When I began to study general relativity seriously (which was in the late fifties), no one knew well what a black hole would be. Even the term itself did not appear in either strictly scientific or popular science publications. This is a stark contrast with what we see today, when almost everyone has read or at least heard about them. The black hole is a product of gigantic gravitational forces. Black holes are born when the gravitational field, growing in the course of catastrophic contraction of a very large mass of matter, becomes so strong that it ceases to let out anything, even light. An object can only fall into a black hole, pulled by its huge gravitational force, but there is simply no way out.

I first read a description of very strong gravitational fields in the Landau and Lifshitz monograph that I have already mentioned. I studied it while still a student, under Zelmanov's guidance. The book gave a very brief but extremely clear presentation of the properties of the gravitational field of a strongly compressed spherical mass. The solution of Einstein's equations for this case was found by the German astronomer Karl Schwarzschild; consequently, this gravitational field is known as the Schwarzschild field.

I remember that this subsection left me rather indifferent. Nevertheless, I did make some evaluations, using the formulas in the book and the knowledge gained from talking to Abram Zelmanov.