Nobel Prize Conversations

“There’s always a little bit of luck involved because you never know what will happen”

Hear physics laureate Pierre Agostini describe how he found his love of science: ”It was only when I started doing research that I discovered the fun of physics.” Together with podcast host Adam Smith, he talks about multiphoton ionisation, Planck time and contradicting Einstein. 

Agostini also tells us about how his life has changed after being awarded the 2023 physics prize and how it has taken him some time to get used to his new role as a Nobel Prize laureate. 

This conversation was published on 11 July, 2024. Podcast host Smith is joined by Clare Brilliant.

Below you find a transcript of the podcast interview. The transcript was created using speech recognition software. While it has been reviewed by human transcribers, it may contain errors.

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Pierre Agostini: When I had the first announcement, I felt bad. There must be something wrong but with the time I got used to that. Anyway, I don’t think I can do anything about it.

Adam Smith: There’s always something especially magical about listening to Nobel Prize laureates. Describe how truly surprised some of them are to receive the call from Stockholm. In this conversation with Pierre Agostini, I got a real sense of his playfulness of spirit as well as an introduction to some pretty deep physics. There’s a lovely moment, for instance, where he describes the panic that set in when he wondered whether he could actually reproduce his groundbreaking result. Then we get to talking about the fact that time is not perhaps infinitely divisible, which is a pretty mind bending concept. Like many of these conversations, it leaves me wondering whether there is any age limit to scientific creativity, which opens up the whole difficult question of how you distribute the jobs between the young and the older. That’s something we go on to discuss. I leave you in the hands of Pierre Agostini. 

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Clare Brilliant: This is Nobel Prize Conversations. Our guest is Pierre Agostini, the 2023 physics laureate. He was awarded the prize for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter. He shared the prize with Anne L’Huillier and Ferenc Krausz. Your host is Adam Smith, Chief Scientific Officer at Nobel Prize Outreach. This podcast was produced in cooperation with Fundación Ramón Areces. Pierre Agostini is emeritus professor at the Ohio State University in Columbus, Ohio. He was a researcher at CEA Saclay in Paris from 1968 to 2002. He speaks to Adam about contradicting Einstein, how forced retirement can sideline scientists at the top of their games and the strange things that happen as you approach the very smallest units of time. But first green cards, transatlantic flights, and feeling at home in two places at once.

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Smith: You live in two cultures. You live in Paris, but also travel to Ohio State University. 

Agostini: Yes, I just spent half of a month half in Iowa and I’m just back now. 

Smith: How do you find this co-existence of living in two countries at once? 

Agostini: That’s a little bit difficult sometimes, especially because I am on the green card program. With the green card we are supposed to spend at least six months in the States and I don’t do that. Each time I go back I have to explain that to the officer, but otherwise it’s okay. I am very used to both places and I feel at home both in Ohio and in Paris. 

Smith: It’s nice to have two homes from a scientific culture point of view. Do you find that there is a different approach at all to science in Paris and in Columbus, Ohio? Obviously science is universal, but the differences are interesting. 

Agostini: Especially in this circumstance of the Nobel Prize. They had organised this month in Ohio state full of ceremonies. It was really very different from the atmosphere from Paris. The people at Saclay are okay, but they’re not doing the same thing as those guys. 

Smith: Maybe the French are more used to having Nobel Prize laureates in optics.  

Agostini: Perhaps, yes. That’s the case. In Ohio they told me that I am the first one. So they celebrated a lot. 

Smith: I’d like to talk about your scientific upbringing, how you started? You were born in North Africa, but you went to a military school in France. When did you discover that you were scientifically minded? 

Agostini: When I moved from Tunisia to military school in Marsailles, I had to choose the scientific option and there was no other solution that this school. I had to turn to science. Although the year before in Tunisia, I was rather classical. I was very much for classical studies like Latin, French and stuff like that. I had to do it and it was okay. I liked it, but my previous life was something else. I turned to science because I had to and it was really a discovery. I really liked it. I liked especially the math program and when I went to Marsailles to start my university classes. I wanted to do math, but I was discourage having people say, oh no, it’s too difficult. You will not make it in math. I took physics and probably physics was just like any other matter. It was only when I started doing research that I discovered the fun of physics. 

Smith: It was a very happy circumstance that you were pushed in this direction. It wasn’t so much a choice. 

Agostini: Yes, I agree. 

Smith: But you never gave up on your interest in the humanities, did you? You read a lot and it’s always been a big part of life. 

Agostini: Yes, I still do that when I can. My schedule is packed. I don’t have time to read a lot, but I try to.  

Smith: That’s a bit sad that the Nobel Prize stops your reading.  

Agostini: Yes, it does after all.  

Smith: Can you describe what the excitement was of discovering the joy of physics through research when you were first exposed to it? 

