The double-slit experiment is a nineteenth-century investigation into the properties of light that has since been found to demonstrate both the duality of photons and the concepts of superposition and quantum interference.
Debate over whether light is made up of particles or waves dates back over three hundred years. In the seventeenth century, Isaac Newton proclaimed that light consisted of a stream of particles. In the early nineteenth century, Thomas Young devised the double-slit experiment to prove that it consisted of waves. While the implications of Young's experiment are difficult to accept, they are illustrative of quantum theory. The noted physicist Richard Feynman claimed that the essentials of quantum mechanics could be grasped from an exploration of the results of the experiment.
In the double-slit experiment, a beam of light is aimed at a barrier with two vertical slits. After the light passes through the slits, the resulting pattern is recorded on a photographic plate. When one slit is covered, a single line of light is displayed, aligned with whichever slit is open.
Intuitively, one might hypothesize that if both slits were open, the resulting pattern would display as two lines of light, aligned with the slits. What occurs in practice, however, is that light passing through the slits and displayed on the photographic plate is entirely separated into multiple lines of lightness and darkness in varying degrees. The result illustrates that interference is taking place between the waves and particles going through the slits in what a layman might expect to be two non-crossing trajectories.
If the beam of photons is slowed enough to ensure that individual photons are hitting the plate, one might expect there to be no interference and a pattern of light would be two lines of light, aligned with the slits. The results of the experiment, however, indicate the presence of interference. Somehow, the single particles are interfering with themselves. On the face of things, this might seem impossible: we expect that a single photon will go through one slit or the other and will end up in one of two possible light line areas.
That expectation, however, is invalidated by the results of the double-slit experiment. What actually occurs is that each photon not only goes through both slits, but also simultaneously traverses every possible trajectory en route to the target. Research into this phenomenon has demonstrated that other elementary particles, such as electrons, exhibit the same behavior.