Abstracts

Stefano Gregorio Giaccari

Causality in nonlocal gravity

The definition of quantum gravity is hindered by the difficulty of reconciling the requirements of renormalizability and unitarity. For example, the Einstein-Hilbert action is non-renormalizable, but, if we include infinitely many counterterms, it is pertubatively unitary. Higher-derivative theories of gravity, while possibly renormalizable, are generically expected to be non-unitary. Recently it has also been pointed out that higher-derivative terms are associated with a violation of causality at scales larger than the Planck length. This can be avoided by adding an infinite tower of massive higher spin particles, which are in fact expected in a weakly coupled string theory. On the other hand this argument seems to rule out causality for quantum gravity theories containing only conventional particles with spin not greater than 2.

In this talk we argue the requirement of causality can still be satisfied for a class of weakly nonlocal gravitational theories which have been proposed as compatible with perturbative unitarity and renormalizability. This result will be justified computing Shapiro's time delay in terms of tree-level scattering amplitudes for nonlocal gravity models with and without matter. We will then argue how generic nonlocal gravity theories consistent with causality can be obtained by a field redefinition from standard local theories. In particular we will discuss a Einstein-Maxwell-Scalar nonlocal field theory and the N=1 nonlocal supergravity.