The Universe is rife with mysteries. From the micro-world of atoms and electrons to the enormous, unimaginable bounds of outer space with its billions of billions of stars, there are many things that still lie beyond our understanding. Among them are black holes – strange, unforgiving entities that even light cannot pass. What immense dangers are black holes brewing? Are they of any use to mankind? And probably the most important question – what is inside them? We take off on a quest to find possible answers to these questions with Nobel Prize winning physicist Professor Gerard ‘t Hooft on Sophie&Co today.

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Sophie Shevardnadze: Professor Gerard ‘t Hooft, Dutch professor, winner of the Nobel Prize in Physics, welcome to the program, it’s really great to have you with us. Wow. So, like, where do I start? I’ve been reading up on black holes, but let me ask you this – so, in galaxy, there are 100,000 wandering black holes – what are the chances that mankind comes across one? Or that a black hole comes across mankind?

Gerard ‘t Hooft: Practically, null, zero. Black Holes are like stars, they are actually even heavier than most stars, they are heavier than the Sun, so they are equipped with a gravitational field just like the Sun or even stronger. Their presence is a big thing in the Universe and they are much more scarce than stars, so there’s only a few of them around between the stars, most of them are even further away that the stars in Universe, which are very far away from us. So the odds that one will just come visit us is practically zero, just like star Sirius or other stars are not just coming much closer to us than where they are now.

SS: Somehow I feel reassured. But, just hypothetically speaking, what would happen to an astronaut, let’s say, if he gets sucked into a black hole?

GH: Imagine, a black hole would come somewhat closer to us - it will be better if it wouldn’t come closer than a few billion of kilometers, because then it would seriously disrupt entire Solar system. Planets would be torn out of their orbits or orbits would be greatly modified. The gravitational pull of black hole, the kind of things that have been detected in Universe, the gravitational pull of those is quite gigantic. But most things that are being attracted by a black hole would go into orbit, just like planets – they are attracted to the Sun, but they are not absorbed by the Sun, but they go in orbit around the Sun. Now, black holes also, when they pull at something, that something will go in orbit. Now, black hole differs from ordinary stars in being very-very small, yet even heavier than most stars – so the pull is gigantic, but things can get dragged on very closely and eventually be pulled into the black hole. You can imagine something going into the black hole, as you can imagine something going into the Sun, every now and then, you might see a the comet disappearing in the Sun, these things happen, and that could happen to an astronaut also. Well, he will be killed by the Sun, but he could also be killed by a black hole if he comes too close. Eventually, there’s an important difference between black holes and stars: it is that black holes have no well-defined surface, they have, what we call instead, a “horizon”, that is, you can go right through that horizon and then what happens, an astronaut will be nearly inconceivable. Actually, to be honest, a black hole wouldn’t even come close to the horizon, without being shredded apart and being hit by other material which happens to come close to a black hole. So, the astronaut would die long before he comes closer to that imaginable surface of a black hole.

SS: So, I’m thinking, if we were able to the center of black hole – what would we find there?

GH: It’s hard to say, because we are not able - for various reasons, we cannot come close to a black hole. One is, as I said, they are very far away, they are many hundreds of light years away from us, so that’s point number one, but point number two, they are very violent objects. They are far too violent even to come close to them. You see, most black holes are accompanied by matter trying to get in, it’s pulling on everything, and so, there’s usually a rocketing disc of material that is surrounding the black hole, and any astronaut trying to get close will first of all be killed by that. So, for practical reasons you would never be able even to come close enough to a black hole, to see all the peculiar features that we talk about. So, we’ll be evaporated. Now, you can ask, what would vapors do when they come too close to the black hole – and, yes, in principle, things – I usually think of elementary particles, or just molecules or atoms – they can go through the horizon of a black hole and survive only a small fraction of a second, and then it would hit what you call the central “singularity”. Now, in principle, things can even get through the singularity, because most black holes will be rotating, and when they rotate, then the singularity in the center is such that you can pass by it and go into even stranger world – in principle, mathematically. So, not physically, because astronaut would be dead long before that, but matter could get past that singularity, very strange region of space and time, a survive a small fraction of a second, but then, even for atoms, the environment would be too violent and they will be destroyed. So, what happens next is very much a topic of speculation – we don’t really know. We don’t even understand the quantum mechanics of black holes well enough to say exactly what happens to things that went way across the singularity.

