What do ripples on puddles, waves on ropes and noise-cancelling headphones have in common?
They all show superposition. When two waves meet they overlap and interact. Sometimes they add to make a wave bigger, sometimes they cancel each other out, and often it’s a combination of both. Noise-cancelling headphones listen to regular and constant noise around you and play the exact opposite sound to cancel annoying noises like jet planes engines.
This phenomenon is known as superposition.
Confusingly, however, in the quantum world superposition can mean something different entirely. At the quantum scale, particles can also be thought of as waves. Particles can exist in different states, for example they can be in different positions, have different energies or be moving at different speeds. But because quantum mechanics is weird, instead of thinking about a particle being in one state or changing between a variety of states, particles are thought of as existing across all the possible states at the same time. It’s a bit like lots of waves overlapping each other. This situation is known as a superposition of states. If you’re thinking in terms of particles, it means a particle can be in two places at once. This doesn’t make intuitive sense but it’s one of the weird realities of quantum physics.
However, once a measurement of a particle is made, and for example its energy or position is known, the superposition is lost and now we have a particle in one known state.