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整理人: jeter(2000-02-11 16:34:59), 站内信件
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以下的文章是关于黑洞的FAQ,下载自伯克利大学的一个网站。
具体域名记不起了，反正在www.yahoo.com上敲下black holes
大约就能找到。
本人读后，觉得不错，但由于水平有限，时间有限，无法完全
译成中文，只好当一回搬运工，在此帖出，希望读得懂的朋友
能喜欢，读不懂的朋友不要当做灌水。
Black Holes FAQ
What is a black hole?
---------------------
Loosely speaking, a black hole is a region of space that has so much m
ass
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 re
sults
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 wil
l
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 h
ard
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 thro
w the
rock in order that it just barely escapes the planet's gravity is call
ed
the "escape velocity." As you would expect, the escape velocity depend
s on
the mass of the planet: if the planet is extremely massive, then its g
ravity
is very strong, and the escape velocity is high. A lighter planet woul
d 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 escap
e
velocity. The Earth's escape velocity is 11.2 kilometers per second (a
bout
25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second (ab
out
5300 m.p.h.).
Now imagine an object with such an enormous concentration of mass in s
uch
a small radius that its escape velocity was greater than the velocity
of
light. Then, since nothing can go faster than light, nothing can escap
e
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 tra
pped
goes all the way back to Laplace in the 18th century. Almost immediate
ly
after Einstein developed general relativity, Karl Schwarzschild discov
ered
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 st
ar
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 r
ules
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 b
lack
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 inexorabl
y
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 eq
uals
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 insi
de
the horizon, then no matter how powerful your rockets are, you can't
escape.
The horizon has some very strange geometrical properties. To an observ
er
who is sitting still somewhere far away from the black hole, the horiz
on
seems to be a nice, static, unmoving spherical surface. But once you g
et
close to the horizon, you realize that it has a very large velocity. I
n
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 g
et
back out. Since the horizon is moving out at the speed of light, in or
der
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 hol
e.
(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 i
s
flying out at the speed of light. It's a bit like Alice in "Through th
e
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 th
is 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, y
ou're
bound to hit the singularity at r = 0. You might try to avoid it by fi
ring
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 Whe
eler,
and seems to have stuck because it was much catchier than previous nam
es.
Before Wheeler came along, these objects were often referred to as 'fr
ozen
stars.' I'll explain why below.
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 death
s of
massive stars, and so we expect those black holes to weigh about as mu
ch 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 us
ing
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. T
hese
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 m
ass
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 wit
h 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 lik
e 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 superma
ssive
black hole has a radius only about four times bigger than the Sun.
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 t
owards
the million-solar-mass black hole in the center of our galaxy. (Actual
ly,
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 awa
y from
the black hole, you just turn off your rockets and coast in. What happ
ens?
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 th
ing
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 stron
ger
as you get closer to the center of the hole, so your feet feel a stron
ger
pull than your head does. As a result you feel "stretched." (This forc
e 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 clo
ser
to the center, and eventually they will rip you apart.
For a very large black hole like the one you're falling into, the tida
l
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 go
t
inside.)
What do you see as you are falling in? Surprisingly, you don't necessa
rily
see anything particularly interesting. Images of faraway objects may b
e
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 cou
rse,
since the light from you can't escape past the horizon.
How long does the whole process take? Well, of course, it depends on h
ow
far away you start from. Let's say you start at rest from a point whos
e
distance from the singularity is ten times the black hole's radius. Th
en
for a million-solar-mass black hole, it takes you about 8 minutes to r
each
the horizon. Once you've gotten that far, it takes you only another se
ven
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 t
he
singularity. Unfortunately, it's hopeless, since the singularity lies
in
your future, and there's no way to avoid your future. In fact, the har
der
you fire your rockets, the sooner you hit the singularity. It's best j
ust
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 fa
ll
----------------------------------------------------------------------
--
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 th
at
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 bla
ck
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 siz
e
just slightly bigger than the Schwarzschild radius.
