1-How would you explain to a neophyte in physics what black holes consist of?

-Black holes are places where you can enter, but from which it is impossible to get out, since the force of its gravity does not let even light escape, which is what travels faster in the universe. Not only that: inside a black hole the universe ends, in a kind of reverse Big Bang in which time, space and matter, instead of starting, come to an end. The most important thing we know about black holes is that, incredible as they may seem, they actually exist, and they are present throughout the universe: most galaxies, including our own, are home to hundreds of millions of moderate size.

2-What would we see if we had one in front of us?

-If we had one in front of us, in front of a starry background, we would not only see its “shadow”, a dark disk, but it would also act as a lens that distorts the image of the stellar background around the shadow. This is because gravity deflects (like a magnifying glass) the rays of light that pass near the black hole but do not fall into it.

3-Why are they produced?

-In order for a black hole to form, a lot of mass must be concentrated in a very small space, for example, as much mass as that of the Sun, compressed within a sphere of only six kilometers. Such concentrations occur when a giant star collapses as its nuclear fuel runs out. We also believe that they were formed in the early universe, which was vastly denser and more violent than it is today.

4-There is talk of a distortion of space-time inside. If something or someone penetrated one of them, what would happen?

Whether an astronaut or a probe falls into a black hole, the force of its gravity will stretch it and compress it so violently that it will disintegrate into pieces. When this happens depends on the size of the black hole: the bigger it is, the longer we can survive. If the size of the black hole is moderate – a few kilometers – we will tear ourselves apart even before entering its interior. But if it were a supermassive black hole, we could survive for several minutes, even hours, inside it.

5-What would we see inside in the case of surviving?

It depends on many details, but we could probably observe objects that fell long before us and for which time has barely elapsed. In any case, the final disintegration would happen so quickly, in milliseconds or less, that we would not have time to feel pain. It is often speculated on the possibility of passing through a black hole to appear elsewhere in the universe. The idea is fascinating, both for science and fiction, but in light of what we know, this is not possible. Personally, if someone invited me to jump into a black hole, I would politely reply, “Come in first.”

6-Would it be possible to develop in the future a technology that allows them to be explored, for example, in a manned mission?

It seems extremely difficult, since the closest known black holes are thousands light years away, which implies that reaching them would require missions of at least thousands of years in duration. But if we ever had the technology to make these trips, then most likely we would also be able to explore the environment near the black hole. Given what we know of the human being, there would probably be no shortage of volunteers to jump inside.

7-What is your relationship with wormholes? What is the difference between them?

Wormholes are a kind of shortcut in the universe: let’s imagine that we are little worms on the surface of an apple, and we want to go to the other end of that apple. We can slowly crawl across the surface, or make a tunnel that takes us to the other side through a shorter path. If they existed in our universe, these tunnels would give us the way to overcome the enormous interstellar distances. Unlike black holes, where there is entry but no exit, a wormhole would allow us to make round trips. However, while today we know that black holes exist in our universe, wormholes still belong to speculation, something that we have only explored with pencil and paper using the equations of Einstein’s theory. Unfortunately, it would not be safe to travel through wormholes: they are so delicate that trying to go through them would quickly close to become black holes where, as we have seen, we would die in pieces. To this day, wormholes are just highly speculative constructions, but in any case fascinating to explore what physics allows us when we push it to its limits.

8-Gravitational waves, can black holes provide us with information?

The discovery, just over two years ago, of the gravitational waves that occurred in the collision and merger of two black holes has provided the most overwhelming evidence of the existence of these objects. Until now we only had indirect evidence of them, traces of their presence as very massive and compact but invisible objects, something like detecting the invisible man by his footprints in the snow. Gravitational waves have revealed black holes to us much more directly. One of the main questions that we now ask ourselves to solve with them is: are these really the black holes that Einstein’s theory predicts, or are they really good “imposters” instead?

9-Was Einstein aware that his theory predicted the existence of black holes?

Einstein never understood that his theory predicted black holes, and he even wrote an article in which he denied their possibility. Despite Einstein’s wonderfully creative imagination, his equations turned out to be smarter than he was, and they went far beyond what he thought possible.