Interview with Amos Ori

0:00 So it's the 18th of August at 5.20pm and I was speaking with Amas Ori. So I guess I'd maybe start by asking if, well, how you first came to work on gravitational waves. Had you worked on it before you were at Caltech? No, I never worked on it. Actually, even in Caltech, most of my work, of course, was not on gravitational waves. So even the work which was related to radiation reaction, I tried to do it in simpler contexts, like scalar field. But I started it when I was in Caltech. Right. And so, was there a particular reason for you to work on, you know, topics that were like radiation action that are more related to gravitational waves? Was that just what was being done at Caltech at the time, or were there other reasons? So I think, okay, when I came to Caltech, nobody worked on this at the beginning, but after maybe half a year or maybe a year, KIP started to tell us that LIGO is, that this program starts to, they start to run it and to think about, to push it, and there will be such a detector and somebody is ought to start working on gravitational waves and so And I think week after week after week, at the group meeting, he told us that somebody is ought to start working on this, on this, and on this. And I think that the accumulating effect of these statements was that some of us started didn't need to work on it these issues after some time yes then the following so then people started to switch to this new and then of course I was influenced by this and also started to think about this and was

2:30 was the fact that these like you were saying these detectors Was that a factor in itself for you, or was it just more that people were getting more interested? Yeah, it was an important piece of motivation. I knew that there was going to be a detector and observation. Of course, it provided most of the motivation, maybe. But actually, it was not the only motivation, because I'm not so much actually interested in stuff related to LIGO, but I'm very interested in the self-force problem. So when the atmosphere in Caltech changed towards this type of gravitational-radiation-related problem, then I realized that I'm attracted to the self-force problem and I started to think about it and to work on it. to that problem for you? So the attraction is that, well, I think it is an extremely interesting problem for several reasons. First of all, we are, we were educated for many years that the basic, one of the basic principles in physics is that a particle cannot apply a force on itself. Suddenly we see that a particle does apply a force on itself, which for me is exciting, that's the first The second thing is that, okay, once you believe that there is a self-force, then you see that when you try to solve the equation of motion, with this force, the solutions are running away. the runaway solution this is a challenge a serious challenge to understand what's going on here does it mean that there is no self force the formula is incorrect I'm talking now about Abraham Lawrence Dirac formula so for me it was very interesting to understand what's going on here and eventually I came to the understanding that Abraham Lawrence Dirac is correct and there is no physically there are no runaway solutions

5:00 it was very interesting to understand how these two facts agree with each other so the runaway solution and what else also the fact the issue of divergences which you must finds a way to remove before you can get a physical result I found it as a very interesting challenge and I think these are the main reasons which attracted me to the runaway problem not the runaway to the self-force problem So it was a combination in a way fundamental issues of principle and the technical challenges Yeah, I think so I guess another thing I'm curious about just to come back to the connection with LIGO and similar detectors is does the fact that these detectors exist make it easier to work on these types of problems like radiation reaction from the practical point of view I mean, is it perhaps easier to get students or postdocs who are interested in funding for that type of work, or it's not? I don't know. As far as my interaction with students is concerned, it didn't make a difference, except that I only had a few students so far. And, of course, one of the questions, which is always in the background, is, okay, suppose I'm working on this, is anybody going to read my paper and to offer me a postdoc and so on so then, of course it makes it easier from this point of view, but the few students which I met were ready to work on the self-force problem even without, I think fact of detection in the near future. Right. They also found it interesting. That's my feeling. But of course, it made things easier, the fact that there is a lot of interest

7:30 in the community currently. And does it have any effect on funding? Is that a factor in Israel? Not as far as I'm concerned. I didn't need funding. I didn't try to get funding for this because I am doing COA, so why do I need it? You don't need particularly. I was just curious. So do you see the detectors themselves as having an impact on your work when they actually begin working or operating? I didn't understand. Do you see the detectors as having an impact on your work when they actually begin operating, when they get results, let's say? So, I'm not attracted to join the massive work which is being done on templates and on very detailed predictions. I'm not attracted to this. I believe that, I do hope that there will be detections. And, number two, that there will be detections which will surprise us. case, of course, I will be strongly attracted to try to understand what's going on. But if there is a waveform which does not exactly fit the prediction, this will not be very attractive to me to find the detailed agreement with theory. For me, I don't think it will be too attractive. It's only if there's some big difference between what's been expected. And I believe there will be. I mean, once we start to to observe the universe in gravitational waves there ought to be a thing which we had not expected then it would be very interesting I think so you're optimistic of there being new challenges brought about I think that once we detect gravitational waves and we have first of all we need to detect and I cannot say I'm optimistic this. I mean, I expect that it will be detected, but I'm also myself am ready to the possibility to the possibility that when it starts to work, we should not detect anything for some period.

