THE ISSUE OF CHOOSING NOTHING: WHAT DETERMINES THE LOW ENERGY VACUUM STATE OF NATURE?

Starting from an (unknown) quantum gravitational model, one can invoke a sequence of approximations to progressively arrive at quantum field theory (QFT) in curved space–time, QFT in flat space–time, nonrelativistic quantum mechanics and Newtonian mechanics. The more exact theory can put restrictions on the range of possibilities allowed for the approximate theory which are not derivable from the latter — an example being the symmetry restrictions on the wave function for a pair of electrons. We argue that the choice of vacuum state at low energies could be such a "relic" arising from combining the principles of quantum theory and general relativity, and demonstrate this result in a simple toy model. Our analysis suggests that the wave function of the universe, when describing the large volume limit of the universe, dynamically selects a vacuum state for matter fields — which in turn defines the concept of particle in the low energy limit. The result also has the potential for providing a concrete quantum...