‘If we find evidence that black holes transform into white holes…it would mean that spacetime can also make quantum jumps’: Carlo Rovelli

In this interview with The Indian Express, Rovelli talks about the book, the possibilities of white holes getting detected, and how it can alter our understanding of the universe.

For most people, black holes themselves are pretty difficult to comprehend. Through this book, you are attempting to introduce the concept of white holes. Would it be accessible to common people?

I think it is, if one puts a little bit of time and is not afraid. Black holes are really holes in spacetime. Imagine you are a little ant, walking in the main square of the town. It is all flat. But then you see a small hole, may be a few millimetre in dimensions, and you decide to peep in. What you see can be surprising, because there might be an enormous space below. Maybe you are staring at a parking lot created below the ground. 

Over the years, we have realised that the universe also is full of similar holes. It is not evident at first, but there is huge amount of space inside. The difficulty in understanding that you mention arises from our education, experience and assumptions. We go to school and are told that space is this three-dimensional thing — x, y and z coordinates, Euclidean geometry — and so on. But that is not the case. You should never trust too much what you are told in school. Space is much more complicated.

Black holes have now been discovered, and sort of ‘photographed’. Are white holes also physical structures, or are they just the conceptual opposites of black holes?

These black holes that have been discovered in the sky, their existence was known much before we could actually detect them or see their effects. They are predicted by Albert Einstein’s General Theory of Relativity. If you follow the equations of General Relativity, you would realise it is about space being distorted. You would understand that if there is a star larger than the Sun and it stops burning after a point, then it collapses on itself. The gravity is so huge that they create deep holes in spacetime.

The mathematics was there but initially, a lot of people, including very good scientists, did not believe such a thing could exist. They were wrong.  

We are more or less in a similar situation with regard to white holes now. These too come out of one of the solutions to the equations of General Relativity. So, they could exist. But unlike black holes, we have not seen them yet, or felt their effects. So we are trying to see if there is a way to detect them, to understand how they can be formed, to calculate how big they can be.

The book that I have written does not say that white holes exist. Rather, it is about the research happening in this direction. It tries to tell you how to think about things that we do not currently know of. That’s what Einstein, and many others, also did — bringing together small pieces and trying to put them together to discover the big picture. Science, most often, works like this. Would it finally lead us to white holes? I don’t know.

Even the name white holes suggests that it is the opposite of black holes. Is white hole anti-black hole in that sense?

Yes, very much so. In fact, the basic idea is not very difficult to understand. Black holes are produced by stars falling in on themselves. And then there are all kinds of matter falling in, which never come out. There is an obvious question here which would be apparent even to a kid. Where does all the matter go? Does the black hole keep eating up matter forever? Does this matter end up in some other universe? What happens? The story we are trying to develop, something that is plausible but not yet established, is that nothing so dramatic happens inside the black hole. What falls in, in this long tube, bounces back at some point of time, and comes out. If you take a ball and let it fall, it bounces back. Conceptually, it is similar. White holes are bouncing back of black holes. White holes are the same thing, the same black holes, which, after some time, start ejecting things.

So, these objects come back and go where? What we have been told about black holes is something very different.

What we have been told is that once something falls inside the black hole, it is gone forever. But nothing is forever. If our view is correct, then it is not true that once something falls inside the black hole, it is gone forever. The idea here is that you bounce back from the same surface, not from any other universe.

The passage from the black to the white hole — the point where the transformation happens — is not described by Einstein’s theory. It is described by quantum mechanics. And Einstein was very well aware of this. In fact, shortly after finishing his General Relativity Theory, he wrote that ‘Here is my theory. I hope it is right, but for sure it is incomplete because there are quantum effects that are not fully accounted for’. So, that theory has to be corrected by incorporating quantum mechanics. My work, and that of many of my colleagues, through most of my life has been exactly trying to do that – trying to correct Einstein’s theory to include quantum mechanics. The quantum property here is the bounce.

Previous descriptions of the black holes suggest that once inside, objects go through the process of spaghettification – they are pulled apart intensely by the huge pressure differential. You present a very different picture of what might be happening inside.

Well, the difference is only in perspective. The story of white hole that my colleagues and I are working on is more or less the same story that you seem to be familiar with. You do get spaghettified, you do feel this enormous pressure difference that will instantly kill you. You get squeezed in some direction, and pulled in another, over and over and over again.

But, and here is the novelty, this does not go on forever. At some point, the object breaks down into such small pieces that they begin to exhibit quantum behaviour. If you use Einstein’s equations, this squeezing and squishing keeps happening till you are basically reduced to just one single line. But not till infinity, as is normally understood. Just before infinity, something else kicks in which makes everything bounce back. And the opposite process of white hole begins.

Do you see a white hole getting detected, or being ‘photographed’ in the same way as a black hole?

This is essential because unless we detect things, we are just speculating. Speculation is a good thing in science but until we also have some measurements that confirms or supports the speculation, we are not going very far.

I see various possibilities of ‘observing’ white holes, both in astronomy and through detectors on Earth. For reasons that are complicated, I would expect that a majority of the white holes out there are very small. We cannot make a picture of them in the same way that a black hole was recently photographed because they interact only with gravity, not with light. But things that can interact with gravity can be used to measure. If we have a sufficiently delicate detector to measure gravitational pulls, it can be detected. A few of my colleagues are already working on a paper explaining how such a detector can be devised and the technology that would be required for it. I think it is not an impossible task. Thirty years ago, when some of my scientist colleagues had proposed that they wanted to build detectors for gravitational waves, not everyone was convinced. But they went ahead and did it and won the Nobel Prize for it. Then there were other colleagues who said let’s create a telescope that can take picture of the black hole. Again, there were people who said it can’t be done. But that too was done. So technology is fantastic. It grows. And if this continues, I think we have a good chance that at some point we would be able to detect a white hole.

The second possibility lies in astronomy. One school of thought is that white holes might be the solution to one of its big mysteries. The biggest mystery in astronomy right now is dark matter. Astronomers are able to sense large amounts of matter but are not able to see them because they seem to interact only gravitationally. And white holes are those little things that interact gravitationally. So, it is possible that there is a relation between white holes and dark matter.

How do you think a confirmation of white holes would alter our view of the universe, specifically, our understanding of time?

If we find evidence that black holes do indeed transform into white holes, then it reaffirms our view of the universe. Remember, this story is largely based on Einstein’s General Relativity, but not entirely. Because the bounce, the transformation from black hole to white hole, is a quantum phenomenon. The bounce is a quantum jump. If this turns out to be true, it would mean that spacetime can also make quantum jumps. It would mean space, as well as time, is discontinuous and discrete. It would be a new way of thinking about space and time, and that is what fascinates me. But I don’t believe we have a final answer by far.

There is a great deal of excitement about artificial intelligence these days. Some people believe machines would be able to outdo humans in all intellectual aspects. Do you think machines would ever be able to imagine a theory like General Relativity?

Well, the power of human mind… I think we should not underestimate it. Because we can visualise things that we have never seen, or which are extraordinarily far away from us. And we have done this repeatedly. 

Now on artificial intelligence,a priori I believe that if our brain can do some things, there is no reason why a machine can also not do them. But, for the moment, I think that most speculations about artificial intelligence are exaggerated. I have interacted with artificial intelligence machines, and I am deeply unimpressed. What they are very good at is to say stupid things just like we humans say most of the times.