Summer students met and discussed with John Ellis about his early career in physics, his current research interests and his views on the relation between philosophy and science...

When did you decide that you would become a theoretical physicist?

When I was a child, my mother often used to take my sister and me to the local library. The adult section was from 14 onwards and when I was about 12 I couldn’t borrow any adult books, but the kid’s fiction was not interesting, so I started reading nonfiction and I read a lot of science and history books.

I was always interested in the most fundamental things, and for me physics was the most fundamental science For a long time I was torn between particle physics, astrophysics and cosmology, though my formal training was in particle physics. Finally, the field in which I am working now is a combination of the two.

Do you think that the influence from your family played a key role in becoming a physicist?

My mother who used to take me to the public library should take a lot of credit for that. On the other hand, neither of my parents was working on anything even remotely resembling science: my father was in the insurance industry and my mother worked at home. I was vaguely aware of the fact that my father had been good at mathematics when he was at school and that was about it. There was nobody in their circle who was a scientist. However, the headmaster in my primary school had been an Antarctic explorer, so maybe just the fact that I knew that made me think that there was something beyond everyday life, and probably that was another important influence.

Why are you doing science? Do you think that personal curiosity is the only factor that should drive scientific research?

I went into science because of my own curiosity. However, I am aware of the fact that science is supported by governments, and naturally they want it to be useful. Value can come out in various different ways.

One is training young people such as yourselves. I am prepared to bet that not all of you are going to finish your careers as research scientists, probably most of you are going to end up doing something different, but just the fact that you have exercised your science, technology, engineering and mathematics skills by being physicists or astrophysicists for a while, and by coming to CERN for a couple of weeks or months, is going to benefit you in your future lives.

I also firmly believe that the research we are doing at CERN does have societal benefit, as knowledge itself contributes to the advance/development of society. For example, I recently had lunch with a colleague who is interested in setting up a hadron therapy facility in Greece. This would be based on technology already developed by particle physicists. Moreover, there is a new design being prepared that uses current CERN expertise. In fact, CERN has helped several groups to build such facilities in their countries with technical advice, designs, training of people etc. This is just one example: I think there is a lot that an organization like CERN can do for the benefit of society.

Of course there is also the question of how the Higgs boson can be useful.  We have no idea but every time in the past when fundamental physicists have selfishly followed their own interests and tried to understand nature better, and made some sort of fundamental advance, it eventually turned out to be useful. Take for example antimatter and medical diagnosis, Maxwell’s equations and mobile phones, the web and so on. Even the Higgs boson, though at first sight it seems to have no practical use, might turn out to be useful in the future.

Is your interest in the fundamental at a scientific or philosophical level?

I was always interested in fundamental science. But when I was an undergraduate student, I did do some reading on philosophy on the side - it was not part of my formal course. I struggled through Bertrand Russell, Karl Popper and other philosophers trying to understand the basic principles of epistemology and the philosophy of science. I don’t find philosophy helpful in my day-to-day research but I am always happy to debate with philosophers, and in fact I have been invited to philosophical festivals where I got the chance to discuss these issues around the table with philosophers. This is not to say that I don’t think physicists are philosophers. Even the absence of formal philosophy is in fact philosophy, but it is sort of a pragmatic minimalist philosophy if you like.

What is the interplay between theory and experiment?

I would not like to get into the chicken versus egg debate as to which is more fundamental: the theory or the experiment. If I had to choose I would have to say that obviously the experiment is more fundamental. On the other hand, a theoretical background is necessary in order to understand and interpret the experiments and predict the outcomes of other observations. If these predictions turn out to be correct then a model or theory emerges.

We scientists aim not just for a description of the data that we already have, we search for something that will give us some insight into the data that we don’t yet have. On the basis of empirical data a model is formulated and then that formulation can be used to make predictions for other experiments. It would be better if the results of those other experiments did not confirm the model exactly, because then we would get some hints how to improve our understanding of Nature. However, theory is just a human construction in order to understand the data: Nature is the boss.

I feel there is a symbiotic relationship between theory and experiment that is a little bit like going around and around the accelerator, with the theories giving you a little kick each time, enabling experiments to understand things a bit better at higher energies.

What is in your opinion the boundary between theoretical physics and pure speculation?

This goes back to one of the existential choices that I had to make when I started  my PhD. One of the questions that my professor asked me was: “Do you want to do theory that has applications, or pure theory?” I said that I want to do theory with applications and, in particular, connections to interpretations of observations or predictions for observations.

Of course, some of the papers that I have written had absolutely nothing to do with experiments, but as time goes on, when I am working on some research project I consider more and more possible experimental astrophysical or cosmological applications of my work. I don’t like doing pure theoretical physics: that is just like mental gymnastics showing how clever one is.

As a theorist, do you think that if mathematics had followed a different path, historically, we could have developed a different formalism?

