Ep 132: Conversation with Suneeta Vardarajan - Exploring Blackholes with Mathematical Physics | IISER-Pune

0:00Welcome to Platidwanee where we try to humanize science. I'm your host, G.V. Pavan Kumar. It is my absolute pleasure and delight to introduce you to my guest today on this episode, Professor Sunita Varadrajal. Sunita is a mathematical physicist and also works on very interesting topics related to gravitational physics. Sunita is a professor at Indian Institute of Science, Education, and Research Pune in the Department of Physics. She did her PhD at Institute of Mathematical Sciences in Chennai. Subsequently, she was a one-room-world fellow at Munich. And then she moved to Canada where she held a position at the University of Alberta, first as a postdoc, and then also in a relativity group in another university. And she also had a tenure-track position at the University of Alberta in the maths department. But we were very fortunate to have Sunita actually moving back to India in 2010, where she has been on the faculty of the physics department

1:04and also mathematics kind of in some capacity. And she has contributed a lot to very interesting questions related to black hole physics and mathematical physics, both the classical aspect of black hole physics and quantum aspect of it. For example, she's been looking at questions related to quasi-normal modes, ring down, mergers of black holes. And also corrections to possible corrections to Einstein's gravitational theory. And within the context of quantum black hole physics, the thermodynamics, information laws, all these questions actually are very interesting to her. And Sunita is also an excellent teacher and a science communicator. With this very brief introduction, I'll welcome Sunita on Pratibadi. Thank you. Yeah. So, yeah, give us a just about your kind of a career trajectory and your upbringing and why you got interested in science. Yeah. So actually both my father and mother

2:05have science backgrounds. My mother is a PhD in mathematics in the actually impure mathematics. So I was always interested in science growing up. And there was a lot of science at home. Everything was explained to me on the using science. So, and I came across Hawking's book, The Brief History of Time. And after that, I decided that that's what I want to really work on. I want to work on black holes. And that's how I did my PhD from Science Defense College Delhi and then moved to IIT, Metras for a University and then moved to the Institute of Mathematical Sciences for a PhD. So after that, as you said, I was a postdoc at the University of Munich. I was a very enjoyable phase. And then I moved to Canada, which is a very strong,

3:06Canada is very strong for GR, for gravitation. Oh, is it? OK. So I moved to a department in which there were people, you know, things are named after them. The first postdoc that I worked with was a former student of Hawking. So it was a very nice group. And so I enjoyed being there actually. So a very good place for gravity, Canada. Wonderful. Wonderful. So one of the aspects, Sunita, why did you come back to India? Is there a specific reason why you wanted to make a book? I came back from the, I came back to India for personal reasons. Because I enjoy being in India. And then the problem is, you know, in India, we have most of the good research institutes that we had when I was doing my PhD, where in stutes where there is no teaching. And I really like the North American model where there is a university, where the professor teaches

4:08and does great quality research. Because then you're able to disseminate things to a wider audience. You're able to influence a lot of people positively. And when the concept of Icer came in India, I was very happy and I jumped at it and passed by him. Wonderful. Wonderful. In fact, I'll actually come to the teaching part subsequently. Because that is one of the crucial components of Icer system. And also, I know that Sunita is a very well-known teacher in Icer Pune. And we are very fortunate to have that aspect of her too. So, Sunita, one of the things which is always important for us to know a little bit more is to understand how do you develop this deep interest in science? Because they, as you clearly mentioned, that you were influenced by books and other kind of interactions. Are there also certain aspects of physics, which really motivated you to get into this?

5:11Because I know you also have a very strong background in mathematics. So much so that you actually were a faculty member in the maths department. Because that is an important part of doing physics. And generally, students have either one extreme new point of looking at everything only from, let's say, some phenomena which can be just observed. Or everything can be just only looked at it as a mathematical aspect of it. But your work kind of brings both of them together. That needs a formative development up to a particular level. Could you please give us a gist of how do you develop that interest? I know each person's experience would be different. But it would be nice to know your own experience in that viewpoint. So, I think mathematics is the language of physics. So, I think it would be foolish for us to block ourselves from that. And just say that we want to give physics explanations for everything.

