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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.
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