I think that there is only one mathematics, in the sense that mathematics is an expansion of logic; it is a construction of “interesting” logical systems. Of course “interesting” is a matter of taste. Some people would replace interesting by beauty. If you formulate it that way mathematics is really a vast subject and there is surely an incredible number of different pathways through it.

There were a couple of instances in the 20th century where physicists were bumping up against the boundaries of what was then known in mathematics. One example was Einstein when he was trying to formulate general relativity and it turned out that non-Euclidian geometry actually did exist. I am not sure how much he was aware of that before he discovered it himself. Another example comes from those working in quantum mechanics, who reinvented matrices for themselves to some extent. Dirac invented a whole new area of mathematics, distributions, because it was what he needed in order to carry out his description of fields.

In those instances, I don’t think that physicists were held out very long by lack of available mathematics. Either it was there, they stumbled across it and then they were able to use it or they started inventing it for themselves and immediately the mathematicians came in and formulated the theory in a more precise, general and useful way. 

One area, though, where I think we lack the mathematical tools today is string theory. We grow up with geometrical ideas inherited from Euclidean idealized points and lines. But in string theory there are, as fundamental structures, extended objects. Originally people thought in terms of objects extended in one dimension, but then they realized in the 1990s that, for the consistency of the theory, they had to consider things extended in more than one dimension, and they started to talk about membranes and M-theory.

It is not clear whether any of these descriptions, in terms of objects extended in some fixed number of dimensions, is sufficiently general. Probably one needs some sort of generalized conception of geometry, which is a long way away from our original ideas of points. At the moment, I think that string theorists are handicapped by not having that deeper insight.

Do you think that understanding the origin of the universe might impact our way of thinking ?

I think that deeper fundamental understanding affects the way people think in many different respects. In the last century, the discovery of quantum mechanics really affected the way people think about nature.  Special relativity certainly had philosophic implications, which I think people are still sorting through, and even read too much into it sometimes.

It was a shock for humankind to realize that we are living on an insignificant planet, going around an insignificant star in an insignificant galaxy and, as we have discovered, we are not even made of the same stuff as most of the matter in universe because most of it is some unseen form of dark Matter. Moreover, most of the universe is actually dark energy, and not any sort of matter at all. The universe has been going on for billions of years, not obviously caring whether we exist or not. I think that this observation is of a profound philosophical importance. Surely there will be more shocks if and when we understand better the origin of the universe.

What is the most effective way to communicate science to the general public? What level of detail and which is the language that should be used?

I don’t think that there is a single simple recipe for this; there are many different ways to engage with the general public and reach different types of people. It is our obligation as scientists, who are funded by the general public, to communicate with them what we are doing, describe our results, and respond to their questions and concerns. It is really imperative to our technology based society that we strive as much as we can to raise the general level of scientific literacy and develop a critical attitude towards assessing evidence. I do a lot of outreach and sometimes I talk to school kids, sometimes I talk to high school students, sometimes to more general audiences. Two weeks ago I did something which I haven’t done before; I participated in a gig at a night club. It was in Manchester in a vault under a railway station. It started off by me being interviewed by a punk rock musician and there was also a presentation about astronomy and extraterrestrial life. The evening finished with a set of punk rock music by a group called the Membranes, played with a backdrop of slides summarizing M-theory. Maybe that reached a new audience? Musically, punk is the ultimate music of defiance, and philosophically I believe that defiance the only possible attitude towards the apparent indifference of the universe.

How do you feel about science teaching in schools? What could change in science education at schools?

I think that a good laboratory for doing experiments is needed in every school, and I was relatively fortunate because I went to a school that allowed us to conduct real experiments. However, I think that one of the problems in high-school science teaching is that usually you know the results of the experiments in advance, and you just verify that the data fit the model. It is probably quite difficult to design activities for high school that give students more open-ended activities than just repeating the same old experiments time after time. Unfortunately, I am aware that many schools don’t have specialized physics teachers. Maybe there are teachers that have some background in science but they are not necessarily physics teachers.

I believe that CERN can do many things to support high-school teachers and therefore high school learners.  Some 15 or so years ago we set up a programme for high-school teachers here. We started off the first year with a dozen participants and now almost a thousand high school teachers visit CERN each year spending a significant amount of time. The programme aims to expose them to current developments in physics, to bring them into contact with each other so that they can exchange ideas in teaching physics and to recharge their enthusiasm.

Which do you think, at this point of your career, is more important outreach, or science?

