Radiation

Radiation is energy travelling through space, in two broad families. Electromagnetic radiation is light in the widest sense — radio, visible, X-rays, gamma rays — and the cosmic microwave background is a whole sky of it. Particle radiation is fast-moving bits of matter flung out when an unstable nucleus undergoes radioactive decay.^[1] The key fact for the rest of this branch: each radioactive isotope decays at its own fixed rate, summed up by its half-life — a clock built into matter itself.^[2]