A central concept in quantum physics is the particle-wave duality, the fact that fundamentalobjects in the physical world, electrons, protons, neutrons, photons and otherleptons, hadrons, and field quanta, all have the same dual nature: they are at the sametime both particles and waves. In some situations the particle aspect may be the dominantfeature, in other vice versa; but the behavior of any given object can never be understood fully by ascribing only one of these aspects to it.

Historically, as indicated, the particle-wave duality was first realized for the electromagneticfield. It is interesting to note that right from the beginning when the first theories of the nature of light was proposed in the 17th century, it was debated whether light were particles (corpuscles), as claimend by Newton, or waves, as claimed by Huygens. The debate appeared to end in the beginning of the 19th century with Young’s famous double-slit interference experiments that demonstrated that light were waves. With Maxwell’s theory (1873) and Hertz’s experiments (1888) the light waves were shown to be of electromagnetic nature. But after nearly one hundred years of wave dominance Planck’s formula for the energy distribution in black-body radiation demonstrated that light possesses some element of particle nature. This particle aspect became more evident with Einstein’s explanation of the photoelectric effect: small energy parcels of light, the so-called light quanta, are able to knock out electrons from metals just like one billiard ball hitting another.

For some years theorists tried to give alternative explanations of the photo-electric effect using maxwellian waves, but it proved impossible to account for the concentration of energy in a small point needed to explain the photoelectric effect without postulating the existence of light quanta – today called photons.