Interview with the 2005 Nobel Prize laureates in physics, Roy J. Glauber, John L. Hall and Theodor W. Hänsch, 6 December 2005. The interviewer is Joanna Rose, science writer.

Dr Hänsch, Dr Glauber and Dr Hall, welcome to this interview and welcome to Stockholm. My congratulations to the Nobel Prize. I thought that perhaps we could start from the beginning, and I’d like to ask you how did you start in science? Dr Hänsch.

Theodor W. Hänsch: I grew up in the city of Heidelberg in a street called Bunsenstrasse, named after the chemist, Robert Bunsen, and we lived in the house that at one time had belonged to Robert Bunsen so as a child, being very impressionable, as I felt that being a chemist must be something important, because you get streets named after you. I asked my father what Bunsen had done, and he, the next day, brought home a Bunsen burner, or Bunsen burner, one of these gas burners that we hooked up to the gas stove in the kitchen. He would put table salt into it and the flame would turn yellow, and my father, who had worked at a pharmacy during the First World War, he knew other powders that one could buy in the pharmacy that would make the flame look red or green. He explained that the atoms have a characteristic colour that one sees there. So that’s something that stirred my interest and my fascination in light and atoms, at the age of six or seven.

Oh, pretty early. Hard heritage from Heidelberg, yes? Doctor Glauber, do you remember your first …?

Roy J. Glauber: Well, I don’t know. I started out thinking I was going to be an artist. I liked to build things. I certainly liked to draw things, but I spent too much time thinking about what it was I wanted to draw, so I began to have suspicions that I might never become a real artist. I’d spend too long pondering what I was going to draw. I began building instruments, a device that polarises light by using black mirrors, because the Polaroid material wasn’t available. I had a certain interest in astronomy and saw in the Encyclopaedia that by putting lenses together you could build a telescope. I put those lenses together alright, but they made terrible images and decided there must be a better way of doing that, so I began grinding telescope mirrors. I was doing that and building big telescopes by the time I was 11 and 12, and then taking pictures through them, and then deciding the optics involved, the physics was more interesting than the astronomy, in fact. The things you could see were all a little disappointing. For altogether different reasons, I began learning mathematics more seriously in intermediate school, so I had a long head start when I went to college on mathematics, and it developed from there. Then the wartime had started, the professors were going away, they announced they were giving graduate courses for the last time in the duration of the war. I took most of those, and I had what amounted to a post graduate education by the time I was 18.

Very early, I would say.

Roy J. Glauber: At that point I went to New Mexico to the nuclear Manhattan project.

To Los Alamos, yes.

Roy J. Glauber: There I worked on the theory division for two years.

Dr Hall, do you remember your first …?

John L. Hall: I had an uncle who perhaps had friends that sold flashlight batteries at a reduced price, I’m not sure. Anyway, I got 20 flashlight batteries and lots of wire and bulbs and some kind of doorbell mechanism, so that was fun. Then I found that if you didn’t use the light bulb in the circuit but only the wire, it becomes red hot. That was pretty nice, because the insulation was burned by that, and then we could make … so what does it burn, so then I studied about that. Then I found that cellulose was just one thing, part of that, and that led me to research on other things, such as black powder, which my father helped to buy the components for. By the time I was in eighth grade I was a researcher for minor explosives, so I feel right at home in the Nobel company.

Did you ever think about becoming a scientist and winning the Nobel Prize?

John L. Hall: No, I have not considered that until the middle of the night a month or two ago.

Oh, I see. Quite shocking.

John L. Hall: I thought of my research, and the national laboratory as being not quite the kind of science and glamour things which would be picked up by this, but it was nice that over 44 years, I think, one little tool with a connector can connect to the next and the next, and finally it can do something. So, that was nice.

Did you think about a Nobel Prize? That you can make science so great? Did you ever dream about a Nobel Prize? Did you think that as a scientist …?

Theodor W. Hänsch: Winning the Nobel Prize?

Yes.

Theodor W. Hänsch: I was at Stanford University when in the early 1980s, -81, Arthur Schawlow, my mentor and friend and colleague for many years, when he was awarded a prize for work that we had done together, so at least I felt that the kinds of research that we’d been doing is not so far away from what they give Nobel Prizes for, and maybe I started to think about things like that. But getting prizes is not the reason why we do science. The joy comes from inventing things that allow you to do what could not be done before or from understanding something for the first time that nobody else has understood, and that’s the real reason why we enjoy science so much.

How do you find the questions or the problems to make your research about?

