Wave-particle duality Guide, Meaning , Facts, Information and Description
In physics, wave-particle duality holds that light and matter simultaneously exhibit properties of waves and of particles (or photons). This concept is a consequence of quantum mechanics.
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2 Einstein and photons 3 De Broglie 4 Doubt over wave-particle duality 5 See also 6 External links |
In the early 1800s, the double-slit experiments by Young and Fresnel provided evidence for Huygens' theories: these experiments showed that when light is sent through a grid, a characteristic interference pattern is observed, very similar to the pattern resulting from the interference of water waves; the wavelength of light can be computed from such patterns. Maxwell, during the late-1800s, explained light as the propagation of electromagnetic waves with the Maxwell equations. These equations were verified by experiment, and Huygens' view became widely accepted.
In 1905, Einstein reconciled Huygens' view with that of Newton; he explained the photoelectric effect (an effect in which light did not seem to act as a wave) by postulating the existence of photons, quanta of energy with particulate qualities. Einstein postulated that light's frequency, ν, is related to the energy, E, of its photons:
In 1924, De Broglie claimed that all matter has a wave-like nature; he related wavelength, λ, and momentum, p:
De Broglie's formula was confirmed three years later by guiding a beam of electrons (which have rest mass) through a crystalline grid and observing the predicted interference patterns.
Similar experiments have since been conducted with neutrons and protons. Authors of similar recent experiments with atoms and molecules claim that these larger particles also act like waves. This is still a contoversial subject because these experimenters have assumed arguments of wave-particle duality and have assumed the validity of deBroglie's equation in their argument.
The Planck constant h is extremely small and that explains why we don't perceive a wave-like quality of everyday objects:
their wavelengths are exceedingly small. The fact that matter can have very short wavelengths is exploited in electron microscopy.
In quantum mechanics, the wave-particle duality is explained as follows: every system and particle is described by state functions which encode the probability distributions of all measurable variables. The position of the particle is one such variable. Before an observation is made the position of the particle is described in terms of probability waves which can interfere with each other.
An intriguingly simple experiment, the double-slit experiment, summarizes the duality: aim an electron gun at a screen with two slits and record their positions of detection at a detector behind the screen. You will observe an interference pattern just like the one produced by diffraction of a light or water wave at two slits. This pattern will even appear if you slow down the electron source so that only one electron's worth of charge per second comes through. "Classically speaking", every electron is a point particle and must either travel through the first or through the second slit. So we should be able to produce the same interference pattern if we ran the experiment twice as long, closing slit number one for the first half, then closing slit number two for the second half. But the same pattern does not emerge. Furthermore, if we build detectors around the slits in order to determine which path a particular electron takes, this very measurement destroys the interference pattern as well. But this is a classical explanation and something much more profound is taking place.
The interference pattern can be explained as a result of the charge wave being diffracted by both slits and interfering with itself. In quantum mechanics, the state function is a complex valued function of space and time. The square of the magnitude of this function describes the probability of finding the electron at a given location at a given time. Interference is due to the fact that the square of the magnitude of the sum of two complex numbers may be different from the sum of the squares of their magnitudes.
The experiment also illustrates an interesting feature of quantum mechanics. Until an observation is made the position of a particle is described in terms of probability waves, but after the particle is observed, it is described as a fixed value. How to conceptualize the process of measurement is one of the great unresolved questions of quantum mechanics. The standard interpretation is the Copenhagen interpretation which leads to interesting thought experiments such as Schrödinger's cat. Due to this confusion, the majority of theorists (including Stephen Hawking and Murray Gell-Mann) believe the many-worlds interpretation is true.
However, this experiment does not invalidate the Heisenberg equations on which the concept of complementarity was based. So similar experiments with other particles would almost certainly be successful. It seems that complementarity was an overly simple conceptual attempt to describe the mathematics, and will need to be revised. Peer review is still required to confirm this.
This is an Article on Wave-particle duality. Page Contains Information, Facts Details or Explanation Guide About Wave-particle duality Fresnel, Maxwell, and Young
Einstein and photons
where h is Planck's constant (6.626 x 10-34 J seconds).De Broglie
This is a generalization of Einstein's equation above since the momentum of a photon is given by p = E / c
where c is the speed of light in vacuum, and λ = c / ν.Doubt over wave-particle duality
There is currently (August 2004) some doubt over complementarity, the current description of Wave-particle duality, due to Shahriar Afshar's 2004 contradictory result using a variation on the double-slit experiment, which also appears to invalidate the Copenhagen interpretation. (See also: Geoff Haselhurst Biography) So far this revised experiment has only been carried out with photons, not electrons, neutrons or protons, and the results have yet to be peer-reviewed. See also
External links