"The most incomprehensible thing about the Universe is that is Comprehensible"

    A Cambridge man , John Miichell pointed out in his paper in 1783, in the 'Philosophical Transactions of the Royal Society of London' that a star that was sufficiently massive and compact would have such a strong gravitational field, that light could not escape. Any light emitted from the surface of the star would be dragged back by the star's gravitational attraction, before it could get very far. Michell suggested that there may be a large number of stars like this. A similar suggestion was made a few years later by the French scientist the Marquis De Laplace, in his book "The System of the World". But the modern concept of black holes comes from  Albert Einstein's theory of General Relativity. His field equations predicts black holes and their properties.

Space-Time, Warped Space Time and Black Holes:

    To understand the nature and properties of a black hole, we should know the modern concept in physics about Space and Time. Since the civilization Philosophers and Scientists have been trying to explain Space and Time. Newton used common sense perspectives adopted by ordinary people to explain space and time. According to Newton's view the world changes from moment to moment. At any given moment the world is 'some well defined state of the whole Universe'.  In fact the world is seen as the totality of all these objects at one particular time , Universal moment. He thought as most people do, that the 'Space' and 'Time' are absolute; concrete or Universal. But Einstein's 1905 paper on relativity 'The Special Theory of Relativity' revolutionized the concepts of space and time. Space and Time can be no longer considered as absolute instead they are flexible, elastic (qualities) and relative. Their values depend on who is measuring them from which frame of reference.  According to this  space and time cannot be regarded as two separate entities anymore. Instead they are one , called 'space-time'. It combines Space and Time into one physical identity. Now Universe is regarded as as series of events in 'space-time'. That is Universe is 'space-time' continuum.

     Special Relativity revolutionized the concepts of Space and Time, but 'The General Relativity' revolutionized the concept of gravity. It describes matter as well as 'space-time'. Newton considered gravity as a force, but according to Einstein gravity is no longer a force, Instead a 'space-time' geometry. The mass of an object curves , warps the 'space-time' world around it. It bends or distorts the fabric of 'space-time'. The geometry of this 'warped' space-time makes bodies like Earth to move to follow the nearest thing to a straight path in a curved space which is called Geodesic,  shortest path between two nearby points. According to 'General Relativity' the earth follows a straight path in four dimensional 'space-time' which appears to us to move along a circular orbit in three dimensional Space. In fact, Black Holes are highly gravitational objects which warp the 'space-time' world around them, so that nothing could escape from their distortion of 'space-time'.
 

Formation of Black Holes:

    So far we have been dealing theoretically about black holes. Now, let us think about about a little bit practically. In reality, how black holes are formed? To understand this , we should know the process of Stellar Evolution.

Stellar Evolution:

    A star is formed when a large amount of gas , Interstellar Medium, starts to collapse , mostly Hydrogen, on itself due to its gravitational attraction. As it contracts the atoms of the gas collide with each other more and more frequently and at a greater speeds. Finally, the gas will be so hot that when the Hydrogen atoms collide they  no longer bounce of each other but instead coalesce to form helium. The heat released in this reaction makes the star shine. THis heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction and the gas stops contracting. 

    Stars will remain stable like this for a long time. However eventually the star will run out of its hydrogen and other nuclear fuels. When a star runs out of fuel, it starts to cool off and contracts. During this contracting process, the burned out core be less than about 1.44 times ( the so called Chandrasekhar limit;Dr. S.C Chandrasekhar, a well known scientist from Tamilnadu, India received nobel prize for his research on 'Black Holes'. )  the mass of our sun, The resultant object is to become a white dwarf. If this mass lies between 1.44 and 3 solar masses, the result is a neutron star. This added mass makes star to crush its matter beyond atomic identity driving the electrons into the protons and making neutrons. If the remnant has  amass greater than about 3 solar masses, gravity wins out. Even tightly packed neutrons cannot withstand the gravitational pull. The objects starts to collapse forever. That is the case of Black Holes. 

    The German physist Karl Schwarzschild found the relationship of critical radius, called Schwarzschild's radius. According to him , any object can be transformed into a black hole by the simple act of shrinking it. The figure is a space-time diagram of the collapsing star. As time passes the star grows smaller and eventually collapses within its Schwarzchild radius. At this instant the horizon forms, inside the horizon the star continues to collapse and shortly thereafter turns into a singularity. Also represented in figure 2 are the paths of light rays. Ray A was emitted from the surface of the star when it was fairly large. Ray B, emitted when the star was smaller is more strongly curved. Ray C, emitted just before the star has reached it Schwarzchild radius. Ray D, was emitted after hole has formed. This ray is trapped and it falls into the singularity.

    Figure 3 shows the curvature of space-time in the hole's vicinity. Figure 4 shows the further collapse. The star grows smaller, the curvature of space-time more distorted and severe. Finally the star entirely vanishes and crushes into a singularity. What remains now is only the curvature. Einstein's gravitational field equations predicts an another object called 'White Holes'. Actually 'White Holes' are just time reversed Black Holes. They emit light all the time; shine steadily. A source from which newly created matter spews forth into the Universe.  Figure 6 shows a black hole ,a white hole and a 'worm hole' which acts as a bridge connecting the black and white holes. The black and white holes may be in the same universe separated by a huge distance or may be in the two different universes. Although Einstein's field equations predict the possibility of 'worm hole' type structures, it is not possible to travel across from black holes to white holes due to the followings reasons:

    One must travel faster than  light in order to pass safely through a worm hole. Secondly, the geometry of 'space-time' represents worm hole exists only for a short period. As time passes, the geometry evolves. Like Black hole a worm hole suffers a similar consequences. In order to pass through it, one must travel very fast otherwise he or she will be caught in the pinch  and engulfed in a singularity.
 

