Black Holes FAQ
(Frequently Asked Questions)
List
by Ted Bunn
------------------------------------------------------------------------
* What is a black hole?
* How big is a black hole?
* What would happen to me if I fell into a black hole?
* My friend Penelope is sitting still at a safe distance, watching
me
fall into the black hole. What does she see?
* If a black hole existed, would it suck up all the matter in
the
Universe?
* What if the Sun became a black hole?
* Is there any evidence that black holes exist?
* How do black holes evaporate?
* Won't the black hole have evaporated out from under me before
I reach
it?
* What is a white hole?
* What is a wormhole?
* Where can I go to learn more about black holes?
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What is a black hole?
---------------------
Loosely speaking, a black hole is a region of space that has
so much mass
concentrated in it that there is no way for a nearby object to
escape its
gravitational pull. Since our best theory of gravity at the moment
is
Einstein's general theory of relativity, we have to delve into
some results
of this theory to understand black holes in detail, but let's
start of slow,
by thinking about gravity under fairly simple circumstances.
Suppose that you are standing on the surface of a planet.
You throw a rock
straight up into the air. Assuming you don't throw it too hard,
it will rise
for a while, but eventually the acceleration due to the planet's
gravity
will make it start to fall down again. If you threw the rock
hard enough,
though, you could make it escape the planet's gravity entirely.
It would
keep on rising forever. The speed with which you need to throw
the rock in
order that it just barely escapes the planet's gravity is called
the "escape
velocity." As you would expect, the escape velocity depends
on the mass of
the planet: if the planet is extremely massive, then its gravity
is very
strong, and the escape velocity is high. A lighter planet would
have a
smaller escape velocity. The escape velocity also depends on
how far you are
from the planet's center: the closer you are, the higher the
escape
velocity. The Earth's escape velocity is 11.2 kilometers per
second (about
25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second
(about
5300 m.p.h.).
Now imagine an object with such an enormous concentration
of mass in such a
small radius that its escape velocity was greater than the velocity
of
light. Then, since nothing can go faster than light, nothing
can escape the
object's gravitational field. Even a beam of light would be pulled
back by
gravity and would be unable to escape.
The idea of a mass concentration so dense that even light
would be trapped
goes all the way back to Laplace in the 18th century. Almost
immediately
after Einstein developed general relativity, Karl Schwarzschild
discovered a
mathematical solution to the equations of the theory that described
such an
object. It was only much later, with the work of such people
as Oppenheimer,
Volkoff, and Snyder in the 1930's, that people thought seriously
about the
possibility that such objects might actually exist in the Universe.
(Yes,
this is the same Oppenheimer who ran the Manhattan Project.)
These
researchers showed that when a sufficiently massive star runs
out of fuel,
it is unable to support itself against its own gravitational
pull, and it
should collapse into a black hole.
In general relativity, gravity is a manifestation of the curvature
of
spacetime. Massive objects distort space and time, so that the
usual rules
of geometry don't apply anymore. Near a black hole, this distortion
of space
is extremely severe and causes black holes to have some very
strange
properties. In particular, a black hole has something called
an 'event
horizon.' This is a spherical surface that marks the boundary
of the black
hole. You can pass in through the horizon, but you can't get
back out. In
fact, once you've crossed the horizon, you're doomed to move
inexorably
closer and closer to the 'singularity' at the center of the black
hole.
You can think of the horizon as the place where the escape
velocity equals
the velocity of light. Outside of the horizon, the escape velocity
is less
than the speed of light, so if you fire your rockets hard enough,
you can
give yourself enough energy to get away. But if you find yourself
inside the
horizon, then no matter how powerful your rockets are, you can't
escape.
The horizon has some very strange geometrical properties.
To an observer who
is sitting still somewhere far away from the black hole, the
horizon seems
to be a nice, static, unmoving spherical surface. But once you
get close to
the horizon, you realize that it has a very large velocity. In
fact, it is
moving outward at the speed of light! That explains why it is
easy to cross
the horizon in the inward direction, but impossible to get back
out. Since
the horizon is moving out at the speed of light, in order to
escape back
across it, you would have to travel faster than light. You can't
go faster
than light, and so you can't escape from the black hole.
(If all of this sounds very strange, don't worry. It is strange.
