Mad About Physics sanjay
Transcript of Mad About Physics sanjay
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MA
A BRIEF HISTO
D ABOUT PHYSICS
Y OF SUPERMASSIVE BLACK
SANJAY KUMAR SHARMA
E-mail: [email protected]
HOLES
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Introduction to Black HoleNeither the layman nor the specialists, in general, have any knowledge of the historical
circumstances underlying the genesis of the idea of the Black Hole. Essentially, almost all
the sundry simply take for granted the unsubstantiated allegations of some ostentatious
minority of the relativists. Unfortunately, that minority has been rather careless with the
truth and is quite averse to having its claims corrected, notwithstanding the documentaryevidence on the historical record.
On the basis of all theories about Black Hole some of the definitions are given as:
A Black Hole is a region of space whose gravity is so strong that it acts like a giant
vacuum cleaner in space. Anything that gets too close gets sucked in. When it hits the
centre, the singularity, it disappears forever. The gravity in black holes is so strong that it
tugs at space and time, slowing down time and stretching out space. Not even light, the
fastest thing in the universe, can escape from a black hole.
In General Relativity a black hole is so dense that nothing can escape from it, not even
light. General Relativity breaks down under such extremes so it cannot describe what goes
on inside a black hole. It is believed that a singularity exists inside and True Relativity alsoshows a singularity where the gravitational force becomes infinitely strong, but the physics
of the True Relativity does not end at the Schwarzschild radius or at the singularity.
Black holesAlmost everyone has heard of black holes. They occupy a special place in the public
imagination and rightfully so, for they are among the most exotic and interesting objects in
nature. However, what exactly are black holes, and how do we know they exist?
Perhaps the simplest and most intuitive definitions of a black hole is an object whose
gravity is so strong that nothing can escape, even at the speed of light. To understand this
better, we can consider the idea of escape velocity. Imagine we are standing on the earth
and throwing a ball up in the air. The faster the initial speed of the ball, the higher the ballwill go before it comes back down. Based on our understanding of gravity, as two objects
move apart the forces between them decreases proportional to the square of the distance.
So as we throw the ball higher and higher, the pull of gravity on the ball becomes weaker
and weaker, and we can imagine throwing the ball with such speed that it leaves the
gravitational field of the earth completely and flies far into space. The speed at which this
happens is called the escape velocity, for the Earth it is approximately 11km/s, which is
why we need such powerful rockets to launch vehicles into deep space.
The equation of gravity tells us that the escape velocity for a spherical object (such as a star
or planet) is given by. Thus, if we make an object more massive (larger M) or compress it
(smaller R), then we increase the escape velocity. Taking this to its extreme, we can imagine
taking an object as massive as the Sun and compressing it down to a radius of only 3km,which implies an enormous density (a teaspoonful of this material on Earth would weigh
many billions of tones!). This object would then be so massive and yet so small that the
escape velocity at its surface would be equal to speed of light. Since nothing in the Universe
can travel faster than speed of light, we infer that nothing can escape this objects strong
gravitational field. Such a remarkable entity is what we call black hole.
The concept of escape velocity gives us a clear and intuitive way of thinking about black
holes, but unfortunately does not provide a full description of the physics behind these
objects. For this we require a more complete description of gravity, which was provided by
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Einstein in his theory of general relativity in the early 20th
century. Einsteins remarkable
insight was that gravity is not simply a force between two objects that acts at a distance,
but instead represents a fundamental curvature in the fabric of space and time in the
universe. Massive objects curve the space-time around them, and the motion of objects
follows straight lines in this curved space, thus the Moon orbits the Earth because it is
following the curvature of space induced by the Earth's gravity. This is an extraordinary
idea, but it makes some robust predictions, chief among which is that light rays (whichhave no mass, and so traditionally were not thought to experience gravity) are deflected as
they pass close to the Sun. The observations of this effect provided among the first
experimental proof of Einsteins theory.
In light of general relativity, to fully understand black holes we must think of them as
objects for which the strong gravity bends space so much that even light cannot escape. In
this sense, black holes truly are black- if we could see a black hole directly; we would see a
black sphere, surrounded by images of background objects that had been deflected or
lenses by the strong gravity of the hole.
SECOND THEORY:
It has frequently been alleged by theoretical physicists that Newtons theory of gravitation
either predicts or adumbrates the black hole. This claim stems from a suggestion originallymade by John Michel in 1784 that if a body is sufficiently massive. All light emitted from
such a body would be made to return to it by its own power of gravity. The great French
scientist P.S. de Laplace made a similar conjecture in the eighteen century and undertook a
mathematical analysis of the matter.
However, contrary to popular and frequent expert opinion, the Michel- Laplace dark
body, as it is actually called, is not a black hole at all. The reason why is quite simple.
For a gravitating body we identify an escape velocity. This is the velocity that must be
achieved by an object to enable it to leave the surface of the host body and travel out to
infinity, where it comes to rest. Therefore, it will not fall back towards the host. It is said to
have escaped the host. At velocities lower than the escape velocity, the object will leave the
surface of the host, travel out to a finite distance where it momentarily comes to rest, and
then fall back to the host. Consequently, a suitable located observer will see the travelling
object twice, once on its journey outward and once on its returning way. If the initial
velocity is greater than or equal to escape velocity, an observer located outside the host,
anywhere on trajectory of the travelling object, will see the object once, and as it passes by
own its outward unidirectional journey. It escapes the host. Now, if the escape velocity is
the speed of light, this means that light can leave the host and travel out to infinity and
come to rest there. It escapes the host. Therefore, all observers located anywhere on the
trajectory will see the light once, as it passes by on its outward journey. However, if the
escape velocity is greater than the speed of light, then light will travel out to a finite
distance, momentarily come to rest, and fall back to the host, in which case a suitably
located observer will see the light twice. Furthermore, an observer located at a sufficiently
large and finite distance from the host will not see the light, because it does not reach him.
