Interview with two of the 2004 Nobel Laureates in Physics, David J. Gross and Frank Wilczek, 9 December 2004. The interviewer is Joanna Rose, science writer.

Dr David Gross and Dr Frank Wilczek, my congratulations to the Nobel Prize. You have been waiting for a long time for this prize, how was it?

David Gross: 63 years!

Maybe not 63, a little over 30?

Frank Wilczek: I thought it was a realistic possibility for the last 20 years or so.

When did you realise that this was a realistic possibility?

Frank Wilczek: I think when the experiments really started to crystallise in the late 1970’s and early -80’s, once that happened I thought it was possible. But they’re very conservative; they want to see very solid experimental evidence. They were telling in fact over lunch today they really wanted to see the curve with the arrow bars, so it took a long time for the experiments to catch up with our theories.

What was a crucial experiment?

David Gross: There are many, it’s really been accumulation. It depends on who you talk to. Many of the original experiments were in 1974, enough to convince people, but then there were the discovery of jets which were really indications you could see quarks and then finally the discovery of jets where you could see gluons. Then, what Frank is really alluding to is the last, especially the last ten years with LAPP and then HERA where you have high precision tests, something I really never thought I would see, tests of detailed predictions and dozens of them to less than 1% accuracy, I still love to see them.

Frank Wilczek: David goes back a little bit further but even when I was a graduate student the concept that the strong interaction, this mysterious thing where the ideas were so vague and where you had the background of nuclear physics which has never become anywhere near as precise even now, that you would be talking about a few percent accuracy and these precise calculations just seemed completely off the radar screen, completely inconceivable. Just ridiculous.

David Gross: You’ll remember one of our mentors …

Frank Wilczek: Yes.

David Gross: … Sam Truman who told me about a month after we made this discovery and started to explore QCD [Quantum chromodynamics] he said, David this is a great theory and maybe you’re right but one thing I’m sure of it’ll never be proven.

What did you think about that?

David Gross: It scared me.

Frank Wilczek: Yes, at first.

It was scary.

Frank Wilczek: I certainly didn’t anticipate that the experimental situation would ever become so critical and precise. We didn’t realise everything at once or we didn’t realise everything ourselves. There was a big community effort to develop the theory and extend it so the concept of what was possible grew, as a function of time both on the theory side and the experiment side.

David Gross: Physicists are always optimistic, my explanation of that is simply natural selection. If you’re not optimistic you’re not in the field. You can only survive by being optimistic. But even optimistic physicists are prone to not to be able to imagine what good experimentalists are capable of. We now do experiments which are a thousand times better than were done 25 years ago. We should have been able to imagine that because that’s been the progress of science for the last 100 years, but it’s still always hard to imagine what can be done if people try hard.

But you are a theoretician so there is not such a problem, if you can prove the idea then the idea is right?

Frank Wilczek: If it’s an idea about the physical world you can have logical coherence and you can have aesthetic congruence, but it reaches an entirely different level when it describes the actual world, real phenomena, at least to me.

David Gross: Yes, and you could be wrong.

Frank Wilczek: Could be wrong.

David Gross: Believe me! I have been wrong, very rarely.

Frank Wilczek: I’ve been wrong maybe more often, but some of my best ideas have proved, what I think my best ideas have proved not to be right or at least not right in the original form they were proposed.

David Gross: There were other issues at the very beginning. Our advance was made on trying to understand what kind of theory could explain the behaviour of quarks at very short distances. The discovery of asymptotic freedom made it immediately clear that you could do that and calculate and test those ideas, but then the converse that the force became strong at large distances and led to the confinement of quarks that was still … When we very tentatively said that the fact that the force grows strong at large distances could explain confinement was very tentative and we had at the time no analogues, no other examples of that and it was such a crazy … I used to have arguments with a very famous physicist at Princeton, Eugene Wigner, for years and years and each time he would say, Quarks can’t exist because you could never produce them. The idea that you could base a theory on objects that you could never see directly seemed to most people – and even to us – dangerous. It took a long time, well, a long time, a few years at least, until one could see how this happened in toy models and then actually discover that there were analogues of that, analogues that people knew very well, like the Meissner effect in superconductivity that said ok well, now it’s not such a strange phenomena, there are examples in ordinary materials. Once those theoretical concepts were clear, I felt a lot more comfortable with the theory.