The horizon
is in a certain sense sitting still, but in another sense it
is flying out
at the speed of light. It's a bit like Alice in "Through
the Looking-Glass":
she has to run as fast as she can just to stay in one place.)
Once you're inside of the horizon, spacetime is distorted
so much that the
coordinates describing radial distance and time switch roles.
That is, "r",
the coordinate that describes how far away you are from the center,
is a
timelike coordinate, and "t" is a spacelike one. One
consequence of this is
that you can't stop yourself from moving to smaller and smaller
values of r,
just as under ordinary circumstances you can't avoid moving towards
the
future (that is, towards larger and larger values of t). Eventually,
you're
bound to hit the singularity at r = 0. You might try to avoid
it by firing
your rockets, but it's futile: no matter which direction you
run, you can't
avoid your future. Trying to avoid the center of a black hole
once you've
crossed the horizon is just like trying to avoid next Thursday.
Incidentally, the name 'black hole' was invented by John Archibald
Wheeler,
and seems to have stuck because it was much catchier than previous
names.
Before Wheeler came along, these objects were often referred
to as 'frozen
stars.' I'll explain why below.
Back to Black Hole Question List
How big is a black hole?
------------------------
There are at least two different ways to describe how big something
is. We
can say how much mass it has, or we can say how much space it
takes up.
Let's talk first about the masses of black holes.
There is no limit in principle to how much or how little mass
a black hole
can have. Any amount of mass at all can in principle be made
to form a black
hole if you compress it to a high enough density. We suspect
that most of
the black holes that are actually out there were produced in
the deaths of
massive stars, and so we expect those black holes to weigh about
as much as
a massive star. A typical mass for such a stellar black hole
would be about
10 times the mass of the Sun, or about 10^{31} kilograms. (Here
I'm using
scientific notation: 10^{31} means a 1 with 31 zeroes after it,
or
10,000,000,000,000,000,000,000,000,000,000.) Astronomers also
suspect that
many galaxies harbor extremely massive black holes at their centers.
These
are thought to weigh about a million times as much as the Sun,
or 10^{36}
kilograms.
The more massive a black hole is, the more space it takes
up. In fact, the
Schwarzschild radius (which means the radius of the horizon)
and the mass
are directly proportional to one another: if one black hole weighs
ten times
as much as another, its radius is ten times as large. A black
hole with a
mass equal to that of the Sun would have a radius of 3 kilometers.
So a
typical 10-solar-mass black hole would have a radius of 30 kilometers,
and a
million-solar-mass black hole at the center of a galaxy would
have a radius
of 3 million kilometers. Three million kilometers may sound like
a lot, but
it's actually not so big by astronomical standards. The Sun,
for example,
has a radius of about 700,000 kilometers, and so that supermassive
black
hole has a radius only about four times bigger than the Sun.
Back to Black Hole Question List
What would happen to me if I fell into a black hole?
----------------------------------------------------
Let's suppose that you get into your spaceship and point it straight
towards
the million-solar-mass black hole in the center of our galaxy.
(Actually,
there's some debate about whether our galaxy contains a central
black hole,
but let's assume it does for the moment.) Starting from a long
way away from
the black hole, you just turn off your rockets and coast in.
What happens?
At first, you don't feel any gravitational forces at all.
Since you're in
free fall, every part of your body and your spaceship is being
pulled in the
same way, and so you feel weightless. (This is exactly the same
thing that
happens to astronauts in Earth orbit: even though both astronauts
and space
shuttle are being pulled by the Earth's gravity, they don't feel
any
gravitational force because everything is being pulled in exactly
the same
way.) As you get closer and closer to the center of the hole,
though, you
start to feel "tidal" gravitational forces. Imagine
that your feet are
closer to the center than your head. The gravitational pull gets
stronger as
you get closer to the center of the hole, so your feet feel a
stronger pull
than your head does. As a result you feel "stretched."
(This force is called
a tidal force because it is exactly like the forces that cause
tides on
earth.) These tidal forces get more and more intense as you get
closer to
the center, and eventually they will rip you apart.
For a very large black hole like the one you're falling into,
the tidal
forces are not really noticeable until you get within about 600,000
kilometers of the center. Note that this is after you've crossed
the
horizon. If you were falling into a smaller black hole, say one
that weighed
as much as the Sun, tidal forces would start to make you quite
uncomfortable
when you were about 6000 kilometers away from the center, and
you would have
been torn apart by them long before you crossed the horizon.