Agostini: First of all, there was some excitement because the subject I started working in multiphoton ionisation and our interaction strong laser fields with matter was nowhere. There was almost nothing on that in the end of the 60s and my PhD advisor, I told him, okay, I’m going to work on multiphoton ionisation. He said don’t do that. He did not believe in this field. The intensity was the parameter that doesn’t matter in those studies, that was the first thing. Then there was the excitement of discovering things that nobody else has seen before. 

Smith: Why did you choose this field that you were advised not to go into? 

Agostini: That’s an interesting question. There was someone from Saclay, actually my old friend Yves Contie, who was a theorist at Saclay at the time. He sort of scouted physicists in Marsailles where I had graduated two years before. He was trying to find people to work on the program. I had a job offer before finishing my PhD and I just took the offer. 

Smith: Pragmatic, yes. 

Agostini: Yes. 

Smith: This work is of course all about the interaction, as you say, of light and matter. It is extraordinary to reflect on the fact that you discovered this phenomenon that was completely unknown above threshold ionisation. 

Agostini: Yes. 

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Brilliant: I might need a bit of help here, Adam. You talk about multi photon ionisation and above threshold ionisation, and I just wondered if you could explain what those are. 

Smith: Yes, I’ll try. The ionisation is referring to the idea of ejecting an electron from an atom, creating an ion. In order to do that, you have to give the electrons sufficient energy to escape from the atom. There’s a certain threshold of energy it needs. Above threshold ionisation just refers to the idea that you are chucking an electron out of an atom with more energy than it actually needs to make that journey from being part of the atom to being expelled. Then multi photon ionisation is one way that can happen, that an electron can absorb the energy not just of one photon, one particle of light, but several on its journey out of the atom. That way it receives more kicks of energy from these photons and is ejected with more than the threshold energy that it needed to get out. 

Brilliant: What’s the consequence of it having more energy? 

Smith: It has a different energy spectrum so that the ejected electron then occupies a different set of energy levels to what it would have if it had received less energy. That has all sorts of consequences that have opened up possibilities such as the creation of these very short pulses of laser light. 

Brilliant: That’s very helpful, thanks. I do feel like I understand it a lot better now. 

Smith: I think it’s very superficial understanding if you’re listening to me, I’m afraid. 

Brilliant: I’m going to test you again now because he mentioned strong field physics and I wondered if you could explain what that is as well. 

Smith: Yes, that’s just the term that physicists use to refer to this field of study of the interactions of laser light and matter. It’s a field that studies ultra-fast processes where light is interacting with matter. 

Brilliant: I think this research extends the work that Einstein did on the photoelectric effect. How does it do this? 

Smith: Einstein proposed an explanation for the observed photoelectric effect, which is that if you shine light at a sheet of metal in a particular way, you can get electrons ejected from it. He proposed that the energy that was necessary to make that happen was being transferred to the electrons through individual photons, which had quanta of energy. They were giving their packets of energy to the electrons. It was his explanation of the photoelectric effect, which was actually the thing that the Nobel Prize Committee identified when they awarded him the prize in 1921. That is the basis of what is going on here. But with multi photon ionisation, it’s not just one photon that is transferring the energy to electron is more than one. It’s absolutely an extension of Einstein. I asked Agostini about following Einstein’s footsteps, let’s listen to what he had to say.  

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Smith: There’s such a tradition here, there’s such a legacy. It must have been so thrilling to feel oneself part of that.  

Agostini: Yes, we were contradicting Einstein a bit with multi photon ionisation. Because then it take several photons to ionise not just one. So the Einstein voice has to be modified a little bit. ATI was one step further. It was interesting because when we had the first result, I remember it was during the summer and then we wrote the paper and it was quickly published. It worked for a long time. We were really worried because we could not reproduce that result. It took really long time because we changed the data, we changed the electron spectrometer and so on. The experience was completely different. It took us some time to realise what we should do to see the ATI again. It was really the worst time in my life because the paper was published, we could not reproduce it. 

Smith: It would’ve been extremely embarrassing to have contradicted Einstein and then been shown to be wrong.  

Agostini: Yes, absolutely. 

Smith: Gosh, I can imagine you remember the sense of relief when the second result came. 

Agostini: Yes. We could sleep at night again. 

Smith: Presumably the environment you found working at Saclay was a very productive and supportive one. I’m interested that eventually you had to leave Saclay. What prompted the move from Saclay? 

Agostini: Oh, you mean in 2002! That was just the rule for retirement. If you were 60 and above, you had to retire. There was no choice. The head of the department had to retire this way. The head of my group had to retire this way. There was no choice. The rule doesn’t exist anymore and people can work until 70 or something. But back then, it was the rule and I couldn’t avoid it. 