Frank Wilczek: It was very nervous making to be proposing a theory all of whose ingredients were unobserved particles and none of whose ingredients were observed particles.

Do you still remember this feeling, how it is to get this crazy idea?

Frank Wilczek: Oh yes.

David Gross: Oh yes.

How was it?

David Gross: It was exhilarating and scary because as Frank says it was clear from the beginning that the stakes were big, this was a big thing. Many of our smart colleagues realised this immediately and many were immediately convinced. But because the stakes were so big and the chances of it being wrong – as often happens – where none zero it was a little giddy and scary. As I remarked in my lecture, we didn’t have time much to think about it because there was so much to do, it just opened up, we got start calculating and physicist theorists really like to calculate. That’s the fun part of our business when you can …

Yes, something to bite.

David Gross: When you can make predictions.

Frank Wilczek: When you know what you’re going to do when you wake up in the morning. You don’t have to start trying to think about something essentially different than you’re meeting technical challenges instead of just being stuck in waiting for a new idea.

David Gross: And you can make predications.

But the idea itself it was so crazy, was it like a revelation or?

Frank Wilczek: It was putting together several, it wasn’t just one idea, it was really putting together several ideas that were formulated in different areas and then also ideas. After the central calculation there was a lot of work in working out its consequences. I don’t know if you’ll agree but I think in many ways the most important idea was not to worry about certain problems, just to go ahead and do the things we could do and not to worry about this problem of confinement.

David Gross: One couldn’t help to worry.

Frank Wilczek: Yes, but we probably didn’t do anything about it!

David Gross: I started to think about it. No, I started to think about it but, and of course being optimists. I originally thought that well, it’ll take us maybe three to four or five years to solve the theory completely. We still haven’t of course, it’s still on-going, a very alive subject to have analytic total control over the theory at large distances where it’s hard. It’s an enormously exciting field that remains. it’s hardly dead. The fact that it got a Nobel Prize does not mean that it’s over by any means, inuclear physics, QCD is still very much alive and exciting.

You said the most important product of knowledge is ignorance.

David Gross: Yes absolutely.

Frank Wilczek: That is what he says.

What do you mean by that?

David Gross: The fuel that drives science are questions, good questions, questions that you can formulate precisely. Actually formulating the right questions in a precise fashion is half the work. Once you have the right question it’s almost inevitable that it will be solved. That’s the hardest thing to teach a graduate student is how to formulate their own questions instead of giving them exam questions or problems. The source of scientific inspiration are good questions which are based on ignorance. But to ask good questions requires a lot of knowledge and that’s what I meant by that. The questions we ask today for example are questions that couldn’t … We ask questions about the spectrum of masses of the elementary particles, 20 years ago we didn’t know what the elementary particles were, we couldn’t ask those questions, we didn’t know what their masses were, we weren’t surprised by the great disparity of masses and so on. We ask now about the cosmological constant, 20 years Einstein had settled it so there was a mistake. The questions we ask today which exhibit our ignorance but exhibit it in an informed way by our knowledge that we accumulate, that’s what I mean by that. If we were to run out of questions we would run out of problems, we would run out, science would come to an end. Luckily, we have plenty of good questions.

What do you like most to be ignorant about?

Frank Wilczek: Maybe I’m not so fond of being as ignorant as David is.

You like to know?

Frank Wilczek: Unsolved problems really annoy me so I can’t let them fester, I really want to know what the dark matter is. I really want to know.

David Gross: Wouldn’t it be more annoying if there weren’t any?

