I would love to fall inside a large black hole

Date of publication
News categories

Professor Veronika E. Hubeny, a leading expert on string theory and quantum gravity, says that rushing is not good for science, and might make us miss tremendous opportunities.  We interviewed the theoretical physicist from the University of California on the occasion of GRAVITY@PRAGUE 2022.

Image of the black hole Sagittarius A* (Sgr A*) (Event Horizon Telescope collaboration et al.)
Image of the black hole Sagittarius A* (Sgr A*) (Event Horizon Telescope collaboration et al.) | photo: Event Horizon Telescope collaboration et al.

“When it comes to playing ball with black holes, you need Gary,” said Jan Zaanen from Leiden University about your academic advisor Gary Horowitz. How did you pick your advisor?

It's a very nice sentence, I haven't heard it before. I chose to do PhD in Santa Barbara, specifically to work with Gary Horowitz. Back in those days, most people in my field were either doing string theory or general relativity; but there were very few people like Gary who really were experts in both, and that was something I wanted. Only after meeting him have I discovered how intrinsically wonderful a person he was. He is incredibly nice; he would always make time to answer my questions. It felt like one could ask about whatever puzzle or confusion one had, and I never felt it was too silly to ask. Also, he would never impose what I should work on; he always gave me a choice, of whether a given problem is something I would like to pursue. And that model I have adopted for my own students: I try to let them be maximally independent, and sort of figure out what they want to do rather than me saying "this is a calculation I want you to do," which I wouldn't have liked myself, as a student.

As a professor there is so much to do and so little time. Do professors in physics have more time for their own research?

Sometimes, but it helps not to have too many students. If you have an especially active one, you don't take on any more till that one becomes independent enough. Some advisors try to pair their students with other students or postdocs. I don't like doing that, I like to give them time individually. There isn't a one size fits all method.  Another question is, what if the mindset of the student is very different from yours. Say, you might think about physics pictorially and the student very algebraically. How do you best explain physics to someone with a different mindset, considerations like that come up all the time.

Gender was completely irrelevant to learning or discussing physics. In some sense we are all in the same boat trying to understand something we don't yet understand, so it is silly to be petty about these irrelevant distinctions.

Veronika Hubeny

Gender gap in physics among highest in science. A study, carried out by researchers at the University of Melbourne, says that at current rates it will be more than two centuries until there are equal numbers of senior male and female researchers in physics.

This is indeed a concern, but I actually never felt in any way discriminated. In fact, as an undergraduate I was essentially always the only girl in the class, and it never felt like that was a hindrance. I was still asking the greatest number of questions, and gender was completely irrelevant to learning or discussing physics. In some sense we are all in the same boat trying to understand something we don't yet understand, so it is silly to be petty about these irrelevant distinctions.

You have participated at the World Science Festival 2017 in a panel discussion moderated by New Yorker contributor Jim Holt. Six man and one woman. Marilee Talkington upset about not giving you fair share of speaking time shouted, “Let her speak, please!” She subsequently posted about it in Facebook and the post went viral. Then came your statement…

Well, I felt that the original post might have been very discouraging for young girls who wanted to go into physics, that they might think it is full of jerks who mansplain all the time, and that maybe it is not worth it. I wanted to make this point before people get discouraged unnecessarily on something they shouldn’t be discouraged about: if you really want to do physics, that is more important than all the rest. We all strive for a better working environment and hope to eventually eliminate the bad sort of behaviour, but just because we're not there yet is not a reason for not doing physics now.

During this panel session I genuinely did not feel affronted or discriminated by the moderator’s behaviour. So, it didn't occur to me that people could worry that I might be uncomfortable. Before we all came to the stage, I was explaining to the moderator what I'm working on, and later on stage he was doing a pretty reasonable job repeating that, for someone who is not physicist.  Nevertheless, I feared his exposition was too incomprehensible for the audience, so most of my attention was taken by trying to figure out how to best interject in a way which would be most useful to the audience, which was a bit of a moving target, since it took some time before he let me speak. So, it was little bit tricky, and focusing entirely on how to best reexplain the physics, I didn't get to be offended by his behaviour.