6:14Then things will not be very quantitative. So, because of my mother being a PhD in mathematics, I was very interested in mathematics from the beginning. The first move that I read when I was a school kid, which is that square root of 2 is irrational. It was amazing for me the whole idea of a proof. So, but I knew that what I was really interested in was real life phenomena. So, my mother is a mathematician, my father is a chemist, but I was more interested in physics. I was interested in all the things I see around me and explain those things. Actually, I found biology also very interesting, but physics I found very interesting. And since I already had some interest in mathematics and proofs and all that. So, I thought theoretical physics would be a good area to work in. But I must say that when I was growing up Carl Sagan's TV series,

7:18Cosmos was showing on Indian television. That's a wonderful series. So, for the eighth or ninth class student to watch Carl Sagan and explain special relativity and things like that was just amazing. It was taken very well. And then I graduated to reading book by Hawking. And so, all these things really motivated me to go into theoretical physics. Very nice, very nice. In fact, you brought up two important science communicators, which we are aware of one, of course, is Carl Sagan. But also, one should not discount Stephen Hawking's. In fact, he was a remarkable communicator of science. And I'll come to them at some point of time when I also discuss about your own interest in communicating science. So, first things, first in trying to understand the kind of foundational aspects of your own work, what was that which you, which intrigued you in your formative years to go towards certain questions related to mathematical physics

8:20and gravitational physics? I know you actually have laid some basic foundation at a level of school or something like that. But subsequently, your ideas would have got a little bit more nuanced and also maybe you would have also asked certain aspects of, or certain questions related to the kind of research what you're doing. So, when did it actually really trigger that you would want to go really into these areas of physics? Yes, so that was more as a master's student. There was a case that was interested in lots of things. I was interested in statistical mechanics also very much. OK. But when I went to my master's, I became interested in gravity. And I was always interested in mathematical physics. So, I just started reading a lot. And you know, one of the problems of India is that we do not have good labs in undergraduate institutions. So, we do tend to gravitate towards theory because of that. But the point is that IIT Madras had a great library.

9:22So, I could look up all the journals that I wanted. So, I read papers about how people were trying to recast gravity as a gauge theory and so on. And these are all old papers, but I read them. I got very interested. And then I went to Institute of Mathematical Sciences, which as you know is just a pure theory. Yes, thank you. Yeah. Nice. So, even there, you know, in the Institute of Mathematical Sciences, we have to give a comprehensive seminar at the end of the first year. And the paper that I had chosen was a paper of written, Edward Witton, which won him the field's pattern. So, that was a beautiful paper. So, you know, once you read papers, research papers, you know, you make the effort of reading a research paper, you get very motivated in that paper. Very nice, very nice. Few things right at the beginning when you actually are getting started,

10:23was there any specific approach you took to really get deep into the mathematical aspects of physics? Because that training would take time, but also a concerted effort towards, you know, getting deep into the subject, was there any kind of courses and what were there any books, which you are consulting during that particular time, during your kind of formative stage of your research career? No, unfortunately, there were not so many courses on the area, which I wanted to work on. So, my PhD thesis was on gravitation. And there was no course on gravitation in the place where I did my PhD. So, I had to learn it all on my own. But, fortunately, what happened at that time was, there was a very important conjecture, being made in the area of string theory, but it was much more than string theory. It was an idea that gravity in some space is due to a key theory,

11:26which lives on the boundary of that space. So, this is a holographic kind of duality, right, because you have a theory with gravity in a certain space. And the boundary has one less dimension. And the gravity in this bulk is described completely by the theory on the path. So, this is a very influential conjecture, which people are still working on now. And fortunately, it came just when I was doing my PhD. So, everything which I worked on was really, you know, all new topics because of the standard. Very interesting. So, could you please define in starting from first principles, what was the questions you were asking right at the beginning of your PhD, and how those questions evolved as you progressed. And as you mean that, let's say you're talking to, let's say, an MSc level student, how would you be able to just give us a gist of your research problems? Yeah. So, the research problems I worked on then were both classical and quantum aspects of

12:32black holes. So, classically, what are the observational signatures of a black hole? So, those were part of the work which I did. And that, you know, has become very important now that we have observational evidence of black holes. And now as a PhD student, we didn't. So, anyway, I worked on these signals and the signatures of black holes. And also, I worked on quantum aspects of black holes. So, simple thermodynamics tells us, for example, that if you have a black hole, a black hole is a region of space time from which nothing can come out. The moment you have a black hole, let us assume you have some stuff outside the black hole. And let us assume that the stuff outside the black hole can be described by thermodynamics. Now, you have a cup of tea and let's say you throw this cup of tea into the black. Now, you know the cup of tea has some thermodynamic entropy. So, there is a thermodynamic entropy of the matter outside.

