turning into neutrons, which cluster so close together that they start to overlap.

But theorists argue about what happens farther in, when densities creep past two or three times higher than the density of a normal atomic nucleus. From the perspective of nuclear physics, neutron stars could just be protons and neutrons — collectively called nucleons — all the way in. “Everything can be explained by variations of nucleons,” said James Lattimer, an astrophysicist at Stony Brook University.

Other astrophysicists suspect otherwise. Nucleons aren’t elementary particles. They’re made up of three quarks. Under immense pressure, these quarks might form a new state of quark matter. “Nucleons are not billiard balls,” said David Blaschke, a physicist at the University of Wroclaw in Poland. “They are like cherries. So you can compress them a little bit, but at some point you smash them.”

But to some, the prospect of a quark jam like this is a relatively vanilla scenario. Theorists have long speculated that layers of other weird particles might arise inside a neutron star. As neutrons are jostled closer together, all that extra energy might go into creating heavier particles that contain not just the “up” and “down” quarks that exclusively make up protons and neutrons, but heavier and more exotic “strange” quarks.

For example, neutrons might be replaced by hyperons, three-quark particles that include at least one strange quark. Laboratory experiments can make hyperons, but they vanish almost immediately. Deep inside neutron stars, they might be stable for millions of years.

Alternatively, the hidden depths of neutron stars might be filled with kaons — also made with strange quarks — that collect into a single lump of matter sharing the same quantum state.

For decades, though, the field has been stuck. Theorists invent ideas about what might be going on inside neutron stars, but that environment is so extreme and unfamiliar that experiments here on Earth can’t reach the right conditions. At Brookhaven National Laboratory and CERN, for example, physicists smash together heavy nuclei like those of gold and lead. That creates a soupy state of matter made up of released quarks, known as a quark-gluon plasma. But this stuff is rarefied, not dense, and at billions or trillions of degrees, it’s far hotter than the inside of neutron star, which sits in the comparatively chilly millions.

Even the decades-old theory of quarks and nuclei — “quantum chromodynamics,” or QCD — can’t really provide answers. The computations needed to study QCD in relatively cold, dense environments are so devastatingly difficult that not even computers can calculate the results. Researchers are forced to resort to oversimplification and shortcuts.

The only other option is for astronomers to study neutron stars themselves. Unfortunately, neutron stars are distant, thus dim, and difficult to measure for anything but the very basic bulk properties. Even worse, the truly interesting physics is happening under the surface. “It’s a bit like there’s this lab that’s doing amazing things,” Alford said, “but all you’re allowed to do is see the light coming out of the window.”

With a new generation of experiments coming online, though, theorists might soon get their best look yet.