Every day we look out upon the night sky, wondering and dreaming of what
lies beyond our planet. The universe that we live in is so diverse and unique,
and it interests us to learn about all the variance that lies beyond our grasp.
Within this marvel of wonders, our universe holds a mystery that is very
difficult to understand because of the complications that arise when trying to
examine and explore the principles of space. That mystery happens to be that of
the ever elusive, black hole.
This essay will hopefully give you the knowledge and understanding of
the concepts, properties, and processes involved with the space phenomenon of
the black hole. It will describe how a black hole is generally formed, how it
functions, and the effects it has on the universe.
By definition, a black hole is a region where matter collapses to
infinite density, and where, as a result, the curvature of space-time is extreme.
Moreover, the intense gravitational field of the black hole prevents any light
or other electromagnetic radiation from escaping. But where lies the “point of
no return” at which any matter or energy is doomed to disappear from the visible
universe?
The black hole’s surface is known as the event horizon. Behind this
horizon, the inward pull of gravity is overwhelming and no information about the
black hole’s interior can escape to the outer universe. Applying the Einstein
Field Equations to collapsing stars, Kurt Schwarzschild discovered the critical
radius for a given mass at which matter would collapse into an infinitely dense
state known as a singularity.
At the center of the black hole lies the singularity, where matter is
crushed to infinite density, the pull of gravity is infinitely strong, and
space-time has infinite curvature. Here it is no longer meaningful to speak of
space and time, much less space-time. Jumbled up at the singularity, space and
time as we know them cease to exist. At the singularity, the laws of physics
break down, including Einstein’s Theory of General Relativity. This is known as
Quantum Gravity. In this realm, space and time are broken apart and cause and
effect cannot be unraveled. Even today, there is no satisfactory theory for
what happens at and beyond the rim of the singularity.
A rotating black hole has an interesting feature, called a Cauchy
horizon, contained in its interior. The Cauchy horizon is a light-like surface
which is the boundary of the domain of validity of the Cauchy problem. What
this means is that it is impossible to use the laws of physics to predict the
structure of the region after the Cauchy horizon. This breakdown of
predictability has led physicists to hypothesize that a singularity should form
at the Cauchy horizon, forcing the evolution of the interior to stop at the
Cauchy horizon, rendering the idea of a region after it meaningless.
Recently this hypothesis was tested in a simple black hole model. A
spherically symmetric black hole with a point electric charge has the same
essential features as a rotating black hole. It was shown in the spherical
model that the Cauchy horizon does develop a scalar curvature singularity. It
was also found that the mass of the black hole measured near the Cauchy horizon
diverges exponentially as the Cauchy horizon is approached. This led to this
phenomena being dubbed “mass inflation.”
In order to understand what exactly a black hole is, we must first take
a look at the basis for the cause of a black hole. All black holes are formed
from the gravitational collapse of a star, usually having a great, massive, core.
A star is created when huge, gigantic, gas clouds bind together due to
attractive forces and form a hot core, combined from all the energy of the two
gas clouds. This energy produced is so great when it first collides, that a
nuclear reaction occurs and the gases within the star start to burn continuously.
The hydrogen gas is usually the first type of gas consumed in a star and then
other gas elements such as carbon, oxygen, and helium are consumed.
This chain reaction fuels the star for millions or billions of years
depending upon the amount of gases there are. The star manages to avoid
collapsing at this point because of the equilibrium achieved by itself. The
gravitational pull from the core of the star is equal to the gravitational pull
of the gases forming a type of orbit, however when this equality is broken the
star can go into several different stages.
Usually if the star is small in mass, most of the gases will be consumed
while some of it escapes. This occurs because there is not a tremendous
gravitational pull upon those gases and therefore the star weakens and becomes
smaller. It is then referred to as a white dwarf. A teaspoonful of white dwarf
material would weigh five-and-a-half tons on Earth. Yet a white dwarf star can
contract no further; it’s electrons resist further compression by exerting an
outward pressure that counteracts gravity. If the star was to have a larger
mass, then it might go supernova, such as SN 1987A, meaning that the nuclear
fusion within the star simply goes out of control, causing the star to explode.
After exploding, a fraction of the star is usually left (if it has not
turned into pure gas) and that fraction of the star is known as a neutron star.
Neutron stars are so dense, a teaspoonful would weigh 100 million tons on Earth.
As heavy as neutron stars are, they too can only contract so far. This is
because, as crushed as they are, the neutrons also resist the inward pull of
gravity, just as a white dwarf’s electrons do.
A black hole is one of the last options that a star may take. If the
core of the star is so massive (approximately 6-8 times the mass of the sun)
then it is most likely that when the star’s gases are almost consumed those
gases will collapse inward, forced into the core by the gravitational force laid
upon them. The core continues to collapse to a critical size or circumference,
or “the point of no return.”
