2015/12/31 by Timothy J. Hollowood, G.M. Shore, Graham M. Shore
Physics and Astronomy · #Action (physics) #Black Holes and Theoretical Physics #Causality (physics) #Classical mechanics #Computer science #Cosmology and Gravitation Theories #Embedding #Gravitation #Graviton #Phase (matter) #Photon #Physics #Planck #Pulsars and Gravitational Waves Research #Quantum mechanics #Scattering #Superluminal motion #Theoretical physics #hep-th
paper · pdf · doi:10.1007/jhep03(2016)129
42 pages, 15 figures, updated references
arxiv created 2016/01/19 · openalex publication_date 2016/03/01 · arxiv updated 2016/04/20 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
The effective actions describing the low-energy dynamics of QFTs involving gravity generically exhibit causality violations. These may take the form of superluminal propagation or Shapiro time advances and allow the construction of “time machines”, i.e. spacetimes admitting closed non-spacelike curves. Here, we discuss critically whether such causality violations may be used as a criterion to identify unphysical effective actions or whether, and how, causality problems may be resolved by embedding the action in a fundamental, UV complete QFT. We study in detail the case of photon scattering in an Aichelburg-Sexl gravitational shockwave background and calculate the phase shifts in QED for all energies, demonstrating their smooth interpolation from the causality-violating effective action values at low-energy to their manifestly causal high-energy limits. At low energies, these phase shifts may be interpreted as backwards-in-time coordinate jumps as the photon encounters the shock wavefront, and we illustrate how the resulting causality problems emerge and are resolved in a two-shockwave time machine scenario. The implications of our results for ultra-high (Planck) energy scattering, in which graviton exchange is modelled by the shockwave background, are highlighted.