To such an observer the host is dark: a Michel-Laplace dark body. But this does not mean
that light cannot leave the surface of the host. It can, as testified by the closer observer.
Now, in the case of black hole, it is claimed by the relativists that no object and no light can
even leave the event horizon of the black hole. Therefore, an observer, no matter how close
to the event horizon, will see nothing.
Since about 1970 there has been an explosion in the number of people publishing technical
research papers, text-books and popular science books and articles on various aspects of
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General Relativity. A large proportion of this includes elements of the theory of black
holes. Quite a few are dedicated exclusively to the black hole. Not only is there now a
simple black hole with singularity, but also naked singularity, black holes without hair,
super massive black holes at the centre of galaxies, black hole quasars, black hole binary
system, colliding black holes, charged black holes, rotating black holes an even white holes!
Black holes are now seen everywhere by the astronomers, even though no one has ever
found an event horizon anywhere. Consequently, public opinion has been persuaded thatthe black hole is a fact of Nature and that anyone who questions the contention must be a
crackpot. It has become a rather lucrative business, this black hole.
Besides the purely mathematical errors that mitigate the black hole, there are also
considerable physical arguments against it, in addition to the fact that no event horizon has
ever been detected.
What does a material point mean? What meaning can there possibly be in the notion of a
material objects without any spatial extension? The term material point (or point mass) is
an oxymoron. Yet the black hole singularity is supposed to have mass and no extension.
Moreover, there is not a single shred of experimental evidence to even remotely suggest
that Nature makes material points. Even the electron has spatial extent, according to
experiment, and to quantum theory. A point is an abstraction, not a physical object. Inother words, a point is a purely mathematical object. Points and physical objects are
mutually exclusive by definition. No one has observed a point, and no one ever will because
it is unobservable, not being physical. Therefore, Nature does not make material points.
Consequently, the theoretical singularity of the black hole cannot be a point mass.
It takes an infinite amount of observer time for an object, or light, to reach the event
horizon irrespective of how far that observer is located from the horizon. Similarly, light
leaving the surface of a body undergoing gravitational collapse, at the instant that it passes
its event horizon, takes an infinite amount of observer time to reach an observer. However
far that observer is from the event horizon. Therefore, the black hole is undetectable to the
observer since he must wait an infinite amount of time to confirm the existence of an event
horizon. Such an object has no physical meaning for the observer. Furthermore, according
to the very same theoreticians, the Universe started with a Big Bang, and that theory gives
an alleged age of 14 billion years for the Universe. This is hardly enough time for the black
hole to form from the perspective of an external observer. Consequently, if the black holes
exist they must have been created at the instant of the Bang. They must be primordial
black holes. But that is inconsistent with the Bang itself, because matter at that time,
according to the Big Bang theoreticians, could not form lumps. Even so, they cannot be
detected by an external observer owing to the infinite time needed for confirmation of the
event horizon. This now raises serious suspicions as to the validity of the Big Bang, which is
just another outlandish theory, essentially based upon Friedmans expanding Universe
solution, not an established physical reality as the astronomers would have us believe,
despite the now commonplace alleged observations they adduce to support it. At first sight
it appears that the idea of a binary system consisting of two black holes, or a hole and a
star, and the claim that black holes can collide, are physical issues. However, this is not
quite right, notwithstanding that the theoreticians take them as well-defined physical
problems. Here are the reasons why these ideas are faulty. First, the black hole is allegedly
predicted by General Relativity. What the theoreticians routinely fail to state clearly is that
the black holes comes from a solution to Einsteins field equations when treating of the
problem of the motion of a test particle of negligible mass in the vicinity of a single
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gravitating body. The gravitational field of the test particle is considered too small to affect
the overall field and is therefore neglected.
It has been recently proved that the black hole and the expanding Universe are not
predicted by General Relativity at all in any circumstances. Since the Michel-Laplace dark
body is not a black hole either, there is no theoretical basis for it whatsoever.
Some interested questions related to Black Holes:
Que 1 How do black holes form ?Ans 1.The most common way for a black hole to form is probably in a supernova, an
exploding star. When a star with about 25 times the mass of the sun ends its life, it
explodes. The outer part of the star screams outward at high speed, but the inner part of
the star, its core, collapses down. If there is enough mass, the gravity of the collapsing core
will compress it so much that it can become a black hole. When its all over, the black hole
will have a few times the mass of the Sun. This is called a stellar-mass black hole, what
many astronomers think as a regular black hole.
2. We think that a black hole form when a very massive star collapses at the end of its
lifetime. Stars become like our sun because of energy from nuclear reactors going on deep
inside the star. They convert hydrogen to helium, and that gives off energy, which becomes
light. Once that hydrogen is used up, they can start using helium in their nuclear furnaces.But eventually any star will run out of fuel. Stars like our sun will collapse and become a
white dwarf. But very massive stars ten times heavier than our sun have such strong
gravity that when they collapse they cant stop. Everything in the star collapses to a tiny
point that still has the mass and gravity of the star but no longer shines. In fact the gravity
is so strong that even light cant escape. That is why we call it a black hole, if you could
visit a black hole (dont get too close!), you would probably see a black ball a few miles
across.
