Does Locality Fail at Intermediate Length-Scales

If quantum gravity implies a fundamental spatiotemporal discreteness, and if its “laws of motion” are compatible with the Lorentz transformations, then physics cannot remain local. One might expect this nonlocality to be confined to the fundamental discreteness scale, but I will present evidence that it survives at much lower energies, yielding for example a nonlocal equation of motion for a scalar field propagating on an underlying causal set. Assuming that “quantum spacetime” is fundamentally discrete, how might this discreteness show itself? Some of its potential effects are more evident, others less so. The atomic and molecular structure of ordinary matter influences the propagation of both waves and particles in a material medium. Classically, particles can be deflected by collisions and also retarded in their motion, giving rise in particular to viscosity and Brownian motion. In the case of spatio-temporal discreteness, viscosity is excluded by Lorentz symmetry, but fluctuating deviations from rectilinear motion are still possible. Such “swerves” have been described in [1] and [2]. They depend (for a massive particle) on a single phenomenological parameter, essentially a diffusion constant in velocity space. As far as I know, the corresponding analysis for a quantal particle with mass has not been carried out yet, but for massless quanta such as photons the diffusion equation of [1] can be adapted to say something, and it then describes fluctuations of both energy and polarization (but not of direction), as well as a secular “reddening” (or its opposite). A more complete quantal ⋆ To appear in Daniele Oriti (ed.),