  The notions of temperature, entropy and `evaporation', usually associated
with spacetimes with horizons, are analyzed using general approach and the
following results, applicable to different spacetimes, are obtained at one go.
(i) The concept of temperature associated with the horizon is derived in a
unified manner and is shown to arise from purely kinematic considerations. (ii)
QFT near any horizon is mapped to a conformal field theory without introducing
concepts from string theory. (iii) For spherically symmetric spacetimes (in
D=1+3) with a horizon at r=l, the partition function has the generic form
$Z\propto \exp[S-\beta E]$, where $S= (1/4) 4\pi l^2$ and $|E|=(l/2)$. This
analysis reproduces the conventional result for the blackhole spacetimes and
provides a simple and consistent interpretation of entropy and energy for
deSitter spacetime. (iv) For the Rindler spacetime the entropy per unit
transverse area turns out to be (1/4) while the energy is zero. (v) In the case
of a Schwarzschild black hole there exist quantum states (like Unruh vacuum)
which are not invariant under time reversal and can describe blackhole
evaporation. There also exist quantum states (like Hartle-Hawking vacuum) in
which temperature is well-defined but there is no flow of radiation to
infinity. In the case of deSitter universe or Rindler patch in flat spacetime,
one usually uses quantum states analogous to Hartle-Hawking vacuum and obtains
a temperature without the corresponding notion of evaporation. It is, however,
possible to construct the analogues of Unruh vacuum state in the other cases as
well. Associating an entropy or a radiating vacuum state with a general horizon
raises conceptual issues which are briefly discussed.
