The Double Slit Experiment explained (?)

Double-slit diffraction is a corner stone of quantum mechanics. It illustrates key features of quantum mechanics: interference and the particle-wave duality of matter. Richard Feynman called the double slit experiment “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 [of quantum mechanics],”

double slit interference

Double-slit diffraction is a corner stone of quantum mechanics. It illustrates key features of quantum mechanics: interference and the particle-wave duality of matter. Here is Professor Jim Al-Khalili explaining the experiment that reveals the central mystery of quantum mechanics, ending his talk with the comment “If you can explain this using common sense and logic, do let me know, because there is a Nobel Prize for you.”

A Potted History

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 wave-fronts. 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.

In 1965, Richard Feynman described a thought-experiment in which individual electrons were fired on a double slit. He predicted that an interference pattern would still become visible, but also warned that the experiment would be impossible to perform because of its small scale.

Feynman originally outlined his thought experiment in volume three of his famous series, The Feynman Lectures on Physics, as a way of illustrating wave–particle duality in quantum mechanics. In the book, he invites the reader to imagine firing individual electrons through two slits and then marking the position where each electron strikes a screen behind the slits.

After many electrons have passed through the slits, the marks on the screen will comprise a diffraction pattern, illustrating the wave-like behaviour of each electron. But if one were to cover up one of the slits so that each electron could only pass through the other slit, the diffraction pattern would not appear, showing that each electron does indeed travel through both slits. Here is Richard Feynman’s lecture on quantum Probability and Uncertainty in the “Messenger” series of lectures on Cornell University 1964, where he elucidates the Double Slit Experiment.

When the third volume of The Feynman Lectures on Physics was published in 1965, physicists already knew that firing a beam of electrons at a double slit results in a diffraction pattern because the experiment had been performed in 1961 by Claus Jönsson at the University of Tübingen in Germany. But while Jönsson’s work clearly illustrated that a beam of electrons can behave as a wave, it did not establish a crucial point of Feynman’s experiment – that an individual electron itself can behave like a wave.

Single-electron double-slit diffraction was first demonstrated in 1974 by Giulio Pozzi and colleagues at the University of Bologna in Italy, who passed single electrons through a biprism – an electron optical device that serves the same function as a double slit – and observed the build-up of a diffraction pattern. A similar experiment was also carried out in 1989 by Akira Tonomura and colleagues at Hitachi’s research lab in Japan.

The first single-electron experiment to use an actual double slit was reported in 2008 by Pozzi and colleagues. The team also performed another experiment in 2012, in which the arrivals of individual electrons from a double slit were recorded one at a time.

Coming right up-to-date, Herman Batelaan of the University of Nebraska-Lincoln, together with colleagues there and at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, now say that they have created a double-slit experiment that follows the precise methodology of Feynman’s thought experiment.

The experiment is described in the New Journal of Physics.

Richard Feynman called the double slit experiment “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 [of quantum mechanics],”

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