A final note on the existence of event horizons
DOI: 10.1063/PT.3.2399
Giddings replies: George Chapline’s letter
Gravastars and other massive remnant variants replace the apparent horizon by a new kind of physical interface. Chapline fails to recognize that the admitted unpopularity of such scenarios stems from significant objections to their underlying physics. Those include, as outlined in my response to Mottola and Vaulin, the extreme form of nonlocality they apparently require—in contrast with the “minimal” nonlocality I have been recently exploring.
Although one would need more of a theory of such an interface to make sharp predictions, clearly, if stationary just outside the would-be horizon, that interface would be very highly boosted with respect to freely infalling matter. Thus it is a significant problem to explain why collisions of infalling matter with the interface don’t lead to substantial outward scattering. Reference 3 of Chapline’s letter places an observational bound that less than 0.4% of the infalling energy is reradiated from a putative surface of Sagittarius A*, assuming surface thermalization. This illustrates how strong such bounds are, and the possibility of extending them to other radiation spectra. In particular, in Chapline’s outlined scenario, one might also expect a significant fraction of accreted energy to be emitted. Although improved data are clearly welcome, I respectfully disagree with Chapline’s statement that the bounds on observed energy radiated from a presumed surface present “no astrophysical evidence” for horizons.
More about the Authors
Steven B. Giddings. (giddings@physics.ucsb.edu) University of California, Santa Barbara.