Causality in Curved Spacetimes: The Speed of Light & Gravity

Abstract

Within the low-energy effective field theories of QED and gravity, the low-energy speed of light or that of gravitational waves can typically be mildly superluminal in curved spacetimes. Related to this, small scattering time advances relative to the curved background can emerge from known effective field theory coefficients for photons or gravitons. We clarify why these results are not in contradiction with causality, analyticity or Lorentz invariance, and highlight various subtleties that arise when dealing with superluminalities and time advances in the gravitational context. Consistent low-energy effective theories are shown to self-protect by ensuring that any time advance and superluminality calculated within the regime of validity of the effective theory is necessarily unresolvable, and cannot be argued to lead to a macroscopically larger lightcone. Such considerations are particularly relevant for putting constraints on cosmological and gravitational effective field theories and we provide explicit criteria to be satisfied so as to ensure causality.

references added, typos corrected

References in corpus (12)

• Causality, Analyticity and an IR Obstruction to UV Completion
• Eikonal phase matrix, deflection angle and time delay in effective field theories of gravity
• Non-local quantum effects in cosmology 1: Quantum memory, non-local FLRW equations and singularity avoidance
• The Refractive Index of Curved Spacetime: the Fate of Causality in QED
• Positivity of Curvature-Squared Corrections in Gravity
• Positivity Bounds and the Massless Spin-2 Pole
• Black Hole Gravitational Waves in the Effective Field Theory of Gravity
• Positivity Constraints for Pseudo-linear Massive Spin-2 and Vector Galileons
• Causality and Micro-Causality in Curved Spacetime
• On the non-local heat kernel expansion
• Generalized positivity bounds on chiral perturbation theory
• The Unbearable Beingness of Light, Dressing and Undressing Photons in Black Hole Spacetimes

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