10:00 I believe that at some point there will be detections. you know, if you have a detection at some time with a resolution of signal tumors of 1 then after one decade you'll probably have a resolution of 100 signal tumor of 110 or presumably this will improve with time and at some point there will start to be we shall have enough sensitivity to see more delicate things and I believe that at some point I am optimistic at some point we should start to see phenomena which we didn't expect that's my expectation it might perhaps we should have to wait 20 years it might take a long time I believe that 20 years this looks realistic to me and do possible detectives like LISA do they interest you in particular because LISA's might see systems like giant black holes with bodies orbiting around them that would be suitable for perturbation analysis does that look particularly attractive those kind of detectives or it's just more something unexpected and it doesn't really matter from what source it comes up. Yeah, these are two separate issues for me. One is the hope to see some unexpected phenomenon. And the other is to observe the perturbative aspects of the self-force. I mean the self-force in the clean perturbative form. So I really do not expect surprises in this context. Of course, if there will be, it will be very interesting. But this looks to me like, I mean, just predicting the orbit and how it decays. For me, this looks like, well, it's hard to believe that what people predict now, we are going to improve the prediction. it's hard for me to believe that

12:30 for such simple systems there will be surprises but of course it might be I'm not sure I answered your question I guess that more or less answers it I was just curious as to I guess I was curious as to whether you saw Lisa as being of interest outside of just something unexpected and I guess there is in the fact that it's relevant to this type of calculation in some sense. Yeah, so LISA also provides relevance to this type of calculation, which makes me happy, but this problem is interesting for me, not because of LISA. So I guess, to leave aside the detectors, what do you see as the remaining challenges in the gravitational radiation reaction problem? For instance, to do the problem where you're doing an in-spiral of a general orbit around a curved black hole, do you see any remaining conceptual obstacles standing in the way to that type of calculation? So, let me talk about something more specific, about calculating the self-force, because this is a necessary condition for calculating the revolution. and so let me try even being more, pick an even cleaner context, suppose to calculate it in the diabatic approximation, suppose we presume that the particle is moving on a geodesic and we want to calculate the cell force, then still there is a challenge I think I mean how are we going to do this One way is by mode some, and then there is a challenge of regularizing it. And I'm now working on an approach which I described yesterday, but so far I didn't try to apply it to care. It still needs to be applied to care, and also from scalar to gravitational. Now, scalar to gravitational, I do not expect much difficulties here.

15:00 to care, it is less obvious to me. I mean, I believe it will be able to do it, but I still think it is a serious challenge to do it. So there is a challenge here. Maybe it may turn out that the mode sum is not the most practical way to do it. Perhaps working in the time domain and in the Green's function in the time domain and just like similar to the line that Alan Weisman described that integrating of the history of the particle perhaps this will turn out to be more practical especially because this allows one to go beyond the adiabatic approximation so this I think using developing this method I think it is also a serious challenge I believe it will be done but I think it will take some time maybe 10 years that's my impression it's also it's both developing the method and also maybe improving the computation especially if you want to deal with integrating over the history in the time domain so we need to calculate the Grinch function for many, many points so in the weak field I think there ought to be analytic ways to do it but in the strong field we need more powerful computers It looks to me that it's realistic to expect that it will take 10 years or something like this until we know how to calculate reliably the self-force for a given orbiting curve. And that's a combination maybe of, as it were, conceptual challenges. That is to say, for instance, you don't know for sure when moving from Schwarzschild to Kerr that there won't be issues of principle which will arise that the performers might not be able to handle and then at the same time there are just technical challenges of how to...

