Four forces of nature, illustration

Illustration conceptualising the four fundamental forces of nature (top-left clockwise): gravity; electromagnetic; strong and weak. Somewhat counterintuitively, the weakest forces operate over the largest distances, and vice versa. Gravity, for example, is by far the weakest force but it binds together entire clusters of galaxies. Electromagnetism works on far smaller scales, that humans can identity with. The weak nuclear force is responsible for nucleons within atoms converting from protons into neutrons and emitting beta radiation in the process. And lastly, the strong force - which is many many times stronger than gravity, operates on the smallest scale of all, holding together the nucleons in an atomic nucleus as well as the quarks within nucleons themselves., by MARK GARLICK/SCIENCE PHOTO LIBRARY