Particle-Wave Duality
Wave-particle duality is a term used to describe the modeling of nature by both wave theory and particle theory. In essence, physical phenomena (e.g. electrons, photons, etc.) can exhibit varying degrees (sometimes determined by the experiment performed) of wave-like behavior and/or particle-like behavior (i.e., the phenomena can best be described by wave related equations or equations that treat phenomena as particles).
In its most usual form, particle-wave duality is a concept generally related to microscopic particles, more specifically their behavior as both particles and waves. This seems like a contradiction in terms because waves and particles are mutually exclusive. This seemingly contradictory statement arises because the concepts of particles and waves were developed from observations of macroscopic objects.
In 1905, German-American physicist Albert Einstein was concerned with explaining the photoelectric effect by extending German physicist Maxwell Planck's concept of energy quantization to electromagnetic radiation. Planck considered electromagnetic radiation to be a wave. During this time, Einstein proposed that in addition to having wavelike properties, light could also be thought to consist of particles, and as such, have properties consistent with particles. Although Einstein's theory of the photoelectric effect agreed with Hungarian physicist and Nobel prize winner Philipp Lenard's work, scientists were reluctant to accept it. Finally, in 1916, American physicist Robert Millikan made definitive tests to prove Einstein's theory. During this time there were also unsuccessful attempts to apply the Bohr theory to atoms with more than one electron, and to molecules. Eventually it became evident that there was a fundamental error in the Bohr theory. In 1923, French physicist Louis de Broglie advanced the idea that just as light shows both wave and particle properties, matter also has a particle-wave dual nature. De Broglie proposed that electrons could show particle-like behavior, but could also show wave-like behavior manifesting itself in the quantized energy levels of electrons in atoms and molecules. In 1927, American physicists Clinton Davisson and Lester Germer confirmed de Broglie's hypothesis by observing diffraction effects when an electron beam was reflected from a crystal of nickel. Diffraction effects were also observed when electrons passed through a thin sheet of metal. Since that time several particles have demonstrated diffraction effects showing that de Broglie's hypothesis applies to all particles and not just electrons.
The particle-like nature of an electron can be understood because it has mass at rest and repels other objects that are like-charged. The wave-like nature of the electron is more difficult to conceptualize although it is important when calculating the energy levels of electrons in atoms. Erwin Schrödinger formalized an equation relating the energy of a system to its wave properties and employs this idea. On the other hand, the wave-like nature of a photon is easy to visualize because of experiences on the macroscopic scale, but its particle-like nature is difficult to fully understand. A photon's rest mass is zero, but photons are never at rest. They are always moving at the speed of light. This apparent particle-wave duality of matter and radiation imposes certain limitations on the information obtainable about a microscopic system reflected in the Heisenberg uncertainty principle.
Under certain conditions an electron behaves like a particle, and under other conditions it behaves like a wave. Although it demonstrates these properties, an electron is neither a particle nor a wave, but something that is not fully understand or describable in terms of macroscopic concepts such as particles or waves.
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