SS: But do we know at least what’s on the edge of the black hole?

GH: Well, a black hole doesn’t have a sharp edge, it has what you call “horizon” – which means you can go right through it, but then there’s a point of no return. The horizon is literally a point of no return. If something gets through, which in principle is possible – don’t think of live astronauts, but think of elementary particles which might go through the horizon – then they have no way of coming back anymore. They are in the black hole forever, they can’t get out, but the particles themselves, they think they get through the horizon and they’ll survive for, maybe, milliseconds, maybe seconds, for a very large black holes it could be even minutes – but then, they’ll enter a region even stranger than the singularity of a black hole. Our ability to describe properly what happens there is not so sure...

SS: Black holes eventually do die, right? Like everything else in the universe?

GH: Yes, but that takes much longer than the age of the universe. I’m talking now about the large black holes that people detect in our Universe. All of them must be heavier than about five times the mass of the Sun. Those black holes, also, in principle, have finite lifetime, but their lifetime is gigantically long, even compared to the present age of the Universe. So, Universe now is nearly 14 billion years old, but for the black hole to decay, to die literally, would take much-much longer than that. Zillions of times or longer than the age of the Universe – that’s nearly an unconceivable period of time. And the reason of why black hole decays is that it emits radiation. We can calculate that and we’re practically sure that’s basically correct – that was Steven Hawking’s great discovery. A black hole emits radiation. But for live black holes, this radiation is so tremendously weak, than the black hole always accumulates more matter than it emits – and that means that black hole will grow rather than decrease in size. But if you would throw absolutely nothing to a black hole and just leave it radiating things, then it will lose mass, it will become smaller, lighter, the radiation becomes more intense, and eventually, after zillions of years, very-very long, compared even to the age of the Universe, the thing will explode in a gigantic explosion. That will be way, way beyond our imagination.

SS: But why is it even called a black hole? Is it a hole? I mean, such an enormous gravitational pull, I mean for something to have pull like that, it has to exist physically, no?

GH: Let’s say a “black hole” is just a name. You want to call it something. What it really is, it is a region of intense gravitational fields that rose when the matter was contracting, a heavy star may contract or two stars might collide, even black holes might collide and make a bigger black hole – the strange thing that happens in such circumstances, is that matter itself will become invisible, it will go through the horizon – but, its gravitational effect remains, so gravitational field of these particles remains there, but the particles become invisible, and it looks a little bit like a hole in your bathtub – the water disappears but the hole remains. So, the word “black hole” its not so badly chosen, it’s also “black” because, in principle, nothing comes out. It was later discovered that yes, things do come out, but with such low intensity that to the human eye, the black hole would still look very-very black, so the word is not so bad.

SS: An explosion of a black hole – is that what we call the Big Bang? Are the black holes the beginning of life?

GH: No. I wouldn’t… Some people think that the Big Bang, which is the beginning of the entire Universe looks a little bit like an exploding black hole, but mathematically, there are important differences, so I would not compare the initial stages of the Universe with a black hole. I think, it’s really something else mathematically. Although, in some idealized circumstances, you can say “Well, they use the same equations”, some regions of space and time look similar, but, there are important differences. So, I would not say that the beginning of the Universe is just like a black hole exploding or something like that. It’s too different from that.

