A four-dimensional universe

A four-dimensional universe has been dynamically generated from the seething, quantum-fluctuating spacetime foam of quantum gravity, in which even the notion of “dimension” is slippery. Just as nonrelativistic quantum mechanics can be formulated with Feynman path integrals, quantum gravity can be expressed with sums over geometries; however, attempts to define the sum over geometries have, to date, yielded degenerate spacetimes that do not resemble our universe. But now, a team of researchers from Denmark, Poland, and the Netherlands have found a highly nontrivial prescription for summing geometries that typically yields a stable, macroscopic 4D spacetime. In their formulation, the sum over geometries can be studied with Monte Carlo simulations. A crucial ingredient is the requirement that contributing geometries be causally well behaved. That requirement drastically cuts down the number of summed geometries, and leads to the qualitatively new behavior. ( J. Ambjørn , J. Jurkiewicz , R. Loll , Phys. Rev. Lett. , in press.)