Gravitational Faraday effect from on-shell amplitudes

Abstract

Effects of massive object’s spin on massive-massless 2 → 2 classical scattering is studied. Focus is set on the less-considered dimensionless expansion parameter λ/b, where λ is the massless particle’s wavelength and b is the impact parameter. Corrections in λ/b start to appear from $$ \mathcal{O} $$ O (G2), with leading correction terms tied to the gravitational Faraday effect, which is a special case of the Lense-Thirring effect. We compute the eikonal phase up to $$ \mathcal{O} $$ O (G2) and extract spin effect on the scattering angle and time delay up to 14th order in spin. The gravitational Faraday effect at linear order in spin [1] is reproduced by λ/b correction terms, which we compute to higher orders in spin. We find that the equivalence principle, or universality, holds up to NLO for general spinning bodies, i.e. away from geometric optics limit. Furthermore, in the black hole limit, we confirm the absence of particular spin structure observed [2–8], along with the associated shift symmetry [7], and argue that it holds to arbitrary spin order at $$ \mathcal{O} $$ O (G2) in the massless probe limit.

Recursion in the classical limit and the neutron-star Compton amplitude

“…Let us end by briefly commenting on previous attempts to construct the Compton amplitude using BCFW recursion on a classical three-point amplitude, namely refs. [25,81]. It was stated in ref.…”

“…The authors of refs. [25,81] missed the former effect, and their accounting of the latter two did not produce O( × 1/ ) terms. 11 Though inconsequential in the black-hole limit, the lack of such effects renders the results of those previous analyses for general spin-induced multipoles discrepant with refs.…”

Wave scattering event shapes at high energies

“…Nevertheless, a comparison between the functions arising in the high-energy limit of [34,35] and [24,[36][37][38][39][40][41][42][43] and their dependence on the rapidity γ ′ = (p 1 • p 2 )/(m 1 m 2 ) shows remarkable similarities with our results expressed in terms of our γ = (n • p)/m. It is tempting to suggest that the quantization of the electromagnetic (and gravitational) field is eventually needed if we are interested in obtaining radiative observables 18 free of mass singularities in perturbation theory, as shown earlier with our analysis of classical and quantum results.…”

“…Other observables for the wave scattering problem have been studied with modern on-shell techniques, mostly working in the geometric optics approximation in order to make contact with the eikonal expansion [13][14][15][16][17] but also beyond [12,18,19]. In particular quantum long-range effects have been taken into account within an effective field theory approach to gravity, with interesting consequences for the equivalence principle [14,[20][21][22][23].…”