13:34But the moment I throw it into a black hole, its thermodynamic entropy has decreased. The entropy about side has decreased because the cup of tea has gone into the black. So, we could have a problem with the second law of thermodynamics. And thus, we postulate the black holes also have an end. Nice. So, this so, black holes just by virtue of a black hole being there, it must have an end. And the sum total of the entropy of the black hole and the entropy outside must always be non-decreasing in any physical process. That will be our new second law. But if black holes have an entropy, we know that thermodynamic entropy can be defined as log of certain number of microscopic states in statistical mechanics of the system. Right? So, what are those micro states of blackness? So, what I worked on, the second part that I worked on in my PhD was on the micro states of blackness.

14:35Despite being the black hole entropy as a log of a certain number of micro states. And we discovered that, you know, in addition to the first term in the entropy of a black hole, there are corrections. The first term in the entropy of the black hole is proportional to something called the ADO of its event horizon. And the first correction is logarithmic in the area and we found out those logarithmic directions. Nice. So, this is a very fascinating aspect of, you know, looking at systems which are really, you know, astrophysical in nature and also detected very, very conclusively in recent years. And also asking questions which are kind of, you know, related to thermodynamics. It's a, it is really one of the very fascinating questions, not just in physics, but also in science. Because, you know, it also gives a great perspective of how important thermodynamics is.

15:36Because here you are applying, you know, the Boltzmann law, the logarithm aspect related to this. And also you're looking at connections to those things. So, how, how, how was this connection really made? So, could you just give us a just, because here is a system which is, you know, very interesting, esoteric and also something which is very profound in a way. And also thermodynamic laws of thermodynamics are, you know, bread and butter of most of the natural sciences. This connection, how did people really make when they came up with this? So, you see a black hole is formed from a collapsing star. Now, of course, stars are thermodynamic objects, you know, that the sun, sun is regating and so on. But the point is the black hole is a very different kind of thermodynamic object. So, the sun, when you say it's a thermodynamic object, what is the temperature of the surface of the sun? It would be something like 6,000. 6,000, yeah, yeah.

16:366,000 degrees centigrade. Okay. Now, the point is the moment it becomes a black hole, it becomes a thermodynamic object at a different temperature. So, the argument which I just gave you five minutes ago, was that a black hole has to be a thermodynamic object because it must have an entropy. Unless it has an entropy and unless the sum total of its entropy and the entropy outside does not increase in a physical process, we will have a problem with the second. Second. But if it is a thermodynamic object, what is its temperature? To answer that question, one must understand that the moment stuff has gone inside a black hole, it can never thumb out. We don't know what is inside the black hole. Even if a star collapses to form a black hole, it will actually scrunch to the center in finite time. So, you can't have a star anyway. You just have this black hole which from outside is only described by its mass and its angular

17:39it could be springing. So, it is described by its mass and by its angular moment. So, it turned out that hawking and many others found out that when you throw something inside a black hole, its mass can change, its angular momentum can change and its entropy can change, but they are all linked by a equation. The equation is called the first law of black hole mechanics. And that equation is very similar to the first law of thermodynamics which is DE is equal to PDS plus PDU. It is very similar to that. Now, if you establish an analogy between these two equations, that suggests you a certain temperature for the black hole. And that temperature is very small, it is in nano-calibre. So, it is not the temperature of the star before it collapse to form a black hole, because you can have a solar mass black stars collapsing to form a black hole. They would have a temperature of something like 6000 Kelvin,

18:41but this temperature that you get by analogy between the law of black hole mechanics and the first law of thermodynamics is in nano-calibre. So, it cannot be the temperature of the star before it became the black. So, what is this temperature? And hawking found the answer. Hawking's answer was that if you have quantum matter outside the black hole, let us assume that you apply the laws of quantum mechanics to the matter outside the black. And you are sure, because everything is quantum-finance. So, when you look at quantum matter in the outside the black hole, then even if the matter is in a vacuum state, which is an empty state with no particles, particles, the black hole will radiate particles. So, classically nothing comes out of the black hole, but quantum-mechanically things do come out of the black hole. In fact, it will radiate all kinds of particles and it will radiate like a black body at a certain temperature. Okay. It is black body radiation. It is thermal radiation.