After a black hole is created, the gravitational force continues to pull
in space debris and other types of matters to help add to the mass of the core,
making the hole stronger and more powerful.
The most defining quality of a black hole is its emission of
gravitational waves so strong they can cause light to bend toward it.
Gravitational waves are disturbances in the curvature of space-time caused by
the motions of matter. Propagating at (or near) the speed of light,
gravitational waves do not travel through space-time as such — the fabric of
space-time itself is oscillating. Though gravitational waves pass straight
through matter, their strength weakens as the distance from the original source
increases.
Although many physicists doubted the existence of gravitational waves,
physical evidence was presented when American researchers observed a binary
pulsar system that was thought to consist of two neutron stars orbiting each
other closely and rapidly. Radio pulses from one of the stars showed that its
orbital period was decreasing. In other words, the stars were spiraling toward
each other, and by the exact amount predicted if the system were losing energy
by radiating gravity waves.
Most black holes tend to be in a consistent spinning motion as a result
of the gravitational waves. This motion absorbs various matter and spins it
within the ring (known as the event horizon) that is formed around the black
hole. The matter keeps within the event horizon until it has spun into the
center where it is concentrated within the core adding to the mass. Such
spinning black holes are known as Kerr black holes.
Time runs slower where gravity is stronger. If we look at something
next to a black hole, it appears to be in slow motion, and it is. The further
into the hole you get, the slower time is running. However, if you are inside,
you think that you are moving normally, and everyone outside is moving very fast.
Some scientists think that if you enter a black hole the forces inside
will transport you to another place in space and time. At the other end would
be a white hole, which is theoretically a point in space that just expels
matter and energy.
Also as a result of the powerful gravitational waves, most black holes
orbit around stars, partly due to the fact that they were once stars. This may
cause some problems for the neighboring stars, for if a black hole gets powerful
enough it may actually pull a star into it and disrupt the orbit of many other
stars. The black hole can then grow strong enough (from the star’s mass) as to
possibly absorb another star.
When a black hole absorbs a star, the star is first pulled into the
ergosphere, which sweeps all the matter into the event horizon, named for its
flat horizontal appearance and because this happens to be the place where mostly
all the action within the black hole occurs. When the star is passed on into the
event horizon the light that the star endures is bent within the current and
therefore cannot be seen in space. At this exact point in time, high amounts of
radiation are given off, and with the proper equipment, can be detected and seen
as an image of a black hole. Through this technique, astronomers now believe
that they have found a black hole known as Centaurus A. The existence of a star
apparently absorbing nothingness led astronomers to suggest and confirm the
existence of another black hole, Cygnus X1.
By emitting gravitational waves, non-stationary black holes lose energy,
eventually becoming stationary and ceasing to radiate in this manner. In other
words, they decay and become stationary black holes, namely holes that are
perfectly spherical or whose rotation is perfectly uniform. According to
Einstein’s Theory of General Relativity, such objects cannot emit gravitational
waves.
Black hole electrodynamics is the theory of electrodynamics outside a
black hole. This can be very trivial if you consider just a black hole
described by the three usual parameters: mass, electric charge, and angular
momentum. Initially simplifying the case by disregarding rotation, we simply
get the well known solution of a point charge. This is not very physically
interesting, since it seems highly unlikely that any black hole (or any
celestial body) should not be rotating. Adding rotation, it seems that charge
is present. A rotating, charged black hole creates a magnetic field around the
hole because the inertial frame is dragged around the hole. Far from the black
hole, at infinity, the black hole electric field is that of a point charge.
However, black holes do not even have charges. The magnitude of the
gravitational pull repels even charges from the hole, and different charges
would neutralize the charge of the hole.
The domain of a black hole can be separated into three regions, the
first being the rotating black hole and the area near it, the accretion disk (a
region of force-free fields), and an acceleration region outside the plasma.
Disk accretion can occur onto supermassive black holes at the center of
galaxies and in binary systems between a black hole (not necessarily
supermassive) and a supermassive star. The accretion disk of a rotating black
hole, is, by the black hole, driven into the equatorial plane of the rotation.
The force on the disk is gravitational.
Black holes are not really black, because they can radiate matter and
energy. As they do this, they slowly lose mass, and thus are said to evaporate.
Black holes, it turns out, follow the basic laws of thermo-dynamics.
The gravitational acceleration at the event horizon corresponds to the
temperature term in thermo-dynamical equations, mass corresponds to energy, and
the rotational energy of a spinning black hole is similar to the work term for
ordinary matter, such as gas. Black holes have a finite temperature; this
temperature is inversely proportional to the mass of the hole. Hence smaller
holes are hotter. The surface area of the event horizon also has significance
because it is related to the entropy of the hole.
