Double-slit experiment (1807)
English physicist Thomas Young's circa 1804 double-slit experiment with light and the interference fringes produced, as diagrammed in his 1807 Natural Philosophy. [1]
In experiments, the double-slit experiment was a circa 1804 experiment, conducted by English physician-physicist Thomas Young, in which he passed a portion of light through two small apertures, and found that they produced light and dark stripes or fringes by their interference, proceeding in the form of hyperbolas, the middle ones becoming more and more dilated, as in section A, as shown adjacent. [1]

Since ancient times, following the invention of glass, people had noticed that whenever broken glass was present the appearance of colors would appear where two refracting surfaces formed a sharp edge.

English natural philosopher Francis Bacon (1561-1626) contended that light was a disturbance in an invisible medium which could be detected by the eye; subsequently, color was caused by objects "staining" the light as it passed.

French natural philosopher Rene Descartes (1596-1650) famously looked through a prism in sunlight and seen a circle of colors. Descartes thought of light as a pressure in the ether and color an effect of the rotation of the ether particles.

English polymath Robert Hooke (1635-1703) objected to the Descartes's view, arguing instead that light was a type of pulse, a vibration of ether, or a wave, like sound.

English physicist Isaac Newton (1643-1727), notably viewing Hooke and Descartes as intellectual giants, objected to the theories of both. He commented, e.g., in the margins of his personal copy of Hooke's 1665 Micrographia:

"Though Descartes may be mistaken so is Mr Hook."

Newton double slit experiment
Sketch of English physicist Isaac Newton's circa 1666 double prism, double perforation experiment, from his notebook, in which he showed that an isolated beam of colored light shined through a prism did not create new colors, contrary to the prevailing theory that prisms somehow added color to white light. [8]
Newton's general objection against the wave ether theory was that light, unlike sound, does not turn corners. He queried to himself: "why then may not light not deflect from straight lines as well as sounds, etc." In its place, in his personal notes during this period, Newton wrote of light as globules, traveling at finite speed, which impinged on the eye. [8]

In circa 1666, English physicist Isaac Newton performed his famous double prism, double perforation experiment, as shown adjacent, in which pure white light (sunlight) enters a dark room through a whole in the window (right), then passes through a first prism (middle), which splits the light into its various colors; each individual colored beam is then sent through a second perforated hole, and into a second prism, after which it is found that the color is pure, meaning that it cannot be split anymore. [8]

Newton's revolutionary theory of light was made public in the 1672 readings of the Philosophical Transactions, which caused such an embroiled turmoil, Newton being embroiled greatly with Hooke, that Newton resigned from the Royal society.

In 1678, Dutch physicist-astronomer Christiaan Huygens had developed an undulatory theory of light, in which light was thought to be a wave spreading out in all directions from a light source, transmitted via a medium known as ether.

This view, however, was in conflict with Newton's corpuscular theory of light, in which light was imaged to be a stream of minute particles or corpuscles emitted by a light source. The final word on Newton's theory is found in his Query 17 to his Opticks (c.1718).

In 1799, English physician-physicist Thomas Young wrote his first paper, with a section on "The Analogy between Light and Sound", in which he addressed the issue of the uniform velocity of light not being consistent with the corpuscular theory, and went on to explain, in objection to the opinion of English mathematician Robert Smith, that waves of sound can interfere with each other, and thus increase and decrease their mutual effect. In this paper, Young had the idea in mind that the analogous phenomenon might explain light.

In May 1801, in a time when Newton's theory was the dominate view, Young was said to have gleaned his first insight into a newer version of the wave theory of light, while reflecting on Newton's experiments with light, particularly on the colors in Newton's rings.

interferencewaves cancelling out
Depiction of Thomas Young's famous circa 1804 double slit experiment. [2] Depiction of two waves producing constructive interferences (left) and destructive interferences (right).
In July 1801, in letter to Nicholson's Journal, Young stated: “light is probably the undulation of an elastic medium.” In a followup paper that year, Young stated his new theory:

“I maintain that when two portions of light are mixed, elevations or depressions in the undulations will result, similar to the way in which waves of colliding water combine in their effects, and I call this the general law of the interference of light.”

