The Double Slit Experiment
Hundreds of years before the double slit experiment, thinkers and scientists argued about the nature of light. In the 17th century, French scientist and astronomer Pierre Gassendi and Sir Isaac Newton argued that light propagates through space as a steady stream of particles. In the early 1760s Leonhard Euler maintained that light was a wave propagating through aether. Euler argued that if the sun were constantly emitting particles it would be rapidly decreasing in size.
The theory behind Thomas Young's experiment is straightforward. Unlike particles, waves obey the mathematical principle of superposition and also unlike particles, waves have the potential to interfere with one another. Young passed monochromatic light through two slits in an otherwise opaque partition onto a dark screen. When light was allowed to pass through only one slit at a time the light was distributed on the screen evenly. When light was allowed to pass through both slits at the same time he observed an interference pattern of light and dark regions of the screen. The distance between the light and dark bands of interference is given by the formula: x = (Lnλ)/d, where x is the distance between the bands, L is the distance from the slits in the partition to the screen, n is the order of maximum observed, λ is the wavelength of light, and d is the separation between the slits.
Young's experiment and the conclusions he drew were met with controversy. British statesman Henry Brougham responded to Young's paper in 1803 calling it "destitute of every species of merit.” Young's experiment strongly supported the wave theory which became the prevalent theory until the 20th century when experimental evidence implied that light also exhibits particle-like properties.
In 1924 Prince Louis de Broglie wrote in his PhD about the wave-particle duality of all matter in the universe. The wavelength associated with a particle is given by the de Broglie relation: λ =h/p. Nobel prize laureate Max von Laue famously responded to de Broglie's thesis: “If that turns out to be true, I'll quit physics.” Shortly after de Broglie's thesis, two variations on the double slit experiment were performed, the results of which have had a profound impact on the modern conception of matter, light, and physics as a whole. In 1927, C. J. Davidson and L. H. Germer reproduced Young's double slit experiment except they passed through the partition electrons boiled off of an electric coil. The points on the screen where the electrons struck were made visible by coating the screen with a luminescent phosphorous. These experiments confirmed de Broglie's hypotheses that even electrons, when passed through two slits, can be observed to produce an interference pattern just like the one Young observed with light. This experiment proved that electrons and by extension matter in general, exhibit wave-like properties. In 1974, a team of researches led by Pier Giorgio Merli at the University of Milan repeated this experiment but did so by sending one electron through the partition at a time. They found that as the electrons accumulated on the screen they fell into the exact interference pattern observed when many electrons were passed at once. This implied that electrons are not constrained to passing through one slit or the other; instead, the electrons are able to pass through both slits simultaneously and interfere with themselves! This experiment would later be reproduced with atoms and even some molecules.
The double slit experiment presents the physicist with an unsettling paradox. Any given electron fired through the partition can assume any number of possible paths before arriving at the screen on the other side. At the end of its trajectory, the electron arrives on the screen as a localized point marked by a spot of illumination in a phosphorous coating. The localized mark on the screen implies that the electron followed one particular path from its source to the screen, not multiple paths. However, the build up of an interference pattern over time implies that as any given electron passes through the partition it does so as a wave propagating through multiple paths. The causal impact of the electron on the screen is manifest as a particle; however, the unobserved trajectory between the electron source and the screen is manifest as a wave. It is as though the electron moves through space as an invisible wave and reverts back to particle when it exchanges energy information with the phosphorous screen.
When detectors are placed at the mouth of each of the slits, the experimenter can determine through which of the two slits the electron passed. However, this knowledge comes at the expense of the interference pattern on the screen. Once the experimenter has defined the particle-like trajectory of the electron, he looses the evidence that it was ever a wave.
This loss of the interference pattern is explained by Heisenberg's famous uncertainty principle: ΔxΔp=ħ/2. This principle states that a more precise determination in the position of a particle necessitates a disturbance in the particle’s velocity. The disturbance induced by the detectors arranged to locate the electron as it passes through the partition is significant enough to erase the interference pattern on the screen. The mathematical description of the quantum world takes into account the existence of an observer in a way that leaves the experimenter with the uncanny feeling that the electrons “know” that they are being observed and are altering their nature accordingly.
Does matter come in waves or particles? How does the physicist negotiate between the phenomenal world of objective observation and the abstract world of mathematics? What is the significance of the observer in the physical world? What is the strength and scope of the ancient “law of the excluded middle?” Is the physical world a deterministic system or a stochastic one? What is the medium through which a quantum wave propagates?
