Two-Slit ExperimentThe two-slit experiment was until recent years for the most part a thought experiment, since it is difficult to build an inexpensive demonstration, but its predictions have been verified in many ways since the 1960's, primarily with electrons. Recently, extremely sensitive CCDs used in photography have been used to collect single-photon events. The two-slit experiment demonstrates better than any other experiment that a quantum wave function is a probability amplitude that can interfere with itself, producing places where the probability (the square of the absolute value of the complex probability amplitude) of finding a quantum particle is actually zero. There is nothing like this in the motion of classical particles, although something like it is well known in the cancellation of crests and troughs in the motion of water and other waves. This interference that creates high and low points in water waves was found to have the same shape as bright and dark areas found in the "fringes" of light at the sharp edges of an object, and were predicted to occur in double-slit experiments, by Thomas Young in the early nineteenth century. The interference of light was thought to deny Isaac Newton's corpuscular theory, in which light is thought to be particles of energy traveling in straight lines or "rays." In his text "Opticks," Newton used the ray/particle theory to explain reflections, refraction, and that different colors of light are bent at different angles by prisms and lenses. The two-slit experiment also demonstrates the famous "collapse" of the wave function or "reduction" of the wave packet, which show an inherent probabilistic element in quantum mechanics that is irreducibly ontological and nothing like the epistemological indeterminacy (human ignorance) in classical statistical physics. Note that probability is pure information. It is neither matter nor energy. When a wave function "collapses" or "goes through both slits" in this dazzling experiment, nothing material is traveling faster than the speed of light or going through both slits. The particle of matter or energy always goes through just one slit, although the popular Copenhagen interpretation of physics . There is something similar in the Einstein-Podolsky-Rosen thought experiments, where measurement of one particle transmits nothing physical (matter or energy) to the other "entangled" particle but only the instantaneous information that has come into the universe. That information, together with conservation of momentum or of spin, makes the state of the coherently entangled second particle certain, however far away it might be. In the two-slit experiment, as in the Dirac Three Polarizers experiment, the critical case to consider is just one photon at a time in the experiment. With one particle at a time (whether photon or electron), a quantum object is mistakenly described as interfering with itself. Indeed, even in the one-slit case, interference fringes are visible, although this is rarely described in the context of quantum mysteries. It is the fundamental relation between a particle and the associated wave that controls its probable locations that raises the "local reality" question first seen in 1905 and described in 1909 by Albert Einstein. Thirty years later, the EPR paper and Erwin Schrödinger's insights into the wave function of two entangled particles, first convinced physicists that there was a deep problem . It was not for another thirty years that John Stewart Bell in 1964 imagined an experimental test that could confirm or deny quantum mechanics. Ironically, the goal of Bell's "theorem" was to invalidate the non-intuitive aspects of quantum mechanics and restore Einstein's hope for a more deterministic picture of an "objective reality" at, or perhaps even underlying below, the microscopic level of quantum physics. At about the same time, in his famous Lectures on Physics at Cal Tech and the Messenger Lectures at Cornell, Richard Feynman described the two-slit experiment as demonstrating what he claimed is the "only mystery" of quantum mechanics. We can thus begin the discussion of the two-slit experiment with a section from Feynman's sixth Messenger lecture entitled "Probability and Uncertainty." We provide the complete video and text of the lecture on this page, and a version starting with Feynman's provocative statement that "no one understands quantum mechanics" here. Open video in a new window so you can read along with the text of the lecture. How, Feynman asks, can the particle go through both slits? We will see that the thing that goes through both slits is only immaterial information - the probability amplitude wave function. The particle always goes through a single slit. A particle cannot be divided and in two places at the same time. It is the wave function that interferes with itself. And the highly localized particle can not be identified with the wave widely distributed in space and determined by the boundary conditions of the measuring apparatus. The immaterial wave function exercises an causal influence over the particles, one that we can jusitifibly call "mysterious", that results in the statistics of many experiments agreeing with the quantum mechanical predictions with increasing accuracy as we increase the number of identical experiments. It is this "influence," no ordinary"force," that is Feynman's "only mystery" in quantum mechanics. Let's look first at the one-slit case. We prepare a slit that is about the same size as the wavelength of the light in order to see the Fraunhofer diffraction effect most clearly. Parallel waves from a distant source fall on the slit from below. The diagram shows that the wave from the left edge of the slit interferes with the one from the right edge. If the slit width is d and the photon wavelength is λ, at an angle α ≈ λ/2d there will be destructive interference. At an angle α ≈ λ/d, there is constructive interference (which shows up as the lightening in the interfering waves in the illustration).
