Light is often described as waves, but it can also be described as a stream of light particles, photons. Matter is also characterised by this dualism. In the 1920s, Louis de Broglie suggested that a particle can also behave as a wave. The lighter and slower the particles are, the longer their de Broglie wavelength is. In a cold gas, if the particles are near enough to one another, their wavelengths start to overlap. Then the separate waves link to become a single wave, common to all the atoms – Bose-Einstein condensation (BEC) occurs. This is a coherent state of matter, i.e. the atomic waves oscillate together, they are in unison.