(An artist's impression of a black hole and its accretion disc, which occurs when large amounts of superheated gas and dust are torn off a
companion star (or left nearby the black hole after the star from whence it came perished), and rotates around the black hole at very fast
velocity).
Que 2 Can only stars become black holes, or could planets and other things become black
holes? If not, why is this ?Ans Technically anything can from a black hole, whether it is a star, a planet, or a human
being! In order for something to become a black hole, it has to be compressed so that its
smaller than its Schwarzschild radius. This radius is a distance from the centre of a star or
another object where math goes crazy (ones an object become a black hole, the
Schwarzschild radius can be called its event horizon. At this distance, space becomes
infinite, and time disappears). Mathematicians call this singular (this is where the word
singularity came from), Most of the time, this radius is much smaller than the object.
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The Schwarzschild radius of the Sun is three km. The formula for finding somethings
Schwarzschild radius is as follows:
R = 1.48 X 10-27 M (where M is the mass of the object in kg.)
Realistically, however, only stars naturally form black holes, and only stars that have more
than 3.2 times the mass of our Sun. Only through some strange supernatural or alien force
could you or I become a black hole, because we just dont have enough mass.
Que 3 What is the best evidence for the existence of black holes? Is it all really just atheory ?
Ans Astronomers have found a half dozen or so binary star systems (two stars orbiting
each other) where one of the stars is invisible, yet must be there since it pulls with enough
gravitational force on the other visible star to make that star orbit around their common
centre of gravity and the mass of the invisible star is considered greater than 3 to 5 solar
masses. Therefore these invisible stars are thought to be good candidate black holes. There
is also evidence that super massive black holes (about 1 billion solar masses) exist at the
centers of the many galaxies and quasars. In this latter case other explanations of the
output of energy by quasars are not as good as good as the explanation using a super
massive black hole.
Que 4 Can anything ever escape from a black hole ?Ans Nothing that falls into a black hole can come back out again at least not in its
original form. But a black hole may lose some of its mass. Quantum theory says that
virtual pairs of the particles sometimes wink into existence from the fabric of space itself.
These particles quickly cancel each other out and vanish. But if a pair of particles appears
just outside a black hole's horizon, one may fall inside, never to make it outside again. If
the one on the outside doesnt fall through the horizon, then the particles cant cancel each
other out. In essence, that steals a little bit of mass from a black hole. Over countless
billions of billions of years, the mass loss could become substantial enough to cause the
black hole to vaporize. Material would come out, but not in its original formonly as
energy and subatomic particles. This energy is known as Hawking radiation in honor of
Stephan Hawking, the physicist who first described it.
Que 5 How many black holes are there ?
Ans To use a technical term, gobs. Astronomers have discovered several dozen likely
super massive black holes in the cores of fairly nearby galaxies, plus many more in the
distant objects known as quasars. They have discovered perhaps a dozen or two likely
stellar-mass black holes in the Milky Way galaxy and a few possible intermediates-mass
black holes in the Milky Way and other galaxies. Yes these dont even qualify as the tip of
the iceberg more like a tiny ice chip. Super massive black holes may inhabit the cores of
all galaxies with central bulges of stars, and thousands of stellar-mass black holes may
inhabit the Milky Way, with thousands more in each of billions of other galaxies. One of
the goals of black-hole researchers is to find as many as possible so they can estimate how
common these objects are.
Que 6 How can a black holes own gravity, but not light, escapes from it ?
Ans In the case of the black hole, its best to think of gravity as Albert Einstein described
it: a warp in space time. Einsteins theory of special relativity says that mass warps the
space around it. For relatively lightweight bodies, like Earth, the effect is tiny. For heavier
objects, like the Sun, the effect is small but detectable. (Scientists confirmed the effect,
among other ways, by measuring the orbit of Mercury, the closest planet to the Sun, which
is dragged forward a bit by the Suns distortion of space-time.) And for the most massive
objects, like black holes, the effect is enormous. Diagrams in astronomy textbooks often
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depict black holes as deep wells in space-time, with matter funneling into the black hole
like pebbles dropped into water well on Earth. So nothing has to escape from the black
hole for it to exert a gravitational influence on the matter and space around it.
Que 7 How do black holes have energy if they form from dead or burned-out stars ?
Ans Black holes form from large stars that collapse when they run out of fuel for their
nuclear reactions. There are many forms of energy heat, light, energy of motion and the
energy of gravity. Black holes have LOTS of gravitational energy!Que 8 - Are there any black holes in our galaxy, and if so, are there any plans to explore
them ?
Ans- Fortunately for us there are no black holes near our solar system. Unfortunately this
means that we cant send a space probe to explore one. We believe that there are several
black holes in our galaxy. Some are about the size of stars. We can tell that a black hole is
there by how it affects a companion star. Of course we cant see the black hole its black!
But we can see that a star is going around in orbit around something else that we cant see,
and we cant see the gases that the black hole has sucked off from the other star. We also
think that most if not all galaxies have a big black hole at each centre. Our own Milky Way
probably has a big black hole at its centre, but its hard to tell for sure because there are
many stars in the way when we look towards the centre. Im not sure what would happento someone who went into a black hole, but I dont think it would be pleasant!