Frank Wilczek: Yes of course, it might be, but then I would have to play chess all the time or computer games.

David Gross: Artificial problems!

Frank Wilczek: Right, make up problems right, but nature poses great problems and there’s certainly no lack of them today. There’s also the other thing that knowledge opens up is opportunities. I think there’s a vast domain of applications of quantum mechanics brought maybe to quantum computing or things that haven’t even been imagined that is a tremendous prospect for the future. As well as problems posed by society, what are we going to do about a source of energy in the long run. It’s a big problem that physics has to face you know. I think it’s going to be physics somehow.

It is about the future of physics. What is your favourite there?

David Gross: There’s so many.

Frank Wilczek: There’s so many.

There is so many.

David Gross: Those are all fascinating things but the things that I really want to know the answer to before I disappear, have to do with the next goal of fundamental physics which is unifying the forces and also understanding the … We can push back the history of the universe to almost its beginning where everything breaks down, even this enormously ambitious and exciting and promising approach based on strength, breaks down so far as well. Something very deep is missing but we have a lot of clues and a lot of very surprising clues and I will be very disappointed if those questions aren’t answered in my lifetime.

Do you think that string theory is the right way to get so far back in time?

David Gross: Yes.

Frank Wilczek: It may be an important direction, I don’t know even know what string theory is but they’re clearly something important. It’s not a well formulated theory in a sense of QCD, with algorithms and clear-cut predictions, there are clearly some important ideas there.

David Gross: QCD is a revolution in the making and so we’re somewhere in the beginning or the middle or who knows where? But as Frank said it’s not yet a well formed … We don’t know what string theory is, in fact we’ve discovered recently that … Frank was somewhat conservative in his middle ages, he’s always been somewhat more sceptical about string theory than I, but we’ve now got the situation where string theory can’t be killed because certain of its aspects is almost the same as the theories that make up the standard model. It’s so continuously part of the physics of the standard model that it can’t be wrong, it might and it might very well be insufficient and at a conceptual level I’ve no doubt that in its present formulation or where we have so far got in understanding what string theory is or should be, its missing some fundamental new concepts. But it’s really not as we’ve learnt much to our amazement, any different than the gauge theories that we use to explain all the fundamental forces that so far observed.

This is where you trust it, somehow?

Frank Wilczek: It can’t be wrong logically because it’s the same theory that’s been so well tested.

What do you say?

Frank Wilczek: To me the question is not so much right or wrong since the theory is vaguely formulated but how fruitful it’s going to be in describing nature and we’ll see.

David Gross: At the moment it’s not really a theory it’s …

Frank Wilczek: It’s somewhere between the desire for a theory and some ideas about a theory.

David Gross: Much more than that, it really is more than that. It’s very well defined in as far as we can use it but for example we believe, some of us do, that QCD itself can be described in a dual formulation as a string theory. We have explicit examples of how that happens in cousins of QCD that are quite close to QCD and exhibit all the dynamics of QCD including confinement and symmetry breaking and all these things. Very close and someday, and this is the hot topic now and one of the hot topics in QCD, is that this is really doing this for QCD itself. In that sense string theory is more of a framework so far for constructing a theory that successfully incorporates gravity and that’s its fantastic success. But something is missing clearly. It isn’t specific enough, it doesn’t so far answer the questions we were looking for answers for and furthermore some of us, certainly me, believe that it hasn’t yet made the probably quite dramatic changes of concept. Conceptual changes that are necessary to truly understand the quantum mechanics of space and time.

What concepts do you have I mind?

David Gross: What is space and what is time.

What is space and what is time?

David Gross: What is space and time?

Frank Wilczek: I mentioned in my lecture yesterday how much physics and several Nobel Prizes in theoretical physics have come out of reconciling special relatively with quantum mechanics. String theory is one aspect perhaps or one attempt to reconcile general relatively with quantum mechanics which poses at least equal problems.

