We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

The difficulty with nailing down where these cosmic rays are coming from is that the Earth is a pretty big target. Even though millions of them hit the Earth on a daily basis, actually placing a detector and catching them at the right time is tricky. We said earlier that hundreds of these hit the Earth every second, and we didn’t lie, but the Earth is a very big place. So the more relevant number is how many cosmic rays hit an area the size of a typical detector, which is counted in square kilometers. Particles at LHC energies (1013 eV) arrive at Earth at a rate of one thousand per square kilometer per second. Particles at absurd energies (1018 eV) arrive more rarely, at a rate of one per square kilometer per year. But the prize jewels, particles above 1020 eV, are much rarer. They arrive at a rate of roughly one per square kilometer per millennium