Que 9 - What is meant by the term associated with black holes called the event horizon?
Ans- Imagine a black hole and a beam of light passing by. The gravity of the black hole is
tugging on the light (and matter) nearby, but the beam of light has energy has energy so it
keeps trying to go on by. The gravity bends the light beam, but if light is not too close it can
still pass by the black hole. If it gets too close, the gravity of the black holes bends the light
into the hole and it cant escape. This would be true no matter what direction the light is
coming from around the hole. So there would be a sphere around the black hole where, if
light went in, it couldnt come out. That sphere is the event horizon. I guess its called
that because its the limit at which you could see an event occur, and a horizon is the limit
of which you can see (usually we mean the sky).
Que 10 How is time changed in a black holes ?
Ans Well, in a certain sense it is not changed at all. If you were entering a black hole, you
would find you watch ticking along the same rate as it always had (assuming both you and
watch survived the passage into the black hole). However, you would quickly fall towards
the centre where you would be killed by enormous tidal forces (e.g., the force of gravity at
your feet, if you fell feet first, would be much larger than at you head, and you would be
stretched apart).
Although your watch as seen by you would not change its ticking rate, just as in
special relativity someone else would see a different ticking rate on your watch than the
usual, and you would see their watch to be ticking at a different than normal rate. For
example, if you were to station yourself just outside a black hole, while you would find your
own watch ticking at the normal rate, you would see the watch of a friend at great distance
from the hole to be ticking at a much faster rate than yours. That friend would see his own
watch ticking at a normal rate, but see your watch to be ticking at a much slower rate.
Thus if you stayed just outside the black hole for a while, then went back to join your
friend, you would find that the friend had aged more than you had during your separation.
Second explanation In general relativity, time and space are a set of variables that can be
used to parameterize the geometry of space-time and the kinds of geodesics that are
possible. But they are not the only kinds of variables that form a set of four coordinates
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that span the dimensionality of space-time. In probing the mathematics of black holes,
physicists have discovered other sets of coordinates that are even better.
Que 11 If black holes are black, how can we find them ?
Ans The black hole itself may be invisible, but the ghostly fingers of its gravity leave
behind fingerprints. Some stars form in pairs, called binary systems, where the stars orbit
each other. Even if one of them becomes a black hole, they may remain in orbit around
each other. By carefully observing such a system, astronomers can measure the orbit of thenormal star and determine the mass of the black hole. Only a few binary systems have
black holes, though, you have to know which binaries to observe. Fortunately, astronomers
have discovered a signpost that points the way to black holes: X- rays.
If a black hole is eating matter from a companion star that matter gets very hot and emits
X- rays. This is like a signature identifying the source as a black hole. Thats why
astronomers want to build spacecraft equipped with special detectors that can see in X-
rays. In fact black holes are so good at emitting X- rays that many thousands can be
spotted this way. EXIST is one such spacecraft, designed to be able to detect tens of
thousands of black holes, some of which may be billions of light years away. EXIST will
create the most sensitive full sky map locating black holes, including those which may be
otherwise hidden from our view by obscuring gas and dust.Que 12 What happens when you fall into a black hole ?
Ans If you fall into a black hole, you are doomed. Sure, once you fall in you can never get
back out, but it turns out youll probably be dead before you get there.
The gravity you feel from an object gets stronger the closer you get. As you approach a
stellar-mass black hole feet-first, the force of gravity on your feet can be thousands of times
stronger than the force on your head! This has the effect of stretching you, pulling you
apart like taffy. Tongue-in-cheek, scientists call this spaghettification. By the time you
reach the black hole, youll be a thin stream of matter many miles long. It probably wont
hurt though: even falling from thousands of kilometers away, the entire gory episode will
be over in few milliseconds.
You may not even make it that far. Some black holes greedily gobble down matter, stealing
it from an orbiting companion star or, in the case of super massive black holes, from
surrounding gas clouds. As the matter falls in, it piles up into a disk just outside the hole.
Orbiting at huge speeds, the matter in this accretion disk gets extremely hot- even reaching
millions of degrees. It will spew out radiation, in particular high energy X- rays. Long
before the black hole could trip you apart youd be fried by the light.
But suppose you somehow manage to survive the trip in, what strange things await you on
your way down into forever?
Once you pass the point where the escape velocity is faster than light, you cant get out.
This region is called the event horizon. Thats because no information from inside can
escape, so any event inside is forever beyond our horizon.
If the black hole is rotating, chaos awaits you inside. Its a maelstrom as in falling matter
turns back on the incoming stream, crashing into you like water churning at the bottom of
a waterfall. At the very core of the black hole the seething matter finally collapses all the
way down to a point. When that happens our math fail us. Its as if the matter has
disappeared from the Universe, but its mass is still there. At the singularity, space and time
as we know then come to an end.
Que 13 How do black holes affect things near them ?
Ans Are we in danger of being gobbled up by a black hole? Actually, no. Were pretty
safe.
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The gravity from a black hole is only dangerous when youre very close to it. Surprisingly,
from a large distance, black hole gravity is no different than the gravity from a star with
the same mass. The strength of gravity depends on the mass of the object and your distance
from it. If the Sun were to become a black hole (dont worry its way too lightweight to ever
to do that), it would have to shrink so much that its event horizon would be only few miles
across. From the Earths distance of 150 million km wed feel exactly the same gravity as
we did when the Sun was a normal star. Thats because the mass didnt change, andneither did our distance from it. But if we got up close to the black hole, only a few
kilometers away, wed definitely feel the difference!
