Gravity and Strings

194 The Schwarzschild black hole
I 0
*
+
*
*
I
I +
*
IV
*
II
III
Fig. 7.2. A Penrose diagram of Schwarzschild’s spacetime.
r = 0
II
I
I
I +
I
+
r = 2M
Fig. 7.3. The spacetime corresponding to the gravitational collapse of a star.
This question can be answered only by inventing a new kind of object, the BH, which
is, by definition, an object giving rise to a spacetime with an event horizon.
How are BHs created in the Universe? In Thorne’s book [885] the story is told of
how, in a process that took almost 50 years, the scientific community arrived at the
conclusion that BHs could originate from the gravitational collapse of very massive
stars and, furthermore, that the gravitational collapse would be unavoidable if the star
had a mass a few times the Sun’s.
It is evident that the spacetime described by the maximally extended Schwarzschild
solution cannot originate from a gravitational collapse (there is no star in the
past). Instead it describes an eternal BH. InFigure 9 the spacetime corresponding
to the spherically symmetric gravitational collapse of a star has been represented in
Kruskal–Szekeres-like coordinates. The dashed region represents the star’s interior
and the exterior is just Schwarzschild’s spacetime in Kruskal coordinates. The BH
appears when the collapsing star has a radius smaller than RS.
I 0