19:43And that temperature is this nano-calibre temperature. Okay. So, black hole is a very strange object. It is thermodynamic, but its temperature is not the temperature of the star that it was before it became a black. It now radates like a black body at a nano-calibre temperature. So, that is the connection. Fabulous. So, it is also intriguing to associate temperature with an object which is generally associated with singularity. Right. Because it is also quite intriguing, because the general definition of temperature has to be expanded maybe in this context, because the average kinetic energy kind of a picture is very difficult to reconcile with this kind of singularity picture. Is there any kind of attempt to really reconcile this or broaden the aspect of definition of the temperature itself in this context? No, the temperature comes about because you exactly get black body radiation. And you derive what there are particles that come out of the

20:46black hole. Okay. But when you see the distribution of energies of the particles, you get exact black body radiation. I mean, all exact. It is likely modified, but actually it is black body radiation, which only cares about the temperature. Temperature. And the temperature I told you is is very different from the temperature of the star before it became a black. So, this is some completely different object. Fascinating. Fascinating. So, it is also means that you are really losing a lot of information into this. Yes. So, the information loss paradoxes precisely this. You see the temperature of the black hole that you get is inversely proportional to its mass. As the black hole radiates, its mass gets smaller and smaller. But because the temperature that this particular temperature is inversely proportional to mass, it gets hotter and hotter, the radiance more and finally, completely radiates away. Okay. So, now you finally have no black

21:47hole, but you have a space filled with thermal radiation, which only cares about a certain temperature. Okay. So, I could have thrown a 100 kg book. It lots of information in it. And a 100 kg block of iron and the black hole radiates the same way, the same way. Okay. So, what happens is that the black hole formed from the collapse of a star. Now, whatever material the star was made of or whatever material collapse to form the star, that material can be described using the laws of quantum mechanics. Quantum mechanics. So, you could start with a pure state, pure quantum state and that collapses and forms this black hole. But now, after the black hole has evaporated away, you are completely left with thermal radiation, which is a mixed state of the matter. Okay. So, a pure state has gone to a mixed state and we know from quantum mechanics that unitary

22:47evolution can never allow. Allow it. Yeah. So, what the evolution, if this is really quantum matter, how did a pure state begin to get a mixed state? Okay. That is the information lost. Nice. So, obviously, there is a connection to entropy because of the fact that this is connected to the information. Yes. This is the quantum and quantum. Quantum and quantum. Quantum mechanical entropy. With a 0 for a pure state and non-zero for a pure state. Non-mixed state. This also brings us to the aspect related to the detection because the LIGO of course played for an enormous role. It was one of the highlights of probably in of the science in the last few decades. So, to speak, it also has motivated new questions. I know that even before these detections were happening and a conclusive proof did emerge of this ringdown and other aspect of it, you have been working on certain questions connected to these kind of measurements too.

23:47Could you give us a just about that? Was there anything what you worked on at the formative stage of your PhD or towards even later part of your PhD? Well, so I have both the observational signatures of black holes. So, what we know very well from LIGO is that first of all, when two black holes are coming close to each other, we can describe the system using corrections to Newtonian gravity. And then they emit gravitational waves and we also are able to see what is the feature of that gravitational wave. Then they come close, they merge and then a final black hole is formed and the final black hole gives off sub gravitational waves. And those waves are at frequencies which are very characteristic of the black hole. They don't care about the, you know, what was the system which formed the black hole, those frequencies are signatures of the final black. So, in my PhD, I worked on those frequencies, computing those

24:50frequencies, those signatures. Very nice. Because this is such an interesting physics problem, more so, you know, whenever you actually have an oscillator and that oscillator is interacting with an environment, there is a kind of an energy dissipation and there is a ringdown of this. This is also very prevalent in optics and various different areas. That ringdown is also a very important measure to characterize the system itself. Because as you mentioned. With the mass amount. Yeah. So, these kind of, you know, inputs, how important are they to really go ahead and ascertain what is observed in LIGO and other kind of detectors? Because I assume there should be a connection which should be made between let's say the measure and the theoretical kind of analysis. Was there also a lot of concrete effort when you were doing PhD in addressing these questions? No, at that time, you know, the since the LIGO measurement had not happened in a very primitive

25:50stage. Now, we are at a very advanced, very refined stage, where we can try to get a theoretical understanding of the entire gravitational wave signal from the inspire of the two black holes towards each other to the merger to the ringdown. Nice. That was not there at that time. You know, at that time, we didn't have the capacity to observe. So, we were looking at the ringdown, which is the most, you know, the thing which we can work without the most theoretically. So, people have worked on the ringdown starting from Chandrishakar and Vishweshwara. Vishweshwara. Yeah. Very nice. So, this is also probably the time when you were also asking questions related to some algebra related to the solutions of the black hole equations. Was that a time where you were already kind of, you know, getting interested in these things? Well, I was interested always in black holes, basically, in quantum aspects of black holes. But then when I went to do my

