|
EPR paradox
|
Albert Einstein
Quantum mechanics
Introduction
Glossary
History
Classical mechanics
Old quantum theory
Bra–ket notation
Hamiltonian
Interference
Complementarity
Decoherence
Entanglement
Energy level
Measurement
Nonlocality
Quantum number
State
Superposition
Symmetry
Tunnelling
Uncertainty
Wave function (collapse)
Afshar
Bell's inequality
Davisson–Germer
Double-slit
Elitzur–Vaidman
Franck–Hertz
Mach–Zehnder
Popper
Quantum eraser (delayed-choice)
Schrödinger's cat
Stern–Gerlach
Wheeler's delayed-choice
Overview
Heisenberg
Interaction
Matrix
Phase-space
Schrödinger
Sum-over-histories (path integral)
Dirac
Klein–Gordon
Pauli
Rydberg
Schrödinger
Overview
Bayesian
Consistent histories
Copenhagen
de Broglie–Bohm
Ensemble
Hidden-variable
Many-worlds
Objective collapse
Quantum logic
Relational
Stochastic
Transactional
Quantum chaos
Quantum computing
Density matrix
Quantum field theory
Fractional quantum mechanics
Quantum information science
Relativistic quantum mechanics
Scattering theory
Quantum statistical mechanics
Aharonov
Bell
Blackett
Bloch
Bohm
Bohr
Born
Bose
de Broglie
Candlin
Compton
Dirac
Davisson
Debye
Ehrenfest
Einstein
Everett
Fock
Fermi
Feynman
Glauber
Gutzwiller
Heisenberg
Hilbert
Jordan
Kramers
Pauli
Lamb
Landau
Laue
Moseley
Millikan
Onnes
Planck
Rabi
Raman
Rydberg
Schrödinger
Sommerfeld
von Neumann
Weyl
Wien
Wigner
Zeeman
Zeilinger
v
t
e
The EPR paradoxof 1935 is an influential thought experiment in quantum mechanics with which Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen ("EPR")claimed to demonstrate that the wave function does not provide a complete description of physical reality, and hence that the Copenhagen interpretation is unsatisfactory;resolutions of the paradox have important implications for the interpretation of quantum mechanics.The essence of the paradox is that particles can interact in such a way that it is possible to measure both their position and their momentum more accurately than Heisenberg's uncertainty principle allows,unless measuring one particle instantaneously affects the other to prevent it, which would involve information being transmitted faster than light as forbidden by the theory of relativity ("spooky action at a distance").This consequence had not previously been noticed and seemed unreasonable at the time; the phenomenon involved is now known as quantum entanglement.While EPR felt that the paradox showed that quantum theory was incomplete and should be extended with hidden variables,the usual modern resolution is to say that measuring one particle does instantaneously affect the other, but that this does not involve transmission of information.A preference for the latter resolution is supported by experiments suggested by Bell's theorem of 1964, which exclude some classes of hidden variable theory.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, and 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 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 of an effect propagating instantly across a distance is in conflict with the theory of relativity. They then concluded that quantum mechanics was incomplete since its formalism does not permit 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, Podolsky 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.
^ Einstein, A; B Podolsky; N Rosen (1935-05-15). "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" (PDF). Physical Review 47 (10): 777–780. Bibcode:1935PhRv...47..777E. doi:10.1103/PhysRev.47.777.
^ Gaasbeek, Bram. "Demystifying the Delayed Choice Experiments", p. 1 (arXiv:1007.3977v1 22 Jul 2010)
^ Bell, John. On the Einstein–Poldolsky–Rosen paradox, Physics 1 3, 195-200, Nov. 1964
^ Aspect A (1999-03-18). "Bell's inequality test: more ideal than ever" (PDF). Nature 398 (6724): 189–90. Bibcode:1999Natur.398..189A. doi:10.1038/18296.
|
Created By:
System
|
|
|
|
|