When a massive star dies, its core collapses into a neutron star, and the outer layers are blown off in an explosion. At that moment, an unimaginably dense cloud of neutrinos is born — particles that barely interact with matter but actively 'talk' to each other. There are so many of them that together they behave like a fluid, with waves rippling across it.
Recently, physicists noticed that stable fast waves in this neutrino sea should decay — much like plasma waves lose energy through the Landau damping effect. But a new study shows that the damping is actually very weak, and in the limit of 'pure fastness' (when particle speeds are nearly identical) it vanishes altogether.
This does not undermine the main point: unstable modes, those that grow and eventually mix neutrino types, still do their job. They determine how neutrinos change flavor, and thus how the explosion's energy is distributed. Understanding which waves survive and which die out helps us model supernovae and neutron star mergers more accurately — cosmic factories of heavy elements like gold and platinum.
Inside a supernova, neutrinos are packed so tightly that their mutual influence is stronger than their interaction with the star's ordinary matter.