🚀 go-pugleaf

From the point of view of external observers, the information about matter
collapsing under the influence of gravity become irreversibly irrelevant.
Therefire, in the sense of information theory, too, black holes should
possess the maximum entropy compatible with the conserved observable
quantities M, J and Q.  The mass of a Kerr-Newman black hole is not
completely lost. Penrose (1969) discovered a mechanism... by which the
rotational and electromagnetic parts of the black hole energy can in
principle be recovered ... by means of boosting a rocket ... in the
ergosphere ..the gas ejected there from the rocket boosters and falling
into the horizon may possess negative energy with respect to an asymptotic
observer (although this energy is locally positive) if its angular momentum
is oriented in the opposite direction to that of the black hole. (p.120)...
This ability to draw energy from a black hole is limited in efficiency
precisely as for a heat engine. ... the area A of a future horizon may
never decrease (Hawking and Ellis, 1973). It can be derived by means of the
Weyl tensor hypothesis ... [ the effective temperature of the black hole is
the surface gravity 1/4M if the area of the surface A is the 4 times the
entropy in analogy to TdS in ordinary thermo JS] ...Bekenstein (1973) ;;;
assumed that this anlogy is not 'accidental'.. He .. proposed a complete
equivalence of thermodynamical and spacetime-geometrical concepts. p.121 ..
A classical black hole would possess temperature T = 0K and infinite
entropy corresponding to an ideal absorber .... According to Hawking, a
black hole of solar mass should instead radiate with a temperature T = 10^-
6K if an exact Sommerfeld radiation condition applied. If this ideal
condition is replaced realistically by the 2.7K background radiation, this
black hole absorbs more radiation than it emits. Only a black hole of less
than 3x10^-7 solar masses in intergalactic space is sufficiently hot to
lose mass at the present temperature of the background radiation. p.122...

It is conceivable... that in a unitary description (not yet known to us in
the absence of a consistent quantum theory of gravitational collapse), the
quantum correlations which originally connected the regions inside and
outside the horizon are somehow revived (Page, 1980) p.122

The Hawking radiation is a specific quantum phenomenon, based on the
irreversible (measurement-like) dynamical separation of components of the
wave function (branching) which originally represented 'virtual'
fluctuations correlating the internal and external regions of the
horizon.... Horizons too depend upon the observer, namely on his state of
motion. There is no horizon for an observer falling into a black hole.
According to the principle of equivalence he might be expected not to
observe any radiation since he is in an inertial state. Similar to the
radiation of an accelerated charge this cannot be decided locally ...
Otherwise observers orbiting black holes would not be able to detect the
Hawking radiaiton either. p.123 [ In general relativity, inertial systems
are equivalent to freely falling systems, According to the principle of
equivalence a freely falling charge should then not radiate, whereas a
charge at rest in a gravitational field should. The problem is more
complex, however, as the equivalence principle is defined locally while the
radiation is calculated from the far field ... for reasons of relativistic
kinematics the reaction of dectectors on radiation must depend on their
acceleration. p.24 .. a freely falling detector will not detect radiation
from a freely falling charge but it will detect radiation from a charge
fixed in a gravitational field. A detector fixed in a gravity field will
not detect radiation from a charge also fixed in the field but it will
detect radiation from a freely falling charge. JS]