So stellar-mass black holes dont go around tearing up stars eating everything in sight.
Stars, gas, planets, and anything else would have to get up close and personal to a black
hole to get trapped. But space is big. The odds of that happening are pretty small.
Things are different near a super massive black hole in the centre of a galaxy. Every few
hundred thousand years, a star wanders too close to the black hole and gets torn apart.
This produces a blast of X-rays that can be visible for decades! Events like this have been
seen in other galaxies, and they are a prime target for satellites such as EXIST to reveal
otherwise dormant black holes.
Astronomers have found another amazing thing about galaxies: the stars in the inner partsof a galaxy orbit the galactic centre faster when the galaxys central super massive black
hole is more massive. Since those stars velocities are due to the mass in the inner part of
the galaxy and even a monster black hole is only a tiny fraction of that mass
astronomers conclude that the total mass of the inner region of a galaxy is proportional to
the mass of its central black hole! Its as if the formation of that black hole somehow
affected the formation of the billions of normal stars around it. EXIST will probe this
suspected feedback between galaxy formation and super massive black holes by
investigating black holes in a very large sample of galaxies.
Que 14 Can black holes be used to travel through space-time ?
Ans Its a science fiction clich to use black holes to travel through space. Dive into one,
the story goes, and you can pop out somewhere else in the Universe, having travelled
thousands of light years in the blink of an eye.
But thats fiction. In reality, this probably wont work. Black holes twist space and time, in
a sense punching a hole in the fabric of the Universe. There is a theory that if this happens,
a black hole can form a tunnel in space called a wormhole (because its like a tunnel
formed by a worm as it eats its way through an apple). If you enter a wormhole, youll pop
out someplace else far away, not needing to travel through the actual intervening distance.
While wormholes appear to be possible mathematically they would be violently unstable,
or need to be made of theoretical forms of matter which may not occur in nature. The
bottom line is that wormholes probably dont exist. When we invent interstellar travel,
well have to go the long way around.
Que 15 If a black hole existed, would it suck up all the matter in the Universe ?
Ans No. A black hole has a horizon which means a region from which you cant escape. If
you cross the horizon, you are doomed to eventually hit the singularity. But as long as you
stay outside the horizons, 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 and 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, the Sun) at sucking in
distant objects.
Que 16 What if the Sun became a black hole ?
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Ans 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 giant star, during which time it will expand to engulf the planets
Mercury and Venus, and make life quite uncomfortable on Earth. After that, the Sun will
end its life by becoming a boring white dwarf star. If I were you, Id make plans to move
somewhere far away before any of this happens. I also wouldnt buy any of those 8-billionyear 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 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 km and as we have observed as long as you
stay well outside the horizon, a black holes gravity is no stronger than that of any other
objects of the same mass.
Que 17 Are we in any danger of falling into a black hole ?
Ans No, although it is believed there is a super-massive black hole at the centre of our
galaxy, we are on the outside of our galaxy, on one of the spiral arms, quite far from the
massive monster.Que 18 Is a black hole really black?
Ans Not according to the British scientist Professor Stephen Hawking. He believes this
radiation can come from virtual particles, particles that are created out of nothing,
thereby defying the laws of physics. As pairs, they can then collide with one another and be
destroyed back into empty space. Normally, we would therefore never even see these
particles before they disappear. If a pair of these particles popped up at the edge of a black
hole, one could be sucked in while the other escaped as a newly created particle coming
from the black hole. This is known as Hawking radiation.
Que 19 Does a black hole ever fill up ?
Ans A black hole never literally fills up. However, a black holes life ends eventually as a
result of matter pouring in, As a result of losing energy and mass through Hawking
radiation, Professor Hawking suggests that there would come a point when the Hawking
predicts that the black hole might then explodes with a force of millions of hydrogen
bombs!
Que 20 Do black holes live forever ?
Ans Accordingly to Professor Hawking, no. He says that as a result of losing energy and
mass through Hawking radiation, there would come a point when the black hole no longer
has enough mass to completely curve the space around it, therefore ceasing to be a black
hole. Hawking predicts that the black holes might then explode with a force of millions of
hydrogen bombs!
Que 21 Is the edge to a black hole a smooth one, or is it a sharp boundary in space ?
Ans As seen from the vantage point of an outside observer, the edge is extremely sharp. It
is a mathematical perfect, spherical surface where light gets infinitely red shifted. To an
observer falling into the black hole, the boundary may be much more complicated than our
axi-symmetric mathematics would suggest. The horizon could be a turbulent surface
rippling with gravitational radiation, or it might dissolve into a fuzzy quantum state at
even finer scales of scrutiny. Someone falling into the horizon would experience NOTHING
PECULIAR, except that once they cross this mathematical surface by even one millimeter,
they can never turn back to escape the black hole. The event horizon is a most peculiar
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concept in physics. Still, it is a theoretical idea which needs to be studied with real data
before we can feel confident that we understand it.
Que 22 Why is it that black holes cant expand but the universe can ?
Ans They are two very different physical phenomena described by two very different sets
of equations in general relativity. The black hole solution is a singularity of the
gravitational field within a pre-existing, 4-d space-time. Black holes are embedded in space,
and there is a coordinate transformation which can be used to take you from the interior ofthe black hole inside the event horizon, to the external space outside the horizon. The Big
Bang solution has no external space in which it is embedded. The space defined by this
solution is a dynamic one which is always in a state of change as the universe expands or
collapses. It is like asking why apples are not bananas since they are both fruits
Que 23 How do astronomers study black holes when they cant see them ?
