Wave-Particle Duality

Wave-Particle Duality, possession of both wave-like and particle-like properties by subatomic objects. The fundamental principle of quantum theory is that an entity that we are used to thinking of as a particle (such as an electron) can behave like a wave, while entities that we are used to thinking of as waves, such as light waves, can also be described in terms of particles (in this case, photons).

This wave-particle duality is most clearly seen in “double-slit” experiments, in which either electrons or photons are fired, one at a time, through a pair of holes in a barrier, and detected on a screen (like a TV screen) on the other side. In both cases, particles leave the gun on one side of the barrier and arrive at the detector screen, each making an individual spot on the screen. However, the overall pattern that builds up on the screen as more and more particles are fired through the two holes is an interference pattern, made up of light and dark stripes, which can only be explained in terms of waves passing through both holes in the barrier and interfering with each other. This gives rise to the aphorism that quantum entities “travel as waves but arrive as particles”.

Wave-particle duality is also related to the uncertainty principle. This says that the exact position of a particle and its exact momentum (essentially, its speed and direction of movement) can never be known simultaneously. Position is a particle property—particles exist at a point. Waves are extended entities by nature, which do not have a position, although they do have momentum. Entities that are both wave and particle are never quite sure either where they are or where they are going.

The wavelength λ and momentum p of a quantum entity are related by the equation pλ = h, where h is a constant known as Planck's constant.