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The universal wavefunction or the wavefunction of the universe is the wavefunction or quantum state of the entire universe.^[1] It is regarded as the basic physical entity^[2] in the many-worlds interpretation of quantum mechanics,^[3]^[4]^[5]^[6] and finds applications in quantum cosmology. It evolves deterministically according to a wave equation.^[7]

The concept of universal wavefunction was introduced by Hugh Everett in his 1956 PhD thesis draft The Theory of the Universal Wave Function.^[8] It later received investigation from James Hartle and Stephen Hawking^[9] who derived the Hartle–Hawking solution to the Wheeler–deWitt equation to explain the initial conditions of the Big Bang cosmology.

Role of observers

Hugh Everett's universal wavefunction supports the idea that observed and observer are all mixed together:

If we try to limit the applicability so as to exclude the measuring apparatus, or in general systems of macroscopic size, we are faced with the difficulty of sharply defining the region of validity. For what n might a group of n particles be construed as forming a measuring device so that the quantum description fails? And to draw the line at human or animal observers, i.e., to assume that all mechanical apparata obey the usual laws, but that they are not valid for living observers, does violence to the so-called principle of psycho-physical parallelism.^[10]

Eugene Wigner and John Archibald Wheeler take issue with this stance. Wigner wrote:

The state vector of my mind, even if it were completely known, would not give its impressions. A translation from state vector to impressions would be necessary; without such a translation the state vector would be meaningless.^[11]

Wheeler wrote:

One is led to recognize that a wave function 'encompassing the whole universe' is an idealization, formalistically perhaps a convenient idealization, but an idealization so strained that it can be used only in part in any forecast of correlations that makes physical sense. For making sense it seems essential most of all to 'leave the observer out of the wave function'.^[12]

References

^ Everett [1956]1973, "Theory of the Universal Wavefunction", introduction, pg 8–9
^ Everett 1957, section 3, 2nd paragraph, 1st sentence.
^ Hugh Everett, Relative State Formulation of Quantum Mechanics, Reviews of Modern Physics vol 29, (1957) pp 454–462. An abridged summary of The Theory of the Universal Wavefunction
^ John Archibald Wheeler, Assessment of Everett's "Relative State Formulation of Quantum Theory", Reviews of Modern Physics, vol 29, (1957) pp 463–465
^ Bryce Seligman DeWitt, Quantum Mechanics and Reality, Physics Today,23(9) pp 30–40 (1970) also April 1971 letters followup
^ Bryce Seligman DeWitt, The Many-Universes Interpretation of Quantum Mechanics, Proceedings of the International School of Physics "Enrico Fermi" Coursu IL: Foundations of Quantum Mechanics, Academic Press (1972)
^ Everett [1956]1973, "Theory of the Universal Wavefunction", chapter 6 (e)
^ Bryce Seligman DeWitt, R. Neill Graham, eds, The Many-Worlds Interpretation of Quantum Mechanics, Princeton Series in Physics, Princeton University Press (1973), ISBN 0-691-08131-X Contains the reprint of Everett's thesis: The Theory of the Universal Wave Function, pp 3–140.
^ Stephen W Hawking, James B Hartle "The Wave Function of the Universe," Physical Review D, vol 28, (1983) pp 2960–2975
^ Everett [1956]1973, "Theory of the Universal Wavefunction", introduction, pg 6
^ Wigner, E. P. (1973). "Epistemological Perspective on Quantum Theory". Philosophical Reflections and Syntheses. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 55–71. doi:10.1007/978-3-642-78374-6_5. ISBN 978-3-540-63372-3.
^ Wheeler, John Archibald (1977). "Include the Observer in the Wave Function?". Quantum Mechanics, A Half Century Later. Dordrecht: Springer Netherlands. p. 1–18. doi:10.1007/978-94-010-1196-9_1. ISBN 978-94-010-1198-3.