IAS Physics Group Meeting

Abstract: In this talk, we'll review the bootstrap method that uses unitarity and causality (analyticity + crossing) to constrain low-energy effective field theories (EFTs), especially those including gravitational couplings. This method allows us to put numerical bounds on the gravitational EFTs, thus sharpening the earlier parametric arguments. To understand the nature of the EFT cut-off, we then consider the scenario where Einstein gravity coupled to a large-N number of low-spin matters. At low energy, this procedure involves summing over those loops that survive by balancing the large-N limit with the gravitational weak coupling limit. By analyzing phase shifts, we find it impossible for low-spin matter loops to be the UV completion of gravity, thus providing a sharp definition of the irrevocable EFT cut-off as the higher-spin gap. We'll also show that causality strongly constrains the higher-spin gap.