The theory behind Thomas Young's experiment is straightforward. Unlike particles, waves obey the mathematical principle of superposition and also unlike particles, waves have the potential to interfere with one another. Young passed monochromatic light through two slits in an otherwise opaque partition onto a dark screen. When light was allowed to pass through only one slit at a time the light was distributed on the screen evenly. When light was allowed to pass through both slits at the same time he observed an interference pattern of light and dark regions of the screen. The distance between the light and dark bands of interference is given by the formula: x = (Lnλ)/d, where x is the distance between the bands, L is the distance from the slits in the partition to the screen, n is the order of maximum observed, λ is the wavelength of light, and d is the separation between the slits.
Young's experiment and the conclusions he drew were met with controversy. British statesman Henry Brougham responded to Young's paper in 1803 calling it "destitute of every species of merit.” Young's experiment strongly supported the wave theory which became the prevalent theory until the 20th century when experimental evidence implied that light also exhibits particle-like properties.
In 1924 Prince Louis de Broglie wrote in his PhD about the wave-particle duality of all matter in the universe. The wavelength associated with a particle is given by the de Broglie relation: λ =h/p. Nobel prize laureate Max von Laue famously responded to de Broglie's thesis: “If that turns out to be true, I'll quit physics.” Shortly after de Broglie's thesis, two variations on the double slit experiment were performed, the results of which have had a profound impact on the modern conception of matter, light, and physics as a whole. In 1927, C. J. Davidson and L. H. Germer reproduced Young's double slit experiment except they passed through the partition electrons boiled off of an electric coil. The points on the screen where the electrons struck were made visible by coating the screen with a luminescent phosphorous. These experiments confirmed de Broglie's hypotheses that even electrons, when passed through two slits, can be observed to produce an interference pattern just like the one Young observed with light. This experiment proved that electrons and by extension matter in general, exhibit wave-like properties. In 1974, a team of researches led by Pier Giorgio Merli at the University of Milan repeated this experiment but did so by sending one electron through the partition at a time. They found that as the electrons accumulated on the screen they fell into the exact interference pattern observed when many electrons were passed at once. This implied that electrons are not constrained to passing through one slit or the other; instead, the electrons are able to pass through both slits simultaneously and interfere with themselves! This experiment would later be reproduced with atoms and even some molecules.
The double slit experiment presents the physicist with an unsettling paradox. Any given electron fired through the partition can assume any number of possible paths before arriving at the screen on the other side. At the end of its trajectory, the electron arrives on the screen as a localized point marked by a spot of illumination in a phosphorous coating. The localized mark on the screen implies that the electron followed one particular path from its source to the screen, not multiple paths. However, the build up of an interference pattern over time implies that as any given electron passes through the partition it does so as a wave propagating through multiple paths. The causal impact of the electron on the screen is manifest as a particle; however, the unobserved trajectory between the electron source and the screen is manifest as a wave. It is as though the electron moves through space as an invisible wave and reverts back to particle when it exchanges energy information with the phosphorous screen.
When detectors are placed at the mouth of each of the slits, the experimenter can determine through which of the two slits the electron passed. However, this knowledge comes at the expense of the interference pattern on the screen. Once the experimenter has defined the particle-like trajectory of the electron, he looses the evidence that it was ever a wave.
This loss of the interference pattern is explained by Heisenberg's famous uncertainty principle: ΔxΔp=ħ/2. This principle states that a more precise determination in the position of a particle necessitates a disturbance in the particle’s velocity. The disturbance induced by the detectors arranged to locate the electron as it passes through the partition is significant enough to erase the interference pattern on the screen. The mathematical description of the quantum world takes into account the existence of an observer in a way that leaves the experimenter with the uncanny feeling that the electrons “know” that they are being observed and are altering their nature accordingly.
Does matter come in waves or particles? How does the physicist negotiate between the phenomenal world of objective observation and the abstract world of mathematics? What is the significance of the observer in the physical world? What is the strength and scope of the ancient “law of the excluded middle?” Is the physical world a deterministic system or a stochastic one? What is the medium through which a quantum wave propagates?
Others apart sat on a hill retir'd,
In thoughts more elevate, and reason'd high
Of providence, foreknowledge, will, and fate,
Fix'd fate, free-will, foreknowledge absolute;
And found no end, in wand'ring mazes lost.
John Milton, Paradise Lost Book II
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