Robert W. Bly
"Because
nothing was, therefore all things are."
-Edgar Allan
Poe
"A black
hole is a cannibal, swallowing up everything that gets in its way."
-John G.
Taylor
Black
holes are among the most interesting and unusual objects to be found in the
universe. If they really exist, they might very well be the most dangerous
objects in the universe as well, since they are capable of destroying anything
that gets in their path, even entire planets. To understand what a black hole
is, we shall take a look at where they come from.
Black
holes are formed from collapsed stars, yet they themselves are not solid
bodies. Rather, a black hole is a region of space into which matter has
fallen and from which nothing, not even light, can escape. Since light cannot
escape the tremendous gravitational attraction of a black hole, it would be
invisible to us, hence the name "black" hole.
Our
sun will probably never become a black hole--it simply isn't massive enough.
Let's say, for convenience, that our sun weighs one solar mass. Stars weighing
1.4 solar masses or more end up as black holes. Let's take a hypothetical star
which weighs between 1.4 and 10 solar masses, and see how it becomes a black
hole.
The
star starts out as a ball of hydrogen. Hydrogen, the lightest atom of all the
elements, consists of a single electron orbiting a proton. Two hydrogen atoms
bonded together form a molecule of hydrogen gas.
The
hydrogen begins to undergo nuclear fusion at the gas ball's center when the temperature
there reaches about four million degrees Celsius. The star's energy comes from
this fusion of four hydrogen nuclei into one helium nucleus. A helium nucleus
consists of two protons and two neutrons.
In
the process, a small amount of mass is converted into energy. In the case of
our own sun, four million tons of its mass are converted into energy each
second. Einstein's well-known mass-energy equation E = mc2 can be used to show that
our sun puts out 4.4 septillion horsepower.
This
energy production tends to make the star expand. This expansion force is
counterbalanced by a contraction force. The contraction force is due to the
weight of the star's outer layers. The outer mass tends to collapse inward
toward the star's gravitational center.
When
all of the star's hydrogen is used up near the core, and the core is mainly
composed of helium, the balance of forces is upset. The gravitational forces
collapse the core because the expansion force is no longer present as the
nuclear fusion of hydrogen into helium has ceased. The fusion reactions still
continue in the layer around the center, and the outermost layers expand.
The
gravitational collapse of the core causes temperatures there to rise until it
is hot enough to initiate helium fusion. Now helium, not hydrogen, is the
star's fuel. The helium fusion produces even heavier elements. The star has a
very hot core, but has expanded tremendously in the outer regions. Stars of
this type are called Red Giants, due to their expanded size and red color.
Let's
go back to our hypothetical star. For a star of the mass range we have chosen,
the nuclear reactions in the core go out of control as that region collapses.
An explosion occurs, and the star blows most of its mass out into space--this
is what is known as a supernova.
What
remains of the star's matter shrinks down into a small, dense body. Protons
collide with electrons to form neutrons, leaving us with a superdense star
whose core is composed of solid neutrons--a neutron star. These stars have
densities as high as ten million tons per cubic centimeter. This is about nine
trillion times more dense than water. Why should these neutron stars be so
dense?
Picture
an atom as electrons orbiting a nucleus. This model isn't quite accurate, but
will do for our purposes here. Now, if electrons were the size of peas and the
nucleus the size of a beachball, the distance between the peas and the
beachball would be about the length of a football field. From this we see that
ordinary matter is mostly empty space.
In
the matter of a neutron star, however, there is virtually no space between the
neutrons. Picture all the beachball nuclei side by side as compared with the
former model of ordinary matter, and it is easy to see why solid neutrons must
be so dense.
If
the mass of a neutron star is greater than or equal to a certain critical mass,
it cannot resist further contraction due to its tremendous gravitational force.
This critical mass is estimated to be between two and three solar masses.
If the
star that we are talking about weighs between 1.4 and 2 solar masses, it ends
its life as a neutron star. But if its mass is greater than two solar masses,
further contraction occurs. The neutron star becomes what is known as a
collapsar, that is, any object collapsing toward its center under gravitational
attraction to become a black hole.
Our
hypothetical star is now a collapsar. It is shrinking, and its radius decreases
until it reaches a certain critical value called the Schwarzschild Radius (Rs). If
our star has a certain mass M, this radius is given by the relation
Rs
= 2GM/c2
where
c is the velocity of light in a vacuum and G is a constant. When the radius
reaches this
value,
the spherical mass becomes so dense that even light cannot escape it. Now the
gravitational field of the star is tremendous, and an event takes place which
no astronomer will probably ever witness.
Suddenly,
all of the star's matter is compressed to the center point. This cannot be seen
because it happens too quickly to observe--the collapse takes place in a time
interval on the order of millionths of a second!
