See also: EPR
The paradox Einstein-Podolsky-Rosen , shortened in EPR , is an experiment of thought worked out by Albert Einstein, Boris Podolsky, and Nathan Rosen, and of which the original intention was to refute the Interprétation of Copenhagen of the Quantum physics.
The interpretation of Copenhagen is opposed to the existence of any state of a quantum system before any measurement. Indeed, there does not exist proof that this state exists before its observation and to suppose it pleasing with certain contradictions.
However, if two particles are emitted and that a relation of conservation exists between one their properties (for example the sum of their Spin S must be 0), the knowledge of one of the properties of informs us instantaneously on the state of the other particle. But the latter can at the moment of measurement be with several kilometers, and informed of the state of the first cannot be ! How to believe under these conditions which this property was not given upon the departure , in contradiction with the representation of Copenhagen?
This paradox was worked out by Albert Einstein and two of his/her collaborators Boris Podolsky and Nathan Rosen to raise what seemed to seem a contradiction in quantum mechanics, or at least a contradiction with at least one of the three following assumptions:
-
impossibility for a signal of exceeding the speed '' C '' (relativistic causality);
- quantum mechanics complete and is entirely described reality (not of variable hidden);
- the two distant particles form elements independent of reality (Non locality).
Intrication: two possible interpretations
Photon S intricate, are two polarized S perpendicularly one compared to the other. These photons are in a state superimposed of two possibilities: 1) The first photon is vertically polarized and the second horizontally 2) the opposite state. Then the measurement of the polarization of a photon necessarily implies that the second photon will be polarized perpendicular to the first, whatever the state of polarization measured for a photon (which one cannot provide).
Indeed, according to quantum mechanics, before measurement, the polarization of these photons is unspecified . The photons are in a state superimposed between polarizations horizontal and vertical, i.e. there is a chance on two to obtain a horizontal polarization during measurement, and a chance on two to obtain a vertical polarization.
Since the state of polarization of each photon seems given by chance at the time of measurement, how to explain that the two photons are always perpendicular? Two interpretations are possible:
- the measurement of the polarization of one of the photons involves a polarization of the second photon perpendicular to that of the first. It is the point of view of Niels Bohr when it discussed it in 1930 with the congress of Brussels.
- the state of the two photons is given before measurement, at the time of their intrication, and is revealed at the time of measurement. It is the position of Albert Einstein, in favor of the determinism. For him, the states of the particles existed before measurement. If two particles are correlated, it is thus because they were it as of the beginning, and not at the time of measurement.
The paradox
It was seemingly only one philosophical debate between two manners of seeing phenomena because these two points of view gave a priori the same result. But under certain particular conditions these two designs were not compatible. Albert Einstein worked out a paradox: paradox EPR, of the name of its inventors.The principle of the paradox is simultaneously to measure (in a sufficiently short time interval so that information does not have time to be propagated from one particle to another) two sizes being excluded, such as the position and speed, which would be in violation with the inequalities of Heisenberg, and who would give more information than than quantum mechanics claims to describe, to prove that this theory is incomplete.
Einstein then proposes to improve quantum mechanics by introducing a theory using of the hidden variables.
The debate between Einstein and Bohr
Niels Bohr immediately answered by rejecting the hidden variables and while insisting on the fact that the quantum states do not exist as much as they were not measured. Before measurement, one can only envisage probabilities of obtaining certain values for a quantum state. There is determinism in a measurement only if the probability of observing it is of 1.The debate between Einstein and Bohr on this paradox lasted 20 years, until the end of their life.
Experiments
See also: Experiment of Aspect
In 1964, John Stewart Bell produced a theorem making it possible to quantify the implications of paradox EPR, opening the way with the experimentation: consequently the resolution of paradox EPR could become an experimental question, rather than an epistemological choice.
The technology of the time did not make it possible to carry out an experiment testing the Inégalités of Beautiful, but Alain Aspect could carry it out in 1981, then in 1982, with Orsay, confirming the validity of the predictions of quantum mechanics in the case of paradox EPR. This Expérience of Aspect proceeded of an idea that it had published as of 1976 but that nobody had begun again since.
In 1988-1989, other experiments (Maryland, Rochester), more improved even, made it possible to test intrications at very long distance and to fill small left experimental faults opened by the experiments of Orsay.
However, if these experiments imply that one gives up the one of the three assumptions (one decided for the locality ), it by no means do not allow the transmission of a signal more quickly than the light (without what besides either causality, or the Relativité would be violated).
A cumbersome enthusiasm
An organized conference in a hasty way with Cordoue - not by physicists, although several were invited - was the occasion for a certain number of “parapsychologists” or specialists in the “occult sciences” to claim this experiment to plead theoretical possibility of phenomena like Télépathie, Télékinésie, Synchronicité and others, in the middle of physicists who could not contradict. This explainable excitation undoubtedly by the innovation of the matter was calmed thereafter, and effect EPR forms today part of the daily newspaper of physics.The researcher Etienne Klein gives a romantic metaphor of effect EPR: Two hearts which interacted in the past cannot be considered same manner any more that if they had never met. Marked forever by their meeting, they form an inseparable whole. This interpretation is not if deprived of scientific interest that it does not appear to with it: Etienne Klein, in his work of popularization, always tried to show how the phenomena of quantum physics held for paradoxes of this discipline resemble for very familiar purposes.
More serene conclusions
The points establish by this experiment are the following:- the Inégalités of Beautiful are violated (what implies that one excludes the theories with local hidden variables more clearly the theories with the shared random variables);
- There thus do not exist local hidden variable (in the direction of locally created before being separate graft participants) as opposed to what hoped for Einstein;
- If one wants to preserve the assumption of a limit at the transmission speed of information ( C , speed of light), it should be admitted that two particles created jointly, even geographically separate, can continue to behave like a single system (not-locality).
Finally, the principle of Causalité remains valid, owing to the fact that one can consider that there is no cause and effect link between the entry of the part has and the result of the part B and vice versa, and that the results of measurement of the two particles are distinct independent events.
References
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