Double-slit experiment

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The double-slit experiment or Young's experiment involves particle beams or coherent waves passing through two closely-spaced slits, after which in many circumstances they are found to interfere with each other.

In quantum mechanics the double-slit experiment demonstrates the inseparability of the wave and particle natures of light and other quantum particles (wave–particle duality). The setup used by Young, and by Newton, differs from the modern version; they passed a beam of light over a thin object such as a slip of card (in Young's case) or a hair (in Newton's case).[1][2] More recently a point light source illuminates a thin plate with two parallel slits, and the light passing through the slits strikes a screen behind them. The beams emerging from the two slits are coherent, in phase, as they are derived from the same source. The wave nature of light causes the coherent light waves passing through the two slits to interfere, creating a pattern of bright and dark bands on the screen. (However, at the screen the light is always found to be absorbed as though it were composed of discrete particles, photons.)[3][4]

Classical particles do not interfere with each other (they can collide, but that is quite different). If classical particles are fired in a straight line through one of a pair of slits they will all strike the screen in a pattern the same size and shape as the slit; if fired through the other slit the result will be similar. If both slits are opened simultaneously, the resulting pattern will simply be the sum of the two single-slit patterns. With light, although in many circumstances it behaves as particles (photons), it has been known for over two centuries that the pattern with two slits is not the sum of the separate patterns—this established the wave nature of light. The actual distribution of brightness can be explained by the alternately additive and subtractive interference of waves.[5]

Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern,[5] illustrating the complementarity principle: that light (and electrons, etc.) can behave as either particles or waves, but not both at the same time.[6][7][8] However, an experiment performed in 1987[9] produced results that demonstrated 'which-path' information could be obtained without destroying the possibility of interference. This showed the effect of measurements that disturbed the particles in transit to a lesser degree and thereby influenced the interference pattern only to a comparable extent.

The double slit experiment can also be performed (using a different apparatus) with particles of matter such as electrons with the same results, demonstrating that light and matter have both particle-like and wave-like properties (particle-wave duality). In the twentieth century small particles such as electrons and protons (diameter about 1.6 fm) were found to exhibit interference when passed through double slits. In 1999 objects large enough to see under a microscope, buckyball molecules (diameter about 0.7 nm, nearly half a million times that of a proton), were found to exhibit wave-like interference.[10][11]

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