Download Apparent versus Event Horizon

BLACK HOLES
Black holes are thought to form from stars or other
massive objects if and when they collapse from their
own gravity to form an object whose density is infinite:
in other words, a singularity.
Singularity, where matter is crushed to infinite
density, the pull of gravity is infinitely strong, and
spacetime has infinite curvature. Here it's no longer
meaningful to speak of space and time, much less
spacetime. Jumbled up at the singularity, space and
time cease to exist as we know them.
Eventually, all possible nuclear fuel is used
up by a star and the core collapses. How far
it collapses, into what kind of object, and at
what rate, is determined by the star's final
mass and the remaining outward pressure
that the burnt-up nuclear residue (largely
iron) can muster. If the star is sufficiently
massive or compressible, it may collapse to
a black hole. If it is less massive or made of
stiffer material, its fate is different: it
may become a white dwarf or a neutron
star.
White Dwarf
When small stars (up to 8
times the size of the Sun)
exhaust their nuclear fuel,
they typically shed large
amounts of matter, leaving a
core that eventually cools
and contracts gravitationally
to about the size of the
Earth. The result is a white
dwarf: the more massive it
is, the greater its inward
gravitational pull, and the
smaller it becomes
A teaspoonful of white dwarf material would weigh
five-and-a-half tons or more in the Earth's
gravity! Yet a white dwarf can contract no
further; its electrons resist further compression
by exerting an outward pressure that counteracts
gravity.
There are many white
dwarfs in our galaxy, but
most are too dim to be
seen. One of the first to be
discovered was Sirius B,
the dense companion star
to Sirius.
Sirius B has another claim to fame. This white
dwarf star fueled a debate in the 1920s
between leading astrophysicists Subrahmanyan
Chandrasekhar and Sir Authur Eddington. At
issue was the following question: How far can a
star possibly collapse? And for a given mass,
what will it collapse into?
Chandrasekhar derived a relation ship between the
star's mass and its radius which sets an upper limit to
the mass a white dwarf can have, beyond which it will
collapse to a neutron star or, if sufficiently massive, to
a black hole. Calculations put the "Chandrasekhar
Limit" at 1.4 solar masses. Decades later
Chandrasekhar's fundamental contributions were
recognized when he won the 1983 Nobel Prize in
Physics.
Neutron Stars
More massive stars tend to burn hotter and faster. Once all
the nuclear fuel has been exhausted, such stars quickly
collapse, shedding much of their mass in dramatic explosions
called supernovae
The most recent event
of this kind was
observed in 1987 when a
star weighing the
equivalent of 20 suns
blew up in a neighboring
galaxy 160,000 light
years away.
If after such an explosion, the remaining material is
greater than 1.4 solar masses, it will contract into an
unimaginably dense core made solely of neutrons.
Neutron stars are so dense a teaspoonful would weigh
100 million tons!
Eventually astronomers
may discover the telltale
signs of a neutron star
exactly where the old star
met its doom, though as
yet none has been
detected.
If the star's final mass exceeds much beyond 2 solar
masses, there is no outward force that can resist gravity.
The core continues to collapse to a critical size or
circumference beyond which there is only one fate: to form
a black hole.
Black Hole at Cygnus X-1
Anatomy of a Black Hole
By definition a black hole is
a region where matter
collapses to infinite
density, and where, as a
result, the curvature of
spacetime is extreme.
Moreover, the intense
gravitational field of the
black hole prevents any
light or other
electromagnetic radiation
from escaping.
The Event Horizon
Applying the Einstein Field
Equations to collapsing
stars, German
astrophysicist Karl
Schwarzschild deduced the
critical radius for a given
mass at which matter would
collapse into an infinitely
dense state known as a
singularity. For a black hole
whose mass equals 10 suns,
this radius is about 30
kilometers or 19 miles,
which translates into a
critical circumference of
189 kilometers or 118 miles.
If you envision the
simplest threedimensional geometry for
a black hole, that is a
sphere (known as a
Schwarzschild black hole),
the black hole's surface is
known as the event
horizon. Behind this
horizon, the inward pull of
gravity is overwhelming
and no information about
the black hole's interior
can escape to the outer
universe.
Apparent versus Event Horizon
As a doomed star reaches its critical circumference,
an "apparent" event horizon forms suddenly. Why
"apparent?" Because it separates light rays that are
trapped inside a black hole from those that can move
away from it. However, some light rays that are
moving away at a given instant of time may find
themselves trapped later if more matter or energy
falls into the black hole, increasing its gravitational
pull. The event horizon is traced out by "critical" light
rays that will never escape or fall in.
Even before the star meets its final doom, the event
horizon forms at the center, balloons out and breaks
through the star's surface at the very moment it shrinks
through the critical circumference. At this point in time,
the apparent and event horizons merge as one: the
horizon.
Beyond the event horizon, nothing, not even light, can
escape. So the event horizon acts as a kind of "surface" or
"skin" beyond which we can venture but cannot see.
Imagine what happens as you approach the horizon, then
cross the threshold.
The Limits of Physical Law
Newton and Einstein may have looked at the
universe very differently, but they would have
agreed on one thing: all physical laws are inherently
bound up with a coherent fabric of space and time.
At the singularity, though, the laws of
physics, including General Relativity, break
down. Enter the strange world of quantum
gravity. In this bizarre realm in which
space and time are broken apart, cause and
effect cannot be unraveled. Even today,
there is no satisfactory theory for what
happens at and beyond the singularity.
It's no surprise that throughout his life Einstein rejected the
possibility of singularities. So disturbing were the implications
that, by the late 1960s, physicists conjectured that the
universe forbade "naked singularities." After all, if a singularity
were "naked," it could alter the whole universe unpredictably. All
singularities within the universe must therefore be "clothed."
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