17:30 At present I do not expect that there will be no conceptual challenges and this... I think that the recent works by Queen and Walt and by the Japanese group by Mino and the other I think they give a very clear conceptual picture which should be valid in any current space-time, in particular in care. So in this respect, I think that conceptually we know what is needed to be done. So rather we are left with technical difficulties of how to implement it. And if you are going to use mode sum, then still there is some conceptual challenge. But once I faced it in the Schwarzschild case, I believe that the transition from Schwarzschild to Kher is more of a technical issue. I must say that when I started to work in this field, my feeling was very different. I felt that there are really conceptual issues to be understood, and I'm happy that in the last decade, then today these issues are much more well understood. So to begin with, you were saying you thought that the conceptual difficulties might take longer to overcome than any, that it was more of a conceptual challenge. For me, there were very many, several, very mysterious conceptual issues at the beginning. And now I feel that I have a much better understanding, not only me, but I think the entire community has a much better understanding of what is the meaning of regularizing the self-force and how do we deal with the runway solutions. So now I think there is an understanding. Yeah, I agree over the last, yeah. You agree with this? Sure, I think, yeah, it seems clear, just at this meeting, for instance. So here, there was not much, people did not talk about the runway solutions, but only because everybody knows that there are no runway solutions, and there are obvious prescriptions to avoid them, and it's not an issue even here. But the regularization is an issue, but people now don't understand it. Yeah, it seems to be at a stage now

20:00 where people are looking more at the technical challenge of how to implement it to that situation. And so now that, say, as you were saying, ten years or so after you started looking, thinking about the problem, it now looks to you as if the technical challenges are actually great enough that it would take a similar amount of time to overcome them as it has, as it were, Well, you know, to overcome the conceptual issues, it took much longer than this. So, because it's already 100 years, I think. Sure. But I was not aware. It took me some time to understand the pieces of work done by Dirac and by De Wittenbrink, maybe 40 years ago or 30, I don't remember. De Wittenbrink was 40, I guess? I guess around 40. More like 40, yeah. So I think that, for example, the Wittenbring do give the resolution to the conceptual problem of how to regularize the self-force. Dirac also gave, but I think, in my point of view, Dirac's prescription, the point of is not attractive. It was good at that time that people dealt with flat space. But the transition from flat to curved space forced people to dig much deeply into this problem of how do we regularize the self-force. And I think that the Witten and Brim did it. And for me, it took some time until I understood it. Also, there was a similar issue regarding the runaway solutions. so it took 100 years I think to understand the conceptual issues here well that's true of course and do you see the problem of the self force as being just the same problem between electromagnetism and gravitation and the scalar force basically, although there is a new issue here in the gravitational case of the gauge freedom which is gauge freedom and gauge ambiguity which makes things much more tricky.

22:30 in the electromagnetic context what we wanted to do is just to calculate the force. In the computational case, calculating the force by itself does not have any meaning because it is gauge dependent and this makes everything more tricky. But to the large extent Basically, my feeling is that it is the same concepts are involved, the same types of problems, even though it's more tricky. So is the main difference between the early work on radiation reaction, the cell force, between that early work and now, just primarily the difference between working in flat space? That's my feeling. So, for example, Dirac's suggestion to use the half-added minus half-advanced as a method for regularizing the self-force does not apply to account space. And these forces want to dig much deeply inside what we want to achieve by regularization and to find the prescription which does this. Right. So, yeah, so just because you can't, because you have this linear combination, you know, these two solutions, which won't really work in... Yeah, it wouldn't be causal, and so I feel that, yeah. Yeah, it's interesting actually because there were these debates which you may be aware of in the 50s when people really started looking at the radiation reaction problem in gravity in which some people like Infel thought that not only should one use half advanced solutions, but that they would actually be a physical solution. Sorry, I'm not can you explain this? Infants saw that this is a physical solution? Yes, and that binary systems wouldn't actually decay because you'd have a system of ingoing ways matching your system of outgoing ways, which would... Only for gravitational radiation, he believed this, or also for the electromagnetic... Only for gravitational radiation, basically,