(That's why we
decided to let you jump into a big black hole instead of a small
one: we
wanted you to survive at least until you got inside.)
What do you see as you are falling in? Surprisingly, you don't
necessarily
see anything particularly interesting. Images of faraway objects
may be
distorted in strange ways, since the black hole's gravity bends
light, but
that's about it. In particular, nothing special happens at the
moment when
you cross the horizon. Even after you've crossed the horizon,
you can still
see things on the outside: after all, the light from the things
on the
outside can still reach you. No one on the outside can see you,
of course,
since the light from you can't escape past the horizon.
How long does the whole process take? Well, of course, it
depends on how far
away you start from. Let's say you start at rest from a point
whose distance
from the singularity is ten times the black hole's radius. Then
for a
million-solar-mass black hole, it takes you about 8 minutes to
reach the
horizon. Once you've gotten that far, it takes you only another
seven
seconds to hit the singularity. By the way, this time scales
with the size
of the black hole, so if you'd jumped into a smaller black hole,
your time
of death would be that much sooner.
Once you've crossed the horizon, in your remaining seven seconds,
you might
panic and start to fire your rockets in a desperate attempt to
avoid the
singularity. Unfortunately, it's hopeless, since the singularity
lies in
your future, and there's no way to avoid your future. In fact,
the harder
you fire your rockets, the sooner you hit the singularity. It's
best just to
sit back and enjoy the ride.
Back to Black Hole Question List
My friend Penelope is sitting still at a safe distance, watching
me fall
into the black hole. What does she see?
-------------------------------------------------------------------
Penelope sees things quite differently from you. As you get closer
and
closer to the horizon, she sees you move more and more slowly.
In fact, no
matter how long she waits, she will never quite see you reach
the horizon.
In fact, more or less the same thing can be said about the
material that
formed the black hole in the first place. Suppose that the black
hole formed
from a collapsing star. As the material that is to form the black
hole
collapses, Penelope sees it get smaller and smaller, approaching
but never
quite reaching its Schwarzschild radius. This is why black holes
were
originally called frozen stars: because they seem to 'freeze'
at a size just
slightly bigger than the Schwarzschild radius.
Why does she see things this way? The best way to think about
it is that
it's really just an optical illusion. It doesn't really take
an infinite
amount of time for the black hole to form, and it doesn't really
take an
infinite amount of time for you to cross the horizon. (If you
don't believe
me, just try jumping in! You'll be across the horizon in eight
minutes, and
crushed to death mere seconds later.) As you get closer and closer
to the
horizon, the light that you're emitting takes longer and longer
to climb
back out to reach Penelope. In fact, the radiation you emit right
as you
cross the horizon will hover right there at the horizon forever
and never
reach her. You've long since passed through the horizon, but
the light
signal telling her that won't reach her for an infinitely long
time.
There is another way to look at this whole business. In a
sense, time really
does pass more slowly near the horizon than it does far away.
Suppose you
take your spaceship and ride down to a point just outside the
horizon, and
then just hover there for a while (burning enormous amounts of
fuel to keep
yourself from falling in). Then you fly back out and rejoin Penelope.
You
will find that she has aged much more than you during the whole
process;
time passed more slowly for you than it did for her.
So which of these two explanation (the optical-illusion one
or the
time-slowing-down one) is really right? The answer depends on
what system of
coordinates you use to describe the black hole. According to
the usual
system of coordinates, called "Schwarzschild coordinates,"
you cross the
horizon when the time coordinate t is infinity. So in these coordinates
it
really does take you infinite time to cross the horizon. But
the reason for
that is that Schwarzschild coordinates provide a highly distorted
view of
what's going on near the horizon. In fact, right at the horizon
the
coordinates are infinitely distorted (or, to use the standard
terminology,
"singular"). If you choose to use coordinates that
are not singular near the
horizon, then you find that the time when you cross the horizon
is indeed
finite, but the time when Penelope sees you cross the horizon
is infinite.
It took the radiation an infinite amount of time to reach her.
In fact,
though, you're allowed to use either coordinate system, and so
both
explanations are valid. They're just different ways of saying
the same
thing.