Why does she see things this way? The best way to think about it is th
at
it's really just an optical illusion. It doesn't really take an infini
te
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 be
lieve
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 cli
mb
back out to reach Penelope. In fact, the radiation you emit right as y
ou
cross the horizon will hover right there at the horizon forever and ne
ver
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 amoun
ts of
fuel to keep yourself from falling in). Then you fly back out and rejo
in
Penelope. You will find that she has aged much more than you during th
e
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 sys
tem
of coordinates you use to describe the black hole. According to the us
ual
system of coordinates, called "Schwarzschild coordinates," you cross t
he
horizon when the time coordinate t is infinity. So in these coordinate
s it
really does take you infinite time to cross the horizon. But the reaso
n 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 terminol
ogy,
"singular"). If you choose to use coordinates that are not singular ne
ar
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 i
s
infinite. It took the radiation an infinite amount of time to reach he
r.
In fact, though, you're allowed to use either coordinate system, and s
o
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 wavele
ngths
as it rises away from the black hole. So if you are emitting visible l
ight
at some particular wavelength, Penelope will see light at some longer
wavelength. The wavelengths get longer and longer as you get closer an
d
closer to the horizon. Eventually, it won't be visible light at all: i
t
will be infrared radiation, then radio waves. At some point the wavele
ngths
will be so long that she'll be unable to observe them. Furthermore, re
member
that light is emitted in individual packets called photons. Suppose yo
u 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 t
o
see you again. (After all, none of the photons you emit *after* you cr
oss
the horizon will ever get to her.)
If a black hole existed, would it suck up all the matter in the Univer
se?
---------------------------------------------------------------
Heck, no. A black hole has a "horizon," which means a region from whic
h you
can't escape. If you cross the horizon, you're doomed to eventually hi
t the
singularity. But as long as you stay outside of the horizon, you can a
void
getting sucked in. In fact, to someone well outside of the horizon, th
e
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 o
bject
(such as, for example, the Sun) at "sucking in" distant objects.
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 t
heir
lives as black holes. The Sun is going to stay roughly the way it is f
or
another five billion years or so. Then it will go through a brief phas
e as
a red giant star, during which time it will expand to engulf the plane
ts
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 reas
on?
The main effect is that it would get very dark and very cold around he
re.
The Earth and the other planets would not get sucked into the black ho
le;
they would keep on orbiting in exactly the same paths they follow righ
t now.
Why? Because the horizon of this black hole would be very small -- onl
y
about 3 kilometers -- and as we observed above, as long as you stay we
ll
outside the horizon, a black hole's gravity is no stronger than that o
f any
other object of the same mass.
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 b
e a
black hole. How can you check whether there is one or not? The first t
hing
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 t
he
1995 edition of "Annual Reviews of Astronomy and Astrophysics"), eight
galaxies have been observed to contain such massive dark objects in th
eir
centers. The masses of the cores of these galaxies range from one mill
ion
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 o
f 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 o
f the
Sun by observing how fast the planets orbit it, and we measure the amo
unt 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 kno
wn as
quasars and active galaxies postulates that such galaxies have superma
ssive
black holes at their cores. If this theory is correct, then a large fr
action
of galaxies -- all the ones that are now or used to be active galaxies
--
must have supermassive black holes at the center. Taken together, thes
e
arguments strongly suggest that the cores of these galaxies contain bl
ack
holes, but they do not constitute absolute proof.
Two very recent discovery has been made that strongly support the hypo
thesis
that these systems do indeed contain black holes. First, a nearby acti
ve
galaxy was found to have a "water maser" system (a very powerful sourc
e 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 fac
t,
they were able to measure the velocity within less than half a light-y
ear
of the center of the galaxy. From this measurement they can conclude t
hat
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 blac
k
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 astro
nomers
have detected a spectral line from one galactic nucleus that indicates
the
presence of atoms near the nucleus that are moving extremely fast (abo
ut 1/3
the speed of light). Furthermore, the radiation from these atoms has b
een
redshifted in just the manner one would expect for radiation coming fr
om
near the horizon of a black hole. These observations would be very dif
ficult
to explain in any other way besides a black hole, and if they are veri
fied,
then the hypothesis that some galaxies contain supermassive black hole
s 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 ar
e
thought to form when a massive star ends its life in a supernova explo
sion.
If such a stellar black hole were to be off somewhere by itself, we wo
uldn't
have much hope of finding it. However, many stars come in binary syste
ms --
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 "acc
retion
disk" of stuff swirling into the black hole. The matter in the accreti
on
disk gets very hot as it falls closer and closer to the black hole, an
d 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 visib
le
star (together with a few other things), you can figure out the mass o
f the
invisible companion. (The technique is quite similar to the one we des
cribed
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 o
f 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 wo
uld
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 ther
e
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 o
n the
basis of slightly less evidence. Furthermore, this field of research h
as
been very active since 1992, and the number of strong candidates by no
w is
larger than three.