In 1802, to elaborate on his theory, by analogy with water waves, Young built a "ripple tank" (below), with a glass bottom and board for creating ripples on one end, arranged such that if light was allowed to illuminate the bottom of the tank, the interference of the ripple patterns could be observed on a board placed above the tank at an angle.
Ripple tankRipple tank
Ripple tank ripples
Ripple tank (top) and interference of two water waves (below) in the ripple tank, as diagrammed in Young's 1807 Natural Philosophy. [1]

In November 1803, Young performed an experiment showing a single beam of light detracted by a narrow card, and the color fringe effects it produced. This experiment was soon attacked by English statesman Henry Brougham, a proponent of Newton's view, who called Young's law of interference "absurdity and one of the most incomprehensible suppositions that we remember to have met with in the history of human hypothesis". This attack was said to have been the main goad or drive behind Young later performing the the now-famous double-slit experiment, so as to prove his theory conclusively.

Single particle interference
The puzzle to the double slit experiment only became more complicated when in the 1980s an experiment was conducted in which single electrons were fired, one by one, through the equivalent of Young’s double-slit apparatus, only to still show, curiously, interference patterns, as if the screen “remembered” the previous spot of impact. [2] Said another way, the previously-fired electron was found to be interfering somehow with the electron in movement through the slits?

This same phenomenon was soon observed for larger more massive particles, such as atoms, dimmers, small van der Walls clusters, and neutrons.

Many, such as American physicist Richard Feynman, have called this the most puzzling experimental finding in all of modern science, one that nobody understands.

In 1999, Viennese researchers, supposedly led by physicist Anton Zeilinger, reported interference in in the movement of Buckyball molecules, a 60 atom molecule, through the double slits, the largest objects to-date ever to show quantum phenomena. [7] In short, by passing Buckyball molecules, one at a time, through a double slit experimental apparatus, Zeilinger showed that single molecules possess wave particle duality. [5]

The implications of the finding that large molecules, such C60, possess wave particle duality, are that single human molecules also possess wave particle duality.

Human chemistry
A loose outline of human particle-human wave behavior was first documented in circa 1885 by Ernst Mach and his noting of what he called "turning tendencies" in troops lost in a snow storm. [3]

A modern approach to the issue of wave particle duality behavior is the the concept of the human molecular orbital, as explained in human molecular orbital theory, in which a person as a bound state point-like molecule (or human particle) moves daily in probabilistic activity orbitals. [4]

The following are related quotes:

“It is daydreaming. What counts is what you see on the screen. Do not ask if the particle did follow some continuous path. We do not know about that. Forget about it.?”
Martinus Veltman (2003), Facts and Mysteries in Elementary Particle Physics [6]

1. (a) Young, Thomas. (1807). A Course of Lectures on Natural Philosophy and the Mechanical Arts: Volume One. Publisher.
(b) Young, Thomas. (1807). A Course of Lectures on Natural Philosophy and the Mechanical Arts: Volume Two. Publisher.
(c) Robinson, Andrew. (2006). The Last Man Who Knew Everything: Thomas Young, the Anonymous Genius who Proved Newton Wrong and Deciphered the Rosetta Stone, among other Surprising Feats (pg. 128). Plume Books.
2. Gribben, John. (2002). Quantum Physics: a Beginner’s Guide to the Subatomic World (pgs. 11, 28-29). DK Publishing.
3. McManus, C. (2002). Right Hand, Left Hand – the Origin of Asymmetry in Brains, Bodies, Atoms, and Cultures. Cambridge: Harvard University Press.
4. Thims, Libb. (2007). Human Chemistry (Volume One) (section: Human molecular spin, pgs. 209-11; turning tendencies diagram, pg. 210; human molecular orbtal, pg. 265; orbital transition state, pgs. 268-69). (preview), (Google books). Morrisville, NC: LuLu.
5. Arndt, Markus; O. Nairz, J. Voss-Andreae, C. Keller, G. van der Zouw, and Zeilinger, A. (1999). "Wave–particle duality of C60" (abs). Nature, 401(6754): 680–682.
6. Veltman, Martinus. (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific Press.
7. (a) Zeilinger, Anton. (2006). "I believe quantum physics requires us to abandon the distinction between information and reality", in: What We Believe, But Cannot Prove (pgs. 223-24). Harper-Perennial.
(b) Anton Zeilinger – Wikipedia.
8. Gleick, James. (2003). Isaac Newton (pg. 80). Vintage Books.

External links
‚óŹ Double-slit experiment – Wikipedia.

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