Over the last century, fundamental physics has undergone a change of scale. To push forward the knowledge of the infinitely small, physicists have been using increasingly sophisticated tools and have formed larger and larger collaborations. The scientists of old, who divided their time between their blackboards and their own small laboratories, have been succeeded by a myriad of specialists. Theorists are no longer experimental physicists, and vice versa.

“100 years ago, no distinction was made between theoretical and experimental physicists. Enrico Fermi was both a genius of theory and an exceptional experimentalist”, says Christophe Grojean, a theorist at DESY who collaborates with the experimental physicists working on the Large Hadron Collider (LHC). ”Today, experimental physics involves such sophisticated technology, and theory requires such a deep knowledge of mathematics, that it’s virtually impossible for one person to cover everything”.

Today, a physicist can be anything from a theorist whose work is far removed from experimental observations to an experimentalist doing R&D on detector components. In the middle of this broad spectrum of specialisations, theoretical and experimental physicists work alongside each other in close collaboration.

“I’m interested in physics theories beyond the Standard Model that can be tested. If I’ve got an idea, I want it to be used to interpret data. I don’t like speculating just for the sake of it” – John Ellis of King’s College London, who has worked at CERN since 1973.

… the theorist or the experimentalist?

Be that as it may, yesterday’s scientists and today’s physicists all have one thing in common: the thirst for a result or, even better, a discovery. The only difference is that they are now divided into two big communities, the theorists and the experimentalists. With which group does the initiative lie?  Are the experimentalists dependent on the theorists’ predictions for their discoveries or, on the contrary, do the theorists rely on the experimentalists’ data to improve their models?

“It’s a bit like the chicken and the egg”, smiles Christophe. But neither theorists nor experimentalists claim to be either the egg or the chicken (not that we really know which came first!). Each relies on the other’s knowledge to advance.

“In an ideal world, experimentalists would be able to understand Nature simply by observing it, but in reality they need the theorists’ input to interpret what they see”, explains Michelangelo Mangano, a theorist at CERN. “Likewise, theorists would love to be able to explain everything without recourse to experiments, finding the theory to rival all others since, in simple terms, Nature could not possibly function any other way. In reality, it’s much more complicated than that. There is always more than one possibility, and the Universe has evolved by taking one of the many paths open to it. This is why theorists also need experimental data to guide them”.

“If we want to understand what we’re measuring, we need a theoretical framework”, adds Chiara Mariotti, an experimental physicist with the CMS experiment at the LHC. "Even if we discover something that the theorists have never predicted, it’s important for us to understand the underlying framework, to have the wider view that the theorists give us”.

Personalities and History of CERN
Chiara Mariotti, an experimental physicist with the CMS experiment, and Christophe Grojean, a theoretical physicist working at DESY, are two of the coordinators of the LHC Higgs cross-section working group. The group brings together theoretical and experimental physicists with the aim of agreeing common theoretical predictions as the basis for analysing the data from the experiments. (Image: CERN)

A frenzy of interpretation

Research at the LHC is a perfect example of the continuous cross-fertilisation between theoretical and experimental physicists that drives scientific progress. The experimentalists' search for the Higgs boson was triggered by theory, but now the theorists are waiting for more data from the LHC to show them which way to go (we will elaborate on this in the next article in this series). Whenever the experiments start to buzz with signs of something new, however weak the signal is, there is a flood of theoretical interpretations. On 15 December 2015, the ATLAS and CMS experiments announced their first results from Run 2 of the LHC, revealing an unexpected (albeit small) signal. A dozen articles containing theoretical interpretations had been posted on the arXiv scientific repository within 24 hours. By April, the number had risen to 312!

“The Standard Model was formulated over 40 years ago. Dozens of models taking it further have emerged in the meantime, but none has gained the upper hand. The experiments will ultimately show who is right”, says Michelangelo. Hence the theorists’ frenzied trawling through the experimental data and the note of urgency in the avalanche of interpretations.

However, the theorists and experimentalists don’t simply stand by and watch like jealous neighbours observing the preparations for a party next door. They sometimes jump over the fence and join in. At CERN, where they’re separated only by a few corridors, cooperation is the name of the game. Michelangelo Mangano is one of the promoters and organisers of this dialogue. When the LHC began operation he set up the LHC Physics Centre at CERN (LPCC), a structure that supports collaboration between experimental and theoretical physicists. “One of my main tasks is to facilitate the interaction between theory and experiments”, he explains.