25:00 only for freely falling systems like binaries and there was this there was this ongoing argument about whether such systems really ought to radiate or really ought to decay in their orbits and part of it was this idea that they because it felt in EIH, it felt that you made use of these these half advanced to have retarded potentials and then began thinking of them in terms of being a real physical solution to them. Do you think that Dirac regarded them as physical solutions? I don't know. I was surprised why Dirac... I mean, I think it's a walking trick in flat space, but I wondered whether Dirac really believed that they have a reality. I'd like to try and find that out and I'll have to try and... can be difficult of course to find out what people were thinking because Einstein in his 1937 paper with Rosen cylindrical gravitational waves he introduces this section a couple of paragraphs in which he says, in which he seems to treat the half-advanced plus half-returned potentials as a physical solution, just again in this case of say a binary system and of course it's difficult from our perspective to know why want to do this I know that at the beginning of the century people did look at these type of potentials as a possible physical solution because they were looking for a reason why electrons didn't decay in their order and so that there was that going back to the 1910s and so on and I don't know if that was part of the reason why so it would be interesting for me to look that up when it came to interact to see So, I just remember now that one, another aspect which I found very attractive when I started to work on radiation interaction in curved space-time is an apparent conflict between a self-force, in several examples, and the equivalence principle. issues like does a particle of a geodesic radiate or not? Does it experience a self-force or not? Because in flat space it does not.

27:30 So people tried to use an equivalence principle to claim that there should be no self-force if it is a free fall. And of course it's false. And it was very interesting to try and understand why the equivalence principle does not work for the self-force in this naive manner. Right. That's another aspect which I found very attractive. So, and what for you is the answer to this problem? So the answer for me, so this is one problem, the other problem, sort of surprise that a particle applies a force on itself, And my answer is that no. There is a particle and there is a field and the particle interacts with the field. And it is interaction between the particle and the field, I mean the electric field, for example, for a charged particle, and the gravitational field, actually, which yields eventually the force on the particle. now the field produced by the particle is not a local entity satisfies an ordinary equation sorry a wave equation and it has a non-local aspect it has non-local aspect that's why it does not obey the equivalence principle in this naive way but I must say that I had arguments in other people in Israel this issue which I thought of course it's trivial that so the issue is this is a charged particle sitting here on the table does it radiate? Of course not it's static but it is accelerating because we have gravitational field it's accelerating so it should radiate I said of course not it's static how about the equivalence principle I said oh it's not relevant because I define radiation by taking a large sphere around the earth and this thing and there is no radiation it's not a local problem but then I realized that this is true but one could take another position one could define a radiation so the particle is very small

30:00 then one could construct so if I define radiation according to flux of energy through a sphere I could take the sphere Not to include the earth and everything, but a local sphere here, which is much larger than the particle. So it will be the wave zone for the particle. And then if I let this sphere to free fall, then it will radiate. So, only recently, because of these dumb arguments, I realized that one could take a point of view in which the equivalence principle works pretty well for radiation issues if one changes the point of view about how to define radiation. However, it's not relevant to the self-force, because self-force, I think, is an issue which does not depend on... So self-force and radiation are not the same, actually. They are related in many cases, in many examples, but actually it's not the same. So I believe that the self-force, you don't have an ambiguity in the definition of the self-force, the monadic self-force, for example. even though for radiation we do have some ambiguities I think yeah I guess you can say that there's a difference between the self-force and the radiation because one can make the argument that the self-force problem arose in the gravitational case long before anyone had thought of gravitational radiation reaction because Laplace did this calculation in the 1770s he was trying to explain the secular acceleration the speed of propagation of gravity is finite, then you would actually have a retarding force on the moon in consequence that would actually cause it to decay. What did you try to explain? So, you know, it's known that Halley was the first to notice that based on ancient eclipse observations, the length of the month seemed to be getting shorter, yeah. I see. Over the centuries. I see. And so they figured, well, the moon seems to be decaying its orbit a little bit and they couldn't explain it on the basis of

32:30 perturbations of just ordinary Newtonian gravity for a long time eventually Laplace did explain part of the effect but it also took, of course, part of the explanation is tidal friction and so on at any rate Laplace came up with what can be argued as the original gravitational radiation reaction calculation So he was retarded, so he thought about a sort of retarded field? Yeah, he actually took a corpuscular viewpoint. He says if the moon is attracted to the Earth, there must be some corpuscle being sent from the moon to the Earth. And if it's an instantaneous force, then the corpuscle will just go radially. I see, that's great. And if not, then there will be an angle. Did he assume that it is a speed of light? well he wanted to calculate how long the speed was well he said what he decided was and of course what's interesting is he had no notion of gravitational radiation he wasn't relating it to any kind of radiation he was just talking about the force what he decided was that to explain the effect that was observed by Halley the speed of propagation of gravity would be 7 million times the speed of life and he of course his 7 million times the speed of flight? But his effect was a border V over C, so he had a relatively large effect. And then he came up with an alternative explanation based on perturbation analysis. And he then determined that the speed of propagation of gravity was at least 100 million times the speed of light. So I gather that in the 19th century, actually, people thought that Laplace had more or less proven that gravity was instantaneous. and it wasn't until the end of the 19th century that people again started to influence by Maxwell's image and now we started to think and relativity started to think well maybe that can't quite be right anyway that made me think of something else if I can remember what it was oh so you were saying about this argument of the the self force and the particle on the table why doesn't it radiate so I'm curious just to know Who was putting the alternative viewpoint? So Amos Harpaz, an Israeli astrophysicist.