Ans We see them by their gravitational effects upon nearby stars which they are often
eating or causing to move at very high velocities just before they enter the black hole. A
star cluster that weigh 1 billion times the mass of the Sun, but only has 1 few million stars,
and has a very small volume producing lots of energy, is a prime candidate for a black hole.
Also, a binary star system where the total mass is 20 solar masses but the only visible star
has a mass of 2 solar masses means a dark companion with a mass of 18 solar masses. Inother words a black hole, we can see them, as the Hubble Space Telescope does, by the
intense core of light they produce in the centers of galaxies. When astronomers study the
speed of the gases in the cores of these galaxies, they find that the gases are orbiting faster
than is possible if only the identifiable stars are accounting for the mass. We can never see
them directly, but we can see what they do to the gas and stars around them. The above
image shows a titled dusty disk at the centre of a galaxy. The bright spot is the den of the
super-massive black hole.
Que 24 Could a galaxy ever collapse into a black hole ?
Ans Yes, at least theoretically. It would take a very long time, however. The stars move in
stable Keplerian orbits which could continue as they are for trillions of years. Long
after the stars they have burned out. But objects on elliptical orbits radiate gravitational
radiation, and eventually through the very weak leakage of gravitational energy away from
the orbiting stars, the orbits around the centre of galaxy begin to slowly drift inwards.
Eventually, the cold stellar cinders merge into one vest super-massive black hole and that
is the end of stellar matter in galaxies. The time this takes is enormoussomething like
10^150 years or more, so this end state is only relevant to galaxy evolution in an infinite
universe destined to expand forever. Sadly, this is the kind of universe we seem to be living
in!
Que 25 How does a black hole make room for all the stuff it sucks up ?
Ans It doesnt have to. It just gets bigger. The radius of a black hole is proportional to its
mass, so as it consumes stars and gas, its mass grows and so does its volume.
Que 26 If nothing can escape a black hole, why do they still emit x-rays ?
Ans It is true that once matter or energy passes within the so-called Event horizon of a
black hole that it can never turn around and get back out. However, in the real world, a lot
can happen to matter as it approaches the Event horizon. Commonly, matter falls into
what is called an accretion disk which orbits the black hole. Materials orbits the black hole
within this disk, but if it happens to be gas and dust, this matter experiences friction and
the disk heats up as some of the orbital energy of the gas is converted into heat. The closer
the disk material is to the black hole, the more rapidly it orbits so that the greater is the
heating effect. Just before it reaches the event horizon, this dark matter can be heated by
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friction to thousands of degrees which is enough to produce X-rays. Even higher
temperatures approaching a million degrees can occur which can produce gamma rays.
This disk radiation, being outside the black hole, is what we detect as we look at black
holes.
Que 27 What happens to matter when it falls into a black hole ?
Ans Outside the black hole, it depends on what form the matter takes. If it happens to be
in the form of gas that has been orbiting the black holes so called accretion disk, the mattergets heated to very high temperatures as the individual atoms collide with higher and
higher speed producing friction and heat. The closer the gas is to the black hole and its
Event Horizon, the more of the gravitational energy of the gas gets converted to kinetic
energy and heat. Eventually the atoms collide so violently that they get stripped of their
electrons and you then have plasma. All along, the gas emits light at higher and higher
energies, first as optical radiation, then ultraviolet and then X-rays and finally, just before
it passes across the Event Horizon, gamma rays. If the matter is inside a star that has been
gravitationally captured by the black hole, the orbit of the star may decrease due to the
emission of gravitational radiation over the course of billions of years. Eventually, the star
will pass so close to the black hole that its fate is decided by the mass of the black hole. If it
is a stellar-mass black hole, the tidal gravitational force of the black hole will deform thestar from a spherical ball, into a football-shaped object, and then eventually the difference
in the gravitational force between the side nearest the black hole, and the back side of the
star, will be so large that the star can no longer hold itself together. It will be
gravitationally shredded by the black hole, with the bulk of the stars mass going into an
accretion disk around the black hole. If the black hole has a mass of more than a billion
times of that of the Sun, the tidal gravitational forces of the black holes are weak enough
that the star may pass across the Event Horizon without being shredded. The star is
essentially eaten whole and the matter in the star does not produce a dramatic increase in
radiation before it enters the black hole. Once inside a black hole, beyond the Event
Horizon, we can only speculate what the fate of captured matter is. General relativity tells
us that there are two kinds of black holes; the kind that dont rotate and the kind that do.
Each of these kinds has a different anatomy inside the Event Horizon. For the non rotating
Schwarzschild black hole, there is no way for matter to avoid colliding with the
singularity. In terms of the time registered by a clock moving with this matter, it reaches
the singularity within a few micro seconds for a solar-massed black hole, and a few hours
for a super-massive black hole. We cant predict what happens at the singularity because
the theory says we reach a condition of infinite gravitational force.
Que 28 Do black hole really exist ?
Ans Probably. Astronomers have discovered quite a few objects that can only be
explained as black holes. These objects are dark, so we cannot see them, but they exert a
powerful influence on the stars, gas, and even space around them. These objects are so
dark, dense, and heavy that they must be either black holes or something even more exotic.
Que 29 Will our universe become a black hole ?