Entropy, for a black hole, can be said to be the logarithm of the number
of ways it could have been made. The logarithm of the number of microscopic
arrangements that could give rise to the observed macroscopic state is just the
standard definition of entropy. The enormous entropy of a black hole results
from the lost information concerning the structural and chemical properties
before it collapsed. Only three properties can remain to be observed in the
black hole: mass, spin, and charge.
Physicist Stephen Hawking realized that because a black hole has a
finite entropy and temperature, in can be in thermal equilibrium with its
surroundings, and therefore must be able to radiate. Hawking radiation, as it
is known, is allowed by a quantum mechanism called virtual particles. As a
consequence of the uncertainty principle, and the equivalence of matter and
energy, a particle and its antiparticle can appear spontaneously, exist for a
very short time, and then turn back into energy. This is happening all the time,
all over the universe. It has been observed in the “Lamb shift” of the spectrum
of the hydrogen atom. The spectrum of light is altered slightly because the
tiny electric fields of these virtual pairs cause the atom’s electron to shake
in its orbit.
Now, if a virtual pair appears near a black hole, one particle might
become caught up in a the hole’s gravity and dragged in, leaving the other
without its partner. Unable to annihilate and turn back into energy, the lone
particle must become real, and can now escape the black hole. Therefore, mass
and energy are lost; they must come from someplace, and the only source is the
black hole itself. So the hole loses mass.
If the hole has a small mass, it will have a small radius. This makes it
easier for the virtual particles to split up and one to escape from the
gravitational pull, since they can only separate by about a wavelength.
Therefore, hotter black holes (which are less massive) evaporate much more
quickly than larger ones. The evaporation timescale can be derived by using the
expression for temperature, which is inversely proportional to mass, the
expression for area, which is proportional to mass squared, and the blackbody
power law. The result is that the time required for the black hole to totally
evaporate is proportional to the original mass cubed. As expected, smaller
black holes evaporate more quickly than more massive ones.
The lifetime for a black hole with twice the mass of the sun should be
about 10^67 years, but if it were possible for black holes to exist with masses
on the order of a mountain, these would be furiously evaporating today.
Although only stars around the mass of two suns or greater can form black holes
in the present universe, it is conceivable that in the extremely hot and dense
very early universe, small lumps of overdense matter collapsed to form tiny
primordial black holes. These would have shrunk to an even smaller size today
and would be radiating intensely. Evaporating black holes will finally be
reduced to a mass where they explode, converting the rest of the matter to
energy instantly. Although there is no real evidence for the existence of
primordial black holes, there may still be some of them, evaporating at this
very moment.
The first scientists to really take an in depth look at black holes and
the collapsing of stars, were professor Robert Oppenheimer and his student,
Hartland Snyder, in the early nineteen hundreds. They concluded on the basis of
Einstein’s theory of relativity that if the speed of light was the utmost speed
of any object, then nothing could escape a black hole once in its gravitational
orbit.
The name “black hole” was given due to the fact that light could not
escape from the gravitational pull from the core, thus making the “black hole”
impossible for humans to see without using technological advancements for
measuring such things as radiation. The second part of the word was given the
name “hole” due to the fact that the actual hole is where everything is absorbed
and where the central core, known as the singularity, presides. This core is the
main part of the black hole where the mass is concentrated and appears purely
black on all readings, even through the use of radiation detection devices.
Just recently a major discovery was found with the help of a device
known as The Hubble Telescope. This telescope has just recently found what many
astronomers believe to be a black hole, after focusing on a star orbiting empty
space. Several pictures were sent back to Earth from the telescope showing many
computer enhanced pictures of various radiation fluctuations and other diverse
types of readings that could be read from the area in which the black hole is
suspected to be in.
Several diagrams were made showing how astronomers believe that if
somehow you were to survive through the center of the black hole that there
would be enough gravitational force to possible warp you to another end in the
universe or possibly to another universe. The creative ideas that can be
hypothesized from this discovery are endless.
Although our universe is filled with many unexplained, glorious
phenomena, it is our duty to continue exploring them and to continue learning,
but in the process we must not take any of it for granted.
As you have read, black holes are a major topic within our universe and
they contain so much curiosity that they could possibly hold unlimited uses.
Black holes are a sensation that astronomers are still very puzzled with. It
seems that as we get closer to solving their existence and functions, we only
end up with more and more questions.
Although these questions just lead us into more and more unanswered
problems we seek and find refuge into them, dreaming that maybe one far off
distant day, we will understand all the conceptions and we will be able to use
the universe to our advantage and go where only our dreams could take us.