The
density and gravitational force of the black hole are infinite at this point
(which is called a singularity) and the matter of the star has literally become
crushed out of existence. Surrounding the singularity is a volume of space into
which matter has fallen and nothing can escape--a black hole. The radius of the
black hole measured from the center is the Schwarzschild Radius.
If
our hypothetical star had been heavier than ten solar masses, we also would
have ended up with a black hole. With a star that heavy, once the core had used
up its hydrogen, nothing can stop its collapse. No supernova outburst need
occur; its own mass will cause it to rapidly collapse into a black hole.
Stars
weighing 1.4 solar masses or less become white dwarf stars when their nuclear
fuel is exhausted. The red giant collapses until it is a white dwarf, and
stabilizes at that point (no further collapse taking place). While not as
compact as neutron stars, white dwarf stars are still quite dense; one
thimbleful of white dwarf material may weigh over a ton. White dwarfs owe their
density to the fact that they are composed of what is known as degenerate
matter.
In
ordinary gaseous matter, the space between atoms is so much greater than the
diameter of the atoms themselves that the atoms may be considered point
particles for all practical purposes. They fly about at random, and the
pressure exerted by the gas is due to these point particles bouncing off other
particles.
In
degenerate gaseous matter, the atoms are
packed together so closely that the distance between the atoms is not much
greater than the diameter of the atoms themselves. In this case the size of the
atoms is significant and they can no longer be considered point particles.
Now, electrons make up most of the volume of the atoms.
The electrons resist being squeezed together in such close quarters. This resistance
is called degeneracy pressure, and is the pressure the gas exerts against
anything trying to compress it. This pressure holds the white dwarf up.
However, just as there was for a neutron star, there is a
certain critical mass for a white dwarf beyond which it cannot resist further
collapse. This is known as the Chandrasekhar Limit, and is 1.4 solar masses. If
a white dwarf should incorporate more mass into its structure and exceed 1.4
solar masses in weight, it would become a collapsar and end up a black hole.
We see that there are many situations which can, and
probably do, give rise to black holes, We call a black hole's center a
singularity in space. At these singular points density and gravity become
infinite.
In the region of a black hole the laws of physics behave contrary
to ordinary experience. The laws of relativity show pronounced effects in these
regions, and next we shall examine in detail the properties of these collapsed
stars.
The boundary surrounding the black hole region of space is
called the event horizon. Outside the event horizon an observer cannot have any
knowledge of what is going on inside the event horizon, i.e. we cannot see what
is going on inside a black hole if we view it from the outside.
The event horizon is a barrier to communication of any
kind, so an observer who has fallen inside a black hole cannot report to those
outside what he is experiencing. He can't ever get back outside the event
horizon to report what he's seen, since nothing can escape from a black hole. Thus,
it seems that we can never directly observe what happens inside a black hole.
Suppose we decide to send an astronaut inside a black
hole, anyway. The astronomer will observe him a safe distance away from the
black hole through a telescope. The first thing the astronomer observes is a
very pronounced redshift in the vicinity of the black hole.
What is a redshift? We noted earlier that the tremendous
gravitational field of a black hole is so strong that not even light can escape
from it. Light is affected by any gravitational field; only in the case of the
black hole the effects are quite dramatic. Now, light loses some of its energy
whenever it has to struggle against gravity. We know that energy is
proportional to the frequency, so when the light loses energy, its frequency
decreases.
Since frequency and wavelength are inversely proportional,
this decrease in frequency causes an increase in wavelength. This
"shift" toward an increase in wavelength is called a redshift because
red light has a longer wavelength than any other visible color.
The redshift, a quantity denoted by the letter z, is
defined as the shift in wavelength per wavelength of light emitted. For
example, consider a beam of light with a wavelength of 5000 angstroms. If the
light is shifted 250 angstroms, z = 250/5000 = 0.05.
At
the center of a black hole gravity is infinite, and so the light trying to
escape loses an infinite amount of energy. Hence the redshift becomes infinite
in the region of the black hole.
Let
us consider an astronaut falling toward the black hole, as observed by an
astronomer from a distance. Each is wearing an identical stopwatch.
As the
falling astronaut approaches the event horizon, we notice that time itself
actually is slowing down for him. This is to be expected, since Einstein's
theory of relativity predicts that time slows down in the region of an intense
gravitational field.
For
each second that the astronaut's stopwatch ticks off, the astronomer's watch
ticks off 1 + z seconds. Since the redshift z increases as one approaches the
event horizon and eventually becomes infinite, the quantity 1 + z also becomes
infinite. This means that as one second passes for the astronaut (who is right
at the event horizon), an infinite amount of time passes in the universe.
