Born rule


The Born rule is a key postulate of quantum mechanics which gives the probability that a measurement of a quantum system will yield a given result. In its simplest form, it states that the probability density of finding a particle at a given point is proportional to the square of the magnitude of the particle's wavefunction at that point. It was formulated by German physicist Max Born in 1926.

Details

The Born rule states that if an observable corresponding to a self-adjoint operator with discrete spectrum is measured in a system with normalized wave function , then
In the case where the spectrum of is not wholly discrete, the spectral theorem proves the existence of a certain projection-valued measure, the spectral measure of. In this case,
Given a wave function for a single structureless particle in position space, implies that the probability density function for a measurement of the position at time is

History

The Born rule was formulated by Born in a 1926 paper. In this paper, Born solves the Schrödinger equation for a scattering problem and, inspired by Einstein's work on the photoelectric effect, concludes, in a footnote, that the Born rule gives the only possible interpretation of the solution. In 1954, together with Walther Bothe, Born was awarded the Nobel Prize in Physics for this and other work. John von Neumann discussed the application of spectral theory to Born's rule in his 1932 book.

Interpretations

Within the Quantum Bayesianism interpretation of quantum theory, the Born rule is seen as an extension of the standard Law of Total Probability, which takes into account the Hilbert space dimension of the physical system involved.
In the ambit of the so-called Hidden-Measurements Interpretation of quantum mechanics the Born rule can be derived by averaging over all possible measurement-interactions that can take place between the quantum entity and the measuring system. It has been claimed that Pilot wave theory can also statistically derive Born's law. While it has been claimed that Born's law can be derived from the many-worlds interpretation, the existing proofs have been criticized as circular. Kastner claims that the transactional interpretation is unique in giving a physical explanation for the Born rule.