In Theory,Personalities and History of CERN
“Theorists and experimental physicists complement each other. They have a different approach to understanding”, explains Michelangelo Mangano, a CERN theorist who facilitates cooperation between the two communities at the LHC. (Image: CERN)

The LHC Higgs cross-section working group is one of the platforms for discussions between theoretical and experimental physicists. The group was set up in 2010 with the aim of making joint predictions for the discovery of the then hypothetical Higgs boson.

“We wanted to prepare for the discovery of the Higgs  and it was important for the experiments to use the same theoretical predictions and analysis criteria”, says Chiara Mariotti, one of the group’s founding members. “That way, once the experiments had observed a signal, it was easier for them to compare their data and check that it really was a discovery”.

Personalities and History of CERN
“The experiments need to speak a common language and also to speak the same language as the theorists”, explains Chiara Mariotti, an experimental physicist at the CMS experiment. (Image: CERN)

Accordingly, on 4 July 2012, the ATLAS and CMS experiments rapidly converged on the same conclusion.  Both had seen a signal, of course, but they were also relying on criteria that had been defined in conjunction with the theorists.

“This cooperation allowed us to focus on the main issues rather than wasting time comparing our methods. It accelerated the discovery”, explains Christophe Grojean, one of the group's current coordinators. The group, which initially comprised around fifty theorists and experimentalists, has now expanded. Between 100 and 200 physicists are active members, and the diverse subjects it covers range from precision Higgs measurements to searches for physics beyond the Standard Model.

Although essential, collaboration between the two communities of physicists is not always straightforward, not least because of the very different social structure of the large experiment collaborations compared to that of the small theory groups.

“A theorist’s work is ultimately much more solitary than that of an experimentalist who is part of a large collaboration and thus inevitably subject to constraints. Theorists are a lot freer, their work is their own, while an experimental physicist is part of a big team", says Christophe.

Confidentiality lies at the heart of the large experiment collaborations, and there’s no question of revealing results if they haven’t been approved. This means that theorists work only on the data that the experiments have already published, unless an experiment turns to them for help with its analysis, in which case they are asked to keep the information to themselves. In such cases, they can add their names to the relevant publication alongside the collaboration members.

Precious autonomy

When you ask theorists if they wish they had taken up a career as an experimental physicist, the answer is usually no. “I enjoy working closely with experimental physicists, but I’m happy to retain my independence as a theorist”, says Michelangelo Mangano. Theorists set great store by their freedom. Freedom of initiative is not always a given in the large experiment collaborations, and the theorists are aware of their privileged position.

“If I’m not satisfied with the results in one field of theory, I can always move to another one where I can put my ideas to good use”, says Slava Rychkov, a CERN theorist. “It’s less easy for experimental physicists. If they don’t get any results from a complex experiment that they’ve spent a lot of time building it can be very demotivating”.

Slava feels that theorists should be more aware of their responsibility when they point the experimentalists in a certain direction.

The relationship between theoretical and experimental physicists is even more complicated when it comes to discoveries. Who should be given the credit for them? The theorist who predicted the phenomenon or the experimentalist who found proof of it? Both, certainly, but the juries who award the prizes are not always so magnanimous.

Alvaro de Rújula, an honorary member of CERN’s theory department, talked ironically about this unspoken competition in an article on the relationship between theorists and experimentalists.  “The relationship between experimentalists and theorists is often one of healthy competition for truth and less healthy competition for fame”, he wrote, illustrating his words with a riddle comparing a theorist, an experimentalist and a discovery with a farmer, his pig and a truffle. 

What is similar and what is different between the following two sets?

A farmer, his pig and truffles A theorist, an experimentalist and the big discoveries
The farmer takes his pig to the woods. The pig sniffs around looking for a truffle. When the pig gets it and is about to eat it, the farmer knocks the pig on the head with his club and steals the truffle. Those are the similarities: a theorist would also claim recognition for an experimenter's discovery (if it has anything to do with her/his theories) even if [s]he did not make it! The difference is that the farmer always takes the pig to woods where there are truffles, while, more often than not, the suggestions by the theorists take the experimentalists to "woods'' where there are no "truffles'' (by suggesting experiments that do not lead to interesting discoveries). Not to be unfair to theorists, one must add that there are notable exceptions to these rules, progress is made by trial and error, and the theorists' guidance is occasionally in the right direction! Even more often, while looking for the theorists' "truffles'' the experimentalists find "gold'': something unexpected but even more interesting! (Nature tends to be more creative than we are).(Image: Alvaro de Rújula/ CERN)

The next article in the “In Theory” series will discuss the theorists' hopes for the future and what the next steps are for the discipline, read the other articles and photoessays in our In Theory series here.