If neutrinos have mass, as my physicist nephew from Fermilab says is now the consensus, how can one explain the virtual simultaneous arrival of neutrinos and photons from the Supernove1987A in a Magellanic cloud about 300,000 light years away?

Asked by: Robert Nelson

Answer

Firstly we need to understand a bit about the photon and the neutrino. The photon is a massless particle that always travels at the speed of light where as the neutrino is a particle with mass and travels a little slower than the speed of light. Now just before a star explodes the stars own gravitational field is crushing the iron core with a huge force then at the point of the star collapsing the there is a huge flux of neutrinos that leave the core and because of their tiny mass they very rarely interact with the stars matter. Then a few seconds later the shock wave produced by the stars core collapse destroys the outer layers sending them off into space; hence we see the visible supernova. The reason why the neutrinos and the photons arrive at virtually the same time is because the neutrino has a small head start when leaving the star over the photons and despite the neutrinos having mass they very rarely interact and even if they do the odd one particle in 10's of billions does not really make a difference.
Answered by: Dan Summons, Physics Undergrad Student, UOS, Souhampton

First, why neutrinos at all from any supernova?

Elements above iron in the periodic table cannot be formed in the normal nuclear fusion processes in stars. Up to iron, fusion yields energy and thus can proceed. But since iron is at the peak of the binding energy curve, fusion of elements above iron dramatically absorbs energy. So to produce heavier elements, enormous amounts of energy are needed. Current opinion is that they must be formed in the cataclysmic explosions known as supernovae. In the supernova explosion, a large flux of energetic neutrons is produced and nuclei bombarded by these neutrons build up mass one unit at a time to produce the heavy nuclei. With large neutron excesses, these nuclei would simply disintegrate into smaller nuclei again were it not for the large flux of neutrinos which make possible the conversion of neutrons to protons via the weak interaction in the nuclei.

The reason the neutrino flux from 1987A reached Earth before its light is because the neutrinos began their journey towards us some 3 hours before the visible light from the explosion itself.

It's estimated that the total neutrino flux energy was on the order of 10 x 10⁴⁶ Joules.
Answered by: Pat Weiler, B.A.