Black holes are a topic of much mystery and wonder to all who are familiar with their existence. Over the years, the astrophysicists have come to know a lot about them but still much remains to be understood. The fact that they are not explained by any conventional theory makes it even more difficult for people to grasp at their bizarre nature.
Fabric of space-time
When Albert Einstein put forward the theory of general relativity in 1915, he combined the three dimensions of space with time to devise an important concept of space-time. This space-time “fabric” can be thought to be similar to an elastic sheet which is stretched yet flexible enough to allow for bends and warping. Since the fabric is smooth and there are no abrupt changes in its condition, if present anywhere, it is referred to as space-time “continuum”. Any object placed on the space-time continuum causes it to bend under the weight of those objects. It can be imagined to be similar to the effect caused when a large and heavy bowling ball is kept over a trampoline. This phenomenon is called geodetic effect. On increasing the weight of the objects, the bending of the space-time continuum also increases.
Every object in space has a gravitational pull with which it attracts every other object towards it. If a person on a celestial object wishes to leave the object to explore other parts of space, they would require a rocket ship which could achieve a velocity sufficient enough to escape this gravitation pull. This is known as the escape velocity for the object.
No escape for light
Now, imagine an object with a very high mass but very small size. It will be incredibly dense. This high mass will create a strong gravitational field for the object. The escape velocity for such an object would reach exceedingly high values. If we keep on making the object smaller and smaller the escape velocity will eventually reach the speed of light. Thus, such an object would not allow matter and even light to escape from its surface.
The atoms are the constituent particles of every object. There are certain nuclear forces which act between these atoms. The repulsive nature of these forces keeps the object from collapsing. But in case of our infinitely dense object, the nuclear forces will be unable to prevent such an object from collapsing under its own weight to an infinitesimal point. The object will become invisible and its existence can be determined only by observing the gravitational effects of such a “singularity” on its neighborhood. A singularity or a gravitational singularity, as in our case, is a point which contains all that huge amount of mass which has been squeezed. Surrounding this gravitational singularity is the “black hole”, a term made popular by the physicist John Wheeler.
A black hole is actually a spherical gravitational boundary which is called “event horizon”. This boundary exists at a certain distance from the singularity. Consider this boundary as the point of no return if you venture further in. Light travels with the fastest speed in the known universe. For a person to leave the surface of the black hole he will have to invent a rocket ship that can travel faster than the speed of light, which is not possible. A black hole is shown in illustrations and movies as a rotating disc because it is actually churning the space-time continuum much like when you put a stick in melted chocolate and rotate it.
Anything or anyone that gets closer to the black hole any more than the event horizon will have their fate warped in the same way that space-time is warped around the singularity. If you are watching your friend disappear into the black hole from outside he will appear to slow down and fade into the oblivion slowly and slowly, taking forever. But your friend will experience himself vanishing from existence in a limited period of time. It is not known as a fact what actually happens to an object that gets sucked in by the black hole. Light that travels close enough to the black hole gets sucked in and if it is a little further way, it gets bent and changes its path. Such is the pull of the black hole. Since we cannot take any gainful measurements of the black hole with the help of any conventional means and equipment, it is said that not even information can escape from the black hole.
Minuscule, huge and ginormous
Black holes usually come in three sizes, at least according to theory. There are miniature black holes, stellar black holes and supermassive black holes.
Miniature black holes would have much less of a mass than that of our Sun. Scientists believe that these might have formed in the beginning of the universe. But till now there is no evidence of their existence. They are thought to contain a mass of Mt. Everest.
Stellar black holes
Stellar black holes are formed when a massive star collapses. Gravitational forces inside the star try to pull everything together. This is matched by the pressure of hot gases that are created inside the star due to the heat from nuclear reactions. But when this massive star runs out of gas, or this “nuclear fuel”, gravitational forces are free to make the star crumble. The outer parts of the star explode while the insides implode. An implosion is a violent inwards collapse. A massive star whose insides have a high mass such as about ten times that of the Sun is sure to form a black hole this way one day.
Supermassive black holes
Supermassive black holes, which supposedly occupy the centers of most galaxies, are black holes which have grown bigger and bigger by engulfing mass. They are thought to have a mass equal to that of a billion Suns. It is firmly believed that one of them exists in the center of our Milky Way galaxy as well.