EPR paradox (nonfiction)

The Einstein–Podolsky–Rosen paradox or EPR paradox of 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 this accuracy, 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 due to the common preparation of the two particles (for example the creation of an electron-positron pair from a photon) the property we want to measure has a well defined meaning only when analyzed for the whole system while the same property for the parts individually remains undefined. Therefore, if similar measurements are being performed on the two entangled subsystems, there will always be a correlation between the outcomes resulting in a well defined global outcome i.e. for both subsystems together. However, the outcomes for each subsystem separately at each repetition of the experiment will not be well defined or predictable. This correlation does not imply any action of the measurement of one particle on the measurement of the other, therefore it doesn't imply any form of action at a distance. This modern resolution eliminates the need for hidden variables, action at a distance or other structures introduced over time in order to explain the phenomenon.

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.

Fiction cross-reference

• Nathan Rosen and The Eel Discuss a Hydrogen Molecule Where None of the Electrons Have a Definite Quantum Number, But the Pair of Electrons has a Pure State

Nonfiction cross-reference

• Albert Einstein (nonfiction)
• Bell's theorem (nonfiction)
• Bell test experiments (nonfiction)
• Boris Podolsky (nonfiction)
• Nathan Rosen (nonfiction)
• Paradox (nonfiction)
• Quantum entanglement (nonfiction)
• Quantum mechanics (nonfiction)
• Uncertainty principle (nonfiction)

External links:

• EPR Paradox @ Wikipedia