Electromagnetic radiation, i.e. light, can be thought of as both a wave and a particle. We’ve touched on this before concerning Planck’s idea that light has a minimum quantity, which is the basis of quantum mechanics. Einstein postulated that this minimum amount traveled in one direction and acted like a particle, called a photon.
Light unquestionably acts like a wave too, though. One property of waves is that they can interfere with each other: if one wave’s peak corresponds to another wave’s trough, they will cancel out. Albert Abraham Michelson invented the interferometer in the 1880’s, in which a beam of light is split in half, sent two different directions, and then recombined at the detector.
By varying the distances to the two mirrors, the peaks and troughs of the light waves can be superimposed in any way when the light beams recombine. For example, you could set them up to cancel out, or to add together again. If a single photon is put through this device, it will interfere with itself (wave behavior) as if it went both directions, but still be detected as a single photon (particle behavior). This makes no sense … that’s part of the point—however, it’s true.
