Y O U N G ' S E X P E R I M E N T
"The nature of light is a subject of no material importance to the concerns of life or to the practice of the arts, but it is in many other respects extremely interesting."
THOMAS YOUNG
THOMAS YOUNG
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In the early 1800's Thomas Young, an English polymath, conducted an experiment with light.
He allowed light to pass through a slit in a barrier so it expanded out in wave fronts from that slit as a light source (under Huygens' Principle). That light, in turn, passed through pair of slits in another barrier (carefully placed the right distance from the original slit). Each slit, in turn, diffracted the light as if they were also individual sources of light. The light impacted an observation screen. When a single slit was open, it merely impacted the observation screen with greater intensity at the center and then faded as you moved away from the center. There are two possible results of this experiment: Particle interpretation: if light exists as particles, the intensity of both slits will be the sum of the intensity from the individual slits. Wave interpretation: if light exists as waves, the light waves will have interference under the principle of superposition, creating bands of light (constructive interference) and dark (destructive interference). When the experiment was conducted, the light waves did, indeed, show these interference patterns. A third image that you can view is a graph of the intensity in terms of position, which matches with the predictions from interference. At the time, this seemed to conclusively prove that light traveled in waves, causing a revitalization in Huygen's wave theory of light, which included an invisible medium, 'ether', through which the waves propagated. Several experiments throughout the 1800s, most notably the famed Michelson-Morley experiment, attempted to detect the ether or its effects directly but they all failed. And a century later Einstein's work in the photoelectric effect and relativity resulted in the 'ether' no longer being necessary to explain the behavior of light. Again a particle theory of light took dominance. Still, once the photon theory of light came about, saying the light moved only in discrete quanta, the question became how these results were possible. Over the years, physicists have taken this basic experiment and explored it in a number of ways. In the early 1900s, the question remained how light - which was now recognized to travel in particle-like "bundles" of quantized energy, called photons - could also exhibit the behavior of waves. Certainly, a bunch of water atoms (particles) when acting together form waves. Maybe this was something similar. It became possible to have a light source that was set up so that it emitted one photon at a time. This would be, literally, like hurling microscopic ball bearings through the slits. By setting up a screen that was sensitive enough to detect a single photon, you could determine whether there were or were not interference patterns in this case. One way to do this is to have a sensitive film set up and run the experiment over a period of time, then look at the film to see what the pattern of light on the screen is. Just such an experiment was performed and, in fact, it matched Young's version identically - alternating light and dark bands, seemingly resulting from wave interference. This result both confirms and bewilders the wave theory. In this case, photons are being emitted individually. There is literally no way for wave interference to take place, because each photon can only go through a single slit at a time. But the wave interference is observed. How is this possible? Well, the attempt to answer that question has spawned many intriguing interpretations of quantum physics, from the Copenhagen interpretation to the many-worlds interpretation. Now assume that you conduct the same experiment, with one change. You place a detector that can tell whether or not the photon passes through a given slit. If we know the photon passes through one slit, then it cannot pass through the other slit to interfere with itself. It turns out that when you add the detector, the bands disappear! You perform the exact same experiment, but only add a simple measurement at an earlier phase, and the result of the experiment changes drastically. Something about the act of measuring which slit is used removed the wave element completely. At this point, the photons acted exactly as we'd expect a particle to behave. The very uncertainty in position is related, somehow, to the manifestation of wave effects. Over the years, the experiment has been conducted in a number of different ways. In 1961, Claus Jonsson performed the experiment with electrons, and it conformed with Young's behavior, creating interference patterns on the observation screen. Jonsson's version of the experiment was voted "the most beautiful experiment" by Physics World readers in 2002. In 1974, technology became able to perform the experiment by releasing a single electron at a time. Again, the interference patterns showed up. But when a detector is placed at the slit, the interference once again disappears. The experiment was again performed in 1989 by a Japanese team that was able to use much more refined equipment. The experiment has been performed with photons, electrons, and atoms, and each time the same result becomes obvious - something about measuring the position of the particle at the slit removes the wave behavior. Many theories exist to explain why, but so far much of it is still conjecture. |
Thomas Young (1773-1829) was the eldest of ten children. At the age of fourteen he had learned Greek and Latin and was acquainted with French, Italian, Hebrew, German, Chaldean, Syriac, Samaritan, Arabic, Persian, Turkish and Amharic.