That sentiment in the audience which the journalist picked up on, though, I could totally understand, because that's happening all over the place.  People are primed to notice it, especially since they anticipate that it happens again and again.  But being immersed in it, it didn't feel like that at all.  It felt more of a nuisance in the background, like if you much prefer light colours but your office is painted black.  Would you let that dissuade you?

Einstein knew that his theory was elegant, so beautiful that such a theory couldn’t be false mentioned Guy Consolmagno in his lectur about Astronomy, God, and the Search for Elegance. How elegant is the description of black holes?

Quite amazingly so. Black holes are extreme in many regards (for instance they are maximally compact, have largest entropy, are fastest in equilibrating and scrambling information; and so on -- these properties are tricky to describe succinctly in lay terms). Black holes are like the hydrogen atom of all physics; indeed, Chandrasekhar called them the simplest macroscopic objects in the universe, and the fact that they happen to be described very elegantly mathematically is a manifestation of that.

The final state of a black hole is independent of the details of what formed it; the description only depends on the total mass, angular momentum, and charge. (In astrophysics we don't even have charged black holes because they would discharge very quickly, so we really need only two numbers for the description.) In contrast, your cup of coffee can’t be described by just two numbers. You have to say more about the coffee, say how was it made, what is the shape of the foam -- so it is a complicated system.

But I'm fascinated by black holes primarily for their multifaceted nature, for having all these connections to different things. The simplicity is a result of what they are, but I like them not because of the simplicity but because of this multifaceted nature.

I try to understand how things work, and if they are simple, then great, because I have a better chance to understand it; but even the simplest thing, if you delve deeply enough into it, you find incredible richness and connection to other things that isn't obvious at first sight. Most things are indeed fascinating, but I think black holes are among the most fascinating ones.


Disk horkého plynu kolem černé díry: proud plynu táhnoucí se vpravo je pozůstatkem hvězdy, kterou černá díra roztrhla. Oblak horké plazmy (atomy plynu s odloučenými elektrony) nad černou dírou se nazývá koróna.
Disk horkého plynu kolem černé díry: proud plynu táhnoucí se vpravo je pozůstatkem hvězdy, kterou černá díra roztrhla. Oblak horké plazmy (atomy plynu s odloučenými elektrony) nad černou dírou se nazývá koróna. | photo: NASA/JPL-Caltech

Astronomers have unveiled the first image of the supermassive black hole at the centre of Milky Way galaxy. Do simulations and pictures of black holes help us visualize them correctly?

How you can visualise black holes changes depending on what aspect you focus on. If you want to see how a black hole looks from far away, you can think of it as a lens which distorts its surroundings, a black disc that deflects light traveling through spacetime around it. The "black hole" is not just that you have flat spacetime, and then something cut out of it.  It is part of the spacetime; the whole of spacetime is curved, and at some point, you reach the "event horizon" from which you can’t escape -- that defines the black hole.  Sometimes such a description might be better conceptualized through diagrams (specifically Penrose diagrams) which indicate the causal structure, namely what happens to light.  The artist's visual conception, that's less useful from the point of view of understanding.

For describing black hole mergers, a nice way of representing what you saw from a distance is the frequency of the gravitational wave that comes from this merger. And then you can even translate the frequency to sound and hear the merger "chirp". It's very cute.

But often you might study a black hole to understand what’s really happening very near its horizon and then the most convenient representation is slightly different;  you want to describe the shape of the horizon. Or imagine you're inside the black hole. Then the interest is more in what happens to the objects there are falling in.

Would you like to have look inside?

Oh, I would love to fall inside, at least into a large black hole (the larger the black hole, the longer it takes to reach the curvature singularity). I know it's once in a lifetime situation.

