Particle astrophysics or ‘‘Looking’’ at the stars

For millennia humankind has looked at the stars and wondered about the universe. Only in recent years have we been able to ‘‘look’’ at the stars by means other than visible light. Not only can we now use photons of other wavelengths—from radio waves to ultra‐high‐energy gamma rays—but we can also detect, or try to detect, other particles. These include (i) cosmic rays—charged particles; probably protons, but perhaps heavier nuclei, which arrive from outer space with energies up to 1020 eV (the Greissen cutoff, a limit due to collisions with the 3 K microwave background radiation); (ii) neutrinos—which can penetrate through large amounts of intervening matter, whereas photons are blocked by less than 100 grams of matter; (iii) gravitational waves—which can be produced by collapsing binaries of neutron stars or black holes, or other large, violent astrophysical events.The talk mentioned some of the weird and wonderful detectors now operating or being built: Cherenkov telescopes, air‐shower arrays; neutrino ...