35:00 He was recently attracted to work on the cell force. So there is a problem on which I think there have been, I believe, hundreds of papers. does a particle in a uniform acceleration experience a self-force and of course Lorenz Dirac said that's not but people say well it radiates it must experience some force due to the radiation so Amosov started to work on this so we had some discussions and some arguments So, he claimed, I think he still believes that a particle, a static particle in a rotational field, like this thing, that if it's charged, it does radiate. And I disagreed, of course, but eventually I came to the understanding that it's sensitive to how one defines what radiates and what does not. So everybody says, hey, it's not a problem. Take a detector, a radiation detector. Okay, how do you construct it? And what road line do you give it? Because, you know, you take a static char and take a detector and move the detector. I mean, even classically, it will detect radiation because the field is varying. So the issue of defining radiation, I think there are several ways to do it. with Amosar Paz, or discussion with Amosar Paz, which made me realize this. But again, the self-force I think is better uniquely defined than the issue of radiation. So you think it's easier to decide that the particle isn't experiencing a self-force? Actually, if it is charged, if it is an electric charge, I think it does experience it. Although it does not radiate according to my simple way of defining radiation is I think it wouldn't radiate but it does experience self-force actually there is a simpler example in which you just take a massive spherical massive shell and you put

37:30 so it is Milkovsky here and Shlavsky outside and you put a charge here so it does not accelerate, it is geodesic so nobody I think will claim that it radiate, it's static there will be a self-force which can be calculated quite easily. Only if it's here, it will not. But if it is here, there will be a self-force. This is a very clean example because this particle moves on a geodesic. Which direction is the self-force? I don't remember. My student, former student, you have so and once calculated it. But then he switched to biology. and he didn't publish it and they didn't we didn't publish it I hope at some point we realize that we have to repeat the calculation but yeah it's not because you know because of this gravitational field here the electromagnetic static field is distorted so here you don't have the 1 over r square decay but you have some additional piece then it's me is a very clear example showing the distinction between issue of radiation and the issue of self-force. Interesting. Yeah, it's both static and on a geodesic. So if the charged particle on the table experiences a self-force distraction, is that moving? So this one? Yeah. I don't remember. So, okay, it was calculated for a black hole, for a static particle in a black hole, Schwarzschild space-time. And Lior Burkow knows the answer, and Alan Weissman knows the answer. Lior Burkow knows the sign. It tells me, but I forget. That's good. I don't know what the sign is. I don't know. So in that case, also, there's no radiation, but there is a cell for us. I would say Also, one could claim that in some definition of radiation there will be radiation here. As you say, depending on how you define it. Yeah, but in the most simple way which I think one should define radiation, I'm not sure. That's the way I was used to think about definition of radiation.

40:00 Take it in a large sphere. So what large is? Arbitrarily large. And take it to be a static sphere. But here there is a distinction between the scale of the particle, which could be a point-like, and the scale of the black hole. So I could take a sphere which includes both, which is infinitely large with respect to both of them, or take one which is infinitely large with respect to the particle, but small compared to the radius of curvature. Then the answer will be completely different. that's I think a very interesting point to realize but I'm used to seeing that there is no radiation in this case so there is no radiation across the distance distance with respect to the gravitational system but maybe there is with the local battery then sure there is radiation as far as local field is concerned because you know a free falling observer perhaps will not feel the curvature of the black hole but it will see a charge accelerating and it will feel radiation I'm interested in this because I know that this was another one of these debates that was gone through in the 50s as well and Ted Newman told me of a meeting at which people were discussing this and according to him John Wheeler got up and asked What was the issue? To say whether they thought... So, you know, the example was, if I have two particles in one in each hand and I drop one and I hold one to the other, which one radiates? And he asked everyone to vote, and according to Newman, about half... It was about 50-50. So did they realize it was the issue of definition? I guess. Well, partly it was related to this problem that many people at the time thought, at the time also thought any particle following G-d6 including a binary system wouldn't radiate so this is connected to this issue that people thought maybe binary systems ought to experience these are gravitational charges or also for electric charges? Well they were thinking of gravitational masses and that is gravitational charges