26:56postdoc, I worked on problems in mathematical physics at that time. So, there were some, you know, equations which come about in theories like string theory, quantum field theory. And those equations had just been used by the mathematician Perilman. Under the old conjecture in mathematics for the Poincarecadet. That happened exactly coincided exactly with the time when I was a postdoc. So, when I heard about this in a mathematics seminar, I realized that those equations were in coming in physics. So, I teamed up with some mathematicians and we worked out all the, we used all the techniques which had been used to prove this conjecture. I used that to answer questions in what a peak. And so, you know. Nice, nice, fascinating. So, what were the questions you were interested in at the postdoc level? Post PhD, now you have already looked at the black holes and also solutions of the black holes,

28:00entropy, thermodynamics. And what were the questions you got interested as you progress? So, as a postdoc, I certainly worked on, you know, applications of the work of Perilman. Questions in what a field can. So, in point of field theory, you see we can, we have a physical system. And we have a way of probing the physical system. But we, you know, do not probe the physical system at all length scales or at all energy scales which are both, you know, two sides of the same coin. So, we have some lab limitations. Okay, our lab can probe up to certain scales. So, we have a coarse-grained description of that system. Okay, so, what you have in point of field theory is something called the renormalization group flow, which is that when you keep coarse-graining, okay, the system that you see has the same Lagrangian, but with coupling

29:04constants in the Lagrangian which flow, okay, as you go towards more and more coarse-graining, those coupling constants change, okay. So, this is called the renormalization group flow. And since it involves coarse-graining, we believe in physics that the flows are irreversible. And you can go from one description back to that system after some time because you're coarse-grained, okay. So, this, so we proved, so I and some mathematicians together, we teamed up to prove the irreversibility of many renormalization group flows using mathematics. Using these techniques, which were used to prove the aparcaic injection. Poincarecence. So, I am just trying to take a small deviation and talk about the flows because flows have been in the news for some very interesting reasons given that Navier's took equations have actually gained a lot of prominence in the context

30:06of certain solutions being also looked at from an AI kind of a machinery viewpoint. Of course, there is also a lot of human work which was utilized. I would want to get your thoughts on that aspect, Surya, because I know you are an expert on this and you probably have a better intuition of understanding this, what it takes to really do this kind of mathematical kind of work. And also, what is the contribution of AI if any in this such a situation? Could you just give us a kind of an overview of that kind of point? Yeah, so I don't know what AI will do in the future in any of these fields, but for example, the Navier's talks problem. I mean, if you look at it, what does it mean? It you know, people are trying to see there is some singular behavior in the fluid in finite time. And this is a common behavior which

31:08you know happens in many flow equations. So, does the flow lead to singular behavior of the fluid? And I believe that if this Navier's talks problem, the breakthrough was actually done by some theoreticians. And once their methods were used, then AI was able to come up with a problem in which in finite time, their singular behavior in the fluid. So, I don't think we have had I mean, for example, as you know, I read Peronance work and I don't think we have had an example so far where the breakthrough, the fundamental breakthrough has come from AI. But we don't know anything can happen to us. Yeah, yeah, yeah. There's so much of interesting developments which are happening. I've been also trying to follow some things what Terry Tao and other people have been writing. Of course, the technical aspects are not very straightforward. If you don't have a background, it would take a little bit more kind of, you know, reading. Nevertheless, it's an

32:11very interesting development. But also, there is a pushback from mathematics community. And also, in certain waters of theoretical physics too, what is your perspective on AI? Do you think it will probably assist people in thinking or we would actually be a situation where it can replace people in doing things? What is your perspective? No, I don't know, that's because you know, although I do use AI, I don't use agents. Is it? There are some recent papers which have come out by people from very good places in the world, Princeton and all that, who have analyzed what happens when you use agents. That's something I don't know anything about. But, you know, I think that you can accomplish a lot if you use agents, but I don't. I mean, it's a small personal preference. Yes, yeah. So, I do use some AI, I don't really use it.