SS: Now, tell me something – would galaxies and planets exist without black holes? I mean, it turns out that they’re present in a lot more locations that previously thought…

GH: Black holes are part of a galaxy, a small part, not so many stars turn to black holes, but some do, and – okay, hundreds of thousands of black holes sounds like a lot, but it dwindles compared to the number of stars in the galaxy – so, the galaxy is predominantly built out of stars, not black holes, so without black holes, the galaxy would look very similar to what it is now. At a center of our galaxy, there’s a very heavy black hole of about 5 million solar masses, but it looks invisible, it looks black, you don’t see a thing in most telescopic images. But – the galaxy would evolve just like it does today, even if were no black hole in the center. So, black holes don’t have a big influence on galaxy’s evolution.

SS: Just let me know, how did we detect the black holes? You’ve said previously, in first part, that some black holes can be detected by us – so how do you detect it when there’s just a black space?

GH: There are various ways. What you see mostly is its gravitational field, so in the center of our galaxy, we see stars moving in orbits around nothing, nothing at first sight, but they orbit around the same point in the sky. At that point, people calculated that it must be a very strong gravitational source, and when they calculated what mass would cause these kinds of orbits, the mass comes out to be nearly 4 million times as heavy as the Sun – but we see nothing there. So, the only thing that can possibly sit there is a black hole. In other places, we do see light, we see lots of x-rays – that means a black hole is there, but there’s accretion disk, which means that matter flows around it in a circular flat pattern and much of that matter is being accelerated so much that it starts to heat up and radiate. So, then we see a very intense sources of radiation, and by analyzing that radiation one can deduce that the only reasonable theory of what causes that radiation is that there must be a black hole sitting there. So, and then, combining theory with observations, we detected many-many black holes. Most of them that are being detected are at the center of the galaxy – that’s where we have the very heavy black holes.

SS: You keep bringing up the Sun as the entity of measurement in comparison with black holes. What happens if our Sun suddenly becomes a black hole? What will happen to our Solar system?

GH: First of all, it won’t happen to our Sun, because our Sun doesn’t have enough mass. If mass is less than about 4 times as heavy as Sun, it won’t make a black hole. We don’t know what astronomical phenomena could cause our Sun to turn into a black hole. Putting all of that aside, and imagining that Sun will be a black hole, then in principle, nothing would change – the planets would continue to move in their orbits as they do today, except there wouldn’t be much light and not much heat – so the planets would freeze, and the temperature of all planets would plummet to very low. We’ll see a source of heavy x-rays, presumably where Sun will be sitting, but the planets would continue in their orbits.

SS: Now, that’s where it gets very interesting for me. Steven Hawking said information – meaning energy – is erased in a black hole. What does that mean?

GH: That’s a very good question, but now you’re touching topics of quantum physics. Difficulty is to reconcile the physics of black holes with the physics of atoms and molecules, which is quantum mechanics. And there, situation gets weird, and since it is so difficult to observe anything there, there are many theories which are in competition with each other. Steven Hawking is one of the experts on the subject, but also, he doesn’t know everything, and one dispute that has been going on for a while is about information, whether the information will be preserved in some abstract manner or really be destroyed. Most particle physicists, like me, say it can’t be really destroyed; it will be very efficiently destroyed as if you burn a book, and you’re only left with the ashes – the information looks as destroyed, but not really, it is still in the ashes. So in that sense the information doesn’t get destroyed. But Hawking could be right, and then we have the problem with energy conservation – does a black hole conserve energy? It has to, because other laws of physics tell you that we would not have been able to phrase them if there was no energy conservation. How would gravity work? We wouldn’t know. So, we think energy is conserved in a center of a black hole – but exactly, how, becomes a mystery we need to search for in quantum mechanics, and this is a hot topic in theoretical physics today.

SS: So the interstellar travel with the aid of black holes, could they theoretically be used for FTL flight?

