Just before the end of the year Professor Pierre van Baal passed away unexpectedly. With him we have lost a unique figure with great passion for research and teaching. In 1980 he came to Gerard 't Hooft as an aspiring PhD student with two diplomas, one in Mathematics and one in Physics. This combination has always played a role in his scientific activity.
His PhD research was on the topic of the strong force, Quantum Chromodynamics (QCD). Quarks are the building blocks of atomic nuclei and they exist in three versions with different "colours". They also carry another quantum number, "flavour", of which six varieties have been found experimentally. They are held together by gluons, that also carry colour. At short distances, much smaller than a femtometer, these couple very weakly to quarks. At greater distances the coupling increases and it causes the confinement of quarks and gluons. The mathematical explanation of this phenomenon would become the focus of Pierre's research.
Pierre had the task to put such a system of weakly coupled quarks and gluons in a small "femto-box" to analyze it. This was a benchmark for computer simulations on the lattice. But the simulations could also explore the regime where the interactions become strong.
Numerical simulations became widely used just in this period and from 1984, when he took a postdoctoral position at the University of Stony Brook, Pierre was asked to speak at many yearly conferences of lattice practitioners. Later in the 80's his mathematical skill resulted in research articles with, among others, Raymond Stora on Witten's topological Yang-Mills theories.
In 1992 he held his inaugural lecture as Professor at the Lorentz Institute with the title "A strong story (a tall tale) of colours and flavours", In those years he stimulated the numerical approach to "Quantumchromodynamics". He was the nucleus of a group of PhD students and postdocs that played an influential role in unraveling the mysterious properties of QCD.
In 1998 he discovered a new excitation, the "caloron". As the name suggests, this excitation plays a role at high temperature. The novelty of Pierre' s caloron was that not only did it share the properties of another excitation already known for two decades, the "instanton" , but it also was a composite of colour-monopoles that was on the whole colour-magnetically neutral. In the last five years this idea has seen an intense revival of activity. It was also in this period that he joined the project HiSPARC, by which secondary school students are taught how to build their own cosmic ray detector.
In 2005 he had a misfortune in the form of a stroke. With much energy he learnt to minimize the physical consequences. His mother and his family contributed strongly to that. In the following years he went back to teaching and his quantum field theory lectures are now published as a textbook. But he was not able to do research again. Half a year ago his closest collaborators came together in a symposium to celebrate Pierre's inspiring research achievements. A published volume with a selection of his articles crowned the occasion. Just in time to see him really happy once again.
Chris P. Korthals Altes