33:11Very nice, very nice. In fact, there is also a lot more fun in actually just using it only for a certain purpose. Because otherwise, you know, there's no real kick out of doing science. Exactly. When it comes to experimental physics, you know, I wish some things were done through AI. Because for every experiment we set up, we will have to go back and do it from the scratch, as you know, because there's no transfer of kind of, you know, a skill which happens immediately. It's a very interesting kind of situation. So, we'll move forward and also just look at some of the questions what you have been asking as an independent researcher. Because I know within Canada, you already started your independent career in the maths department. And then you also gradually moved towards a match science physics kind of departments. Tell us a little bit about the kind of

34:12problems you were starting to think about during that stage and also then look at the specific aspects in ICERPUNE. Well, actually during my post-structural work, it was mainly in mathematical physics that I did, which was, you know, analyzing these flows and analyzing what they mean for physics. Because, you know, that was very new. The elements were very new. And there were lots of things in this work which could have, could be applied immediately to physics. So, I was doing that. But then after I moved to India, I of course became more interested in gravity because what I started out doing as a PhD student. And so now I work almost all my time on lactation. On lactation. That's right. You know, the LIGO discovery was so exciting. So after that, you feel very motivated because there's clear observation. Absolutely. Absolutely.

35:14One of the things which you have done very successfully is to integrate your research interest and also your teaching into kind of in a in a co-sphere sort of SP. I know that a lot of students do get motivated to study gravitation if they have a good teacher. You know, I'm glad that you also interact with students and the ISA community has benefited. Could you please let us know how this kind of tradition, you know, has has been there for a long duration in theoretical physics. Getting back to even John Wheeler, whether he's beautiful diagrams which he wrote, down to even the stage where a lot of people across India and the world have been communicating and teaching this aspect. Because it's not a very straightforward subject because it needs quite a bit of work because a lot of times people get influenced by nice things in the media. But once you get into the nitty gritty of the subject, it needs quite a bit of understanding.

36:16Even mathematical physics has to be brought to be bought in. And also your intuition has to be updated. That is my feeling even when I got exposed to even rudimentary general theory of relativity. It is absolutely kind of, you know, a very intriguing aspect of science. And added to that, you bring in quantum mechanics, it becomes even more richer. Tell us a little bit about this aspect, Sanita, how you have been able to gradually integrate your own research and the teaching aspect of gravitation. So as you know, Pavan, we have two courses on gravitation in ISA now. And the first course is actually something which talks about the mathematics behind gravity general relativity, how Einstein built it up. But it also emphasizes that all this theoretical physics would be new use if you did not have experimental evidence. So as you know, we have these two courses in gravitation. And the first course makes a connection between an

37:18abstract theory which was made using mathematics at that time and something which you observe experimentally or observationally, you know, the discrepancies in the orbits of planets around Sun. So that's a very important theory. I mean, it tells you that even if you have fancy mathematics, you must also observations. And sometimes if you want to answer observations, you have to actually build up using pancina. So the second course is basically a course which suburises the achievements of people like the Nero's and Hawking. And the amazing thing that they did was that they used topology, which is a field of branch of mathematics. They used that to answer questions which are very fundamental to physics. So one of the questions is that you know that a star explodes into two many times. A star explodes into many pieces that's called

38:21a supernova. Now we know that black holes are formed from stars, stars collapse to form black holes, but a black hole can never break into it. Interesting. And how to prove this result? See this result is a you know, if this result can be proved using five lines into a course by Hawking. And so the amazing thing is you know people like Hawking and Penbrills, they believed that all the knowledge that humility has created until now, which could be all the mathematics also, is all there for me to answer the physics question that I want to ask. Neemly the question can black holes break into two and they cannot, but stars can break into stars can break up into a million track. And we have seen so many binary black hole mergers. Okay, LIGO has seen so many mergers, not a single instance where a black hole has broken into two. So this is like the power and so this for example this particular result and its five line proof,

39:26we do in the theory in the second course on the gravitational. So I think it is more you know we have to know the great work which which people have done in this field. I mean it's amazing, it's amazing work. There are courses on this work in Cambridge and you know many other good places. So my motivation was for the undergrad students in ICER to actually you know learn this. Learn. No, no, no about this. Very nice, very nice. In fact, that is an important part of actually also understanding how science is done. You know there is a lesson there because ideas actually can come and influence other ideas in a very interesting way. And most of the time if one is really looking at it only from a slightly more smaller perspective broadening the horizons literally I and also you know figuratively really helps to speak in a very interesting way.