In practice, you will actually become invisible to Penelope
before too much
time has passed. For one thing, light is "redshifted"
to longer wavelengths
as it rises away from the black hole. So if you are emitting
visible light
at some particular wavelength, Penelope will see light at some
longer
wavelength. The wavelengths get longer and longer as you get
closer and
closer to the horizon. Eventually, it won't be visible light
at all: it will
be infrared radiation, then radio waves. At some point the wavelengths
will
be so long that she'll be unable to observe them. Furthermore,
remember that
light is emitted in individual packets called photons. Suppose
you are
emitting photons as you fall past the horizon. At some point,
you will emit
your last photon before you cross the horizon. That photon will
reach
Penelope at some finite time -- typically less than an hour for
that
million-solar-mass black hole -- and after that she'll never
be able to see
you again. (After all, none of the photons you emit *after* you
cross the
horizon will ever get to her.)
Back to Black Hole Question List
If a black hole existed, would it suck up all the matter in
the Universe?
---------------------------------------------------------------
Heck, no. A black hole has a "horizon," which means
a region from which you
can't escape. If you cross the horizon, you're doomed to eventually
hit the
singularity. But as long as you stay outside of the horizon,
you can avoid
getting sucked in. In fact, to someone well outside of the horizon,
the
gravitational field surrounding a black hole is no different
from the field
surrounding any other object of the same mass. In other words,
a
one-solar-mass black hole is no better than any other one-solar-mass
object
(such as, for example, the Sun) at "sucking in" distant
objects.
Back to Black Hole Question List
What if the Sun became a black hole?
------------------------------------
Well, first, let me assure you that the Sun has no intention
of doing any
such thing. Only stars that weigh considerably more than the
Sun end their
lives as black holes. The Sun is going to stay roughly the way
it is for
another five billion years or so. Then it will go through a brief
phase as a
red giant star, during which time it will expand to engulf the
planets
Mercury and Venus, and make life quite uncomfortable on Earth
(oceans
boiling, atmosphere escaping, that sort of thing). After that,
the Sun will
end its life by becoming a boring white dwarf star. If I were
you, I'd make
plans to move somewhere far away before any of this happens.
I also wouldn't
buy any of those 8-billion-year government bonds.
But I digress. What if the Sun *did* become a black hole for
some reason?
The main effect is that it would get very dark and very cold
around here.
The Earth and the other planets would not get sucked into the
black hole;
they would keep on orbiting in exactly the same paths they follow
right now.
Why? Because the horizon of this black hole would be very small
-- only
about 3 kilometers -- and as we observed above, as long as you
stay well
outside the horizon, a black hole's gravity is no stronger than
that of any
other object of the same mass.
Back to Black Hole Question List
Is there any evidence that black holes exist?
---------------------------------------------
Yes. You can't see a black hole directly, of course, since light
can't get
past the horizon. That means that we have to rely on indirect
evidence that
black holes exist.
Suppose you have found a region of space where you think there
might be a
black hole. How can you check whether there is one or not? The
first thing
you'd like to do is measure how much mass there is in that region.
If you've
found a large mass concentrated in a small volume, and if the
mass is dark,
then it's a good guess that there's a black hole there. There
are two kinds
of systems in which astronomers have found such compact, massive,
dark
objects: the centers of galaxies (including perhaps our own Milky
Way
Galaxy), and X-ray-emitting binary systems in our own Galaxy.
According to a recent review by Kormendy and Richstone (to
appear in the
1995 edition of "Annual Reviews of Astronomy and Astrophysics"),
eight
galaxies have been observed to contain such massive dark objects
in their
centers. The masses of the cores of these galaxies range from
one million to
several billion times the mass of the Sun. The mass is measured
by observing
the speed with which stars and gas orbit around the center of
the galaxy:
the faster the orbital speeds, the stronger the gravitational
force required
to hold the stars and gas in their orbits. (This is the most
common way to
measure masses in astronomy. For example, we measure the mass
of the Sun by
observing how fast the planets orbit it, and we measure the amount
of dark
matter in galaxies by measuring how fast things orbit at the
edge of the
galaxy.)
These massive dark objects in galactic centers are thought
to be black holes
for at least two reasons. First, it is hard to think of anything
else they
could be: they are too dense and dark to be stars or clusters
of stars.
Second, the only promising theory to explain the enigmatic objects
known as
quasars and active galaxies postulates that such galaxies have
supermassive
black holes at their cores. If this theory is correct, then a
large fraction
of galaxies -- all the ones that are now or used to be active
galaxies --
must have supermassive black holes at the center. Taken together,
these
arguments strongly suggest that the cores of these galaxies contain
black
holes, but they do not constitute absolute proof.
