Only one neutron star collision has ever been observed. That means that there is little data on the cosmic phenomena. We have a few answers to the many questions that arise when wondering what happens when such massive objects collide.

Thankfully, the conditions can be partially simulated on Earth, thanks to the GSI's heavy ion accelerator.

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Colliding stars, colliding particles

Scientists at the Technical University of Munich and the GSI Helmholtz Centre for Heavy Ion Research in Germany (the HADES Collaboration) recently used the GSI heavy ion accelerator to simulate a neutron collision, right here on Earth.

As Science Alert reports, some of the conditions in heavy ion collisions are similar to those of neutron star collisions. The densities and temperatures, in particular, resemble the enormous impact of two neutron stars.

In the same way, virtual photons are produced in neutron star collisions, these particles can also appear when two heavy ions collide at velocities close to light speed.

HADES detector with digital camera that records light patterns originating from virtual photons. Source: Thomas Ernsting/GSI

However, virtual photons appear very rarely and are quite weak.

"We had to record and analyze about 3 billion collisions to finally reconstruct 20,000 measurable virtual photons," said TUM physicist Jürgen Friese in a press release.

Detecting Cherenkov radiation

To detect the weak particles, the team designed a large custom camera - 1.5 square meters - that can detect the faint Cherenkov radiation patterns generated by the decay products of virtual photons.

"Unfortunately the light emitted by the virtual photons is extremely weak. So the trick in our experiment was to find the light patterns," Friese said.

"They could never be seen with the naked eye. We, therefore, developed a pattern recognition technique in which a 30,000 pixel photo is rastered in a few microseconds using electronic masks. That method is complemented with neural networks and artificial intelligence."

Through their research, the team determined that two colliding neutron stars, each with a mass 1.35 times larger than the Sun, would emit temperatures of 800 billion degrees Celsius. Such collisions, therefore, fuse heavy nuclei.

Insights into the early universe

Not only that, the experiment is providing insight into quark matter (QCD matter) that was prevalent in the universe moments after the Big Bang.

"A plasma of quarks and gluons transitioned into nucleons and other hadronic bound states in the early universe," the researchers explain in their paper.

"Similar states of matter, at lower temperatures, are believed still to exist in the interior of compact stellar objects, such as neutron stars. The formation of such cosmic matter in heavy-ion collisions provides access to studies of the microscopic structure of QCD matter at the femtoscale."

The research has been published in Nature Physics.