In the previous section the two main classical
properties of black holes -- the total area of event horizons can only
increase, and the surface gravity is constant over each event horizon.
We call these classical properties because they were
discovered by solving the Einstein equations, which are equations that
do not use quantum mechanics.
What happens when we add quantum mechanics to
the analysis of classical black holes?
The easiest way to combine quantum mechanics
with classical general relativity is to look at particle scattering
in curved spacetime, where the spacetime curvature can't react to the
scattering quantum particles. This is a bit of a fake, because we're
keeping the gravity part classical and only using quantum physics for
the particles. But even so, stuff happens that is very astounding and
important to the theory of black holes.
Black holes decay!
Theoretical physicists who studied quantum particle
scattering in a curved spacetime discovered that the definition of particle
and antiparticle depends on the observer, which is against the usual rules
of the theory of general relativity. The implication of this is that the
number of particles being counted depends also on the observer
The picture that emerged from all of these studies
is that if a physicist were tossed into a black hole, he wouldn't see
anything special happen at the event horizon. He would just be crushed
by the huge gravitational forces at the center. However if he were held
just outside the event horizon by a rope attached to his thumbs, his toes
would be burning from a hot soup of particles being emitted from the black
But how can particles get out of the black hole?
Even light can't get out of a black hole. (Light is made of massless particles,
and if massless particles can't escape, then neither can the particles
with nonzero mass.)
In classical black hole physics, the event horizon
is an absolute barrier to everything trying to get back outside. However,
quantum mechanics brings with it quantum uncertainty,
and quantum vacuum fluctuations where particle-antiparticle pairs are
always being created, then destroying one another, virtually, in the vacuum.
In the animation above, P stands
for particle and A stands for antiparticle.
A particle-antiparticle pair is created for a brief instance just outside
the black hole event horizon. Before the pair can destroy one another
as usual, the antiparticle is sucked behind the event horizon, while the
particle is ejected in the opposite direction. (Or vice versa.)
According to the physicist observing the event
horizon by hanging from a rope by his thumbs, the black hole has emitted
a particle through the event horizon. To a distant observer, the black
hole's mass has now decreased by the mass of the emitted particle, and
the area of the event horizon has gotten smaller!
But how can this happen? This means that the total
area of black holes can and will decrease in time, and black holes can
decay, contrary to the classical prediction using the Einstein equations
and neglecting quantum physics.
Where are the quantum microstates?
Not only does the black hole decay, but the particles
it spits out when it decays have a thermal distribution.
These decaying black holes start to look like thermal objects that classical
physicists have studied in thermodynamics since the 19th century.
But in the 20th century, in the quantum
revolution, it was discovered that all 19th century classical thermodynamics
could be described as the bulk limit of sums of quantum microstates. The
thermodynamics of steam power plants reduced to understanding the quantum
microstates of water and air molecules, for example.
So then, what are
the quantum microstates that give rise to black hole thermodynamics?
String theorists believe they have the answer.