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The universal wavefunction or universal wave function is a term introduced by Hugh Everett in his Princeton PhD thesis^{[1]} The Theory of the Universal Wave Function, and forms a core concept in the relative state interpretation^{[2]}^{[3]} or many-worlds interpretation^{[4]}^{[5]} of quantum mechanics. It has also received more recent investigation from James Hartle and Stephen Hawking^{[6]} in which they derive a specific solution to the Wheeler-deWitt equation to explain the initial conditions of the Big Bang cosmology.
Everett's thesis introduction reads:
Since the universal validity of the state function description is asserted, one can regard the state functions themselves as the fundamental entities, and one can even consider the state function of the entire universe. In this sense this theory can be called the theory of the "universal wave function," since all of physics is presumed to follow from this function alone.^{[7]}
The universal wave function is the wavefunction or quantum state of the totality of existence, regarded as the "basic physical entity"^{[8]} or "the fundamental entity, obeying at all times a deterministic wave equation."^{[9]}
Ray Streater writes:
The idea of the wave-function of the universe is meaningless; we do not even know what variables it is supposed to be a function of. [...] We find the laws of Nature by reproducible experiments. The theory needs a cut, between the observer and the system, and the details of the apparatus should not appear in the theory of the system.^{[10]}
Hugh Everett's response:
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 aparata obey the usual laws, but that they are not valid for living observers, does violence to the so-called principle of psycho-physical parallelism.^{[11]}
Digital object identifier, Quantum mechanics, Albert Einstein, Epistemology, Classical mechanics