by Carlo Rovelli
originally published in italian by Il sole 24 ore on August 24 2014
There is something paradoxical in what we know about black holes. On the one hand, they have become normal objects for astronomers. They observe them, they count them, they measure them. If there is still surprise, it is only as far as they behave exactly as the theory written by Einstein a century ago, when no one dreamed that such funny things could exist. On the other hand they are still mysterious; a window open to the mystery. On the one hand a beautiful theory, the general relativity of Einstein spectacularly confirmed by astronomical observations, and an amazing playground for astronomers and astrophysicists, where to observe and study these monsters that swallow stars, spin around, produce powerful rays, and similar devilries . The universe is surprising, variegated, full of things that we could not even imagine before, but understandable. On the other, a but. A little question of what children do when grown-ups get too excited: But where does the material that we see fall into black holes end up?
Here the trouble starts. Einstein's theory gives a precise and elegant mathematical description even from the inside of black holes, so we can use it to know the path of the matter that falls into the hole. The equations tell us that matter always falls faster to the central point. And then ... then Einstein's equations lose all meaning. They no longer tell us anything. They melt like snow in the sun. All the variables become infinite and nothing makes more sense. Ohi. What happens to the matter that falls into the center of the hole? Einstein's theory cannot answer. Simply: we don't know. We see it falling with telescopes, then we follow its path with thought until almost to the center, and then we don't know it anymore. We must recognize our ignorance. We know how black holes are made outside and inside, but one detail is missing: the center. Only it's not a small detail, because everything that falls (and in the black holes that we see in the sky keep falling things) everything ends up in the center. The sky is full of black holes where we see things falling ... that we don't know where they end up. The roads explored to answer this question are the most risky: perhaps the matter emerges in another universe? Perhaps our universe too was born of a black hole that opened in a previous universe? Perhaps in the center of the black hole everything merges into a cloud of probability where space, time and matter no longer mean anything? Or do black holes radiate heat and mysteriously the matter that enters turns into heat in the zillions of years to come? If there seem bizarre hypotheses, well, they seem so to me and to several of my colleagues; but there are very serious scientists in prestigious research centers who try the same to study them.
In Marseille, in the research group where I work, together with colleagues in Nijmegen in the Netherlands and in Grenoble, we are exploring a different path, which seems simpler and more plausible to us. Our idea is that matter slows down and stops before it reaches the center. When it is very concentrated, we think, a very strong pressure develops that prevents it from collapsing further. This pressure is similar to the pressure that prevents the electrons from falling on the atoms: it is a quantum phenomenon. The matter stops falling and forms a kind of very small and very dense star. A Planck star. And then? Then he does what the matter always does in these cases: it bounces.
The matter that falls into the hole then bounces, like a ball on the floor. And get out. As the bouncing ball follows the path of fall but in reverse in time, so the black hole is transformed (in jargon it is said for tunnel effect) in its inverse: a white hole. What is a white hole? It is another solution to Einstein's equations (like black holes) of which my university book said that there is nothing like it in our real world.... A region of space where nothing enters and gives where things come out. A black hole in reverse. An exploding hole. But then why do we see the matter fall into black holes and don't we see it immediately bouncing off? The answer, and here is the busillis of the matter, is in the relativity of time. And it looks beautiful to me.
We know that time does not pass everywhere at the same speed. Clocks, like all physical phenomena, go slower on the plains than in the mountains. If I'm in the mountains and look at a clock down in the plains, I see it go slower than the clock I have with me. Time slows down if they are lower, where gravity is more intense. Within a black hole the gravity is very strong, and this slowing of time is fierce. The rebound of the falling matter happens quickly, seen by someone nearby (assuming someone has the guts to go and see a black hole from the inside). But seen from the outside everything is slowed down. Huge slowed down. We see things disappear, and vanish from our sight for an extremely long time. Seen from the outside, everything seems frozen for millions of years: just like the black holes we see in the sky. But a very long time is not an infinite time, and after the necessary wait we will see the matter that comes out. A black hole then in the end is nothing but a star that collapses and then bounces, seen from the outside, in slow motion. This is not possible in Einstein's theory, but Einstein's theory overlooks quantum effects. Quantum mechanics allows matter to escape from the black trap. After how long? After a very short time for the matter that fell into the black hole, very long for us that we observe from outside.
Here is the whole story: When a star like the Sun, or a little bigger, stops burning because it has consumed all the hydrogen, it starts to cool down and the heat does not generate enough pressure to counterbalance the weight. The star sinks on itself and if it is heavy enough it produces a black hole and falls into it. A star the size of the Sun, that is thousands of times the Earth, would generate a black hole with a diameter of one and a half kilometers. Think: the whole Sun compressed into the volume of a hill. These are the black holes we observe in the sky. The matter of the star continues its run inside, sinking more and more until it reaches the monstrous compression that triggers the rebound. At this point, the entire mass of the star is concentrated in the space of a molecule. Here the repulsive quantum force turns on and immediately the star bounces and begins to explode. Only a few hundredths of a second have passed through the star. But the expansion of time generated by the enormous gravitational field is very strong: when matter begins to emerge outwards, tens of billions of years have passed in the rest of the universe. This is the hypothesis we are working on.
It's true? That's it? I do not know. I think yes. More than anything else because the alternatives seem to me even less plausible. But I could be wrong, I know.
Trying to understand, however, is beautiful.