40:28So one more question Sunita related to you know getting trained in this area. You know there are there are there are a lot of students who are interested in doing this kind of physics. And also a lot of students who may want to actually also learn more about this particular topic. What is the kind of you know training it is required? Especially I'm talking more about the research level because undergraduate level of course there are good books and other things. But research level how should one really get started in this area? It's an important part of getting trained not only at a graduate level even at undergraduate level. There are certain things you will have to think and also do could you please give us a brief overview of that? Yeah so whether you want to do classical black holes or quantum black holes the training will be different between. Classical black holes you need to not just learn the basics but also know how what like

41:29or what what it can do how to you know the basic problem in classical black holes is how to get the signal all the way from the spiral to the merger to the ring dot. And this question is very difficult because you know the way it's done now is that at the early spiral case you can study the problem using corrections to Newtonian gravity. At the late stage when the black hole rings down you can use gravitational perturbation theory to solve the problem. But in the intermediate stages if people use numerical delete. So you know a student in classical gravity I mean who the person who is not running codes should be a person who wants to get an analytical picture of the entire signal from the spiral to the merger to the ring dot. And that's very complex. And reason why it's not an

42:32analytical picture because simulation cannot do every single initial condition. You know for every single initial condition where two black holes with so many so much mass and so much spin comes spiraling towards each other you cannot run the simulation for each one of them. Okay so it's computationally expensive. So you need an analytical you know approximation scheme to actually study the problem entirely from the spiral to the merger to the ring dot. And this involves of course learning all the work which people have done before. And understanding what like I can observe and what it cannot observe and then try to solve the problem. So this requires certain skills you know certainly and then quantum black holes require completely different skills because there you cannot just get by with your knowledge of gravitation. You have to know all the theories of quantum gravity for example string theory or other theories

43:33and you have to understand what they predict and you have to just work out for a black hole well if I am in this theory what does it predict for the black hole does it answer you know problems like the information loss padded off and so on. So it has study quantum matter in the presence of black which is completely different. Very very interesting. The two skills are completely different. Completely completely different. So there's also a lot of interesting information processing questions because it's also a relativistic perspective which comes into picture which means that there's also a questions related to quantum gravity which has to be addressed when you include the quantum aspect of it. Has there been also kind of efforts in trying to address questions because quantum gravity by itself also is a very big area of research so many interesting aspects are yet to be you know you know unveiled so to speak and there's a lot of effort which is going on in terms of a variety of theories which have been hypothesis and also a lot

44:39of work has already been done. What is that current stage and the connection to the gravitation so to speak. Yeah so see the one the most important questions is is there any way I can observationally test the various theories of quantum gravity. So there exists a system which has been just you know analyzed recently. By recently I mean the last five years. This system is a black hole which is spinning at the maximum permissible limit. Okay so a black hole for a certain mass cannot have arbitrarily high angular momentum. The angular momentum is kept off to be some maximum back. Now you can easily like if you just study classical processes you can have two black holes which are spinning and which are orbiting with very high orbital angular momentum and when they merge they waveform of black hole which is at nearly its maximum angular

45:40moment. So it seems like such a system would be very easy to form a black hole which is spinning at the maximum allowed limit should be very easy to form. However what we have found in the last few years is that such black holes receive very large quantum corrections. Now you normally expect that the effects of quantum gravity are very small but in these particular class of black holes which are spinning at nearly the maximum limit the quantum corrections are believed to be very large. Interesting. So the thing is is there such a thing as that black hole. I mean such black holes are called extremal black holes. Is there such a thing as an extremal black because there's always quantum effects in we can never escape quantum effects in this world. If quantum effects are actually you know changing that particular space that particular space time then is that space time there. I mean can I actually form it you know seems to me that there's no problem forming it but the problem

46:40is that the quantum corrections are very large for such a time. So there are some questions like this which are very recent which are really you know getting very close to whether we can observe quantum effects of gravity. And that I think is very exciting. So that's something which I'm working on now. So I think yes it's a very exciting time for quantum effect. Fabulous. So questions about mentoring where you also have been you know training band of students are both undergraduate and you know PhD level students. What has been your experience in working with them especially on these kind of topics. I know there are very good students who have graduated from your group. I think I have had a lot of fun dealing with students and I don't believe that any AI can replace a good student. Okay and so I had a very good PhD student with whom I worked on black hole for what dynamics. Momodali momat is now in ICTS

47:47Bangalore and I have fantastic undergrad students. I mean some of the undergrad students the amazing level of complexity which they can read and learn and work on. For example I'm working with an undergraduate student scientist and checker-burti on this particular problem of large quantum corrections of these black holes which are spinning at the maximum allowed limit and it's just amazing what science is doing. So I have had a very great experience with ISA students especially. Wonderful. Wonderful to know. It also is a kind of attribute to the system because yes it's a bit twice. It's certainly a bit twice. Yeah because it gives room for undergraduates to do research which I think is one of the very better aspects of this system and I'm hoping that this is going to continue as things progress. So a few questions related to the science communication aspect Sunita. I know you are very passionate about communicating science. You also