Ans Unlikely. Recent developments that show our universe is expanding at an even-
increasing rate. The cause of the expansion, called dark energy, is not understood, but it
appears that the universe is destined to undergo a slow and cold death. If there were
enough mass in the universe, and if dark energy did not exist, then it might have been
possible for the universe to collapse in on itself, condensing all matter and energy to an
almost infinitely small point, like a black hole.
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Que 30 Are any black holes close to Earth ?Ans The closest black holes yet discovered are several thousand light-years away. They
are so far that they have no effect on Earth or its environment. A super-massive black hole
appears to inhabit the centre of the Milky Way galaxy, about 27,000 light-years away.
Although it is several million times the mass of the Sun, its great distance insures that it
wont affect our solar system.
Que 31 Do black holes grow ?
Ans Yes they do! They grow up accreting matter that falls into them (it doesnt get
sucked into a black hole!). Stellar-mass black holes (black holes with the suns mass or
perhaps up to 50 times as much) can double their mass by accreting material from a
companion star. Super-massive black holes may grow by absorbing millions stars over the
course of billions of years.Que 32 How can gravity escape a black hole ?
Ans To observe outside the black hole, the gravitational field of a black hole escapes the
black hole not only because portions of the star are still outside the event horizon, but
because there is more to the gravitational field than just the part that is produced by
matter. The thing to always keep in mind with relativity is that there are always two
different observing frames in any relativistic process. For the black hole, an observer at
great distances will see one thing happen, and an observer actually falling into the black
hole will see something quite different. Suppose the person falling in, emitted regular
electromagnetic pulses during the whole trip. In her reference frame, nothing strange was
happening, except that once she crossed that invisible, mathematical surface called the
event horizon, she would then be snuffed out of existence within the few milli seconds ittakes to fall from there and into the singularity.
Que 33 If a black holes pull is faster than the expansion of the universe, will a new Big
Bang happen ?
Ans - What you are asking is, could the gravitational pull from a single black hole
overwhelm the expansion of the universe and send it into collapse. To do this, the mass
inside the black hole would have to equal the mass outside of it in order for the
gravitational field to be cosmologically important. If you have a black hole that has the
mass of the universe, you already do not have a universe that is expanding. The material
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would be dragged into the black hole before it could get very far away, and this condition
would have to have taken place when the universe was very young. Now according to
physicists such as Stephan Hawking, it may be possible that matter flowing into a black
hole could form a separate universe in another part of our space-time, or even in another
dimension. We have no way to know if this is possible but in that case a black hole could
form another Big Bang in a separate universe. This is all speculation, and makes for good
science fiction but not much else.
Que 34 What happen when black holes collide ?
Ans There is no hing to prevent two black holes from orbiting each other just as the
Moon orbits earth. So long as they are at a distance of a few dozen time; their event
horizon, they can orbit each other for a very long time. If they are much closer, then
gravitational forces deform them into football like shapes and they start to emit gravity
waves in huge amounts. This causes the orbits to evolve and decay rapidly so that the black
holes eventually merge together. The Chandra X-ray observatory recently discovered two,
super-massive black holes in the core of the galaxy NGC 6240 in the last stages of this
million-year death spiral. A team of astronomers led by Stefanie Komossa at the Max-
Planck Institute announced this discovery in November 2002. At the present time, the two
black holes are 3000 light years apart, but within a few hundred million years, there willonly exist one, even more stupendous black hole. In another galaxy called Arp 220, a
similar pair of super-massive black holes has also been spotted by Chandra. Astronomer
David Clements at the Imperial College London and his colleagues announced this finding
in April 2002. This galaxy is well known to astronomers as one of the most powerful
infrared galaxies in the universe. The bottom line is that when two black holes collide and
merge, they emit huge amounts of gravitational radiation. This energy is lost to the black
holes, and causes the black hole system to lose about five percent of its mass in the process
of formation.
Que 35 What is the Singing Black Hole ?
Ans Astronomers have known since the 1970s that the X-ray light produced by some
black holes isnt steady in intensity. It actually flickers at many different times, from
milliseconds to several seconds. Astronomers call some of the more regular of these flicking
quasi-periodic oscillations or QPOs. If the process producing this flickering were
completely random, every frequency of flickering should be present with about the same
intensity, like the hiss you hear on a blank recording tape. But this turns out not to be the
case. Instead for some black holes, slower flickering is more common that fast flickering,
and in a particular way called one over f noise. What this jargon means is that whatever
is going on in regions closest to a black holes horizon, it has some kind of memory or
correlation. Astronomers Phil Utley and Lane McHardy of the University of
Southampton have analyzed this flicker noise and now conclude that it has a common
cause among a vast number of different black hole systems. They have used the NASA,
Rossi X-ray Timing Explorer satellite for the last six years, listening-in to the X-ray sounds
from a variety of black holes. What they think they are hearing is astonishing. Near the
inner edge of an accretion disk, gases are turbulent and form cells of plasma. When these
plasma cells of varying size pass across the black hole horizon, they emit a burst of X-rays.
When added together, the X-ray light from these gas cells produce the one over f noise
that is detected by the NASA Rossi XTE satellite. What is even stranger is that the same
black hole will suddenly change its style of making this noise. This kind of noise is also
interesting because if you took classical, popular or jazz music and counted its frequency
content in the same way, it would produce a similar flicker noise. So, although we may not
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identify the tune, black holes do have a song to sing, and like whales in the ocean, they
change their song from time to time. Even more surprising is that super-massive black
holes in the cores of distant galaxies also produce this same kind of noise, but slowed down
a million times. The conclusion is that stellar and super-massive black holes lead to exactly
the same observations and accretion disk physics once the differences in their masses are
factored out.