Smith: It seems a terrific waste of talent to let people go so young. Of course, as you say, it’s changed. What do you think about that? What do you think about the balance between people who have long track records and the new people coming in? How do you get that right? 

Agostini: I’m not sure what’s the best solution, but certainly being sort of retired at 60 or 61 for me was probably the worst thing possible. I suppose there is always more ways to hire young people without retiring old people. 

Smith: Sure. In your case, you’d only quite recently done the work, which is cited by the Nobel Prize Committee in their award producing at a second pulse trains and defining them. 

Agostini: We did this kind of experiments. There’s always a little bit of luck I think, involved because you never know what will happen. 

Smith: Were you very surprised that your experiments yielded that pulse train? 

Agostini: We’re happy. The surprise was first because we didn’t find anything at all in the results. After we analysed, we divided by the total crowns and then the oscillation, almost as a miracle, appeared at the time. We were happy, we were expecting that. It was not a surprise. The surprise was because at the beginning we didn’t see the oscillation.  

Smith: Right. Do you remember what you did when you realised that you were seeing the oscillation? 

Agostini: I think we wrote the paper very quickly. 

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Brilliant: Adam, I know we’ve talked about attosecond in previous episodes, but I think I could do with a little bit of reminder. What is an attosecond? 

Smith: An attosecond is 10 to the minus 18 seconds. That’s a zero followed by a decimal point and 17 zeroes in a row before you get to a one. It’s a very short space of time. 

Brilliant: I can remember telling my kids about it because I just thought it sounded so cool. What was the specific research that Pierre Agostini did a round attosecond and that he was awarded the prize for? 

Smith: He got the prize for producing the first pulse of light that was in the attosecond time domain. He produced a pulse of laser light. In fact, we had a little sequence of pulses, but the little sequence of pulses lasted just I think 250 attoseconds. He also was able to demonstrate that that was the case, which was far from trivial. But this was the beginning of the race to get shorter and shorter trains of pulses of laser light. Now they’re down to a few tens of attoseconds in length. 

Brilliant: Amazing. 

Smith: Basically the production of incredibly short pulses of light, 

Brilliant: Which at the time that must have been quite an amazing thing to sort of realise the ability to do that. 

Smith: Yes. I think what was opening out was a whole world of processes which last roughly that long. Suddenly you can isolate them and see them as never before. 

Brilliant: We’re about to dive into some fairly complicated topics that I haven’t heard of before. I wondered if you could help explain some of them. 

Smith: Sounds scary, yes. 

Brilliant: This is not a test. The first is a term positronium. 

Smith: Yes, positronium is like hydrogen, which is an electron orbiting a proton. But in the case of positronium, proton is replaced by another positively charged entity, which is the positron, the anti metaparticle to the electron. Since these two annihilate each other when they meet, the positronium is very short-lived. Fascinating, but only briefly there. 

Brilliant: I see. We’ve talked about attosecond, but what are zepto and yocto seconds.  

Smith: Things tend to go down in threes. If you go down a thousand folds from an attosecond, you get to a zeptosecond, which is 10 to the minus 21 seconds and another thousand fold down will take you to the yoctosecond 10 to the minus 24 seconds. I believe a zeptosecond is the shortest period of time ever measured so far. A yoctosecond is approximately at the time it takes light travel across the distance of the atomic nucleus. We’re down to really, really, really small things. Tiny amounts of time. 

Brilliant: I can’t get my head around. It’s such a short amount of time. 

Smith: Physicists have it all. They get to study the shortest things and also the biggest things like the cosmos. As you said, it’s all pretty cool. 

Brilliant: Yes, very. Does the Planck time help us to understand this better? That’s another term I’m not so familiar with. 

Smith: Think that Planck time takes us into dimensions of beyond our understanding, but the Planck time is the shortest time that anybody can talk about. It’s defined as the time it takes light to travel the Planck length, which doesn’t help you very much. But the Planck length is one of those fundamental constants defined by Max Planck over a hundred years ago when he was seeking to establish a set of constants that were based not on things that we measure around us as humans, the meter and the second, but rather units based on the fundamental constants of the universe, like the speed of light. The Planck time equates to 10, to the minus 43 seconds. Physicists tell us that there’s no point in talking about anything shorter but this was something that I was interested to explore with Agostini, whether that’s the end of time or whether you can go still lower. Let’s listen to that conversation.  

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Smith: At the beginning of your Nobel Prize lecture you ask a question, which I suppose is one that physicists debate often, but is not something that most people think about, which is; is time infinitely divisible? The attosecond is as people keep saying very, very short, but can you just go shorter and shorter and shorter? Is there a limit to how much you can divide time up? Could you talk about that a little because it’s such an exciting question. 