He began to study medicine in London in 1792, moved to Edinburgh in 1794, and a year later went to Göttingen, Lower Saxony, Germany where he obtained the degree of doctor of physics in 1796. In 1797 he entered Emmanuel College, Cambridge. In the same year he inherited the estate of his granduncle, Richard Brocklesby, and became financially independent. In 1799 he established himself as a physician at 48 Welbeck Street, London. Many of his first academic articles were published anonymously.
In 1801 Young was appointed professor of natural philosophy (mainly physics) at the Royal Institution. In 1802, he was appointed foreign secretary of the Royal Society, of which he had been elected a fellow in 1794. He resigned his professorship in 1803, fearing that its duties would interfere with his medical practice. His lectures were published in 1807 in the Course of Lectures on Natural Philosophy and contain a number of anticipations of later theories.
In 1811 Young became physician to St. George's Hospital, and in 1814 he served on a committee appointed to consider the dangers involved in the general introduction of gas into London. In 1816 he was secretary of a commission charged with ascertaining the precise length of the second's or seconds pendulum (the length of a pendulum whose period is exactly 2 seconds), and in 1818 he became secretary to the Board of Longitude and superintendent of the HM Nautical Almanac Office.
He is famous for, amongst many other achievements, having partly deciphered Egyptian hieroglyphics (specifically the Rosetta Stone) before Jean-François Champollion, eventually expanding on his work.
A few years before his death he became interested in life assurance, and in 1827 he was chosen one of the eight foreign associates of the French Academy of Sciences. In 1828, he was elected a foreign member of the Royal Swedish Academy of Sciences.
Thomas Young died in London on 10 May 1829. Later scholars and scientists have praised Young's work. His contemporary Sir John Herschel called him a 'truly original genius'. Albert Einstein praised him in the 1931 foreword to an edition of Newton's Opticks. Other admirers include physicist Lord Rayleigh and Nobel laureate Philip Anderson.
Thomas Young's name has been adopted as the name of the London-based Thomas Young Centre, an alliance of academic research groups engaged in the theory and simulation of materials.
He began to study medicine in London in 1792, moved to Edinburgh in 1794, and a year later went to Göttingen, Lower Saxony, Germany where he obtained the degree of doctor of physics in 1796. In 1797 he entered Emmanuel College, Cambridge. In the same year he inherited the estate of his granduncle, Richard Brocklesby, and became financially independent. In 1799 he established himself as a physician at 48 Welbeck Street, London. Many of his first academic articles were published anonymously.
In 1801 Young was appointed professor of natural philosophy (mainly physics) at the Royal Institution. In 1802, he was appointed foreign secretary of the Royal Society, of which he had been elected a fellow in 1794. He resigned his professorship in 1803, fearing that its duties would interfere with his medical practice. His lectures were published in 1807 in the Course of Lectures on Natural Philosophy and contain a number of anticipations of later theories.
In 1811 Young became physician to St. George's Hospital, and in 1814 he served on a committee appointed to consider the dangers involved in the general introduction of gas into London. In 1816 he was secretary of a commission charged with ascertaining the precise length of the second's or seconds pendulum (the length of a pendulum whose period is exactly 2 seconds), and in 1818 he became secretary to the Board of Longitude and superintendent of the HM Nautical Almanac Office.
He is famous for, amongst many other achievements, having partly deciphered Egyptian hieroglyphics (specifically the Rosetta Stone) before Jean-François Champollion, eventually expanding on his work.
A few years before his death he became interested in life assurance, and in 1827 he was chosen one of the eight foreign associates of the French Academy of Sciences. In 1828, he was elected a foreign member of the Royal Swedish Academy of Sciences.
Thomas Young died in London on 10 May 1829. Later scholars and scientists have praised Young's work. His contemporary Sir John Herschel called him a 'truly original genius'. Albert Einstein praised him in the 1931 foreword to an edition of Newton's Opticks. Other admirers include physicist Lord Rayleigh and Nobel laureate Philip Anderson.
Thomas Young's name has been adopted as the name of the London-based Thomas Young Centre, an alliance of academic research groups engaged in the theory and simulation of materials.