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Despite its counter-intuitive nature, the case for wave particle duality appears, at face value, strong and convincing: The concept was born on twin pillars of hypothesis and experiment.
Through history, the model went from corpuscular, to wave then finally, a composite: duality.
Some important steps were:
Early on, Isaac Newton found that it was possible to model many properties of light with ray optics and proposed that light be of a corpuscular nature. In the 19th century, this idea was overturned because of the need to account for the observation of diffraction and interference patterns. From there, light was shown to be a form of electromagnetic radiation and could be described with the electromagnetic wave theory developed by James Clark Maxwell.
At that point, the notion of light as a wave phenomenon looked to a closed case. However, new discoveries demanded that a corpuscular approach be revisited.Then there was a major development in 1923, when Louis De Broglie took the brilliant step of postulating that the relationships that held for light could be applied to material particles. Working from relativistic relationships regarding wavelength and momentum, he predicted a universal relationship (for photons and matter) between a particle's momentum and its wavelength (L) being:
Max Planck's model for thermal radiation required radiation to be emitted and absorbed in discrete quantities related to a constant "h" now called Planck's constant. Early this century in 1905, Einstein postulated that experimental observation of photo-electricity provided convincing evidence that light interacted with matter as though it consisted of distinct particles. His findings showed that light exchanges energy (e) with matter in discrete amounts related to the frequency (f) of the light by the formula e = hf where h, again, is Planck's constant. Observations of atomic spectra showed atoms exchanged only particular frequencies of light. In 1915, the Danish physicist Neils Bohr developed a simple model of atomic states. The model involved electrons orbiting the nucleus in stable orbits that only occurred at discrete energies. He showed that the allowed orbits were characterized by an orbital angular momentum (L) that satisfied the relationship L=nh/2pi . Where n is an integer and h, yet again, is Planck's constant. The experimental confirmation followed in 1927, when Davisson and Germer produced what is taken to be very clear cut experimental confirmation by detecting the diffraction of electrons by a nickel crystal and then determining that the required wavelength matched De Broglie's prediction. Subsequently, many more experiments with diverse particles (including whole atoms) confirm the relationship predicted by De Broglie. L = h/P Where h is Planck's constant and P is the momentum of the particle.
Finally, Erwin Schrödinger, starting from De Broglie's relationships, developed his famous wave mechanics that described the states of motion that particles adopt with respect to one another.
In an effort to reconcile the obvious difficulty in reconciling wave and particle properties, the Danish Physicist, Neils Bohr proposed the Copenhagen interpretation.
Bohr's interpretation employs the notion of "complemetarity" in which wave properties and particle properties were exclusive, in that one cannot simultaneously observe both properties. To accomodate this duality, the model invokes ad hoc requirements in that the act of observation "determines" reality and that the "wave function" "collapses" when the particle is observed.
If you take the model literally, then a particle that appears to be localised, in that it travels in straight lines and impacts in a very small area (less than a millionth of a meter for a photon or an electron), also has a distributed wave-like presence that acts in domain that extends over many meters. Nevertheless, the concept is incorporated most current quantum interpretations because there is a clear set of mathematical relationships and the observations match those predictions.
As far as I can tell, this way of relating to these dual types of behavior underpins all orthodox, and most unorthodox, interpretations of quantum theory in this one respect:
- Particles, both of matter as well as those of light (photons), always "show up" in measurements as discrete, corpuscular entities. Phenomena such as the photoelectric effect show that light always delivers discrete localised packets of energy as though it is made up of a stream of distinct particles.
- Particles also clearly exhibit patterns of interaction and scattering that look to be identical to the patterns that waves produce when propagating and interacting. Beams of photons or particles (eg. electrons) produce scattering patterns that are identical to the interference and diffraction patterns made by travelling waves.
All contain the notion that particles possess wave-like properties, or are associated with a sort of "wave function", that propagates with the particle (light or matter) and plays an active part in their interactions.This assumption runs deep, even within statistical interpretations of quantum theory (along the lines of Max Born's where explicit references to "waves" are avoided) there is still an underlying presumption that wave processes such as interference and diffraction are the cause of the observed behaviors.
In the experiment, a parallel beam of photons/particles is used to illuminate a pair of narrow, parallel, slits cut in an absorbing screen. The particle/photon beam is prepared such that all the particles/photons have the same momentum. The emerging beam of particles/photons is made visible by placing a suitable detecting screen downstream from the slits. For light, a typical arrangement would involve shining a laser beam at two hairline slits with the image on the detecting screen being captured on photographic film.
When one of the slits is obscured then the fine pattern of bars disappears and the broader background of the pattern remains as shown below. The form of this "single slit" pattern is also exactly like the pattern made by a wave process. The process is called diffraction, the process by which a short segment of wave breaks up and spreads as it leaves a single slit. Note that the twin slit pattern above is superimposed on this "single slit" pattern when both slits are opened.
Furthermore, the "interference" patterns are not caused by one photon going through one slit "interfering" with another going through the other slit. The interference and diffraction patterns are a result of effects that apply to individual particles/photons. When the beam is dimmed to the point that photons traverse the slits one at a time, the pattern remains unaltered.
If a wave process is responsible then it would be as though each photon "interferes and diffracts" by, and of, itself. However, I propose that there is an alternative, in which particles are scattered by quantum process that occurs as the particles interact with, and depart, the slit screen.
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