Agostini: Okay. After 2001 and the attosecond demonstration, there were a few papers, five or six papers from a lab group in Heidelberg trying to figure out how we could make even shorter pulses than attoseconds, yoctosecond and even later zeptosecond were in the picture. The solution they proposed is to use positronium, or stuff like that, to replace the atoms in the harmonics. It’s not completely obvious how could do that practically, but attoseconds is certainly not the limit and at least up to or down to zepto or yocto it’s certainly not physically impossible. On the other hand, there is a limit. The limit is the Planck time and conceptually you can’t go beyond that limit. That limited is 10 minus 44. We are still very far from it. 

Smith: I know that the Planck time is the time that light takes to travel the Planck distance. You can define the speed of light by that. But please, why is that the limit? 

Agostini: The Planck time is the limit as far as I understand it, because if you look at the spectrum corresponding to this short time. The spectrum that is the energy. The spectrum and the frequency and the energy is the same. This energy is of all the universe so there’s no way you can go beyond that. Then the time is 10 minus 44 seconds. 

Smith: I guess most people don’t think of time and energy being related, but this shows very clearly that they are. 

Agostini: Yes. 

Smith: It’s just mind expanding to think of it even though it becomes too complicated to think about very quickly.  

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Smith: Let me change direction a little bit, please onto something non-scientific. Just how it has been since the announcement of the Nobel Prize. Because I am sure that it’s increased the amount of demands on your time and you must be receiving invitations to be all over the world all the time. How do you cope with the increased attention? 

Agostini: First, let me tell you something. When I had the first announcement I felt there must be something wrong there. I couldn’t feel at all like being on the list of the Nobel Prize laureates, people like Serge Haroche. I felt that there must be something wrong somewhere. But with time I got used to that and I was always trying to figure out that, okay, there must be other cases where other people who didn’t feel like they really deserved the Nobel Prize. After that, especially after my trip to Ohio State and all the ceremonies and the things that we had there and meeting the president of the university, I felt a little bit more confident. Anyway, I don’t think we can do anything about it. 

Smith: That’s true. It reminds me of a comment made by a laureate I was traveling with last summer. An audience member asked him, does receiving the Nobel Prize make imposter syndrome better or worse? He answered yes and no. I guess most new Nobel Prize laureates wonder how they fit into the general picture of the list of people who’ve received it. At the same time, as you say, it also maybe has the effect of building confidence. 

Agostini: Yes. For some people it’s almost natural. If I think of Serge Haroche, I mean their life has been working in one direction and compared to them, what I did was not comparable at all. However, if you think that the prize was given not just for attosecond but for things that go back to the 70s then I have some reason to believe that it was a reasonable choice. 

Smith: It’s the development of an idea. It’s the realisation of a potential, isn’t it? 

Agostini: Yes. I’d say it’s an accumulation of affect time in a certain field. 

Smith: I understand. Coming to terms with this, it was one aspect of it, but then just the purely practical point of how many people want you and want your time, how do you handle that? 

Agostini: I am trying to fight for my time. I have to help leaders from Ohio and we are trying to get things in the reasonable limit. For instance, I’m declining, not most, but at least a lot of those invitations and keeping just a few. That’s one way of doing it.  

Smith: How does the family feel about your prize? 

Agostini: They are very happy. My daughter will give me the news that I got the Nobel Prize. She had heard about it on the internet or something. The others were happy and everybody was happy to see my grandchildren in Stockholm, especially my grandson. 

Smith: How old was he when he went to Stockholm? 

Agostini: He is turning 22 this year and she’s 24. 

Smith: It’s really fascinating to hear your reflections on it because, like it or not, it is a life changing event.  

Agostini: Yes, for a moment it has been a life changing event, and it’ll be like that until at least the end of this year. After that I hope it at least slowly return to normal. It’s difficult to manage. We still have problems with referees that with writing papers in Ohio, and I need five. 

Smith: It doesn’t make any difference to that side of life at all. Maybe it makes life even harder with referees now. I don’t know. It’s been so very nice to talk with you. I’ve enjoyed it very much. Thank you. 

Agostini: Thank you. 

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Brilliant: You just heard Nobel Prize Conversations. If you’d like to learn more about Pierre Agostini, you can go to nobelprize.org where you’ll find a wealth of information about the prizes and the people behind the discoveries.

Nobel Prize Conversations is a podcast series with Adam Smith, a co-production of Filt and Nobel Prize Outreach. The producer for this episode was Karin Svensson. The editorial team also includes Andrew Hart, Olivia Lundqvist, and me, Clare Brilliant. Music by Epidemic sound. If you’re interested in another conversation about the extraordinary things that happen as you approach singularity, check out our episode with 2020 physics laureate Roger Penrose. You can find previous seasons and conversations on Acast or wherever you listen to podcasts. Thanks for listening. 

Nobel Prize Conversations is produced in cooperation with Fundación Ramón Areces.

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