# The basis problem in many-worlds theories

Henry Stapp

Lawrence Berkeley National Laboratory, University of California, Berkely

Stapp criticises the many-worlds theory for not being able to specify how the evolution of the Schrödinger equation can by itself, and without some other process, create the discrete features of the world of classical physics. The significance of this is that Everett’s many-worlds proposal allowed determinism into quantum theory, and in a deterministic world there is no place for freewill, and possibly no function for consciousness.

Stapp suggests that there is a problem as to how Everett’s many-worlds theory works. Everett claimed that the memory records of an observer in a many-worlds scenario would be the same as those witnessing a standard form of wave-function collapse. In the simplest example of the Everett universe there could be two worlds, one where a particular electron had ‘spin up’, and another where this electron had ‘spin down’. The reality of such a universe is deterministic because there is no need for a random decision between ‘spin up’ and ‘spin down’, since both forms are allowed to continue into the future.

The physicist, David Deutsch, looked at the many-worlds idea in the 1980s. He studied the problem of specifying an instant at which a measurement is completed. This has to be defined in terms of the Schrödinger equation. Stapp suggests that Deutsch rather fudged this area. He assumed a model world with a finite number of states, without explaining how it carried over into the real world with its infinite possible number of states.

Stapp argues that left to itself the evolution of the Schrödinger equation would leave the universe in a smeared-out state, and features such as the planet Earth would lack a well defined location. The classical world that we experience requires the singling out of particular subspaces that are not provided for in the Schrödinger equation. The continuous action of the equation on a smeared-out universe does not account for the well defined states that are experienced.

The original Copenhagen interpretation of quantum theory does pick out the particular subspaces of the classical world from a continuum of logically possibly alternatives. However, the participant-observer in the Copenhagen interpretation, who chooses a particular experiment, and observes a particular outcome, is able to do this because they are not part of the Schrödinger evolution, and are not part of the quantum universe, but instead act upon it and observe it from outside. Any theory, such as the Everett many world theory that suggests that there is nothing except the Schrödinger evolution needs to explain how the continuous evolution can pick out the discrete realities that are observed.

The more modern approach to wave function collapse in terms of decoherence in the environment rather than measurement by observers also assumes the conversion of a wave that could extend over a large distance into something more limited with highly localised classical properties. It is not clear to Stapp how this can be achieved without looking to something outside the Schrödinger process. The Copenhagen interpretation had at least that advantage that it pre-specified a classical system that would collapse the wave function.