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Quantum indeterminacy is the apparent necessary incompleteness in the description of a physical system, that has become one of the characteristics of the standard description of quantum physics. Prior to quantum physics, it was thought that (a) a physical system had a determinate state which uniquely determined all the values of its measurable properties, and conversely (b) the values of its measurable properties uniquely determined the state. Albert Einstein may have been the first person to carefully point out the radical effect the new quantum physics would have on our notion of physical state.

Quantum indeterminacy can be quantitatively characterized by a probability distribution on the set of outcomes of measurements of an observable. The distribution is uniquely determined by the system state, and moreover quantum mechanics provides a recipe for calculating this probability distribution.

Indeterminacy in measurement was not an innovation of quantum mechanics, since it had established early on by experimentalists that errors in measurement may lead to indeterminate outcomes. However, by the latter half of the eighteenth century, measurement errors were well understood and it was known that they could either reduced by better equipment or accounted for by statistical error models. In quantum mechanics, however indeterminacy is of a much more fundamental nature, having nothing to do with errors or disturbance.

Indeterminacy and incompleteness

Quantum indeterminacy is the assertion that the state of a system does not determine a unique collection of values for all its measurable properties. Indeed in the quantum mechanical formalism, for a given quantum state, each one of these measurable values will be obtained non-deterministically in accordance with a probability distribution which is uniquely determined by the system state. Note that the state is destroyed by measurement, so when we refer to collection of values, each measured value in this collection must be obtained using a freshly prepared state.

This indeterminacy might be regarded as a kind of essential incompleteness in our description of a physical system. Notice however, that the indeterminacy as stated above only applies to values of measurements not to the quantum state. For example, in the spin 1/2 example discussed above, the system can be prepared in the state ψ by using measurement of σ₁ as a filter which retains only those particles such that σ₁ yields +1. By the von Neumann (so-called) postulates, immediately after the measurement the system is assuredly in the state ψ.

However, Einstein did believe that quantum state cannot be a complete description of a physical system and, it is commonly thought, never came to terms with quantum mechanics. In fact, Einstein, Boris Podolsky and Nathan Rosen did show that if quantum mechanics is correct, then the classical view of how the real world works (at least after special relativity) is no longer tenable. This view included the following two ideas:

1. A measurable property of a physical system whose value can be predicted with certainty is actually an element of reality (this was the terminology used by EPR).
2. Effects of local actions have a finite propagation speed.

This failure of the classical view was one of the conclusions of the EPR thought experiment in which two remotely located observers, now commonly referred to as Alice and Bob, perform independent measurements of spin on a pair of electrons, prepared at a source in a special state called a spin singlet state. It was a conclusion of EPR, using the formal apparatus of quantum theory, that once Alice measured spin in the x direction, Bob's measurement in the x direction was determined with certainty, whereas immediately before Alice's measurement Bob's outcome was only statistically determined. From this it follows that either value of spin in the x direction is not an element of reality or that the effect of Alice's measurement has infinite speed of propagation.

External link

• Common Misconceptions Regarding Quantum Mechanics See especially part III "Misconceptions regarding measurement".

References

• A. Aspect, Bell's inequality test: more ideal than ever, Nature 398 189 (1999).
• V. Braginski and F. Khalili, Quantum Measurements, Cambridge University Press, 1992.
• G. Bergmann, The Logic of Quanta, American Journal of Physics, 1947. Reprinted in Readings in the Philosophy of Science, Ed. H. Feigl and M. Brodbeck, Appleton-Century-Crofts, 1953. Discusses measurement, accuracy and determinism.
• J.S. Bell, On the Einstein-Poldolsky-Rosen paradox, Physics 1 195 (1964).
• A. Einstein, B. Podolsky, and N. Rosen, Can quantum-mechanical description of physical reality be considered complete? Phys. Rev. 47 777 (1935).
• G. Mackey, Mathematical Foundations of Quantum Mechanics, W. A. Benjamin, 1963 (paperback reprint by Dover 2004).
• J. von Neumann, Mathematical Foundations of Quantum Mechanics, Princeton University Press, 1955. Reprinted in paperback form. Originally published in German in 1932.
• R. Omnès, Understanding Quantum Mechanics, Princeton University Press, 1999.

• Quantum mechanics
• Determinism