Two very recent discovery has been made that strongly support
the hypothesis
that these systems do indeed contain black holes. First, a nearby
active
galaxy was found to have a "water maser" system (a
very powerful source of
microwave radiation) near its nucleus. Using the technique of
very-long-baseline interferometry, a group of researchers was
able to map
the velocity distribution of the gas with very fine resolution.
In fact,
they were able to measure the velocity within less than half
a light-year of
the center of the galaxy. From this measurement they can conclude
that the
massive object at the center of this galaxy is less than half
a light-year
in radius. It is hard to imagine anything other than a black
hole that could
have so much mass concentrated in such a small volume. (This
result was
reported by Miyoshi et al. in the 12 January 1995 issue of Nature,
vol. 373,
p. 127.)
A second discovery provides even more compelling evidence.
X-ray astronomers
have detected a spectral line from one galactic nucleus that
indicates the
presence of atoms near the nucleus that are moving extremely
fast (about 1/3
the speed of light). Furthermore, the radiation from these atoms
has been
redshifted in just the manner one would expect for radiation
coming from
near the horizon of a black hole. These observations would be
very difficult
to explain in any other way besides a black hole, and if they
are verified,
then the hypothesis that some galaxies contain supermassive black
holes at
their centers would be fairly secure. (This result was reported
in the 22
June 1995 issue of Nature, vol. 375, p. 659, by Tanaka et al.)
A completely different class of black-hole candidates may
be found in our
own Galaxy. These are much lighter, stellar-mass black holes,
which are
thought to form when a massive star ends its life in a supernova
explosion.
If such a stellar black hole were to be off somewhere by itself,
we wouldn't
have much hope of finding it. However, many stars come in binary
systems --
pairs of stars in orbit around each other. If one of the stars
in such a
binary system becomes a black hole, we might be able to detect
it. In
particular, in some binary systems containing a compact object
such as a
black hole, matter is sucked off of the other object and forms
an "accretion
disk" of stuff swirling into the black hole. The matter
in the accretion
disk gets very hot as it falls closer and closer to the black
hole, and it
emits copious amounts of radiation, mostly in the X-ray part
of the
spectrum. Many such "X-ray binary systems" are known,
and some of them are
thought to be likely black-hole candidates.
Suppose you've found an X-ray binary system. How can you tell
whether the
unseen compact object is a black hole? Well, one thing you'd
certainly like
to do is to estimate its mass. By measuring the orbital speed
of visible
star (together with a few other things), you can figure out the
mass of the
invisible companion. (The technique is quite similar to the one
we described
above for supermassive black holes in galactic centers: the faster
the star
is moving, the stronger the gravitational force required to keep
it in
place, and so the more massive the invisible companion.) If the
mass of the
compact object is found to be very large very large, then there
is no kind
of object we know about that it could be other than a black hole.
(An
ordinary star of that mass would be visible. A stellar remnant
such as a
neutron star would be unable to support itself against gravity,
and would
collapse to a black hole.) The combination of such mass estimates
and
detailed studies of the radiation from the accretion disk can
supply
powerful circumstantial evidence that the object in question
is indeed a
black hole.
Many of these "X-ray binary" systems are known,
and in some cases the
evidence in support of the black-hole hypothesis is quite strong.
In a
review article in the 1992 issue of Annual Reviews of Astronomy
and
Astrophysics, Anne Cowley summarized the situation by saying
that there were
three such systems known (two in our galaxy and one in the nearby
Large
Magellanic Cloud) for which very strong evidence exists that
the mass of the
invisible object is too large to be anything but a black hole.
There are
many more such objects that are thought to be likely black holes
on the
basis of slightly less evidence. Furthermore, this field of research
has
been very active since 1992, and the number of strong candidates
by now is
larger than three.
Back to Black Hole Question List
How do black holes evaporate?
-----------------------------
This is a tough one. Back in the 1970's, Stephen Hawking came
up with
theoretical arguments showing that black holes are not really
entirely
black: due to quantum-mechanical effects, they emit radiation.
The energy
that produces the radiation comes from the mass of the black
hole.
