Extremal Love: tidal/electromagnetic deformability, logarithmic running and the weak gravity conjecture

TL;DR

This paper investigates how quantum corrections and higher-derivative terms affect the tidal deformability of extremal charged black holes, revealing non-zero Love numbers and constraints from unitarity and the Weak Gravity Conjecture.

Contribution

It computes the static linear response of extremal charged black holes in Einstein-Maxwell EFT, showing non-zero tidal Love numbers with logarithmic running constrained by fundamental principles.

Findings

Tidal Love numbers are non-zero for extremal charged black holes.

Logarithmic running of susceptibilities is observed, indicating quantum effects.

Constraints from unitarity and the Weak Gravity Conjecture determine the sign and behavior of deformations.

Abstract

In General Relativity, the tidal Love numbers of black holes vanish, implying they are resistant to tidal deformation. This "rigidity" is easily broken in the presence of higher-derivative corrections. Focusing on extremal charged black holes in Einstein-Maxwell EFT, we compute the static linear response for both the vector ($\ell=1$) and parity-odd tensor ($\ell \ge 2$) sectors. We find that the resulting tidal Love numbers are non-zero and exhibit logarithmic running, a hallmark of quantum corrections. Crucially, we show that the sign of these deformations is not arbitrary; the induced electric and magnetic susceptibilities and their log runnings in the $\ell=1$ sector are constrained by unitarity and the Weak Gravity Conjecture. Furthermore, due to gravito-electromagnetic mixing, we find the cross log runnings and show that they are the same, which we explain through the worldline effective field theory.