Inside a neutron star, matter is squeezed so tightly that a teaspoon of its material would weigh as much as a mountain. We can't directly see what happens to particles under such conditions, but stars tell us about themselves through gravitational waves and light when two such stars merge. Each observation is like a fingerprint, depending on how neutrons and protons interact.
Previously, to connect an observation with theory, scientists had to solve incredibly complex quantum physics equations millions of times. Each calculation took hours on a supercomputer — for just a single scenario. It was like trying to guess a recipe by tasting a dish blindfolded, having to start from scratch each time.
Now, researchers have built "smart" simplified models — emulators that learn from accurate but slow calculations. One model quickly predicts the behavior of neutron matter; another predicts how that leads to a star. By combining them with data from gravitational-wave observatories, scientists have for the first time directly constrained two key numbers describing particle interactions.
It turns out that even observations of distant stars can narrow down the allowed values of these numbers. This means astrophysics is becoming a full-fledged tool for testing nuclear physics, as if we could look inside a star without leaving Earth.
Neutron stars spin so fast that some make hundreds of revolutions per second — faster than a kitchen blender.