Born rule

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Not to be confused with Cauchy–Born rule.

The Born rule (also called the Born law, Born's rule, or Born's law) is a law of quantum mechanics which gives the probability that a measurement on a quantum system will yield a given result. It is named after its originator, the physicist Max Born. The Born rule is one of the key principles of quantum mechanics. There have been many attempts to derive the Born rule from the other assumptions of quantum mechanics, with inconclusive results; for example, while it has been claimed that Born's law can be derived from the Many Worlds Interpretation, the proofs have been criticized as circular.[1] However, within the Quantum Bayesianism interpretation of quantum theory, it has been shown to be an extension of the standard Law of Total Probability, which takes into account the Hilbert space dimension of the physical system involved.[2] The Born rule can, however, be derived in the ambit of so-called Hidden-Measurements Interpretation of quantum mechanics, by averaging over all possible measurement-interactions that can take place between the quantum entity and the measuring system.[3][4]

The rule[edit]

The Born rule states that if an observable corresponding to a Hermitian operator A with discrete spectrum is measured in a system with normalized wave function \scriptstyle|\psi\rang (see bra–ket notation), then

  • the measured result will be one of the eigenvalues \lambda of A, and
  • the probability of measuring a given eigenvalue \lambda_i will equal \scriptstyle\lang\psi|P_i|\psi\rang, where P_i is the projection onto the eigenspace of A corresponding to \lambda_i.
(In the case where the eigenspace of A corresponding to \lambda_i is one-dimensional and spanned by the normalized eigenvector \scriptstyle|\lambda_i\rang, P_i is equal to \scriptstyle|\lambda_i\rang\lang\lambda_i|, so the probability \scriptstyle\lang\psi|P_i|\psi\rang is equal to \scriptstyle\lang\psi|\lambda_i\rang\lang\lambda_i|\psi\rang. Since the complex number \scriptstyle\lang\lambda_i|\psi\rang is known as the probability amplitude that the state vector \scriptstyle|\psi\rang assigns to the eigenvector \scriptstyle|\lambda_i\rang, it is common to describe the Born rule as telling us that probability is equal to the amplitude-squared (really the amplitude times its own complex conjugate). Equivalently, the probability can be written as \scriptstyle|\lang\lambda_i|\psi\rang|^2.)

In the case where the spectrum of A is not wholly discrete, the spectral theorem proves the existence of a certain projection-valued measure Q, the spectral measure of A. In this case,

  • the probability that the result of the measurement lies in a measurable set M will be given by \scriptstyle\lang\psi|Q(M)|\psi\rang.

If we are given a wave function \scriptstyle\psi for a single structureless particle in position space, this reduces to saying that the probability density function p(x,y,z) for a measurement of the position at time t_0 will be given by p(x,y,z)=\scriptstyle|\psi(x,y,z,t_0)|^2.


The Born rule was formulated by Born in a 1926 paper.[5] In this paper, Born solves the Schrödinger equation for a scattering problem and, inspired by Einstein's work on the photoelectric effect,[6] concluded, 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.[7] John von Neumann discussed the application of spectral theory to Born's rule in his 1932 book.[8]

See also[edit]


  1. ^ N.P. Landsman, "The conclusion seems to be that no generally accepted derivation of the Born rule has been given to date, but this does not imply that such a derivation is impossible in principle.", in Compendium of Quantum Physics (eds.) F.Weinert, K. Hentschel, D.Greenberger and B. Falkenburg (Springer, 2008), ISBN 3-540-70622-4
  2. ^ Fuchs, C. A. QBism: the Perimeter of Quantum Bayesianism 2010
  3. ^ Aerts, D. (1986). A possible explanation for the probabilities of quantum mechanics, Journal of Mathematical Physics, 27, pp. 202-210.
  4. ^ Aerts, D. and Sassoli de Bianchi, M. (2014). The extended Bloch representation of quantum mechanics and the hidden-measurement solution to the measurement problem. Annals of Physics 351, Pages 975–1025 (Open Access).
  5. ^ Zur Quantenmechanik der Stoßvorgänge, Max Born, Zeitschrift für Physik, 37, #12 (Dec. 1926), pp. 863–867 (German); English translation, On the quantum mechanics of collisions, in Quantum theory and measurement, section I.2, J. A. Wheeler and W. H. Zurek, eds., Princeton, New Jersey: Princeton University Press, 1983, ISBN 0-691-08316-9.
  6. ^ "Again an idea of Einstein’s gave me the lead. He had tried to make the duality of particles - light quanta or photons - and waves comprehensible by interpreting the square of the optical wave amplitudes as probability density for the occurrence of photons. This concept could at once be carried over to the psi-function: |psi|2 ought to represent the probability density for electrons (or other particles)." from Born's Nobel Lecture on the statistical interpretation of quantum mechanics
  7. ^ Born's Nobel Lecture on the statistical interpretation of quantum mechanics
  8. ^ Mathematische Grundlagen der Quantenmechanik, John von Neumann, Berlin: Springer, 1932 (German); English translation Mathematical Foundations of Quantum Mechanics, transl. Robert T. Beyer, Princeton, New Jersey: Princeton University Press, 1955.

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