Our notion of reality is built on everyday experiences. But wave-particle duality is so strange that we are forced to re-examine our common conceptions.

Wave-particle duality refers to the fundamental property of matter where, at one moment it appears like a wave, and yet at another moment it acts like a particle.

To understand wave-particle duality it’s worth looking at differences between particles and waves.

We are all familiar with particles, whether they are marbles, grains of sand, salt in a salt-shaker, atoms, electrons, and so on.

The properties of particles can be demonstrated with a marble. The marble is a spherical lump of glass located at some point in space. If we flick the marble with our finger, we impart energy to it – this is kinetic energy, and the moving marble takes this energy with it. A handful of marbles thrown in the air come crashing down, each marble imparting energy where it strikes the floor.

We are all familiar with particles, whether they are marbles, grains of sand, salt in a salt-shaker, atoms, electrons, and so on.

The properties of particles can be demonstrated with a marble. The marble is a spherical lump of glass located at some point in space. If we flick the marble with our finger, we impart energy to it – this is kinetic energy, and the moving marble takes this energy with it. A handful of marbles thrown in the air come crashing down, each marble imparting energy where it strikes the floor.

In contrast, waves are spread out. Examples of waves are the big rollers on the open ocean, ripples in a pond, sound waves and light waves.

If at one moment the wave is localised, some time later it will have spread out over a large region, like the ripples when we drop a pebble in a pond. The wave carries with it energy related to its motion. Unlike the particle the energy is distributed over space because the wave is spread out.

Why waves are so different from particles

Colliding particles will bounce off each other but colliding waves pass through one another and emerge unchanged. But overlapping waves can interfere – where a trough overlaps a crest the wave can disappear altogether.

This can be seen when parts of a wave pass through closely spaced holes in a screen. The waves spread out in all directions and interfere, leading to regions in space where the wave disappears and regions where it becomes stronger.