Theodor W. Hänsch: I think it’s different for everybody. I like to play, I like to do experiments that require nice toys. I’m also very curious, so constantly new questions come to mind, and I don’t have a very long attention span. I follow my inclinations and every once in a while one finds something good.

How often do you not?

Theodor W. Hänsch: Oh, many, many times.

John L. Hall: But just one good toy left.

Are you also the kind of scientist that are playing around?

John L. Hall: Oh, I think so.

With toys?

Roy J. Glauber: There’s an awful lot to do with play. I try and explain that in the elementary class I teach, that it looks as though we are just a bunch of kids doing demonstrations and playing with toys, but anybody who follows the psychology knows that playing with toys is the way kids learn about the world. It’s certainly the way we learn about the world. As far as the Nobel Prize is concerned, I’d like to add to what Ted has just said. The notion that one engages in science as a competition to win the prize is absolutely ridiculous, an even nasty, because what it does is to make failures of thousands of people who are doing the most constructive things they can. It’s, to some degree at least, accidental who wins the prize. It’s correlated, perhaps, with talent, but it’s correlated with contribution and everybody is trying to make contributions.

Yes. But not everybody who’s playing is a good scientist, so I wonder, you have lots of students and young PhD researchers, can you recognise the talent, the exceptional talent? Can you see that?

Roy J. Glauber: We’re pretty good at. If you want to know where there’s competition, it’s to sign up the talent. It’s to gather in those talented people.

How do you see that people are talented in science? How do you recognise that?

Theodor W. Hänsch: Sometimes in a ten-minute conversation, it’s just what makes them excited, what are the questions that they ask. It gives one a feeling. It’s not a science, it’s an art, but after dealing with so many young people I feel that I can decide rather quickly whether this is somebody I like to work with.

It’s kind of intuition you have.

Roy J. Glauber: The point that Ted is making that it’s an art is absolutely right. Too many kids in school get the notion that science is deductive, and deductive science is almost never creative. Real ideas arrive via intuition, via guesswork, and we’re guessing all the time. You can quickly judge people who make interesting guesses.

John L. Hall: I think also the talent that people have is a little bit involved with their personality. Some who are quiet and maybe have not shown up so nicely on the exams turn out to be just power packed researchers when they’re in the lab, because some person’s mother owned a greenhouse, and then there’s water pipes to take care of and motors and pumps and things, and it’s where you learn stuff about that. If you have a little bit of family resources then a boy can go to auctions and bid on scientific apparatus. In one case, a frequency counter which one of my students had bought from the surplus, was better than anyone which I had in the laboratory, so his thesis was done with that. You can tell these young people just have a sparkle.

So it’s very important to have the right personality somehow.  Can you tell us, Dr Hänsch, how you for the first time got the idea of the frequency comb?

Theodor W. Hänsch: It’s a long story. Way back in the 1970’s at Stanford University we had a mode-locked picosecond dye laser and it already made a frequency comb that could be used to measure the distances between two spectral lines, but it was not a frequency comb that could be used to compare and upgrade the frequency to a microwave frequency and to make an absolute frequency measurement. That was just the technical state of the art, but in that context, together with my student, Jim Eckstein, we already worried about the phase slips from pulse to pulse, the shift that you could not really tell from the repetition frequency of the laser where the comb lines are, what one would need to do to find that out, but then we left it at that. Then in the early -90s, suddenly making ultra short pulses became very much easier with the invention of Kerr-lens mode-locking and titanium sapphire lasers. Suddenly, what required several PhDs, you could buy as a box and turn it on. We actually bought a femtosecond laser in 1994 for our laboratory, with the intention of trying to measure optical frequencies, but we didn’t go after it very seriously.

What I think was the point that triggered my intense interest was one afternoon in Florence, Italy, I was working with a young researcher, Marco Bellini, who had an amplified femtosecond laser and he would focus the light into a crystal plate and white light would emerge, which is common and used in many ultrafast laboratories as a broadband broad beam, and so I asked the question, and together we answered it. What would happen if we take our laser beam and split it in two and focus at two different spots? These two white light sources, will they form interference stripes? Is this rendered white light or is the face of this white light linked to the laser? We did that experiment, we saw beautiful interference stripes and that made me realise that it should be possible to make a train of pulses of white light pulses that have a comb spanning more than an octave, and then it’s easy to figure out where the comb lines are. At that time I wrote down a five or six page detailed proposal for what I had called a universal optical frequency comb synthesiser, and then we went to work with Thomas Udem and a little bit later Ronald Holzwarth to try and turn it into reality.

This was this crazy idea, as I understand you Dr Hall, you didn’t believe that it was possible to realise?