Collapse of the Wave FunctionBut how do we interpret the notion of the "collapse" of the wave function? At the moments just before a particle is detected at the CCD or photographic plate, there is a finite non-zero probability that the photon could be detected anywhere that the modulus (complex conjugate squared) of the probability amplitude wave function has a non-zero value. If our experiment were physically very large (and it is indeed large compared to the atomic scale), we can say that the finite probability of detecting (potentially measuring) the particle at position x1 on the screen "collapses" (goes to zero) and reappears as part of the unit probability (certainty) that the particle is at x2, where it is actually measured. Since the collapse to zero of the probability at x1 is instantaneous with the measurement at x2, critics of quantum theory like to say that something traveled faster than the speed of light. This is most clear in the nonlocality and entanglement aspects of the Einstein-Podolsky-Rosen experiment. But the sum of all the probabilities of measuring anywhere on the screen is not a physical quantity, it is only immaterial information that "collapses" to a point. Here is what happens to the probability amplitude wave function (the blue waves) when the particle is detected at the screen (either a photographic plate or CCD) in the second interference fringe to the right (red spot). The probability simply disappears instantly.
Animation of a wave function collapsing - click to restart
HistoryThe first suggestion of two possible directions through a slit, one of which disappears ("collapses?") when the other is realized (implying a mysterious "nonlocal" correlation between the directions), was made by Albert Einstein at the 1927 Solvay conference on "Electrons and Photons." Niels Bohr remembered the occasion with a somewhat confusing description. Here is his 1949 recollection:
At the general discussion in Como, we all missed the presence of Einstein, but soon after, in October 1927, I had the opportunity to meet him in Brussels at the Fifth Physical Conference of the Solvay Institute, which was devoted to the theme "Electrons and Photons." At the Solvay meetings, Einstein had from their beginning been a most prominent figure, and several of us came to the conference with great anticipations to learn his reaction to the latest stage of the development which, to our view, went far in clarifying the problems which he had himself from the outset elicited so ingeniously. During the discussions, where the whole subject was reviewed by contributions from many sides and where also the arguments mentioned in the preceding pages were again presented, Einstein expressed, however, a deep concern over the extent to which a causal account in space and time was abandoned in quantum mechanics.Although Bohr seems to have missed Einstein's point completely, Werner Heisenberg at least came to explain it well. In his 1930 lectures at the University of Chicago, Heisenberg presented a critique of both particle and wave pictures, including a new example of nonlocality that Einstein had apparently developed since 1927. It includes Einstein's concern about "action-at-a-distance" that might violate his principle of relativity, and anticipates the Einstein-Podolsky-Rosen paradox. Heisenberg wrote:
In relation to these considerations, one other idealized experiment (due to Einstein) may be considered. We imagine a photon which is represented by a wave packet built up out of Maxwell waves. It will thus have a certain spatial extension and also a certain range of frequency. By reflection at a semi-transparent mirror, it is possible to decompose it into two parts, a reflected and a transmitted packet. There is then a definite probability for finding the photon either in one part or in the other part of the divided wave packet. After a sufficient time the two parts will be separated by any distance desired; now if an experiment yields the result that the photon is, say, in the reflected part of the packet, then the probability of finding the photon in the other part of the packet immediately becomes zero. The experiment at the position of the reflected packet thus exerts a kind of action (reduction of the wave packet) at the distant point occupied by the transmitted packet, and one sees that this action is propagated with a velocity greater than that of light. However, it is also obvious that this kind of action can never be utilized for the transmission of signals so that it is not in conflict with the postulates of the theory of relativity.Clearly the "kind of action (reduction of the wave packet)" described by Heisenberg is the same "mysterious" influence that the wave function has over the places that the particle will be found statistically in a large number of experiments. Apart from the statistical information in the wave function, quantum mechanics gives us only vague and uncertain information about any individual particle. This is the true source of Heisenberg's uncertainty principle. And it is the reason that Einstein correctly describes quantum mechanics as "incomplete. Quantum mechanics does not prove that the particle actually has no position at each instant and a path that conserves its momentum, spin, and other conserved properties. In EInstein's view of "objective reality," the particle has those properties, even if quantum mechanics prevents us from knowing them without a measurement that destroys their interference capabilities or "decoheres" them.
Some Other Animations of the Two-Slit Experiment with CommentsAnimation Source in Context One good thing in this animation is that it initially shows only particles firing at the slits. This is important historically because Isaac Newton thought that light was a stream of particles traveling along a light ray. He solved many problems in optics by tracing light rays through lenses.