David Gross: Personally I really think that we’re in a pre-revolutionary state and that the next revolution that will deal with the deeper meaning of space and time, quantum space and time which is what general relativity is, and string theory already suggests some, will be greater than that of quantum mechanics because a deterministic view of the world was, is sort of what you naturally think of although it’s not so clear from talking to ordinary people. But our macroscopic view of space and time is truly built into not only to the way we lay people think about the world, but the way we formulate physics. Physics after all is supposedly the science of taking the present and predicting the future. If time itself is just an approximate concept which works for large time how do we formulate the laws of physics? For me it’s the most interesting question.

And you hope to hear the answer.

David Gross: I have very little faith that I will be able to answer that myself, but some young person might and I hope to be around to hear about the answer, yes.

There is one severe problem with the string theory, if I may talk about the theory, the question is how to test it.

David Gross: Yes, but it’s probably because we don’t understand it. It’s hard to test a theory you can’t say, as Frank said, what it is. In particular it’s more of a framework and we don’t know how the theory selects that world that it predicts and therefore we don’t know how to test it. Aside from the fact that we are faced – string theory or not – with the fact that the next threshold of physics is very likely to be far removed from where we can directly do experiments.

Frank Wilczek: You’re setting very high standards for threshold. We’re going to learn a lot at the LHC [Large Hydron Collider]. That will be I think a new golden age in physics. Perhaps not revolutionising the basic concepts of space and time but certainly enriching them perhaps with super-symmetry heading quantum dimensions to the macroscopic dimensions we’ve known about and teaching us a lot about fundamental physics.

So, we have to wait like another four years?

Frank Wilczek: Yes, that may very well happen on a timescale of three or four years, so well within our productive lifetime, so that’s a really exciting prospect.

David Gross: That might allow us to indirectly write test theories like string theory.

Frank Wilczek: Sometimes the crucial clues come from domains where they’re not anticipated, that certainly happened for QCD. The crucial experiments were these …

David Gross: Absolutely.

Frank Wilczek: … SLAC [SLAC National Accelerator Laboratory] experiments which were not highly anticipated by the large body of theoretical community.

David Gross: Almost no-one wanted them.

Frank Wilczek: They were these kind of funny, no-one wanted to do them or think about them!

David Gross: My favourite example is the year of Einstein, 1905, he wrote three papers one proving the existence of atoms to even Ernst Mach’s agreement after Einstein’s work on Brownian motion Mach denied the existence of atoms, agreed that they must exist. He didn’t actually observe atoms, they didn’t have scanning, tunnelling microscopes, he explained a phenomena that was discovered by a botanist 50 years before the motion of little grains by the random movement of atoms. What more indirect evidence for atoms could you imagine but it was sufficient to explain, to convince the most die-hard, positivist of all Ernst Mach.

Frank Wilczek: It was very quantitative and non-trivial, that’s what made it convincing.

David Gross: I still have faith that if we had enough understanding of the theory which Einstein did at that time, of atomic theory, that we could indirectly test a theory like string theory but the theory has to do better. Unless, we can’t always rely on experimentalists to give us the clues.

I understand that you have to have patience as a physicist.

David Gross: Optimism and patience.

Frank Wilczek: And resilience, you have to keep trying.

You’ve kept trying for a long time; you’ve been waiting for the Nobel Prize for a long time.

Frank Wilczek: Yes, but we just haven’t been twiddling our thumbs waiting. David and I have done different things, we’ve diverged, but both of us have kept very active.

David Gross: Yes, it’s silly to wait for prizes.

Now you know you received it, what is the difference before and after?

David Gross: It’s a bit of a relief.

Frank Wilczek: It’s a relief. An honest relief, we’ll sleep better in October now and feel better about ourselves.

David Gross: I used to say October is, to paraphrase T.S. Eliot, October is the cruellest month!

Not any longer.

Frank Wilczek: No, not at all.

Thank you very much.

Frank Wilczek: Thank you.

David Gross: Pleasure.

Thank you for sharing your time with us.

Watch the interview

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