Interview with Jorge Pullin (contd.)

0:00 Well, while I'm not thinking of the other question that I was going to ask, I know that you're also interested in quantum gravity, quantification, and when I was looking at the earlier type of history, the earlier project that you read in the paper about, I noticed that one of the big attractions of doing gravitational wave research back in the 50s at a time when nobody really felt it could be detected or anything was that it might be a useful handle towards quantizing gravity. And, well, my sense was that, on the whole, subsequently gravitational waves don't seem to have played a big role in the quantization effort, but I was wondering what you thought and whether it's still... I would, well, it depends how deep you want to go into these things, but think about the following way. These new variables that Ashtickar invented derive from the work he was doing on asymptotic issues of space-time, asymptotia, derived from self-duality notions that Ted Newman was using, Freud and Roger Penrose were trying to use to understand the asymptotic structure of space-time, by trying to properly define what a gravitational wave is. So the amount of understanding of relativity that occurred by the work of Bondi and collaborators in the 60s, I think it stand amount to a revolution. I think we didn't understand almost anything. Einstein didn't understand relativity almost at all. I mean, he certainly didn't know what a black hole was or didn't care very much for it. And he did even, I mean, he, as you, in your text point out, he was back and forth on the issue of even gravitational waves existed. So this work in the 60s revolutionized relativity as a whole and the perspective we have in the field as a whole. And the people who are approaching now issues of quantizations are certainly approaching them with knowledge that was developed in the 60s. So I think it's, in a contorted way, I think it played a crucial role. But, I mean, in a direct way, these new variables that people are using around here, a direct brainchild of all these techniques developed for studying gravitational waves, so, I mean, loosely speaking, gravitational waves, not the ones that, not from the perspective of them being detected by LIGO, but just studying what their intrinsic properties are, so.

2:30 Well, just as you say that, we thought, yeah. That's probably not what people were thinking in the 50s when they were thinking gravitational waves are going to be useful for quantization, they were probably thinking more linearized theory and stuff like that, but in the end, it ended be in this way. That's an interesting point, yeah. Well, so given that there is a strong connection, maybe not what people originally expected, but as you say, you should make a good point, a good case for it. Do you see, obviously LIGO is likely, you know, I'm assuming it's successful, to have a big impact on numerical relativity and a lot of work done in relativity, such as your work that you're discussing. Do you think it's likely to have a big impact on... I frankly can't even fathom yet what LIGO is going to entail. I don't know if this is, you know, if LIGO detects gravitational waves, and this becomes the hottest issue in physics, and then, you know, instead of spending, whatever, 60% or 40% of the budget in physics and particle physics like we're doing now, we start spending it in gravitational wave detection, I can't The field will be transfixed, reconfigured, changed in such a way that it's very difficult for me to imagine. But on broad terms, what I could expect, you know, more people are going to get attracted to the field in general. And it's a Gaussian, therefore, you know, a lot of people will be attracted to whatever life is actually doing, you know, detecting gravitational waves and stuff like that. But I could imagine other people get attracted to these other side issues that are kind of more fundamental, and that would be good for that. It could be very different, though. exactly know what's going to happen. That's the bottom line. It could happen that, you know, boom, LIGO discovers gravitational waves, suddenly everything becomes like a tool, and then just becomes one more tool that astrophysicists use, and nothing much, I mean, it's a boom in astrophysics if you want, but relativity as a field is not that impact. I mean, the analogy I typically give is, well, think of, you know, I don't know, radio astronomy. Was this a boost for radio research? Not really, right? I mean, it was a boost for astronomy, for astrophysics, but the people who actually designed the radio telescopes and the nuances of the radio telescope didn't become a big field in physics. So it could happen that, you know, the same happens to gravity, that yeah, gravitational waves are the way people observe the universe, but they just need to