In
other words, time is frozen at the event horizon. The astronomer peering
through his telescope would see the astronaut falling slower and slower toward
the event horizon, until he seemed frozen forever at the event horizon. To the
outside observer, the astronaut will never pass through the event horizon into
the black hole. To our eyes, he has achieved a kind of immortality, for he will
remain as he is while the universe grows old and dies.
But
wait a minute. How can a black hole be said to eat up all matter in its path if
time is frozen at its boundary and nothing passes through it? Well, Einstein
showed us that time is a subjective thing, depending on your point of view, or
your reference frame, as it's called.
For
example, to a man standing still, a rocket ship which weighs ten tons at rest
would weigh almost twenty-three tons if travelling at ninety percent the speed
of light. But if the man was to fly alongside the rocketship and measure it, he
would find it weighed ten tons. If he is in the same reference frame as the rocket, it doesn't gain any mass. In a
different reference frame, it does.
By
the same reasoning, the astronomer and astronaut are in different reference
frames, this time the difference being due to gravitation and not velocity.
Yes, to the astronomer outside the black hole's pull, it appears as if the
astronaut is frozen at the event horizon. But what is it like in the
astronaut's frame of reference?
As
he approaches the event horizon, time runs perfectly normally from his point of
view. As he falls, he sees nothing in front of him; a black hole looks like
just that--a hole. At this point he might be killed or at least seriously
injured by something which only affects oceans on Earth, namely, the tide.
A
tidal force is exerted by a mass on a body. We know that the gravitational
force decreases in strength with distance. It follows that the gravitational
attraction on a body is strongest at that point on the body nearest the mass,
and weakest at that point on the body farthest away from the mass.
In
the case of the moon acting on the Earth, this causes the water to bulge toward
the moon (at the point nearest the moon) and away from the moon (at the point
farthest away). This gives us two tidal humps on opposite sides of the Earth,
as if the gravitational forces were "stretching" the water.
A black hole exerts a great deal more gravity on the
astronaut than the moon exerts on the Earth. For a black hole of ten solar
masses, the tidal force at the event horizon is about a hundred million million
times greater than the tidal force one would feel standing on the Earth. The
tremendous tidal force does to the astronaut something similar to what happens
with the ocean.
If the astronaut approaches the hole feet first, the
gravitational pull on his feet will be considerably stronger than that on his
head. So strong is the force that the astronaut will be stretched out into a
long cylinder; he will be torn apart on a sort of cosmic torture rack. If the
astronaut isn't killed by tides at the event horizon, the infinite
gravitational force at the black hole's center will surely do the trick.
But whether he does it dead or alive, the astronaut will pass through the event horizon.
Once inside the black hole, he is pulled toward the center point where all the
mass of the hole is contained.
The density of matter here is infinite; this is the
singularity discussed before. Here. all objects lose their identity since the
mater is crushed out of existence. Our astronaut is no longer an astronaut; he
is merely mass that has been added to the black hole. He no longer exists in
our universe. But it doesn't really matter, since once past the event horizon
he is out of the picture forever.
Or is he? Some speculation has been made to the effect
that what goes into a black hole comes out somewhere else through something
called a "white hole." Keep in mind that this notion, while
aesthetically pleasing, may be more science fantasy than science fact.
Nature loves symmetry and order. She is supposed to make
sense. The whole idea of science is to find some unity and order in nature. We
believe that there are regions in space where matter is devoured, and the
picture of the universe would be more symmetric if there were other regions
where matter is produced. There is nothing in Einstein's Theory of General
Relativity which prohibits the existence of such white holes.
These white holes would form like a collapsar in reverse,
starting with a point of infinite density and ending with matter erupting
outward into the universe. Thus, matter devoured by a black hole may be
expelled by a white hole somewhere else in the universe. This smacks of being a
sort of "space warp," where you go from one point to another
instantaneously without having to travel between them.
Another model of white holes pictures matter from our
universe passing through a black hole, and erupting through a white hole into
an alternate universe. In this picture, the black hole-white hole pair form a
sort of "gateway" to another universe.
Keep in mind, however, the fact that the alternate
universe is a mathematical model only. It can never be proven that it exists,
since someone or something travelling through a black hole into such a universe
would never be seen in our own world again.
The actual observation of a black hole in space would
strongly confirm our present theories of relativistic physics. But how can we
possibly detect a black hole far out in space? It is invisible, emitting no
radiation whatsoever. Unless we are close enough to feel its gravity, we won't
know it it's there or not.
Stars and starlike objects exist not only as single
objects, but also in pairs. A system of two stars orbiting one another is
called a binary star. It turns out that if one of the stars is a black hole, we
should be able to detect it through its interaction with the other star.