Jacob Bekenstein showed that black holes set a theoretical maximum on information storage, which applies to any quantum computer.

The amount of information you can store is related to the entropy, or maximal amount of disorder.  Curiously, unlike familiar systems in thermodynamics, for black holes this quantity does not scale with the volume inside the black hole but only with the event horizon area. (This comes from thought experiments regarding the 2nd Law of thermodynamics in presence of black holes.) Nevertheless, any other physical system which could fit inside the same region would have smaller entropy, so in that sense you could store less information with it.

But it's not that information storage in a black hole would be particularly useful. Black holes are also the fastest scramblers -- they set the upper bound on how fast that information could be scrambled and therefore effectively rendered useless. One pictorial way of visualizing it, due to Susskind and Uglum, is to think of a fundamental string falling into a black hole. Viewed from outside it appears as though the string spreads over the entire horizon on a very short timescale (comparable to the light crossing time). So, one cannot recover the information in any local region. It's like putting a drop of milk in your coffee: after you stir it, it is no longer manifest where that drop first fell. From another viewpoint, if something falls into a black hole, the horizon locally deforms, but it settles back down likewise on this very fast time scale. 

Publish or perish that is today’s rule in science. But the research needs some time to flourish, should scientist succumb to the pressure and publish their theories in quick succession?

I would say no if you can afford it. Rushing is not good for science and might make us miss tremendous opportunities. Indeed, I recently finished a paper with Matt Headrick for which the original idea dates back some eight years, when we started discussing it at a workshop. We kept getting distracted by other projects and trying to finish papers with junior collaborators and suchlike, but whenever we were at the same conference, we sort of revived the idea. But during all this intervening time, our ideas were germinating, and I think our paper, when it finally appeared, was much nicer than if we had written it earlier, because we had a more well-rounded perspective on it.

It often takes a while to understand a theory well enough, and especially for people to realise that there is some connection to other descriptions.  For instance, we now believe there is a deep connection between quantum information and gravity, and it is very exciting. In fact, there is no time throughout the history of physics I would rather be than today, not even at the beginning of quantum mechanics.  With all these new realizations, there is chance that many long-standing puzzles might be finally explained.  And building on the new understanding might take us in a very surprising directions. It feels magical, realising the connection with something else that was previously thought to be completely different, that way of mentally repackaging of what is going on in a way that suddenly you understand, and things make sense. You try to resolve some paradox, or you try to generalise some theory, and in the process you understand something you're never even asked about, or maybe never even realised that there was a puzzle in the first place, but suddenly things fall into place. I expect that sort of scientific revolution to happen.

I actually don't even like the delimitation of self-identifying as a string theorist or relativist or a condensed matter physicist, because all these things are interlinked. Nowadays we might be learning about condensed matter by working on black holes.

But when we try to classify what we are working on, or fit ourselves into a particular subfield, we have a sort of tunnel vision, and we're in danger of understanding the subject in only a limited sense. To me perhaps the most fascinating thing about physics is its interconnectivity.  Usually this is not manifest, and only gets explained when we understand the underlying structures. In the case of dualities, the same physics can have multiple completely different descriptions, and things which are difficult in one description may be easy in the other. But finding the underlying structures often requires broad perspective, and time.

In the grant proposals they ask, what is the perfect application? 

The generic observation is that progress of science is typically accompanied by practical applications, but at the time of scientific development, we don't yet know what these will be. Statistically, we know that it is likely to eventually have practical applications, and we are internally confident of its utility, but unfortunately that doesn't suffice for grant proposals. Yet in the early days of quantum mechanics, nobody knew that we would use it for our cell phones, much less in early days of general relativity did anyone suspect that we'll need it for our GPS.  In quantum information theory there is lots of excitement and optimism, but the timescale for everyday application is not clear either.  I think we just have to acknowledge the uncertainty and enjoy the adventure.