Are ripples caused by waves or particles? Science Photo Library
Quantum physics is our basic theory of how particles and the forces that act on them work. It is the foundation of the hugely successful standard model of particle physics – the most exhaustively tested theory ever. But quantum theory is also notoriously inscrutable: to get it to work, you must first assume some very basic, rather counter-intuitive things about the way nature works on its smallest scales. Complementing our grand tour of the universe in Reality guide: The essential laws of cosmology, here we explore the laws of the quantum world.
LAW 4: Quantisation
Things come in bite-size chunks
The origin of quantum theory was, quite literally, a light-bulb moment. In 1900, Max Planck was trying to describe mathematically the energy output of light bulbs, and so make better ones. Existing theories failed to match reality. After a few false starts, Planck found he could bridge the gap by making a radical assumption: the electromagnetic energy given out by a radiating body was emitted not continuously, but in indivisible packets.
Planck initially thought these “quanta” were a limitation of the theory, not a description of reality. But in 1905, Einstein showed that the way some metals expel electrons when light shines on them – the photoelectric effect – could also be explained by assuming that light is made of discrete particle-like quanta, which he called photons. This was just the beginning. As quantum theory developed, it became clear that not just energy, but many other properties such as electric charge and spin, come in …