I try to find a balance between the two. I think that it is natural for young scientists to be more focused on actually doing science, and I believe that older scientists have to provide them with some sort of framework that makes that possible. When I was head of the Theory Division at CERN I used to think that my job was to make the external world invisible to theorists working here. They shouldn’t have to worry about money or working conditions; they should just get on with the physics. Outreach is part of what we must do to make that possible. That said, I think that young scientists have also something to contribute in terms of outreach, because they are closer in age to young people, who are perhaps more susceptible to having their career choices affected. I also think that young scientists are more likely to be responsive to the people out there. This can benefit the young scientists themselves giving them new ways to look at their work. I am unlikely to change what sort of science I do after giving a public lecture, but maybe younger scientists would. However, I wouldn’t tell them that they have to spend x% of their time in outreach.

Have you ever had a "Eureka moment" in your years of theoretical work?

I can recall two such moments.The first was in 1975. At the time we believed that the quarks were held together inside protons and neutrons by gluons. There were many good theoretical reasons for thinking that quarks were real, and also that these gluons were real, but nobody had found direct experimental evidence for the gluon.

I had just had a coffee downstairs in the cafeteria and was walking back to my office when, as I was rounding the corner by the library, I had an idea for how you could demonstrate directly experimentally the existence of gluons.  That was a "Eureka moment" when I felt I was the only one with this insight. Then I got together with a couple of my colleagues, we wrote a paper, we told the experimentalists about it, and some 3 years later they found the gluon in the way that we described.

At that time, we had a whole bunch of data about what happens when you scatter electrons off protons and there were different theories for what might be going on. This gluon theory fit the data but there were other things that fit the data equally well. However, with this theory there was a more fundamental understanding of what was going on and that enabled us to make predictions.

That happened here at CERN, and I was wearing clothes, so it wasn't the traditional "Eureka moment" of Archimedes. But some years later I did have an idea while I was lying in the bath, and I did jump out and run around the house shouting "Eureka", just to do it properly.

Do you think that string theory could be a theory of everything?

Once I was asked to write a review article for Nature, which had the title String theory: a theory of everything or of nothing?. This is still an open question; although I would now say that it is at least a theory of something, because it is useful for understanding heavy-ion collisions and some aspects of condensed-matter physics. I still think that string theory is the best and possibly the only candidate that we have for a theory of everything. But we are a long way away from finding any experimental proof of that.

What do we mean by referring to a theory of everything?

That is another debate. We have a Standard Model that describes data very well, and we have a theory of how quarks gather inside protons and neutrons that we can use to calculate on the lattice the properties of strongly-interacting particles. But that is not always the case when you go to the next layer of the cosmic onion. For example, we have an understanding of protons and neutrons but we still can’t say that we can describe the nuclei in great detail. And we understand how electrons bind with nuclei to form atoms but we don't always understand in a precise way how the properties of materials emerge. Of course we can always make simplifications and models, but we are not in the position to calculate their properties from first principles.

When I talk about a theory of everything, what I have in mind is that we discover what the first principals are, and run experimental checks to be reasonably sure that this is actually the right answer, not that we calculate every physical phenomenon in detail. In the past, by discovering the gluons, we could have a reasonably good conviction that the quarks and gluons were the underlying theory of protons and neutrons. But this did not necessarily mean that we could (or ought) to calculate everything in nuclear physics.

From that point of view, "theory of everything' is perhaps rather too heuristic a term, and we should never abuse it. It might be that we will be lucky and eventually come up with some formulation, but there would still be things that we couldn't calculate. Maybe we come up with a theory that we are convinced is right but we cannot actually calculate things at the next level up in the onion. As I mentioned, there are historical precedents for that.

Which is the most important thing that you expect to find out from the upgrades of the LHC? Which property of the Higgs boson do you expect to be more crucial for understanding Nature?

If Nature is kind, the upgrades of the LHC may reveal a whole set of new particles, as in supersymmetric theories. As for the Higgs, I cannot give a specific answer to that as there are so many different measurements to be made at the LHC. For example, how Higgs decays into different particles, different ways to produce the Higgs particle, how the cross section changes with energy, angular distributions and so on. The Standard Model makes a very specific set of predictions, and other theories may disagree with the energy dependence, or they may suggest that its decay into muons is going to be different, etc.. I think that you should keep on searching through all these things … production mechanisms, decay mechanisms are not predicted in the standard model. Of course you can never prove that it is the Standard Model Higgs boson, but you may be lucky enough to prove that it is not. I sincerely hope that the outcome of these experiments will be the discovery that it is not the Standard Model Higgs boson, and that there has to be some new physics. If a discrepancy is discovered with the Standard Model, then the new physics would have to be at accessible energies.

John Ellis: I would like to finish with a question to the students which you don't have to answer but you just go away and think about it. The question is whether you are doing what you really want to do. Physics takes dedication, but rewards it.

John Ellis was interviewed by Silvia Manconi, Maria Brigida Brunetti, Cristina Martin Perez, Rodrigo Gaston Cortinas, Catherine Hsu and Gudmundur Stefansson.

We would like to thank Cian o 'Luanaigh (DG-CO) for his useful comments.