Black holes and white holes

If black holes exist, could they have white counterparts?
18 April 2023

Interview with 

Carlo Rovelli, Centre de Physique Theorique de Luminy


An artist's impression of a supermassive black hole


White holes are, for now, the theory that if a body can exist from which nothing can escape - that’s black holes - then there could be something into which nothing can enter. And if you subscribe to the theory that black holes do not destroy information, there might be a way of it coming back out. Perhaps our best chance of understanding this might lie with theoretical physicist and author of the upcoming book White Holes, Carlo Rovelli.

Carlo - What is a white hole? It's a black hole as if it would look like if you could film it and show the film backwards. So it's sort of a black hole coming back. So the idea here is that a white hole could be like a ball falling to the ground and bouncing up. So everything that falls in bounces and comes out through the white hole.

Will - How would a white hole form then? Is it just a black hole that's almost collapsed in on itself and then for lack of a better word, exploded back outwards again?

Carlo - Yes, exactly. So one way of visualizing a black hole, what's going on inside is that there is a horizon, a surface. The thing of the black hole we see on the outside, but inside there's a long tube that becomes longer and longer and longer and narrower until the point in which it's so narrow that quantum effects make it bounce back. And so this long tube starts becoming thicker again and shorter comes back. So the formation of this tube bounces into a coming back of the tube itself, and that's a white hole from the outside. First we see the horizon where everything comes falling in, and then we see the white hole, which is the same horizon, the same position in the sky, the same star if you want. But now instead of an horizon where everything can only go in, it's an horizon for which everything can only go out. So whatever is inside now can bounce out, and that's a white hole.

Will - And what would cause the black hole to sort of flip in a quantum sense to start throwing stuff back out?

Carlo - One of the things that quantum mechanics tell us about the world is that many things that we think are continuous are actually granular. So light is made by photons. It's not a continuous wave, it's just a few grains, which are the photons. And if applied quantum mechanics to space itself, it tells us that space is granular. So a black hole cannot squeeze things down to infinity. There's no infinity small. At some point you get to the minimum size, and once you get to the minimum size, the fall is blocked, so to say. And you cannot fall it more and it has to stop. And when things stop falling, typically they bounce. If you want black holes are a solution of Einstein's equations, which we understand very well except how they end. And white holes are a solution of Einstein equations that we understand very well except how it starts. So it's extremely plausible to imagine that when a black hole ends, that's exactly the quantum position to a white hole being born.

Will - What kind of matter would it spit out? Would it be recognizable? Would it just be pure energy? What kind of things would we be looking at?

Carlo - We would expect that what spits out is just ordinary matter radiation, more likely like electromagnetic radiation. Very much unrecognizable with respect to what fell in because things are gonna be squashed horrendously by falling into the black hole. Calculations suggest that what could come out is very low energy radiation. So like light of very, very low energy, very low frequency, which is one of the signals. There are many of these things. It could be another of the signals that one might search to identify the existence of this object.

Will - We have to stress at this point that we haven't actually observed a white hole yet. This is all theoretical for now. But is that simply because we don't really know what to look for yet?

Carlo - No, it's more than that. We don't know if this scenario is correct or not. We don't know if white holes exist in the universe or not. We have to remember that for almost a century, black holes were predicted by general relativity and many people didn't believe they existed. It was an open question. When I was a student, I studied black holes and my teacher in my textbook said, well, it's implausible that these things exist in the universe. So we are in the same situation now with white holes. They're predicted by generativity, but we don't know whether they exist in the universe or not. To me, it seems very plausible that they do, and it seems very plausible that the end of a black hole is the birth of the white hole. But until we are actually seeing them or seeing an effect of them or a consequence of them, we have to consider white hole hypothetical as an idea.

Will - There's lots of theories going on about white holes and black holes and their relationship. And one of them is that white holes don't exist in this universe. They exist in another universe, and there's some form of door that exists between black holes and white holes. Do you subscribe to that theory or do you think there's something else going on?

Carlo - I think there's something else going on. I think that when the black holes die, three things can happen. One is that just everything disappears magically, but that seems implausible to me. The second possibility is that it ends up in another universe. Somehow. It creates another universe. This is a beautiful, enchanting idea, but it doesn't seem plausible to me. I mean, creating another universe might not be so simple as collapsing a star. And the third possibility we just talked about namely, that whatever falls in can come out because a black hole becomes a white hole. So the entire process of formation of a black hole evaporation and then formation of white hole and coming out is just like a ball that falls to the ground and then bounces up. One of the fascinating aspects of this story for me is that in general activity, time goes at a very different speed. As we know, time goes faster in the mountains than near the sea on earth, but that's a tiny difference. Now, if you work out the speed of time inside the black hole and the white hole is enormously lower than outside. So the process seen from the outside of a star collapsing and then becoming white hole and coming out may last billions of years. But if you fall inside, it takes just a few seconds or minutes for you to get to the center and come out as a white hole. So black hole to white hole might be just like a shortcut to the very distant future. If you survived, you wouldn't be washed by the enormous forces inside the black hole. You could just jump to the black hole and come out from a white hole. But very, very far in the future, millions or even billions of years in the future.


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