John L. Hall: Another way to describe Hänsch idea or of Chebotayev’s explanation was something like, have something which produces white light and then do it 100 million times per second. Then no matter what’s the character of this, if it works the same each time then this light will have some comb character in it, and then, as Ted’s already mentioned, some issue of what the phases are. Combs have been used for a long time, I did my thesis with a comb. It was a commercial system produced for World War II, because there would be people on two teams and they would like to listen each to their home base and which frequency do I adjust this variable capacitor to? There would be a crystal and its harmonics could be found then by tuning the receiver.

In radio waves?

John L. Hall: In radio waves, and now I don’t know whether I’m on number 56 or 57 of the harmonics, but if I have another crystal, with just some offset, then it can be there, and so I can figure out which harmonic I have, and finally, with some interpolation, so now I have to only move an interpolation a much smaller distance. This development of crystal oscillators that had stability but only at low frequencies, in some of the radiation lab books it’s explaining why this was such a decisive circumstance for one of the teams.

How was your proposal received, then?

Theodor W. Hänsch: I did not publish it. I had it witnessed by a few friends, and then showed it to some experts, including John and I think at that time John felt that was a goofy idea, he didn’t want to study it carefully.

John L. Hall: I agree with that.

But you were somehow sure this will work.

Theodor W. Hänsch: At that time I was sure it would work.

In spite of John’s scepticism.

Theodor W. Hänsch: John is smart, so I think a year later he no longer felt that was stupid.

John L. Hall: You just postulated things that didn’t exist, and when those come into existence then OK, now you have the tools and they can work. So that’s why come always to discussion about what tools do we have at hand, and why an increment in that space is the most powerful thing about doing science.

But it’s not always … What I’m thinking about is that you have more ideas that you can realise and some of them are more crazy than the others, but maybe it’s good to have crazy ideas.

John L. Hall: Absolutely.

That’s why you are here. I understand. I have another question. You’ve been at Stanford, as you mentioned before, and ten years ago you moved to Europe. You came back to Germany.

Theodor W. Hänsch: It was 20 years ago now.

Almost 20 years ago. Can you compare the style of doing science in the States and in Europe? Is there similarities or differences?

Theodor W. Hänsch: In some ways it’s a different style. Of course, science is so international nowadays that we all meet at conferences, we visit each other and it’s no longer isolated, the US and Europe. Nevertheless, of course their styles are a little bit different. At Stanford, of course that was a hotbed of the creation of Silicon Valley, a lot of interesting people and a lot of excitement about innovation. In Germany, at least, I think innovation, people are at first sceptical, they feel life is good the way it is, why do you want to innovate? The spirit is, at least of the general public, is maybe not so enticing for doing research, but of course we live in our own world, in our circle of students and colleagues, who share our excitement. One thing that actually might be better in Germany, at least in the Max Planck Society, is that for basic research we don’t have to declare what we are going to do next year or the year after that, but we have a stable level of support, quite comfortable, that makes it possible to pursue also risky long-term research projects.

And this is the difference to what is the American …

Theodor W. Hänsch: This is the difference.

Do you recognise that? You have short-term financial?

John L. Hall: There is maybe less long-term investment made in the United States now, but it did exist. The Science Foundation was particularly visionary, and as well the Office of Naval Research, in accepting people and I think they are graded not by what they propose, but after some experience the men with responsibility for the money and the researcher on the other side have a respect and the money is transferred on the basis of thinking this guy hasn’t done anything fantastic for two years, but I guess he’ll do another thing like he did four years ago, and let’s give him money.

You two know each other well, you three, actually, but I thought about you’re colleagues but you’re also competitors in science. How do you deal with that? Do you have secrets?

John L. Hall: Not so much.

No? You are a theoretician from the beginning, and I have a question. What is the relation between the theory, the quantum theory of light, and your experimental work? Where do you meet?

Theodor W. Hänsch: If you would like me to say something from my perspective. Roy is the one who explained how light can behave like a classical wave. Of course, we know since Einstein and Planck that light is made up of quanta of photons, so how can a laser wave behave like a classical wave? Roy was the first to explain that in mathematical terms, and to show that there are very many intriguing aspects of light that are lost if you think of it as a classical wave. Many correlation effects.

John L. Hall: And as well, if you think of it as only photons, you need them both.

Roy J. Glauber: Well, go ahead. You’re saving me a lot of trouble.

But this is a problem with the light, that you say it’s both waves and particles.

