Carlo Rovelli: Where does the stuff that falls into a black hole go?

By Carlo Rovelli

Mark Garlick/Science Photo Library

THERE is something paradoxical in what we know about black holes. They have now become “normal” objects for astronomers. Astronomers observe them, count them and measure them. They behave exactly as Einstein’s theory predicted a century ago, when no one dreamed that such peculiar objects could actually exist. So, they are under control. And still, they remain utterly mysterious.

On the one hand we have a beautiful theory, general relativity, confirmed in spectacular manner by astronomical observations, which accounts perfectly well for what the astronomers see: these monsters that swallow stars revolve in vortices and produce immensely powerful rays and other devilry. The universe is surprising, variegated, full of things that we had never foreseen or imagined the existence of, but comprehensible. On the other hand, there is still a small question of the kind that children specialize in when adults are overly enthusiastic: “But where does all the material that we see falling into a black hole go?”

And this is where things become difficult. Einstein’s theory provides a precise and elegant mathematical description even of the inside of black holes: it indicates the path that material falling into a black hole must follow. The matter falls ever faster until it reaches the central point. And then… then the equations of Einstein lose all meaning. They no longer tell us anything. They seem to melt like snow in sunshine. The variables become infinite and nothing makes sense. Ouch.

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What happens to matter that falls into the centre of the hole? We don’t know.

Through our telescopes we see it falling, and we mentally follow its trajectory until it nearly reaches the centre, and then we have no knowledge of what happens next. We know what black holes consist of, both outside and inside, but a crucial detail is missing: the centre. But this is hardly an insignificant detail, because everything that falls in (and into the black holes that we observe in the sky, things continue to fall) finishes up at the centre. The sky is full of black holes into which we can see things disappear… but we don’t know what becomes of them.

The roads taken to explore answers to this question have so far been hazardous. Perhaps, for instance, the matter emerges in another universe? Perhaps even our own universe began this way, emerging though a black hole opened in a preceding one? Perhaps at the centre of a black hole everything melts into a cloud of probability where spacetime and matter no longer mean anything? Or perhaps black holes irradiate heat because the matter that enters them is mysteriously transformed, over zillions of years, into heat.

“What happens to the matter that falls into the centre of the hole? We don’t know”

In the research group I work with in Marseille, together with colleagues at Grenoble and at Nijmegen in the Netherlands, we are exploring a possibility that seems to us both simpler and more plausible: matter slows down and stops before it reaches the centre. When it is most extremely concentrated,a tremendous pressure develops that prevents its ultimate collapse. This is similar to the “pressure” that prevents electrons from falling into atoms: it is a quantum phenomenon. Matter stops falling and forms a kind of extremely small and extremely dense star: a “Planck star”. Then something happens that always happens to matter in such cases: it rebounds.

It rebounds like a ball dropped on the floor. Like the ball, it rebounds along the trajectory of the fall, in temporal reverse, and in this way the black hole transforms itself (by “tunnel effect”, as we say in the jargon) into its opposite: a white hole.

A white hole? What is a white hole? It is another solution to the equations of Einstein (like black holes are) about which my university textbook says that “there is nothing like it in the real world”… It is a region of space into which nothing can enter, but from which things emerge. It is the time reversal of a black hole. A hole that explodes.

But then why do we see matter fall into black holes but do not see it immediately bouncing back out again? The answer – and this is the crucial point about what we are dealing with – lies in the relativity of time. Time does not pass at the same speed everywhere. All physical phenomena are slower at sea level than in the mountains. Time slows down if I am lower down, where gravity is at its most intense. Inside black holes the force of gravity is extremely strong, and as a result there is a fierce slowing of time. The rebounding of falling matter happens rapidly if seen by someone nearby, if we can imagine someone venturing into a black hole to see what it’s like on the inside. But seen from outside, everything appears to be slowed down. Enormously slowed down. We see things disappear and vanish from view for an extremely long time. Seen from outside, everything looks frozen for millions of years – exactly how we perceive the black holes we can see in the sky.

But an extremely long time is not an infinite time, and, if we waited for long enough, we would see the matter come out. A black hole is ultimately perhaps no more than a star that collapses and then rebounds – in extreme slow motion when seen from outside.

This is not possible in Einstein’s theory, but then Einstein’s theory does not take quantum effects into account. Quantum mechanics permits matter to escape from its dark trap.

After how long? After a very short time for the matter that has fallen into the black hole, but after an extremely long one for those of us observing it from outside.

So here is the whole story: when a star such as the sun, or a little bigger, stops burning because it has consumed all its hydrogen, the heat no longer generates enough pressure to counterbalance its weight. The star collapses in on itself, and if it is sufficiently heavy it produces a black hole and falls into it. A star of the dimensions of the sun, that is to say thousands of times bigger than Earth, would generate a black hole with a diameter of one and a half kilometres.

Jamie Stoker

Imagine it: the whole of the sun contained within the volume of a foothill. These are the black holes that we can observe in the sky. The matter of the star continues on its course inside, going ever deeper until it reaches the monstrous level of compression that causes it to rebound. The entire mass of the star is concentrated into the space of a molecule. Here the repulsive quantum force kicks in, and the star immediately rebounds and begins to explode. For the star, only a few hundredths of a second have elapsed. But the dilation of time caused by the enormous gravitational field is so extremely strong that when the matter begins to re-emerge, in the rest of the universe, tens of billions of years have passed.

Is this really the case? I don’t know for sure. I think it might well be. The alternatives seem less plausible to me. But I could be wrong. Trying to figure it out, still, is such a joy.

In a further extract, “Copernicus and Bologna”, Rovelli writes about the value of a university education

…I also found something else in Bologna, when I studied there in the seventies: an encounter with that spirit of my generation, a generation that was intent on changing everything, that dreamed of inventing new ways of thinking, of living together and of loving. The university was occupied for several months by politically engaged students. I got involved with the friends of Radio Alice, the independent radio station that had become the voice of the student revolt.

In the houses we were sharing, we nourished the adolescent dream of starting from zero, of remaking the world from scratch, of reshaping it into something different and more just. A naive enough dream, no doubt, always destined to encounter the inertia of the quotidian; always likely to suffer great disappointment. But it was the same dream that Copernicus had encountered in Italy at the beginning of the Renaissance. The dream not only of Leonardo and of Einstein but also of Robespierre, Gandhi and Washington: absolute dreams that often catapult us against a wall, that are frequently misdirected – but without which we would have none of what is best in our world today.

“A black hole is perhaps no more than a star that collapses and rebounds in extreme slow motion”

What can the university offer us now? It can offer the same riches that Copernicus found: the accumulated knowledge of the past, together with the liberating idea that knowledge can be transformed and become transformative.

This, I believe, is the true significance of a university. It is the treasure-house in which human knowledge is devotedly protected, it provides the lifeblood on which everything that we know in the world depends, and everything that we want to do. But it is also the place where dreams are nurtured: where we have the youthful courage to question that very knowledge, in order to go forward, in order to change the world.

These excerpts are taken from the book There Are Places In The World Where Rules Are Less Important Than Kindness, published by Allen Lane on 5 November in the UK. A review follows overleaf

Now watch Carlo Rovelli speak about the nature of time in our science talks series on YouTube

https://www.youtube.com/watch?v=NrjFE_Rd2OQ