Toss a handful of coins on a table: each one lands heads or tails on its own. Now imagine the coins start flipping over, nudging each other. That's roughly how neutrinos behave in a very dense environment.

Neutrinos have a tiny mass, and because of it, a particle can spontaneously switch from 'left-handed' to 'right-handed' spin — physicists call this a helicity flip. Under normal conditions, this happens rarely. But when neutrinos gather in huge numbers, like in the depths of an exploding star or during the merger of neutron stars, they start to feel each other through the weak interaction. A collective dance emerges: the particles exchange not only energy but also spin direction.

Previously, such processes were calculated one particle at a time, as if each coin lay still on the table. But in reality, neutrinos constantly collide and exchange momentum. The authors built a model with several particles in precisely defined states and saw that it's exactly this exchange of nudges between them that triggers an avalanche of flips. Conventional methods that only account for forward motion completely miss this effect.

If neutrinos in a dense cloud flip their helicity en masse, they carry energy away from the hot core differently. That means the explosion proceeds differently, and the newborn neutron star cools in a different way. Perhaps such collective flips explain why some stars explode brightly while others fade quietly. For now, this is just a hypothesis, but it forces us to revisit old scenarios.