Consequently, the black hole gradually shrinks. It turns out
that the rate
of radiation increases as the mass decreases, so the black hole
continues to
radiate more and more intensely and to shrink more and more rapidly
until it
presumably vanishes entirely.
Actually, nobody is really sure what happens at the last stages
of black
hole evaporation: some researchers think that a tiny, stable
remnant is left
behind. Our current theories simply aren't good enough to let
us tell for
sure one way or the other. As long as I'm disclaiming, let me
add that the
entire subject of black hole evaporation is extremely speculative.
It
involves figuring out how to perform quantum-mechanical (or rather
quantum-field-theoretic) calculations in curved spacetime, which
is a very
difficult task, and which gives results that are essentially
impossible to
test with experiments. Physicists *think* that we have the correct
theories
to make predictions about black hole evaporation, but without
experimental
tests it's impossible to be sure.
Now why do black holes evaporate? Here's one way to look at
it, which is
only moderately inaccurate. (I don't think it's possible to do
much better
than this, unless you want to spend a few years learning about
quantum field
theory in curved space.) One of the consequences of the uncertainty
principle of quantum mechanics is that it's possible for the
law of energy
conservation to be violated, but only for very short durations.
The Universe
is able to produce mass and energy out of nowhere, but only if
that mass and
energy disappear again very quickly. One particular way in which
this
strange phenomenon manifests itself goes by the name of vacuum
fluctuations.
Pairs consisting of a particle and antiparticle can appear out
of nowhere,
exist for a very short time, and then annihilate each other.
Energy
conservation is violated when the particles are created, but
all of that
energy is restored when they annihilate again. As weird as all
of this
sounds, we have actually confirmed experimentally that these
vacuum
fluctuations are real.
Now, suppose one of these vacuum fluctuations happens near
the horizon of a
black hole. It may happen that one of the two particles falls
across the
horizon, while the other one escapes. The one that escapes carries
energy
away from the black hole and may be detected by some observer
far away. To
that observer, it will look like the black hole has just emitted
a particle.
This process happens repeatedly, and the observer sees a continuous
stream
of radiation from the black hole.
Back to Black Hole Question List
Won't the black hole have evaporated out from under me before
I reach it?
---------------------------------------------------------------------
We've observed that, from the point of view of your friend Penelope
who
remains safely outside of the black hole, it takes you an infinite
amount of
time to cross the horizon. We've also observed that black holes
evaporate
via Hawking radiation in a finite amount of time. So by the time
you reach
the horizon, the black hole will be gone, right?
Wrong. When we said that Penelope would see it take forever
for you to cross
the horizon, we were imagining a non-evaporating black hole.
If the black
hole is evaporating, that changes things. Your friend will see
you cross the
horizon at the exact same moment she sees the black hole evaporate.
Let me
try to describe why this is true.
Remember what we said before: Penelope is the victim of an
optical illusion.
The light that you emit when you're very near the horizon (but
still on the
outside) takes a very long time to climb out and reach her. If
the black
hole lasts forever, then the light may take arbitrarily long
to get out, and
that's why she doesn't see you cross the horizon for a very long
(even an
infinite) time. But once the black hole has evaporated, there's
nothing to
stop the light that carries the news that you're about to cross
the horizon
from reaching her. In fact, it reaches her at the same moment
as that last
burst of Hawking radiation. Of course, none of that will matter
to you:
you've long since crossed the horizon and been crushed at the
singularity.
Sorry about that, but you should have thought about it before
you jumped in.
Back to Black Hole Question List
What is a white hole?
---------------------
The equations of general relativity have an interesting mathematical
property: they are symmetric in time. That means that you can
take any
solution to the equations and imagine that time flows backwards
rather than
forwards, and you'll get another valid solution to the equations.
If you
apply this rule to the solution that describes black holes, you
get an
object known as a white hole. Since a black hole is a region
of space from
which nothing can escape, the time-reversed version of a black
hole is a
region of space into which nothing can fall. In fact, just as
a black hole
can only suck things in, a white hole can only spit things out.
White holes are a perfectly valid mathematical solution to
the equations of
general relativity, but that doesn't mean that they actually
exist in
nature. In fact, they almost certainly do not exist, since there's
no way to
produce one. (Producing a white hole is just as impossible as
destroying a
black hole, since the two processes are time-reversals of each
other.)
Back to Black Hole Question List
What is a wormhole?
