1. What Is Inside Black Holes, and How Can This Be Proved by Observations?

My first scientific discussion about black holes took place in the autumn of 1975, when I had just entered my second year at the Physics Faculty of Tashkent University.

Rashid Sunyaev of the Space Research Institute in Moscow gave a review lecture on the discovery of black-hole candidates. He was still young, but already known as a major astrophysicist. Only later did I learn another curious detail: he, too, was from Tashkent.

For more than an hour Sunyaev showed slides and described satellite observations that, over the preceding several years, had revealed many compact stars emitting powerful X-rays. Some of these objects had to be so massive and compact that they could no longer be ordinary neutron stars. There was therefore great hope that the long-sought black holes had finally been found among them.

As one of the creators of the theory of accretion disks around such objects, Sunyaev explained in detail the processes occurring in matter falling toward them.

After the lecture he spent almost another hour answering questions. There were many. Professors, lecturers, and researchers from scientific institutes were in the hall. They discussed details of spectra, portions of observational curves, features of satellite instruments, and properties of X-ray radiation.

But all that time I was occupied by a completely different question.

No one asked it.

When the audience finally began to tire and it became clear that the discussion was about to end, I understood that if I kept silent I would never get an answer.

So I raised my hand.

I stood up and asked:

— What is inside these black holes?

The whole hall burst into laughter.

People who had listened seriously and intently to the authoritative guest for almost two hours suddenly seemed to have been given a chance to relax. Some laughed to tears; others began applauding through the laughter.

I looked around and could not understand what was so funny. I did not think I had said anything particularly foolish.

Annoyed and confused, I sat down and looked only at the speaker.

It was then that I was first struck by Sunyaev's reaction.

He proved to be not only a genuine researcher who understood the importance of the question, but also a remarkably considerate person. I was sitting close and could see his reaction clearly. He looked at the laughing audience with surprise, smiled slightly, and before the hall had fully quieted he came closer and began to answer with a completely serious expression.

He explained that the available observations provided only indirect information. They allowed us to judge what was happening outside a very compact object: to measure its mass, estimate its rotation, study the motion of infalling matter, and observe radiation from the accretion disk.

But according to the accepted theory itself, direct information about what lies inside a black hole cannot be obtained, because nothing should come out from there.

One could, he said, look for signs that an ordinary material surface was absent. When matter falls onto a neutron star, interaction with its surface can produce observable effects. A black hole should have no such surface.

As for the inner region, theorists had constructed highly unusual models for it. As I remember the conversation, the ordinary roles of space and time were said to change radically there, and the description became so unfamiliar that the usual physical picture almost disappeared.

Sunyaev noted that all this could be found in well-known courses on theoretical physics, but he did not want to go deeply into the details of such constructions. He was more interested in what could actually be observed.

The answer was thorough.

But it did not satisfy me.

I stood up again and asked the second part of the question:

— But how can observations prove that these are specifically black holes and not something else?

The audience laughed again, though not as loudly. There was some ironic applause as well.

This time it was the speaker who looked slightly uncomfortable.

He acknowledged that indirect data were indeed not a direct proof of the internal structure of an object. Observational astrophysics had to work with what could be observed, and he saw no way, in the foreseeable future, to look inside a supposed black hole.

As for the numerous theoretical models of the interior, he added, that was the theorists' business.

But the conversation did not end there.

Sunyaev began to consider what observations might in principle confirm the existence of the event horizon predicted by the theory. He went through possible signatures, limitations, and indirect evidence.

Suddenly I noticed that the hall had long since become completely silent.

I turned around and saw an entirely different scene.

The people who only minutes earlier had laughed at a student's naive question were now listening with intense attention to a leading specialist seriously discussing one of the deepest questions in physics: what exactly do we observe when we speak of a black hole, and how can we establish the nature of an object from whose inner region, by definition, no information can emerge?

The discussion had unexpectedly risen above the details of spectra, instruments, and individual observational curves.

A fundamental question was now before us.

At the end Sunyaev advised me to read the latest papers and reviews on the subject in the leading scientific journals and named several of them.

That ended the discussion.

Then the hall applauded again — this time without irony. The applause was for the speaker's fascinating professional answers and, it seemed to me, also for the questions themselves, which had allowed everyone for a moment to rise above the routine of everyday scientific work.

That was my first peculiar initiation into the scientific community.

Through misunderstanding and coincidence, about a hundred and fifty intelligent, educated people first laughed at me; a few minutes later the same people listened to a discussion of my questions in complete silence and ended by applauding gratefully.

Neither I nor anyone else in that hall could then have imagined that many years later those two simple questions would prove far from accidental for me.

What is inside a black hole?

And, even more importantly:

how can one prove that the observed object is specifically a black hole?

For many years, like most physicists, I accepted the existence of black holes almost as an established result of theory. I returned to these questions much later, when I began to work seriously on general relativity and gravitational collapse.

It then became clear that the problem was much deeper than it had seemed to me in 1975.

Gradually the question itself changed.

It was no longer a matter of what is inside a black hole, but whether such an inner region is formed at all during the real gravitational collapse of a star in general relativity.

Nor was the first question any longer how to prove the existence of a black hole. There was a more elementary scientific requirement:

what does Einstein's own theory actually predict for a collapsing star?

The answer I reached decades later was completely different from the one assumed in that auditorium in the autumn of 1975.

It was this path that eventually led me to the theory of frozars.

Later in my scientific life I repeatedly found myself in situations where my conclusions met distrust, indifference, or sharp rejection. But from that student episode onward, a simple rule gradually took shape for me: the reaction of the majority proves nothing by itself.

First laughter, then silence, then discussion.

In science something else must be decisive: how consistently the question is posed, how rigorously the answer follows from the initial principles, and how well that answer corresponds to observed reality.

Truth is not determined by a vote.

And if a simple question exposes a contradiction in the familiar picture of the world, it is worth asking regardless of how naive it may seem to others.

In 1975 I merely wanted to know what was inside black holes.

I did not yet know that many years later I would have to ask a much more radical question:

do black holes form at all?