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 ene
rgy
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 contin
ues
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 bla
ck
hole evaporation: some researchers think that a tiny, stable remnant i
s
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 impossibl
e to
test with experiments. Physicists *think* that we have the correct the
ories
to make predictions about black hole evaporation, but without experime
ntal
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 be
tter
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 en
ergy
conservation to be violated, but only for very short durations. The Un
iverse
is able to produce mass and energy out of nowhere, but only if that ma
ss
and energy disappear again very quickly. One particular way in which t
his
strange phenomenon manifests itself goes by the name of vacuum fluctua
tions.
Pairs consisting of a particle and antiparticle can appear out of nowh
ere,
exist for a very short time, and then annihilate each other. Energy
conservation is violated when the particles are created, but all of th
at
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 t
he
horizon, while the other one escapes. The one that escapes carries ene
rgy
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 par
ticle.
This process happens repeatedly, and the observer sees a continuous st
ream
of radiation from the black hole.
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 wh
o
remains safely outside of the black hole, it takes you an infinite amo
unt
of time to cross the horizon. We've also observed that black holes eva
porate
via Hawking radiation in a finite amount of time. So by the time you r
each
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 y
ou
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 rea
ch
her. If the black hole lasts forever, then the light may take arbitrar
ily
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 th
at
you're about to cross the horizon from reaching her. In fact, it reach
es
her at the same moment as that last burst of Hawking radiation. Of cou
rse,
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.
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 y
ou
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 equatio
ns 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 destr
oying
a black hole, since the two processes are time-reversals of each other
.)
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 ar
e 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. I
n
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 t
he
black hole and pop out of the white hole. This combination of black an
d
white holes is called a wormhole.
The white hole may be somewhere very far away from the black hole; ind
eed,
it may even be in a "different Universe" -- that is, a region of space
time
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 tr
avel
to another Universe. Maybe the exit to the wormhole would lie in the p
ast,
so that you could travel back in time by going through. All in all, th
ey
sound pretty cool.
But before you apply for that research grant to go search for them, th
ere
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 equatio
ns
doesn't mean that it actually exists in nature. In particular, black h
oles
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 in
to
one of those, you're not going to pop out anywhere. You're going to hi
t a
singularity, and that's all there is to it.
Furthermore, even if a wormhole were formed, it is thought that it wou
ld
not be stable. Even the slightest perturbation (including the perturba
tion
caused by your attempt to travel through it) would cause it to collaps
e.
Finally, even if wormholes exist and are stable, they are quite unplea
sant
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 fri
ed by
these X-rays and gamma rays.
Where can I go to learn more about black holes?
-----------------------------------------------
Let me begin by acknowledging that I cribbed some of the above materia
l
from the article about black holes in the Frequently Asked Questions l
ist
for the Usenet newsgroup sci.physics. The sci.physics FAQ is posted mo
nthly
to sci.physics and is also available by anonymous ftp from rtfm.mit.ed
u
(and probably other places). The article about black holes, which is
excellent, was written by Matt McIrvin. The FAQ contains other neat th
ings
too.
There are lots of books out there about black holes and related matter
s.
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 a
lso
worth reading. R. Wald's "Space, Time, and Gravity" is an exposition o
f
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 bes
t
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 c
hapter
discusses the general theory.) Taylor and Wheeler have been threatenin
g 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" d
oes
not assume that you know vast amounts of physics, but it does assume t
hat
you're willing to work hard at understanding this stuff. It is not lig
ht
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 an
y of
several introductions to general relativity. B. Schutz's "A First Cour
se in
General Relativity" and W. Rindler's "Essential Relativity" are a coup
le 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 boo
k
"General Relativity" is at a comparable level to "Gravitation," althou
gh
the styles of the two books are enormously different. What little I kn
ow
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.
September 1995
--
在宇宙中流浪了一百亿年的光子，
希望有一天飞进你的眼中，在你
的脑海深处的某个神经末梢上，
引起一丝难以察觉的颤傈。
※ 来源:．月光软件站 http://www.moon-soft.com．[FROM: 202.100.28.39]
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