Professor Tony Bell from Oxford University led our discussion on cosmic rays.

These are the highest energy particles in the universe. Their energies reach about ten million times that of particles accelerated at CERN. Mainly protons, one single particle can have the same energy as a well-hit tennis ball with a velocity departing from the speed of light by one part in 1022.

Recent observations indicate that the highest energy cosmic rays probably gain their energy in relativistic jets ejected by black holes in quasars and active galaxies.

The highest energy cosmic rays are very rare and require enormous detectors since they arrive at the surface of the earth at the rate of about one per century per square kilometre. Protons with energies comparable to those reached with the Large Hadron Collider are produced by supernova explosions. Using gamma-ray telescopes we can observe them as they are being accelerated.