Isaac Newton propounded in his great work Opticks (1704) that light consists of a flow of particles. Generations of students and researchers have accepted this idea. Light was believed to be composed of large numbers of billiard-ball type material bodies, visualized as spherical objects that traveled through vacuum and non-opaque regions with no luminiferous medium required.
Thomas Young’s work in the early nineteenth century contradicted this established view. He apparently started with a pro-wave agenda, which he communicated to the scientific community by means of a series of well-constructed experiments. First, he built a ripple tank to demonstrate the nature of wave propagation, specifically the phenomenon of interference. When the surface of calm water was disturbed at two separate points, waves emanated in widening concentric circles. Where the waves from the two separate sources intersected, a distinctive interference pattern could be observed. Each wave consisted of successive high ridges and low troughs. Where the ridges coincided, still higher perturbations could be observed and where the troughs intersected, lower dips appeared. This unique behavior arises from constructive and destructive interference, where the highs and lows reinforce one another, and it is quite obviously a result of wave propagation.
A second experiment conducted by Young succeeded in demonstrating that light energy must similarly propagate in the form of wave-like oscillations, presumably in some luminiferous medium that pervades all of space. This demonstration took place between 1801 and 1805 (the exact date is uncertain). Young set up a light source adjacent to an opaque barrier in which he had cut two parallel slits. Passing through them, the two beams of light struck a nearby screen. The unmistakable interference pattern with fringes similar to the water waves in Young’s ripple tank, was seen to establish the wave theory of light, as opposed to Newton’s particle theory.
Young’s narrative was widely accepted until late in the nineteenth century, when the Michelson-Morley experiment, which also made use of interference patterns, seemed to call into question the existence of a luminiferous medium. With no substance to vibrate, how could there be vibrations?
Albert Einstein’s Special Theory of Relativity appeared in 1905 and seemed to resolve the conflict with the notion that light (and by extension all electromagnetic energy) had a dual nature, both particle and wave.
Meanwhile, the double-slit experiment returned, providing some answers and simultaneously raising still more questions. If powerful coherent light emitted by a laser is aimed at two parallel slits, a pronounced fringe-like interference pattern will be observed on a nearby screen. There is a series of light and dark bands, more pronounced when the slots are made narrower.
Extending this idea, it is now possible to build equipment that emits only a single photon at a time. When a succession of them separated by a definite amount of time is directed through both slots so as to strike a photosensitive screen, single dots appear. This is as expected because the single photons are fired through the slots at a rate of about 1/20 of a second apart, so they cannot give rise to an interference pattern. And yet what is odd about all this is that if the process is made to continue until a great many dots accumulate on the screen, a definite discernable interference pattern arises.
The explanation has to be that individual electrons, separated in time, pass through both slots. This physical impossibility is the only way to account for the interference pattern. It gets even stranger. If a detector is set up to determine through which slot the photon passes, the interference pattern disappears! The act of observing the wave aspect of light “collapses” it (using quantum terminology) into its particle aspect.
The double-slit experiment has been performed numerous times with variations in technique and equipment. In addition to using photons, researchers have fired electrons, atoms and whole molecules through the two slits. It has become abundantly clear that the act of measurement transforms the probability wave aspect of any quantum-scale entity into its particle version. This is not a question of human knowledge or perspective.
The transformation takes place anytime the wave is pinned down by an act of machine measurement though humans aren’t involved. Ingenious experiments have verified this quantum dualism even to the extent of observing strange, faster-than-light phenomena associated with quantum entanglement.
Greg Marlow says
Can all the weirdness of double slit experiment be explained by the fact that we are only looking at it from a stationary frame perspective? If you look at the special relativity equation for total energy you see that it has a velocity of the speed of light. For an observer in an orthogonal rest frame to that perspective it looks entirely like a wave of light and would defract and interfere just like light. In our reference frame we observe the momentum energy component and the rest energy component of the total energy separately as the velocity of a particle with rest mass. So it looks like you can have a wave and a particle at the same time. How much wave-like and how much particle-like it looks only depends from what perspective you are looking.