Roy J. Glauber: That’s been true for a long time. Historically the wave picture turns out to be very valuable in designing instruments and all optical instruments, the whole world of optics, if you looked at the American Journal of Optics for example, you’d only find applications of wave theory through the 1950s. In fact, if anybody were so daring as to send them a manuscript with light behaving like particles, they would probably not print it, they’d send it off to a journal that deals with atomic physics. So there’s been a kind of renaissance in optics as well, as they’ve come to deal with problems, that is the theory of problems, involving many quanta at once. They usually had stuck to the simpler problems involving only one quantum at a time, or two quanta at a time.

By the 1950s, there were some new experiments involving two quanta at a time, in which funny statistical results began to appear that weren’t easily understood, and I was intrigued by those. At the same time, the laser was being developed and the laser was developed on the basis of what we call semi-classical theory. It didn’t even take the theory of light quanta as fully seriously as it should have. If they had done that, we might still be waiting for the invention of the laser. But these things, as we’ve said, are often done more than half by intuition, and using half-baked theories. But sooner or later, something like the laser was going to make altogether new experiments possible that you could never begin to explain by the wave theory alone, and indeed that has changed the whole field. The optics journals are now full of papers in which light does consist of photons.

It’s strange anyhow. Can it be that light is neither particles nor waves, so to say, it’s something that we just don’t have any idea …?

Roy J. Glauber: I think that’s true.

John L. Hall: I think you’d need an operational discussion. What is the box you’ve put on the table that’s your detector, and if it registers clicks then you’re going to detect a photon character, and if it measures frequency then you’d see some other parameter.

Roy J. Glauber: It does depend on the experiment you perform, but that’s the fascinating thing about this entity. It has different faces, if you like, that it shows to different kinds of experiments.

Theodor W. Hänsch: The wave aspect and the particle aspects are things that we know from our everyday world. We know what are waves, and sound waves, and we know things like objects, but you’re certainly quite right, that the quantum world does not really conform with these classical concepts that we depend on, from which angle we look. It mimics particles or waves, but it’s something different altogether.

Something we don’t understand.

Roy J. Glauber: No, we do understand it …

Theodor W. Hänsch: We have formulae, we have equations, but we don’t know why these equations apply.

John L. Hall: It’s still easy, even for professional scientists, to get to a place that they’re a little surprised at what the result is, because one has correlation experiments where there is huge distance between them, and you wouldn’t have thought that could happen.

Theodor W. Hänsch: We believe that there is at least a mathematical formula that in principle should describe everything, even though very quickly things are so complex that there is no computer in the world that could make predictions based on these equations, so we have …

Roy J. Glauber: I would say we do understand them. The notion that this is an abiding mystery because it’s unfamiliar in daily life is just wrong. We understand very well by dealing with these things in relatively simple mathematical terms, but it’s a kind of behaviour that one simply doesn’t have in everyday life. You have an entity which you cannot describe, uniquely, either one way or the other, and what you see depends on how you look.

John L. Hall: Let me offer you a challenge, then. How does that differ from religion?

Roy J. Glauber: Oh my! Well, for one thing it’s, I don’t know quite what you mean by religion, because in a sense mathematics is a kind of religion to me, but it’s verifiable, it works. There’s an awful lot of religion, I’m afraid, that’s not that verifiable.

If we go back to science, do you have any favourite problems that you would like to have solved, before you quit science? What is the dream ahead?

Theodor W. Hänsch: Something where we might actually have a chance to find an answer with the frequency combs, is whether fundamental constants are truly constant, or maybe it’s slowly changing with the evolution of the universe.

Very slowly.

Theodor W. Hänsch: To my mind, it is a deep problem. It wouldn’t matter for everyday life, but for understanding how the universe works, whether God had any choice in the constants, whether He had to work with some …

Roy J. Glauber: What he’s saying is that these … He’s not saying what he’s looking for, he’s just saying these are marvellous hunting tools, and that’s part of the historic progression of science.

John L. Hall: When one had first a microscope and looked in water and became aware that there was some life form that we didn’t know, Galileo found the bottom of Coca-Cola bottles and looked and found that another planet had itself a moon, and that revolutionises the way one thinks about things. I think tools is where it’s at, and as far as what to search for, one of the things which physics doesn’t address is why the numbers are what the numbers are, and it’s just weird that if you talk about energy that’s associated with electric effects, and you talk about energy that’s associated with a charge that’s moving, the ratio between that is 137.0339997. Why? Where did that number come from? And what about energy that’s associated with the rest mass of some nuclear system? That’s some other number. Where do those numbers come from? We haven’t a clue.

One answer would be that they are changing so just now they are …

John L. Hall: That’s the business for the next years, is to look hard at everything that we can.

I’m looking forward to you. Thank you very much for sharing your time with us, and I hope you will have a great time in Stockholm. Thank you.

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