Double-slit Experiment - Overview

Overview

If light consisted strictly of ordinary or classical particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit. However, when this "single-slit experiment" is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the angle of spread. The top image in the image on the right show the central portion of the pattern formed when a red laser illumiminates a slit.

Similarly, if light consisted strictly of classical particles and we illuminated two parallel slits, the expected pattern on the screen would simply be the sum of the two single-slit patterns. In actuality, however, the pattern changes to one with a series of light and dark bands (See the bottom photograph to the right.) When Thomas Young (1773-1829) first demonstrated this phenomenon, it indicated that light consists of waves, as the distribution of brightness can be explained by the alternately additive and subtractive interference of wavefronts. Young's experiment, performed in the early 1800s, played a vital part in the acceptance of the wave theory of light, vanquishing the corpuscular theory of light proposed by Isaac Newton, which had been the accepted model of light propagation in the 17th and 18th centuries. However, the later discovery of the photoelectric effect demonstrated that under different circumstances, light can behave as if it is composed of discrete particles. These seemingly contradictory discoveries made it necessary to go beyond classical physics and take the quantum nature of light into account.

The double-slit experiment (and its variations), conducted with individual particles, has become a classic thought experiment for its clarity in expressing the central puzzles of quantum mechanics. Because it demonstrates the fundamental limitation of the observer to predict experimental results, Richard Feynman called it "a phenomenon which is impossible ... to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery .", and he was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment. Časlav Brukner and Anton Zeilinger have succinctly expressed this limitation as follows:

he observer can decide whether or not to put detectors into the interfering path. That way, by deciding whether or not to determine the path through the two-slit experiment, he/she can decide which property can become reality. If he/she chooses not to put the detectors there, then the interference pattern will become reality; if he/she does put the detectors there, then the beam path will become reality. Yet, most importantly, the observer has no influence on the specific element of the world that becomes reality. Specifically, if he/she chooses to determine the path, then he/she has no influence whatsoever over which of the two paths, the left one or the right one, nature will tell him/her is the one in which the particle is found. Likewise, if he/she chooses to observe the interference pattern, then he/she has no influence whatsoever over where in the observation plane he/she will observe a specific particle. Both outcomes are completely random.

The Englert–Greenberger duality relation provides a detailed treatment of the mathematics of double-slit interference in the context of quantum mechanics.

A low-intensity double-slit experiment was first performed by G. Taylor in 1909, by reducing the level of incident light until photon emission/absorption events were mostly nonoverlapping. A double-slit experiment was not performed with anything other than light until 1961, when Clauss Jönsson of the University of Tübingen performed it with electrons. In 2002, Jönsson's double-slit experiment was voted "the most beautiful experiment" by readers of Physics World.

In 1999, objects large enough to be seen under an electron microscope — buckyball molecules (diameter about 0.7 nm, nearly half a million times larger than a proton) — were found to exhibit wave-like interference.

The appearance of interference built up from individual photons could seemingly be explained by assuming that a single photon has its own associated wavefront that passes through both slits, and that the single photon will show up on the detector screen according to the net probability values resulting from the co-incidence of the two probability waves coming by way of the two slits. However, more complicated systems that involve two or more particles in superposition are not amenable to such a simple, classically intuitive explanation.