48:52recognize the importance of science in society and you have also been writing very nice things related to some things are connected to your work but also slightly more broader in context. For example whenever you travel for in recent case where you went to Italy you also wrote about Galileo and you visited that particular place and the museum. What really makes you to really you know communicate science it's an important part of scientists life. Not many do it and a few do it we should hear them and I'm trying to ask you why do you do it. I will actually you know since I came to into science because of the videos of Carl Sagan or the book of Hawking or many other popular science books which I have read as a DSS student. I think it's very important it's very important to communicate science and there are things I'm

49:57passionate about even though I don't work in them. For example at some point of time when I was on X somebody developed a game called Quantum Chess which uses the principles of entanglement which teaches entitlement very nicely. I mean when you make a move you your next move is a superposition of moves and so on. It's a very interesting game. So when I read such things I like to you know spread them amongst the community of people you know could be students or people in other jobs and whenever I've done any such outreach activities you know I really have find it fun find it fun interacting with people talking about science. Wonderful wonderful yeah I will also link in the in the references some of the YouTube lectures and also a TED Talk Sunita has given you should actually listen to and also watch that it is a very interesting kind of

51:02discussion related to some of the topics which we have also discussed about. So Sunita finally some recommendations to our listeners and viewers some aspects of science art any kind of books or any kind of other sources which you think has also inspired you and also probably can inspire other people. Well science I have already answered yeah and I have other interests you know which are I think are very important to make us complete you know that we shouldn't just be you know machines doing science. So I have an interest in art and I have an interest in art cinema. So you know there's lots of art cinema is more like moving art and you know there are lots of sites for example there's a site called movie MULR and movie shows lots of great cinema to us

52:08in our homes. So you can find streams in your homes. So that's something which you know I would recommend people try. We have had some great cinema even from India. Maybe yeah yeah. So you know the great greats of Malayalam cinema such as it dry and so on. So yeah sure so I would say movie and if somebody is lucky to be in Pune then the National Film Archive of Auditorium where there's green great cinema. So yeah so I'm it and if someone goes to TIFR I think it's one of the best art galleries of India. So there's paintings in every new can corner of the great painters. Absolutely thanks to Romi Janki Baba for those wonderful wonderful collections. I haven't been to the other one because the TIFR one is breathtaking it's very very very good.

53:13Great Sunita this has been a wonderful conversation you know I kind of you know I was expecting this kind of you know wonderful ideas to actually emerge out of this kind of conversation. And also very importantly you have described to us in a in a broad way what it is the research area what you've been working on and is there any specific aspects you would want to just give a message to especially students who would want to get into this area and then very close. Yeah so I I think you know in classical gravity we've come because of LIGO we have come to a stage where we can test whether there are corrections to Einstein's theory. You know there is no derivation of Einstein's regression as such it's basically something which he post related and it agrees very well with experiment at the solar system level but when you have strong gravity then there may be deviations wrong. Furthermore there are other things which people have proposed which are

54:18like for example there are no black holes but there are certain objects which mimic black holes from very very far out these are called fastballs and when you come very close they are not like black holes so they don't suffer from any of the paradoxes of black holes. So if you want to ask for example how these some two objects like this merge what is the signal that comes gravitational wave signal that comes out from these objects which are not black holes or which mimic black holes. Now all these questions are now at a stage where we can answer them because of LIGO. I mean especially in the last year there was a very very powerful LIGO event in which many of these questions can be asked. So it's a great time for students to enter this area of classical gravitational physics and quantum black holes of course as I told you there are some systems in which the quantum gravity effects can be very large. So these are written this is again a very exciting idea.

55:22Excited wonderful. I had a message for people who are not in science who would want to learn what about it because this is also very fascinating why should only people in science have all the fun. In fact in earlier times in earlier centuries we have people who had a very healthy interest in science. I would just say that there's great videos in YouTube you know just learn how black holes what black holes are what is the observational evidence for black holes everything you can just pick up from YouTube. So I think we should keep in touch with science. I think listen to music or they go and watch cinema absolutely wonderful wonderful on this wonderful note we will conclude this episode. Thank you Professor Sunita Rajan. Thank you.