Some Other Characteristics of Black Holes:

    Black Holes, as Eric Chaisson says in his book 'Relatively Speaking' are not cosmic vacuum cleaners. They don't go out of their way to absorb matter. But if some matter does happen to fall in through the normal pull of gravity, there is no way to escape.

     Black Holes permit matter to flow in only one direction inward by absorbing , swallowing matter, they constantly increases their mass and size. Event  horizon depends on the amount of mass entombed inside.

    The famous theoretical physicist Stephen W. Hawking's studies on Black Holes show the nature of the event horizons and the immediate environment of the black holes. According to Hawkings, the area of the event horizon might stay the same or increase with time; but it could never decrease since the paths of light rays which form the event horizon always move parallel to or away from each other. Also he pointed out that the area would increase whenever matter or radiation fell into the black hole or if two black holes collide and merge together to form a single black hole. In the latter case the area of the event horizon of the final black hole would be greater than or equal to the sum of the areas of the event horizons of the original black holes. Hawkings explained the above mentioned effect of the event horizons with the laws of Thermodynamics. Another important prediction (and property) of Black Holes by Hawkings is its emission of particles.  According to quantum mechanics' uncertainty principle  rotating black holes should create and emit particles. But Haw king's calculations shows even non rotating black holes create and emit particles at a steady rate.

    We know nothing could escape from black holes. In this situation how emission is possible from black holes? Hawkings showed using quantum theory that the particles do not come from within the black holes but from the space just outside the black hole's event horizon.

    Another important property of black hole is its tidal stress effect, which means whatever falls into a black hole is subject to great tidal forces or stress. Suppose, a person falling feet first into a black hole would find himself stretched enormously in height while being squeezed laterally. He would not stay in one place for more than a fraction of a second after passing the event horizon. This falling matter is the reason for Black holes' radiation. In falling matter emit radiation on its own due to the rapid heating of matter by tides and collisions, before the submersion below the hole's event horizon. Also, quantum mechanics predicts that black holes might not last forever. They slowly evaporate and at last explode, scattering their contents into the vast interstellar space. Their life time just like an ordinary stars depends on their mass as well.

Is it Possible to Travel Near Black Hole?

    Space Travel near black hole is not possible for human since the human body cannot withstand physiological stress more than ten times the present gravitational pull. We normally feel earth's surface. But we can send a mechanical robot in a spaceship designed to withstand stressful conditions normally intolerable to humans. In this way we may be able to study the vicinity of a black hole. But once it is  inside the event horizon, it is  gone forever. It is lost in the space. Eternally trapped inside  different universe where our laws of physics do not work. This is the big question of man kind. What is inside a black hole? What lies within the event horizon? No one knows....
 

Some Observational Evidence....

    Are these black holes really exist? Or they are just a mathematical expression. But observations confirmed the existence of black holes. The discovery of Cygnus X-1, in 1973 in the binary system may have the clue to solve the black hole puzzle. This discovery provided a valuable information to the theoretical physics. Binary stars travel in pairs and orbits a common centre of gravity. Of one of the stars collapsed into a black hole, the black hole would exert a gravitational force on its partner. What astronomers found in the constellation Cygnus is that kind of similar effect. We cannot rush to a conclusion. The invisible mate may  a star that is too faint to be seen.

    Cygnus X-1 is a strong source of X-rays. According to Hawkings, the explanation is the matter has been blown off the surface of the visible star due to its invisible partner's strong gravitational pull. As it falls towards the invisible mate, it develops a spiral motion and gets very hot , thus  starts to emit X-rays. However in this situation, the unseen mate may be a small white dwarf, or may be a  neutron star or may be a black hole.  But it is possible to determine the lowest mass of the unseen mate from the orbit of the visible star. In the case of Cygnus X-1, this mass is about six times the mass of the sun. According to Chandrasekhar limit, this fits only for a black hole. It is too great for White Dwarf or Neutron Star. So it must be a black hole.

    Now we have more evidence for other black holes  in our galaxy and in the neighboring galaxy , 'Megellanic Clouds'. We also have some evidence of a larger black hole , with a mass of about 100,000 times of our sun at the centre of our galaxy.

Mini Black Holes:

    According to Hawkings there is a possibility such that there may be more mini black holes than visible stars in our galaxy. The extra gravitational attraction of these black holes may explain why our galaxy rotates at the rate it does.

    Another suggestion by Hawkings and some other Scientists is that the pressure in the early Universe might be able to form a significant number of 'Primordial' black holes. Discovery of these 'Primordial' black holes will provide more clues about the early stages of the Universe.
 

Bibliography:

Barry Prker, Einstein's Dream, Pllenum Press, NY 1986.
Bertrand Russell, ABC of Relativity, Unwin Paper backs, London 1985.
Eric Chaission, Relatively Speaking, W.W.Norton & Co., NY 1988.
George Greenstein, Frozen Star, New American Library, 1983
George Camow, One Two, Thre..Infinity, Dover Publications, 1974.
John Boslough, Stephen Hawking's Universe, Avon Books, NY 1985.
John Gribbin, Cosmic Coincidence, Bantom Books
Jayant. V. Narlikar, The Lighter side of gravity, W.H.Freeman & Co., Sanfrancisco
Paul Davis, Other Worlds, A  Touchstone Books.
Stephen W. Hawkings, A Brief History of Time, Bantom Books, 1988