Quantum nonlocality is a paradox that was described first by Einstein, Podolsky, and Rosen (EPR), who published the idea in 1935. The EPR paradox draws attention to a phenomenon predicted by quantum mechanics known as quantum entanglement, in which measurements on spatially separated quantum systems can instantaneously influence one another. As a result, quantum mechanics violates a principle formulated by Einstein, known as the principle of locality or local realism, which states that changes performed on one physical system should have no immediate effect on another spatially separated system.

Our "local realistic" view of the world assumes that phenomena are separated by time and space and that no influence can travel faster than the speed of light. Quantum nonlocality proves that these assumptions are incorrect, and that there is a principle of holistic interconnectedness operating at the quantum level which contradicts the localistic assumptions of classical, Newtonian physics.

• Note: Quantum nonlocality does not prove that signals travel faster than light. Rather, it shows that at a deep level of reality the speed of light as a limiting factor is irrelevant because phenomena are instantaneously connected regardless of distance.

The Hows and Whys of Nonlocality

At the quantum level, "particles" do not possess definite, deterministic qualities until they are measured:

At the subatomic level, matter does not exist with certainty at definite places, but rather shows "tendencies to exist," and atomic events do not occur with certainty at definite times and in definite ways, but rather show "tendencies to occur." (Capra, 1982, p. 80)

Despite the fact that the quality of particles is indeterminate until a measurement is made, any two photons or electrons that originate from a common source will possess a total spin of zero once they are measured. Thus:

• If particle 1 has a spin of "up," particle 2 will have a spin of "down."

• If particle 1 has a spin of "down," particle 2 will have a spin of "up."

In 1935 Schrodinger published an essay describing the conceptual problems in Quantum mechanics. A brief paragraph in this essay described the cat paradox:

"One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The Psi function for the entire system would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a ``blurred model'' for representing reality. In itself it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.

We know that superposition of possible outcomes must exist simultaneously at a microscopic level because we can observe interference effects from these. We know (at least most of us know) that the cat in the box is dead, alive or dead and not in a smeared out state between the alternatives. When and how does the model of many microscopic possibilities resolve itself into a particular macroscopic state? When and how does the fog bank of microscopic possibilities transform itself to the blurred picture we have of a definite macroscopic state. That is the measurement problem and Schrodinger's cat is a simple and elegant explanation of that problem."

References:

1 E. Schrodinger, ``Die gegenwärtige Situation in der Quantenmechanik,'' Naturwissenschaften. 23 : pp. 807-812; 823-823, 844-849. (1935). English translation: John D. Trimmer, Proceedings of the American Philosophical Society, 124, 323-38 (1980), Reprinted in Quantum Theory and Measurement, p 152 (1983).