GH: Unfortunately, I am a sober scientists, I am looking at the facts, and what you’re saying to me now is very romantic, and science-fiction authors have a lot ideas in this respect – but I’m pretty sure nothing of that can ever materialize. We can’t use black holes for travel. If we try to get too close, you’ll evaporate, and if you try to get even closer still, you will not only evaporate, but you will be torn apart by gravitational forces which are far too strong, and even then if that all will be neglected or ignored, you wouldn’t be able to get out, no matter where. So, the black hole is not a good thing for interstellar travel, and that’s where science fiction authors… they need something to go faster-than-light for stories to go on, otherwise you can’t travel from one star to the next, like in their books. But, unfortunately, reality is more stubborn and we can’t travel faster than light, and certainly, not using black holes. I’m sorry to be not very romantic, but that’s the way it is.

SS: You’ve just killed every single romantic idea that I had about the black hole, but here’s something less romantic. Steven Hawking, again, he has warned against experimenting with the The Large Hadron Collider, saying that it could bring about the creation of a black hole. Are those worries warranted?

GH: Well, basically, not, because first of all, most people do not believe that black holes can be made by machines which are as small as LHC – LHC is the biggest machine we have for smashing particles, but it’s nothing compared to what you need to remake a real black hole. So, most scientists say that no, it’s not in today’s physics that a black hole will be formed. But, even if you imagine that a black hole would be formed, then the fact is that it will not be as drastic or as dangerous as some people think. The black hole would not start sucking up things, because it will be too hot, it will emit enormous amount of radiation and decay as quickly as it was formed, so maybe what would call a black hole looks like a particle that is created and then decays, and that’s it. So, not very romantic, not very dangerous either. It would just be a form of matter, just like other forms of matter – but very unstable and very short-lived like any other forms of matter that are generated in machines such as LHC, so there’s no reason to worry.

SS: So, tell me something, if there are these black holes that consume everything, are there holes and entities that spit everything out, that do the opposite?

GH: That is the idea of inverting time – but ordinary black holes already do that, because as Hawking correctly, we think, calculated, black holes do emit things, they do radiate particles, and they radiate that with certain probabilities. In principle, there’s a probability that something more complicated than a particle comes out – maybe an astronaut comes out, and you can speculate about that. But if you compute the odds, it’s tremendously small, so small that no sane physicist would believe that. An astronaut also doesn’t come out of your teapot when you boil a tea for yourself. So, in principle, it could happen, but in practice it doesn’t – and so nothing comes out of black hole in real world, in only comes out in your imagination. So, we can imagine time reversal of black hole, but then we can compute the odds that something comes out – and see that no, only things that come out are elementary particles, and that’s it. Again, not very romantic. You can’t have anything coming out of a black hole.

SS: Alright, so you’re saying that the chances of mankind coming across a black hole are equal to null, and that our Sun will never turn into a black hole. But, I read about these super-massive black holes and millions of Suns there are squashed together, and there’s one located right in the center of the Milky Way, our galaxy. Is it going to consume the galaxy eventually?

GH: It will consume a single star every now and then. What we see right now at our galaxy is a gas cloud, and probably part of that gas is going into the black hole in a few years’ time and they are all very eager to observe it. But, then, every now and then, a star will be absorbed by a black hole. Our galaxy contains hundreds of billions of stars. For all of those to be consumed by a black hole – that will take many hundreds of billions of years, so, the galaxy will evolve, stars will change, stars age and they age faster than what a black hole can do to consume stars. So, we’re quite safe here, our galaxy will live for many more billions of years.

SS: Well, thank you very much, professor, for clarifying for me and for my viewers that we’re not going to be sucked into a black hole anytime soon. It has been really great talking about black holes.

GH: I’m sorry we couldn’t be more romantic about black holes.

SS: Well, at least you reassured me that we’re going to live, we’re going to overcome the black holes. Thanks a lot for this amazing insight, we were talking to professor Gerard ‘t Hooft, Nobel Prize-winning scientist, professor of physics. We were talking about black holes and if they are actually a dangerous thing for our galaxy and what they are all about. That’s it for this edition of Sophie&Co, stay tuned for the next one.