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In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior associated with both classical particles and classical waves.

This type of experiment was first described by Thomas Young in 1801 when making his case for the wave behavior of visible light. The experiment belongs to a general class of "double path" experiments, in which two diffracted waves reconverge, creating an interference pattern. In the basic version of this experiment, a coherent light source, such as a laser beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles. However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves); the interference pattern appears via the varying density of these particle hits on the screen. Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave). However, such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through. These results demonstrate the principle of wave–particle duality. The experiment can be done with entities much larger than electrons and photons, although it becomes more difficult as size increases. Among the larger objects which the double-slit experiment has been performed were molecules that each comprised 2000 atoms (whose total mass was 25,000 daltons) and nanoparticles composed of 5000-10000 sodium atoms. The double-slit experiment (and its variations) has become a classic for its clarity in expressing the central puzzles of quantum mechanics.

There is some question as to whether he ever actually performed a double-slit interference experiment. A low-intensity double-slit experiment was first performed by G. A slit interference experiment was not performed with anything other than light until 1961, when Claus Jönsson of the University of Tübingen performed it with coherent electron beams and multiple slits. In 1974, the Italian physicists Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi performed a related experiment using single electrons from a coherent source and a biprism beam splitter, showing the statistical nature of the buildup of the interference pattern, as predicted by quantum theory. In 2002, the single-electron version of the experiment was voted "the most beautiful experiment" by readers of Physics World.

In 2002, an electron field emission source was used to demonstrate the double-slit experiment. The experiment demonstrates wave interference using only quantum atoms and photons.

In the double-slit experiment, the two slits are illuminated by the quasi-monochromatic light of a single laser.

The double-slit experiment can illustrate the path integral formulation of quantum mechanics provided by Feynman. The differences in the cumulative action along the different paths (and thus the relative phases of the contributions) produces the interference pattern observed by the double-slit experiment.

Like the Schrödinger's cat thought experiment, the double-slit experiment is often used to highlight the differences and similarities between the various interpretations of quantum mechanics.

Niels Bohr interpreted quantum experiments like the double-slit experiment using the concept of complementarity. Certain pairs of classical properties will never be observed in a quantum system simultaneously: the interference pattern of waves in the double slit experiment will disappear if particles are detected at the slits.

In the Copenhagen interpretation, complementarity means a particular experiment can demonstrate particle behavior (passing through a definite slit) or wave behavior (interference), but not both at the same time.

So while a single particle will travel through one particular slit in the double-slit experiment, the so-called "pilot wave" that influences it will travel through both.

Quick Facts

• Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave).
• Among the larger objects which the double-slit experiment has been performed were molecules that each comprised 2000 atoms (whose total mass was 25,000 daltons) and nanoparticles composed of 5000-10000 sodium atoms.
• In the basic version of this experiment, a coherent light source, such as a laser beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate.
• The experiment belongs to a general class of "double path" experiments, in which two diffracted waves reconverge, creating an interference pattern.
• The double-slit experiment (and its variations) has become a classic for its clarity in expressing the central puzzles of quantum mechanics.

Source material: Wikipedia - "Double-slit experiment". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.