Quark Stars

Wow — evidence that very massive neutron stars may have cores made of deconfined quark matter! The idea of a ‘quark star’ is not new, but I didn’t know it was a serious possibility.

An ordinary neutron star has a core made mostly of densely packed neutrons. A matchbox-sized chunk of this stuff weighs about 3 billion tonnes. But if you squeeze this stuff hard enough, eventually the neutrons break. Each neutron consists of 3 quarks held together by gluons. So when the neutrons break you get ‘quark matter’ — a sea of quarks and gluons, no longer confined in neutrons.

We’ve made something similar here on Earth: at CERN and Brookhaven, physicists smack atomic nuclei at each other so hard that the protons and neutrons break and momentarily form a ‘quark-gluon plasma’. But the conditions in a neutron star core are different: cooler, but more pressure — and not just temporary.

This new paper tries to take the measured properties of massive neutron stars and see if they fit a model where the inner core is made of deconfined quark matter:

• Eemeli Annala, Tyler Gorda, Joonas Hirvonen, Oleg Komoltsev, Aleksi Kurkela, Joonas Nättilä and Aleksi Vuorinen, Strongly interacting matter exhibits deconfined behavior in massive neutron stars, Nature Communications 14 (2023), 8451.

They say it does with about 80% probability! I’d take this with a big grain of salt, but it’s an exciting possibility. It’s not every day we find quintillions of tonnes of a new state of matter.

For me, the coolest part is that deconfined quark matter may have an extra symmetry, called ‘conformal symmetry’. This means that if you zoom in on it, it looks almost the same. An atom looks like a blob with some specific size. So does a neutron. But a system with conformal symmetry is just a blur spread out everywhere — and if you zoom in or zoom out, you see something very similar. This is crazy.

We see matter with conformal symmetry elsewhere too, like at ‘critical points’. As you know, when you heat water it boils. There’s a sharp divide between liquid and gas. But as you keep boosting the pressure, the boiling point increases. And at 647 degrees kelvin and 220 times ordinary atmospheric pressure something weird happens: the distinction between liquid and gas ends! This is called a ‘critical point’.

Right at the critical point, water looks weird. It look like a blur of droplets floating in gas. But if you look at any droplet you’ll see it’s full of bubbles of gas. And if you look in any of these bubbles you see it’s full of droplets. As you keep zooming in, you keep seeing basically the same thing…. droplets of liquid in gas, bubbles of gas in liquid…. until you get down to the scale of atoms. So we say this stuff has ‘conformal symmetry’.

The math of this is incredibly cool: it’s called ‘conformal field theory’, and a lot of mathematicians and physicists study it. So it would be really neat if extremely heavy neutron stars turn out to be made of deconfined quark matter with conformal symmetry… or even just approximate conformal symmetry.