5:00 to do all the interesting stuff, and therefore it doesn't happen as I said at the beginning, so I don't know what's going to happen. But in the first scenario, that is, if this issue becomes hot and a lot of people get attracted to gravity research, naturally, you know, you can just see it happening now. The people who now come and want to work in quantum gravity are probably the people who want to work in particle physics 20 years ago, you know, unifying the forces were the hot thing, and doing quantum variety was something that was dead, so. You mentioned contact with Andrew Abrams and the North Carolina group. In general, do you Do you have much contact with the American people or...? Yeah, we are in very good terms with the XCNCSA group that now is in Potsdam. It's just an issue of, you know, I teach here and I have all sorts of responsibilities at Penn State, and I also do with Law and Gravity. So the amount of time I have to go to these things is not terribly big. So progress here is slow, and we tend to contact them when we get something. So it's not like we're working on a day-to-day basis, but we say, well, let's do the collision of two spreading black holes. We spend a year doing that, and then we go see them, hey, you guys, you probably did this or can do it. And sometimes it's been mutual. We've been able to guide them a little bit, like Steve Brandt at NCSA had done a PhD thesis studying distortions of black holes in numerical codes, just like a warm-up to doing full numerical productivity. Then, you know, I approached him and Ed Seidel and said, look, since you can do any distortion of black hole you do, you want not imperturbation, but, you know, full evolution of Einstein equations, and we're treating collisions as distorted black holes, we could provide initial data. You could run them with full code. It won't be a real collision because you're still creating the problem in a single black hole. issues of, you know, giving data on two horizons and stuff is not appropriately done, but you could run a full code and we could run the perturbations and compare. And this is actually happening. Steve Brunt and Hans-Peter Knowlers are now trading notes in these collisions and spinning black holes very closely. And these collaborations are great because they

7:30 allow each other to find all these factors of two that are missing in the calculations. We go and compare, the waveforms are the same and they're a factor of 16 off. So then someone comes back and says, oh, I forgot a fact or two here, still doesn't work, so we go look back at ours, and finally they converge in that way. I think it's useful. Do you get any sense of there being any crucial or overt differences between the way that, say, the American people do their work and the way it works with more traditional people? I would say proceed on a more intuitive basis whereas when you're doing a full 3D numerical code at the end of the day you evolve and you get something and unless you have something to compare it's very difficult to to understand what you're doing so so for them I guess it's a big big challenge to do that I mean what they're doing is for instance comparing 3D codes with 2D codes and stuff like that which is very good whereas we try to build you know a little block like we with no momentum, let's do it with momentum, now let's do it with spin. Whereas, for instance, they haven't quite done all these intermediate steps, they're just going for the full code somehow. So it is true that the issues that arise there are different and it's worthwhile just pushing that way and learning those issues, but it's slightly different in that aspect. So you were saying that with your approach with more of the building blocks you can develop an intuition as you go along? I would like to do that, yeah. I would like to develop an intuition and understand, for instance, you know, look at the waveforms and be able to account, you know, to an audience that doesn't know very much about this, why things are happening, like, you know, this dip we had. By looking at how cancellations were occurring in the different perturbations approaches, we were able to come up with this idea of horizon jumping out and other things that you just run numerical code, you get the result, and it's very difficult to get intuition It's not that it's possible, you can go and study how variables were working and stuff like that, but it's sometimes a little bit more difficult. Yeah, that was one... I mean, occasionally now talking to people... Today, for instance, Paolo Laguna was speaking to this point,

10:00 mentioned that he thought, well, numerical work was a bit like neither doing an experiment or doing theory, something a little bit different again. And so I'm curious as to this aspect of him, not knowing much about numerical work myself. And interestingly, and I'm going to hopefully speak to him tomorrow, when I spoke to Jeff Winokur about some of the earlier history, and all of these controversies and debates over, say, the quadruple formula, for instance. And he had done some work on that, of course, which is what I was speaking to. And he said that he thought that the numerical work that was being done on the development of the numerical side of the field, that that might lead to situations in which things like the quadruple formula wouldn't really be relevant anymore for people interested in that. I don't think it would just make that kind of a question, not of any particular interest. You'd be sort of, I don't know, somehow taking a more holistic approach or something. Sure, well, there are many situations where you can imagine that a quarter four or more doesn't apply. But I guess if you have a code and you run in a situation where it does apply, it better work, right? it would seem that, certainly for the moment anyway, for the foreseeable future, when the numerical people actually have the code up and running there, they certainly want something to check, I guess. Right. Because it's so difficult to tell whether it actually came out right after all that. Well, I guess the other thing I was going to quickly ask in was, you were sort of biographical because you were a graduate student at the Institute that's there. The way it works is, I did my Master's Science there, and then I went to do my PhD at this other place called Cordoba, where Gleiser is. I did my PhD work there, but I still submitted the thesis back formally to the Paul Sabre Institute. thesis work I did in Cordoba. And then I came as a postdoc to Syracuse and I was a postdoc in Utah and then I came here as an assistant professor first and associate. Who were you working with in Syracuse? In Syracuse I came to work in Ashtager's group. When I came from Argentina I wanted to work on

12:30 these new variables or what they were about and stuff like that. That was very interesting, thank you. Okay, well I hope it helps. No, it does, it does.