Uses and abuses of decoherence

Uses and abuses of decoherence

Ulrich Mohrhoff

If objective existence is attributed to the state vector of the universe, then the world experienced by us can only have a relative objective existence, and this not only in the sense that it lacks the absolute objective existence of the universal state vector. Decoherence theorists consider the quantum state of a physical system objective if it is “monitored” by the environment, so that it can be known indirectly (i.e., by observing the state of the environment) without perturbing it or re-preparing it in the process. But while environmental “monitoring” causes the interference terms to be “not there,” it does not cause the universal state vector to collapse. The latter remains a superposition of terms corresponding to different pointer orientations, which means that each of the possible pointer orientations indicates something that is objective relative to a branch of the universal wave function.

On the other hand, if one starts from human experience — the universal context of science — then definiteness and objectivity are givens. The pointer points either left or right; the cat is either dead or alive. Objectivity is implied by the existence of common referents of a common language, and it need not amount to more than that. The question then is: to what extent is the definiteness inherent in direct sensory experience attributable to what is not directly experienced? How far can it be objectivized? And the answer to this question is: to the extent that observables are “monitored” by the environment.

The extent to which observables are “monitored” by the environment depends on the extent to which time and space are objectively differentiated. It depends on the extent to which the distinctions we make between regions of space or between intervals of time can be objectivized. If we partition space or time into smaller and smaller regions or increasingly short intervals, then, according to Wallace, “we will eventually reach a point where interference between branches ceases to be negligible, but there is no precise point where this occurs.” There is no shame in not knowing the extent to which definiteness can be objectivized, or the extent to which the differentiation of space and time into smaller and smaller regions or increasingly short intervals can be carried, before it ceases to be capable of objectivation. On the contrary, any attempt to draw a sharp line would be highly suspect.