CERN Accelerating science

This website is no longer maintained. Its content may be obsolete. Please visit for current CERN information.

Home | Sitemap | Contact us
this site all CERN
Patrick Janot

Theorists, experimentalists and the like, at CERN

by Patrick Janot

For obscure reasons, physicists - at CERN as everywhere else - like to categorize themselves in different classes, such as theorists, experimentalists, accelerator physicists, technicians,... Of course, each class is convinced to be dominant over all the others, and that physics is best served by them. Worse! Theorists often think they do create physics with their theory, experimentalists often think they do discover or invent physics with their experiments, accelerator physicists often think they make it possible with their instruments,... and they all think that the other categories exist only to provide them with the technicalities they either do not want - or are not able - to work out: long but straightforward calculations, subtle but repetitive measurements, powerful but dirty tools?

Well... all this cannot be true at a time. So, what is the difference between, say, a theorist and an experimentalist? From the point of view of an experimentalist, it could be summarized with the following two jokes:

1. An experimentalist has worked out an empirical equation that seems to explain his data. He asks the theorists to have a look at it. A week later, the theorist says that the equation is invalid, but the experimentalist used his equation to predict the results of further experiments and gets excellent results. So he asks the theorists to look again. Another week goes by and they meet once more. The theorist tells the experimentalist that the equation does work, "but only in the trivial case where the numbers are real and positive."

2. The experimentalist comes running excitedly in the theorist's office, waving a graph taken off his latest experiment. "Hmmm", says the theorist, "that's exactly where you'd expect to see that peak. Here is the reason." A long logical explanation follows. In the middle of it, the experimentalist says "Wait a minute", studies the chart for a second and says "Oops, this is upside down." He fixes it. "Hmmm," says the theorist, "you'd expect to see a dip in exactly that position. Here is the reason…"

It may give an idea to the outsider of what a theorist really is! More seriously, is there a noticeable difference between a theorist and an experimentalist? Theorists and experimentalists seem to insist on it. Apparently, according to them, something essential would be lost in the description of Nature without this distinction. Some of them make commendable efforts to try to go beyond this artificial opposition, talking of complementarities, for instance, while such a distinction simply does not exist.

To understand it, let's just define the word experiment in such a way that it be no longer the realization of some theory. An experiment is the place, the people, the groups, the instruments, the laboratories, the procedures, the documents, etc., which allow any human activity to take place. Whether it is a quantity to measure with an accuracy of 0.01%, or a mathematical equation to reformulate, or an ultra-fast electronic device to produce, there is always practice, and therefore experimental work in it. Practice (or experiment) is therefore a word that refers to the action of doing something. It has no contrary!

There we go. Theory is therefore not the contrary of experiment. In theory, a theory does not exist. A theory is only the product of some practice, with blackboards, colleagues, symbols, computers, discussions, books. The work place of a theorist is very much alike an experimental laboratory. A theory is never produced in a theoretical manner (whatever it may mean!): facing the theory, a theorist is not more than a farm-labourer facing his field, or a technician in front of his electronic device.

People (mostly theorists) will tell you that theory is more than (i.e., above) practice because it allows the result of forthcoming experiments to be predicted, from the knowledge of a few constants of Nature. How, indeed, could the theory of relativity depend on any practice? The question assumes again the identity between the product - the theory of relativity - and the process of producing the theory. The thinking process that generates the theory is far from being theoretical: it requires documents, discussions, practice and training. The predictive power of a theory is never located in the brain of a theorist at the time the theory is produced. It comes only when other theory technicians (i.e., the theorists) work the theory further out or reformulate it.

Theory, when defined as a thinking process, is therefore an artefact. It is not more abstract than other thinking processes, like designing an accelerator or an experiment, which in turn lead to the ultimate equation. If one aspect is missing, there is no theory, but at most a summary to which there is lot to add before it becomes something tangible.

Similarly, a theorist, when defined as a person mastering this (non-existing) thinking process, is an artefact. A theorist, an experimentalist, or an accelerator physicist, are all technicians aimed at serving physics, and are therefore to be only placed on the same footing. There is no difference in essence between theoretical and experimental knowledge. Experimentalists are bound to understand the theory as well as theorists should understand the experiment, so as to allow the ultimate equation to be produced in an efficient way.

More importantly, physics is independent of all these trivialities, these practical details. How it works is just here to be understood by all of us (theorists and experimentalists) with our limited intellectual means and limited lives. Why it works is not a question that can be worked out by scientists, whether they define themselves as theorists or not. It is only a matter of religion.

Let me conclude with this last related joke: An engineer, an experimentalist, a theorist, and a mystic, are asked to name the greatest invention of all times. The engineer chooses fire, which gave mankind power over matter. The experimentalist chooses the wheel, which gave mankind power over space. The theorist chooses the alphabet, which gave mankind power over symbols. The mystic chooses the thermos bottle. "Why?" the other ask. "Because it keeps liquids hot in winter and cold in summer." The others: "Yes, so what?" The mystic: "Think about it. That little bottle: how does it know?"

Isn't the mystic the only theorist of the group? Whether he is useful or not to the group is another debate.