Are quarks real? A philosophical Interlude.

The Nobel-Prizewinning physicist and sometime bongo-player Richard Feynman famously said

Philosophy of science is about as useful to scientists as ornithology is to birds.

I have actually always liked philosophy. As a physics undergraduate I regularly attended philosophy seminars and lectures. Later during my PhD in elementary particle physics at the national laboratory DESY, in Hamburg, I often got together with students from all kinds of backgrounds to drink wine and discuss philosophy. While I can't remember what kind of books we read and talked about, I always enjoyed the debates, especially the different points of view. Unfortunately, while pursuing a career in particle physics, I lost touch with philosophy. However two years ago my colleague and friend Robert Harlander told me about a working group on particle physics, philosophy and history which had formed in Wuppertal.
Robert invited me to one of the meetings, and I was very impressed how the group interacted and worked jointly on various topics at the interface of LHC physics, philosophy of science and contemporary history of science. Recently, the Wuppertal group organized an international spring school on particle physics and philosophy, which I found very exciting and enjoyable. It included a mixture of lectures by physicists, philosophers and historians, as well as working groups where students debated topics like the "theory–ladenness of experiments" and the "reality of quarks". Everybody was very enthusiastic, and the talks and tutorials triggered plenty of discussion between lecturers and students. There was a good feel about the school, with some memorable late night conversations, where I learned about the role of shoes in Heidegger's philosophy, Berlin's street art scene, and the magic of the Bergisches Land.

Murray Gell-Mann, captain quark, by Toyah Walker, from Lily's quarks.
Back to philosophy! Paul Hoyningen-Huene from Hannover presented a stimulating introductory lecture on positions and limits of physical knowledge. One position which is quite popular among physicists is that of "convergent realism". Simply speaking, a convergent realist believes that physics theories over time approach the correct theory and that entities in these theories (like quarks) are real. Well, I did assume every physicist is some kind of a convergent realist: why else would one do fundamental research if not to understand more and more about the underlying truth of nature!? And doesn't everybody agree that science has made dramatic progress from the time of ancient Greek philosophy, where it was believed that the fundamental building blocks of matter are fire, earth, air and water? Today we know that chemistry can be described in terms of atoms, that atoms are made of protons, neutrons and electrons, and that protons and neutrons are made up of very few basics constituents, i.e. quarks. And who would dispute that atoms are real? Atoms can not only be seen, but they can also be individually manipulated.
I mentioned this to Robert Harlander who was sitting next to me during the lecture. To my surprise Robert answered that he does not believe in the reality of atoms – or in the reality of anything, for that matter. We argued for a while and tried to place our beliefs into the philosophical categories at hand. I finally settled for "progression realist", not least because the alternatives of "instrumentalist" or "anti-realist" sounded too negative to me. Robert called himself an "anarchist" which gave me the impression that he did not take the reality discussion very seriously. In any case, one of the good things about philosophical labels is that there are arguments and counter-arguments for almost every point of view, so you can easily change your position when you get tired of it.
Now, I think the reality of things is a serious matter, so let me mention one more point of view which I found particularly attractive and which was presented by Holger Lyre from Madgeburg. Holger introduced "structural realism" which says that all that exists and that can be known about the world is structure. Now this sounds very appealing to me as a theoretical particle physicist, as there is a very special structural principle at the heart of our description of nature, i.e. symmetries. Specifying the symmetries, specifically Lorentz and gauge symmetries, determines the structure of the three fundamental interactions: the strong, the weak and the electromagnetic force. A very powerful and very beautiful principle. But then wait, who first introduced symmetries to describe the fundamental structure of matter? It was the Greek philosophers. Plato's building blocks of matter correspond to polyhedrons (the Platonic bodies), geometric figures which are special because of their high symmetry and which actually represent mathematical symmetry groups. Of course, these are not the symmetry groups we use in modern physics. But if you think that symmetries shape the laws of nature, well, that has been proposed more than 2000 years ago. So maybe our progress is not quite as steady as I thought and there is something to be learned about physics even from ancient philosophers? Well, I'm not so sure, but it is certainly a lot of fun to think about...
Wuppertal is actually the only University Feynman ever visited in Germany and despite his scepticism, I think he would have enjoyed our discussions.