How to Detect a Black Hole🕳
One of the greatest scientist of our era prof. Stephen Hawking with Roger Penrose discussed the idea for defining a black hole as the set of events from which it was not possible to escape to a large distance. It means that the boundary of the black hole, the event horizon, is formed by rays of light that just fail to get away from the black hole. Instead, they stay forever, hovering on the edge of the black hole. It is like running away from the police and managing to keep one step ahead but not being able to get clear away.
How could we hope to detect a black hole, as by its very definition it does not emit any light? It might seem a bit like looking for a black cat in a coal cellar. Fortunately, there is way, since as John Michell pointed out in his pioneering paper in 1783, a black hole still exerts a gravitational force on nearby objects. Astronomers have observed a number of systems in which there is only one visible star that is orbiting around some unseen companion.
One cannot, of course, immediately conclude that the companion is a black hole. It might merely be a star that is to faint to be seen. However, some of these systems, like the one called Cygnus X-I, are also strong sources of x rays. The best explanation for this phenomenon is that the x rays are generated by matter that has been blown off the surface of the visible star. As it falls toward the unseen companion, it develops a spiral motion-rather like water running out of a bath-and it gets very hot, emitting x rays. For this mechanism to work, the unseen object has to be very small, like a white dwarf, neutron star, or black hole.
Now, from the observed motion of the visible star, one can determine the lowest possible mass of the unseen object. In the case of Cygnus X-I, this is about six times the mass of the sun. According to Chandrasekhar’s result, this is too much for the unseen object to be a white dwarf. It is also too large a mass to be a neutron star. It seems, therefore, that it must be a black hole.
There is much other evidence for black holes in a number of other systems in our galaxy, and much other bigger one in the center of other galaxies and quasars. One can also possibly say that there are black holes with a much smaller mass than our sun. Such black holes could not be formed by gravitational collapse, because their masses are below Chandrasekhar’s limit. Stars of this low mass can support themselves against the force of gravity even when they have exhausted their nuclear fuel. So, low mass black holes could form only if the matter were compressed to enormous densities by very large external pressures. Such conditions could occur in a very big hydrogen bomb. The physicist John Wheeler once calculated that if one took all the heavy water in all the oceans of the world, one could build hydrogen bomb that would compress matter at the center so much that a black hole created. Unfortunately, however, there would be no one left to observe it.
A more practical possibility is that such low mass black holes might have been formed in the high temperatures and pressures of the very early universe. A black hole could have been formed if the early universe had not been perfectly smooth and uniform, because then a small region that was denser than average could be compressed in this way to form a black hole. But we know that there must have been some irregularities, because otherwise the matter in the universe would still be perfectly uniformly distributed at the present epoch, instead of being clumped together in stars and galaxies.