Que - 36 We see black hole, e.g. the one in M87. Does this preclude the idea that universe isa black hole, or could one black hole exist inside another black hole ?
Ans Actually a black hole could exist inside a black hole. Imagine turning a whole galaxy
into a black hole by bringing all its stars extremely close together. All these stars might
themselves be black holes. The individual black holes wont even be touching when they are
all surrounded by a larger event horizon with a radius corresponding to the mass of the
galaxy. However, once they are all surrounded by a larger event horizon with a radius
corresponding to the mass of the galaxy. However, once they are all within the event
horizon they will end up merging together in a final collapse together in a final collapse to a
singularity.
Que 37 Can black holes be used to explain the missing mass in the universe ?
Ans Black holes would be hard to detect, and a great deal of unseen mass could be storedinside black holes. However, based on how we think black holes must form (from
collapsing cores of relatively rare super-massive stars) there cant be that many black holes
in comparison to more normal stars. Hence we expect that the total mass in black holes is
only a small fraction of the mass we can see, so that they dont constitute a major
component of the unseen, or missing mass in the universe.
Que 38 Is it possible that there is a black hole in our galaxy ?
Ans It is almost certain that there are black holes in binary systems in our galaxy.
Another question is whether or not there is a really big black hole in the centre of our
galaxy. In recent years the answer has emerged; yes! Direct observations of the centre of
the galaxy have been able to follow the orbits of the massive stars over a number of years.
Using Keplers laws we can calculate the mass of the thing that they are orbiting and it
comes out to about 3 million solar masses. Thus, although we dont directly observe the
black hole, we see the effect of gravitational field.
Que 39 What is a white hole ?
Ans The equation 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 youll 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 the 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 doesnt mean they actually exist in nature. In fact,
they almost certainly do not exist, since theres 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.)
Que 40 What is a wormhole ?
Ans 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 or have charge things get more complicated. In
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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 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 thereforeprovide a convenient and rapid way to travel very large distances or even to travel to
another universe, may be exist 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 of
white holes, just because something is a valid mathematical solution to the equations
doesnt mean that is actually exists in nature. In particular, black holes that form from the
collapse of ordinary matter do not form wormholes. Furthermore even if a wormhole were
formed, it is thought that it would not stable. Even the slightest perturbation would cause it
to collapse. Finally even if wormholes exist and stable, they are quite unpleasant to travel
through. Radiation that pours into the wormhole gets blue shifted to very high frequencies.As you try to pass through the wormhole, you will get fried by these X-rays and gamma
rays.
Que 41 Can black holes be used to travel through space-time ?
Ans Its a science fiction clich to use black hole to travel through space. Dive into one,
the story goes, and you can pop out somewhere else in the Universe, having traveled
thousands of light years in the blink of eye.
But thats fiction. In reality, this probably wont work. Black hole twist space and time, in
a sense punching a hole in the fabric of the Universe. There is a theory that if this happens,
a black hole can form a tunnel in space called a wormhole (because its like a tunnel
formed by a worm as it eats its way through an apple). If you enter a wormhole, youll pop
out someplace else far away, not needing to travel through the actual intervening distance.
While wormholes appear to be possible mathematically, they would be violently unstable,
or need to be made of theoretical forms of matter which may not occur in nature. The
bottom line is that wormholes probably dont exist. When we invent inter-stellar travel,
well have to go the long way around.
ABOUT EXIST
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed NASA
Satellite that will look at the energetic X-rays emitted from black holes and other exotic
astronomical objects. It is a strong candidate to be the Black Hole Finder Probe, one of the
three Einstein Probes in NASAs Beyond Einstein program. EXIST could be launched
early in the next decade, and with unparalleled sensitivity, will be used to study black holes
of all sizes.
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Recapitulation and Conclusion The black hole and the Big Bang model are mutually exclusive. The black hole has no valid
basis in General Relativity or Newtons theory of gravitation. The alleged escape velocity of
a black hole and the radius of its event horizon (Schwarzschild radius) are obtained from
Newtons expression for escape velocity which is inserted post hoc into Hilberts solution in
order to obtain a material source: this is a two body relation in what is alleged to be the
solution for a one body problem, and so it is inadmissible. The idea of multiple black holesviolates the defining boundary condition of space-time asymptotic flatness of the alleged
black hole, which necessarily excludes the possibility of multiple black holes. The principle
of Superposition is invalid in General Relativity and so additional masses and radiation
cannot be superposed upon any solution to Einsteins field equations in order to obtain
multiple masses and photons. When there are no materials sources present for the
gravitational field Einsteins field equations must vanish. The total energy of Einsteins
gravitational field is always zero so that Einsteins field equations violate the usual
conservation of energy and momentum and cannot localize energy to produce Einstein
gravitational waves. Einsteins pseudo tensor representing the energy of the gravitational
field is a meaningless collection of mathematical symbols because it implies the existence of
a 1
st
order intrinsic differential invariant which does not in fact exist. The HawkingPenrose Singularity Theorem is invalid. The cosmological constant has no physical
meaning and so expansion of the Universe.
It was once told as a good joke upon a Mathematician that the poor man wentMad and mistook his symbols for realities; As M for the moon and S for the Sun OLIVER HEAVISIDE (1893)
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