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EPR paradox
Albert Einstein Quantum mechanics Introduction Glossary · History Bra–ket notation Classical mechanics Hamiltonian Interference Old quantum theory Complementarity Decoherence Entanglement Energy level Nonlocality Quantum state Superposition Tunnelling Uncertainty Wave function Wave function collapse Symmetry Measurement Afshar Bell's inequality Davisson–Germer Delayed choice quantum eraser Double-slit Elitzur-Vaidman Franck-Hertz Mach-Zehnder inter. Popper Quantum eraser Schrödinger's cat Stern–Gerlach Wheeler's delayed choice Formulations Heisenberg Interaction Matrix mechanics Schrödinger Sum over histories Phase space Dirac Klein–Gordon Pauli Rydberg Schrödinger Interpretations (overview) Bayesian Consistent histories Copenhagen de Broglie–Bohm Ensemble Hidden variables Many-worlds Objective collapse Quantum logic Relational Stochastic Transactional Quantum chaos Quantum field theory Density matrix Quantum statistical mechanics Quantum information science Quantum computing Scattering theory Fractional quantum mechanics Relativistic quantum mechanics Bell Blackett Bohm Bohr Born Bose de Broglie Candlin Compton Dirac Davisson Debye Ehrenfest Einstein Everett Fock Fermi Feynman Heisenberg Hilbert Jordan Kramers von Neumann Pauli Lamb Landau Laue Moseley Millikan Onnes Planck Raman Rydberg Schrödinger Sommerfeld von Neumann Weyl Wien Wigner Zeeman Zeilinger v t e The EPR paradox is an early and influential critique leveled against the Copenhagen interpretation. Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen (known collectively as EPR) designed a thought experiment which revealed a consequence of quantum mechanics, which today is called quantum entanglement. To that end, they hypothesized a consequence of quantum mechanics that its supporters had not noticed but looked unreasonable at the time. According to quantum mechanics, under some conditions, a pair of quantum systems may be described by a single wave function, which encodes the probabilities of the outcomes of experiments that may be performed on the two systems, whether jointly or individually. At the time the EPR article discussed below was written, it was known from experiments that the outcome of an experiment sometimes cannot be uniquely predicted. An example of such indeterminacy can be seen when a beam of light is incident on a half-silvered mirror. One half of the beam will reflect, the other will pass. If the intensity of the beam is reduced until only one photon is in transit at any time, whether that photon will reflect or transmit cannot be predicted quantum mechanically.The routine explanation of this effect was, at that time, provided by Heisenberg's uncertainty principle. Physical quantities come in pairs which are called conjugate quantities. Examples of such conjugate pairs are position and momentum of a particle and components of spin measured around different axes. When one quantity was measured, and became determined, the conjugated quantity became indeterminate. Heisenberg explained this as a disturbance caused by measurement. The EPR paper, written in 1935, was intended to illustrate that this explanation is inadequate. It considered two entangled particles, referred to as A and B, and pointed out that measuring a quantity of a particle A will cause the conjugated quantity of particle B to become undetermined, even if there was no contact, no classical disturbance. The basic idea was that the quantum states of two particles in a system cannot always be decomposed from the joint state of the two. An example (in bra-ket notation is: Heisenberg's principle was an attempt to provide a classical explanation of a quantum effect sometimes called non-locality. According to EPR there were two possible explanations. Either there was some interaction between the particles, even though they were separated, or the information about the outcome of all possible measurements was already present in both particles.The EPR authors preferred the second explanation according to which that information was encoded in some 'hidden parameters'. The first explanation, that an effect propagated instantly, across a distance, is in conflict with the theory of relativity. They then concluded that quantum mechanics was incomplete since, in its formalism, there was no room for such hidden parameters.Violations of the conclusions of Bell's theorem are generally understood to have demonstrated that the hypotheses of Bell's theorem, also assumed by Einstein Poldolsky and Rosen, do not apply in our world. Most physicists who have examined the issue concur that experiments, such as those of Alain Aspect and his group, have confirmed that physical probabilities, as predicted by quantum theory, do exhibit the phenomena of Bell-inequality violations that are considered to invalidate EPR's preferred "local hidden-variables" type of explanation for the correlations to which EPR first drew attention. Cite error: There are tags on this page, but the references will not show without a {{reflist}} template (see the help page).
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