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A paradox is a statement which has either an apparent or a real self-inconsistency (such as “A and non-A”), thus contradicting the logical principle of excluded third (`tertium non datur')
In the early stages of quantum theory development there were at least two suggested paradoxes, one reported by Schrödinger and the other reported by Einstein-Podolsky-Rosen. These quantum `paradoxes' are as follows:
1. The `Schrödinger cat paradox' in a thought (or `gedanken') experiment that supposedly shows that a “cat is neither alive nor dead” but in a superposition of the so-called `live' and `dead' states. A related version of this `paradox' is the two-slit experiment in which, for example, `an electron passes through two slits at the same time, thus interferring with itself and resulting in a diffraction pattern at the detector behind the two slits'; however, if the electron position is determined before reaching the two slits the diffraction pattern, of course, disappears, as only one electron possible state exists–that which was already measured or observed.
The wave-particle duality proposed by Louis deBroglie, and readily accepted by Albert Einstein, is thought to remove this type of quantum `paradox', although it simply shifts the argument to the quantum logic realm where quantum micro-entitites such as an electron or any other quantum `particle' can simultaneously possess an associated wave (or `character'); the existence of the associated
wave of a quantum particle was later elaborated in quantum field theories (QFT) in the form of `virtual photons' that mediate the electromagnetic interactions between charged quantum particles such as electrons, protons, ions, etc.
2. The Einstein-Podolsky-Rosen (or EPR) `paradox' is a thought experiment that reveals the non-local character of quantum theory, and was presented initially as `proof that Quantum Mechanics' is incomplete, because quantum non-locality was proposed to contradict both Special and general relativity theories.
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