42:30 so it's interesting to see that the same month many of the same ideas are still being debated And, uh... So, let me see... I guess maybe the last question is, so if you see it as being maybe 10 years' time to really work through the challenges and the self-force problem and working out radiation reaction for a particle and the curve-space-time, orbiting the curve-space-time. Do you see yourself pursuing the problem for that length of time? Well, you know, even at present, I'm not working intensively on the radiation reaction. I'm thinking a lot about this. And I am encouraging my students to work on this. But I am primarily working on other problems, on the singularity problems as a whole. But from time to time, I am doing the work on this. I believe I will continue to work on this style for several years. But I think part of the progress which needs to be achieved, part of it is rather on the technical or even computational level and I was never involved in this that's why I don't think I will be involved intensively in this part of the problem and as I said to you I think my feeling is that conceptual issues are much more clear now so on this level work to do but there still is a problem a work to do even on the conceptual level I do see myself working on this for more, a few years

45:00 I was going to ask in fact if you see the contrast in styles of working between different areas of relativity if you like as you say you work on problems of singularities and black holes Some of the other people here have worked on similar issues. Is there a contrast in styles between the methods needed to attack those problems? So is there a contrast? Between the method of working or the style of working that's needed in looking at issues, problems of singularities and so on, and these problems of radiation reaction? or is it, for instance, you mentioned that in the case of this radiation reaction problem, you know, numerical methods may be increasingly important. Yeah, I think in both areas there is important. There is a need to do numerical work. People are starting, I think, to do it more and more in both areas. I see maybe the similarity between the two areas. Yeah, they're not that much different. Is there an overlap between the people working in the different areas, or is it mostly different? I understand. I mean, the actual people who are working in the two areas, is there a considerable overlap between them? Are there many like yourself who are working in both, or is it mostly different? Let's see. So I think, so there are much more people working on gravitational, on radiation, gravitational radiation issues than on singularities, on internal black hole singularities. But I think among the people which went, which spent some time in Caltech recently, then there is an overlap. So Eric Poisson and Patrick Brady and now Leo Burko and I think in terms of Caltech community then there is an overlap but besides this I don't see an overlap. yeah i'm just interested in this idea of you know different communities of

47:30 physicists and whether there's differences between the way that they that they work in that probably there is a difference but i nothing nothing that i can identify very clearly yeah there may not be be a big difference that's interesting too i think I don't know how to identify the difference. There may not be any. Of course they both come I suppose from within a broader relativity tradition anyway. That's right. And both types of problems can be thought of in my view a very clean problem very fundamental problems. So this is, I think, common to both, as opposed to many other problems, like the PTA, maybe in other areas of walking. These are two problems which are very clean. You can define them in very clean ways, sort of isolated, unlike other problems as physics, which everything is related to many, many other problems and issues. These are very problem which can be very cleanly isolated formulated of course in the practical context of binary systems it's not just radiation reaction the self force problem is much more than this self force is just a toy model but in terms of this problem of self force I think it's a very clean problem and it's the same I have a same feeling about the singularity problem problem so that's why they're both attractive to you for me I know I'm attracted to this type of problem which are not you can work quietly on a problem without being influenced by many other pieces of you know you do not have to worry too much about new information about the structure of this star, of another star, of metallicity. You know, I just give an example of in astrophysics, everything is influenced by many, many factors. Here, both problems

50:00 you know, you can focus on it and you can isolate yourself from all the noise outside, concentrating on that. That's what I like in these two problems. And more generally I think in general relativity. and they are classical both are classical problems right, so that's attractive as well for me it's very attractive I mean, classical is the same classical physics as opposed to quantum physics but also the self-force is also a hundred years old problem so it's classical also in this sense that's right, it's a very long standing problem that's interesting well thank you very much Thank you, it's very interesting.