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Abstract |
The open question of whether a black hole can become tidally deformed by an
external gravitational field has profound implications for fundamental physics,
astrophysics and gravitational-wave astronomy. Love tensors characterize the tidal
deformability of compact objects such as astrophysical (Kerr) black holes under an
external static tidal field. We prove that all Love tensors vanish identically for
a Kerr black hole in the nonspinning limit or for an axisymmetric tidal
perturbation. In contrast to this result, we show that Love tensors are
generically nonzero for a spinning black hole. Specifically, to linear order in
the Kerr black hole spin and the weak perturbing tidal field, we compute in closed
form the Love tensors that couple the mass-type and current-type quadrupole
moments to the electric-type and magnetic-type quadrupolar tidal fields. For a
dimensionless spin ~ 0.1, the nonvanishing quadrupolar Love tensors are ~ 0.002,
thus showing that black holes are particularly "rigid" compact objects. We also
show that the induced quadrupole moments are closely related to the physical
phenomenon of tidal torquing of a spinning body interacting with a tidal
gravitational environment. |
