The quantum physics principle of wave-particle duality states that matter and light both show the behavior of waves and particles, depending on the circumstances of an experiment. The main importance of wave-particle duality is that all of the behaviors of light and matter can be explained by using a differential equation representing the wave function, usually in the form of the Schrodinger equation. Through the work of Max Planck, Albert Einstein, Louis de Broglie, Arthur Compton, Niels Bohr, Erwin Schrodinger, and many others, the present theory of science holds that all particles display wave-like properties and vice versa. To explain the structure and behavior of atoms, the assumption is thought to be necessary that particles possess wave-like properties. [Sources: 1, 3, 8]
Louis de Broglie speculated that particles might acquire the properties of waves. In 1924, Broglie developed his electron wave theory, the idea that matter might possess the properties of waves at the atomic scale, which was rooted in early theories by Albert Einstein. Based on the idea that light and all other electromagnetic radiation can be considered either of particle or wave nature, physicist Louis de Broglie proposed in 1923 that the same sort of duality should apply to matter. [Sources: 6, 11]
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The particle theory of light survived all challenges that were made to it, and any remaining naysayers were put to rest in 1873, when Maxwell’s electromagnetism theory was confirmed by experiments performed by Heaviside and Herz, including a prediction that electromagnetic radiation acted just like light. For the next 200 years, Isaac Newton was perhaps the best-known advocate for the particle theory of light, though he admitted that individual particles of light can interfere with one another as waves do when they collide, and because of Newton’s reputation, that view prevailed for nearly a century. [Sources: 5]
Thomas Youngs Double Slit Experiment produced apparent wavelike behavior and seemed to strongly favor a wave theory of light over Isaac Newton’s particle theory. Scientists were not in agreement on whether light acted as a particle or as a wave. In 1801, British scientist Thomas Young was convinced light was a wave, and Thomas Young designed his famous double slit experiment. By validating the theory that particles can behave like waves at the same time, physicists discovered that streams of electrons act the same as streams of light. The idea came that waves of light behave as particles — those particles are called photons. [Sources: 1, 3, 9, 11]
The light thus came to be called photons (quanta of light) because it has properties of a wave as well as of a particle. Since wave energy and light energy are not identical, light is said to be a particle that has the properties of a wave. Light is like waves because it can diffract, refract, reflect, and interfere. Observing light is one of the easiest ways to demonstrate the duality of a particle and a wave. [Sources: 0, 6, 11]
Wave-particle duality is used in electron microscopy, in which a smaller wavelength associated with an electron can be used to see objects far smaller than those seen using visible light. Diffraction and interference phenomena show the wavelike nature of light, but photoelectric effects, atom absorption, and emission also demonstrate that light has a particle-like nature. Double-Slit experiment In the 17th century, Newton demonstrated that similar to waves, light beams could also be diffracted and interfered with by shining a white light through a prism to pick up seven different colors, then combining them with a second prism to make white light. [Sources: 6, 7, 8]
Einstein later interpreted this result as an indication of the particles’ nature of light, as quantization of the light energy in discrete packets suggested each packet was a light particle or photon. Albert Einstein’s models explained the photoelectric effect, proposing that energy is delivered to electrons from particles of light, later called photons, that have an energy E=hf and strike electrons. [Sources: 2, 5]
The answer was to suggest that energy associated with its movement was not continuous, but came in fixed amounts, like if composed of large numbers of particles, such as our handfuls of marbles. Just as with light, the matter seems to display wave-like as well as particle-like properties in the appropriate circumstances. Throughout this whole scientific quest for a few centuries, the assumption, and the experimental support, was that everything was composed of particles at quite small levels, except for light (discussed later). [Sources: 1, 2, 3]
The experiment which falsified our conception of reality involved two particles bound together in a single wave. As optical instruments have grown increasingly sensitive over time, the evidence continues to pile up that light does travel in waves, not in streams of particles. The inability to truly pinpoint a medium for transmission was a major point of disagreement between scientists during the ensuing 200-year-long debate about whether light is made up of particles or waves. [Sources: 3, 5]
The observation of light having particle qualities came about with the emergence of quantum mechanics, with Planck’s 1900 solution to the Black-Body radiation problem. Photons would not be called photons until 1925, but even back in 1905, they represented a typical instance of the wave-particle duality. In 2007, the physicist Alain Aspect (1947-) described wave-particle duality as the main component of the first quantum revolution, i.e., the explosion of new knowledge about physics at the turn of the 20th century, which ultimately led to transistors, lasers, and other microelectronic devices that made possible our information-dependent society. [Sources: 2, 8, 10]
One of physics’ more counterintuitive concepts — the idea that quantum objects are complements, that they act as waves in some situations, but like particles in others — has just gained new, quantitative support. This ability to describe reality as waves is the basis for quantum mechanics. Indeed, the contemporary interpretation of the paradox of the double helix, which expands the Copenhagen interpretation originally proposed by Bohr and Heisenberg, depends on the wavelike character of a particle even more centrally. In other words, the probability that a particle is at some position is a wave, but the actual physical appearance of this article is not. [Sources: 1, 4, 8]
In the resultant picture, also called the de Broglie-Bohm theory or Bohmian mechanics, the wave-particle duality disappears, explaining wave behavior as scattering, with a wave-like appearance, since the motion of the particles is governed by the guidance equations or quantum potentials. The pilot wave model, first developed by Louis de Broglie, and later developed by David Bohm as hidden variables theory, suggests that no duality exists, but instead a system that displays both particle properties and waves properties at once, with particles being guided, deterministically, by the pilot wave (or by its quantum potential, which would guide them towards regions of constructive interference as opposed to regions of destructive interference. It is thus a limitation of humans that we cannot precisely describe the precise nature of light, but instead have to characterize it using terms like particle and wave, a situation that further forces us to accept that light has characteristics of both particle and wave, however difficult that situation may be to depict visually. [Sources: 8, 10]
Sources:
[0]: https://photonterrace.net/en/photon/duality/
[1]: https://www.thoughtco.com/wave-particle-duality-2699037
[2]: https://brilliant.org/wiki/wave-particle-duality/
[3]: https://theconversation.com/explainer-what-is-wave-particle-duality-7414
[4]: https://physicsworld.com/a/wave-particle-duality-quantified-for-the-first-time/
[5]: https://www.cantorsparadise.com/wave-particle-duality-d7b7243dd3e3
[6]: https://byjus.com/physics/wave-particle-duality/
[8]: https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
[9]: https://www.aps.org/programs/outreach/physicsquest/wave-particle.cfm
[10]: https://www.encyclopedia.com/science/science-magazines/physics-wave-particle-duality
[11]: https://interestingengineering.com/louis-broglie-and-the-idea-of-wave-particle-duality
