Carlo Rovelli interview | ‘Universe may have resulted out of a “Big Bounce”’

From what I understand of your book, a white hole is like a black hole in reverse — or as you would describe it — akin to a movie running backward. Could you explain why they are significant?

We’ve seen black holes, taken pictures of them colliding. We see matter around them spiralling and falling inside them. But then what? If we fell into them we would be squeezed and stretched but say we resisted it, what happens next? Are whatever falls into it eaten by monsters, do they go into a different universe? One can speculate in all kinds of ways. What I describe in my book is that something very simple happens. Things just fall and fall and then they bounce back. Like a ball bouncing back up from the ground, it passes on the same path while going back up but with a reduced velocity. What interested me and my colleagues is that the bouncing out is also predicted by Einstein’s theories. This possibility of things coming out is what we call a white hole. Einstein’s theories however don’t accommodate the possibility of a bounce, but quantum mechanics [The science and laws governing the subatomic world] does. So we use the equation of ‘loop quantum gravity’ – our theory of quantum mechanics and gravity together — and this suggests that a black hole itself bounces and becomes a white hole. If this is true, this implies that space and time undergo ‘quantum jumps’ and suggests that we have to change our understanding of what space-time is. Electrons undergo quantum jumps and that is what makes all our electronic devices possible. These are quantum jumps of matter and don’t do anything to space or time. But here, the shape of space itself is a quantum jump. It implies that time is no longer this uniform, local thing and performs tiny ‘jumps.’ — akin to a stretching rubber band.

Is it possible that our universe is emerging out of a white hole, and before this was one that was swallowed by a black hole?

Well, it is the same but there are some differences. The ball that is going up after bouncing is slower and has less velocity and energy. Where’s the energy gone? It dissipates as heat on the ground. The black-hole-to-white-hole transition is also similar and that’s why studying the transition is complicated. The dissipated energy of the white hole was studied by Stephen Hawking; it is his great contribution to physics. Black holes emit heat or ‘Hawking radiation’ and this information isn’t lost but can come out from a ‘white hole.’ There is a very interesting theory that a universe that will fall into a black hole will bounce and generate something very similar to the Big Bang. We don’t know if it is so but it is possible. The equations that predict a black to white and a bounce are quite similar. So you and I, in this universe, might be coming out of a ‘big bounce’ through what looks to us like a Big Bang 14 billion years ago. Stephen Hawking towards the end of his life used to say that if you ever feel that you are in black hole and you will be stuck in it forever, do not worry; even black holes aren’t forever.

How must someone in 2023 understand time? Is it an abstract but fixed entity – like the marker between two events. Or is it something physical, made up of particles, that can stretch and squeeze, and influenced by forces such as gravity?

We’ve discovered something very factual; that time doesn’t pass the same for everybody. It seems unusual only because we aren’t used to it or don’t experience it often enough. If we could travel near light speeds, it would be usual for us to go on a long trip (at those speeds) and come back to see that a younger sibling is now older than you. This is as much a fact as that the earth is round. Albert Einstein was the first to suggest that time wasn’t something fixed like the ticking of a clock, but something more flexible like a rubber band. What we need to know is what makes time stretch, unevenly, in this way.? Is it something like an electric field? There are electric and magnetic fields around us all the time but we don’t feel it. Similarly, there is also the ‘time field’ that Einstein called the gravitational field. Einstein imagined us as being trapped in a large jellyfish, which is ‘time’ itself. In Einstein’s theory, there are no particles of time but rather an entity called ‘space time. Mass distorts space time and the bigger ones more. Near a black hole, time passes super-fast. Someone at the edge of one, looking at earth, can see a year go by in a minute. But viewed from earth, everything is going very, very slowly and that is time-distortion. It’s easier to think of space and time as a single-entity. This was all Einstein, 110 years ago. But we know that is limited and we also have to account for quantum effects; even though they may be tiny. They become important inside a black hole, near the Big Bang. If we account for quantum effects, space is granular. Not continuous and smooth and this granular aspect of space is time. This however is speculation and results from what we think should happen when combining quantum mechanics and gravity. That’s what I have been working on, all these years. (We think) there are ‘time-steps.’ There may be atoms in space but they aren’t moving in space. They themselves are space.