An
X-ray binary is a system in which a normal star is bound to a superdense star.
The superdense star may be a white dwarf, a neutron star, or a black hole.
Matter
streams from the normal star and follows a spiral path until it falls into the
denser star. As the matter falls into the dense star energy is gained and
converted into X-rays. This is because the matter is ionized plasma, and has an
electrical charge. The X-rays are emitted into space (hence the name
"X-ray" binary) and can be detected by the astronomer's instruments
in orbit above the Earth.
It
is assumed that the superdense star has a magnetic field. For a neutron star
this field is not necessarily aligned with the axis of rotation of the star. A
black hole possesses axial symmetry, meaning its magnetic field aligned with
its rotational axis.
The
X-ray signal will be a regular pulse if the magnetic field is skewed, so
regular pulses indicate a neutron star. If the pulse is sharp and irregular,
the magnetic field is aligned with the axis of rotation and we have a black
hole. Each burst of X-rays is caused by matter spiralling toward the superdense
companion star, and the bursts get narrower as the spiral orbit gets smaller.
Using
this information, astronomers scanned the skies for black holes, and believe
they may have found one. Remo J. Ruffini, a Princeton astronomer, believes that
the star Cygnus X-1 is a black hole. Its companion is a blue supergiant, star
HDE 226868.
A
rocket flown by the Goddard Space Flight Center accumulated data on the system
which Ruffini has carefully interpreted. The data indicates that Cygnus X-1 is
definitely a collapsed object, and almost certainly a black hole since the
X-ray bursts were irregular in nature. Also, its mass is estimated to be at
least five solar masses, and we know that any collapsed object heavier than two
to three solar masses is a black hole.
Singularities
in space, if found, might tell us a great deal about our own beginnings. This
is because some theorists are now speculating that the entire universe began as
a singularity
in space!
Most astronomers support the Big Bang theory of the
creation of the universe. This theory says that the universe began as a chunk
of hot, superdense matter small enough to fit in the volume of our own solar
system.
The explosion of this chunk of matter (the big bang) was
the start of the creation of the universe. It is possible that this creation
may have been a singularity where matter poured outward; in other words, our
universe was formed from a tremendous white hole.
The big bang sent matter spewing outwards at great speeds, and even
now our universe is still rapidly expanding, with galaxies moving away from us
a appreciable fractions of the velocity of light. This may continue until the
universe ends, but many astronomers feel that this expansion may eventually
come to a halt.
At
that point, gravitation will cause the universe to contract. Galaxies would
come together and eventually become squeezed into such a small volume that a
black hole may be formed, and all the matter in the universe would be crushed
out of existence.
In
fact, the universe may even now be nothing but a gigantic black hole. The
universe is estimated to have a radius of ten billion light years. One light
year is the distance something travelling at the speed of light covers in one year.
From estimates of the amount of mass in the universe, the
Schwarzschild Radius of the universe has been calculated to be roughly ten
billion light years. If the radius of the universe is equal to its Schwarzshild
Radius, then we are all living inside the ultimate black hole, or more
accurately, the ultimate collapsar.
Since the volume of the universe is so large, its density
is relatively low. In the continuing collapse, of course, it would eventually
reach a value of infinite density at the center point.
Finding a real black hole would confirm the theory, This
is justification enough for the search, since the mathematical treatment of
such objects is one of the chief glories of modern science. But some are always
looking for immediate practical applications of the subject in question. Are
there any in the case of black holes?
Three physicists at the Lawrence Livermore Laboratory have
a suggestion. They feel it would be possible to find and capture a mini-black
hole. (Such miniature black holes may have been formed in the original big
bang.) Once captured, we would shoot thermonuclear fuel, such as hydrogen,
right at the black hole.
As the fuel falls toward the black hole, the tremendous
gravitation compresses, heats, and ionizes the hydrogen. Nuclear fusion, a
reaction discussed earlier, is initiated. The fuel explodes in a flash, and
helium is formed.
Some matter is converted into energy. The fuel mass is
blown back by the explosion along with the energy away from the black hole. The
energy is beamed back to Earth via microwaves where it is put to work.
This system is totally nonpolluting, and has no moving
parts (save the fuel itself). The idea may seem a bit wild, but the Livermore
scientists are seriously urging that we search out and capture such a black
hole for this purpose.
Black hole energy sources, alternate universes, space
warps, and frozen time ... the physics of the black hole sounds more like
science fiction that modern astrophysics. But J.B.S. Haldane spoke accurately
when he said: "The universe is not only queerer than we imagine, it is
queerer than we can imagine."
###