-------------------
So far, we have only considered ordinary "vanilla"
black holes.
Specifically, we have been talking all along about black holes
that are not
rotating and have no electric charge. If we consider black holes
that rotate
and/or have charge, things get more complicated. In particular,
it is
possible to fall into such a black hole and not hit the singularity.
In
effect, the interior of a charged or rotating black hole can
"join up" with
a corresponding white hole in such a way that you can fall into
the black
hole and pop out of the white hole. This combination of black
and white
holes is called a wormhole.
The white hole may be somewhere very far away from the black
hole; indeed,
it may even be in a "different Universe" -- that is,
a region of spacetime
that, aside from the wormhole itself, is completely disconnected
from our
own region. A conveniently-located wormhole would therefore provide
a
convenient and rapid way to travel very large distances, or even
to travel
to another Universe. Maybe the exit to the wormhole would lie
in the past,
so that you could travel back in time by going through. All in
all, they
sound pretty cool.
But before you apply for that research grant to go search
for them, there
are a couple of things you should know. First of all, wormholes
almost
certainly do not exist. As we said above in the section on white
holes, just
because something is a valid mathematical solution to the equations
doesn't
mean that it actually exists in nature. In particular, black
holes that form
from the collapse of ordinary matter (which includes all of the
black holes
that we think exist) do not form wormholes. If you fall into
one of those,
you're not going to pop out anywhere. You're going to hit a singularity,
and
that's all there is to it.
Furthermore, even if a wormhole were formed, it is thought
that it would not
be stable. Even the slightest perturbation (including the perturbation
caused by your attempt to travel through it) would cause it to
collapse.
Finally, even if wormholes exist and are stable, they are
quite unpleasant
to travel through. Radiation that pours into the wormhole (from
nearby
stars, the cosmic microwave background, etc.) gets blueshifted
to very high
frequencies. As you try to pass through the wormhole, you will
get fried by
these X-rays and gamma rays.
Back to Black Hole Question List
Where can I go to learn more about black holes?
-----------------------------------------------
Let me begin by acknowledging that I cribbed some of the above
material from
the article about black holes in the Frequently Asked Questions
list for the
Usenet newsgroup sci.physics. The sci.physics FAQ is posted monthly
to
sci.physics and is also available by anonymous ftp from rtfm.mit.edu
(and
probably other places). The article about black holes, which
is excellent,
was written by Matt McIrvin. The FAQ contains other neat things
too.
There are lots of books out there about black holes and related
matters. Kip
Thorne's "Black Holes and Time Warps: Einstein's Outrageous
Legacy" is a
good one. William Kaufmann's "Black Holes and Warped Spacetime"
is also
worth reading. R. Wald's "Space, Time, and Gravity"
is an exposition of
general relativity for non-scientists. I haven't read it myself,
but I've
heard good things about it.
Both of these books are aimed at readers without much background
in physics.
If you want more "meat" (i.e., more mathematics), then
you probably start
with a book on the basics of relativity theory. The best introduction
to the
subject is "Spacetime Physics" by E.F. Taylor and J.A.
Wheeler. (This book
is mostly about special relativity, but the last chapter discusses
the
general theory.) Taylor and Wheeler have been threatening for
about two
years now to publish a sequel entitled "Scouting Black Holes,"
which should
be quite good if it ever comes out. "Spacetime Physics"
does not assume that
you know vast amounts of physics, but it does assume that you're
willing to
work hard at understanding this stuff. It is not light reading,
although it
is more playful and less intimidating than most physics books.
Finally, if "Spacetime Physics" isn't enough for
you, you could try any of
several introductions to general relativity. B. Schutz's "A
First Course in
General Relativity" and W. Rindler's "Essential Relativity"
are a couple of
possibilities. And for the extremely valiant reader with an excellent
background in physics, there's the granddaddy of all books on
general
relativity, Misner, Thorne, and Wheeler's "Gravitation."
R. Wald's book
"General Relativity" is at a comparable level to "Gravitation,"
although the
styles of the two books are enormously different. What little
I know about
black-hole evaporation comes from Wald's book. Let me emphasize
that all of
these books, and especially the last two, assume that you know
quite a lot
of physics. They are not for the faint of heart.
Back to Black Hole Question List
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September 1995
Science
& Mathematics
The Uncle
Taz Library
Uncle
Taz Home Page 2
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