vendredi 20 mai 2016

CERN - In Theory: Is theoretical physics in crisis?

CERN - European Organization for Nuclear Research logo.

May 20, 2016

Image above: “The way physics develops is often a lot less logical than the theories it leads to -- you cannot plan discoveries. Especially in theoretical physics.” Gian Giudice, Head of CERN’s Theory Department (Image: Sophia Bennett/ CERN).

Over the past decade physicists have explored new corners of our world, and in doing so have answered some of the biggest questions of the past century.

When researchers discovered the Higgs boson in 2012, it was a huge moment of achievement. It showed theorists had been right to look towards the Standard Model for answers about our Universe. But then the particle acted just like the theorist’s said it would, it obeyed every rule they predicted. If it had acted just slightly differently it would have raised many questions about the theory, and our universe. Instead, it raised few questions and gave no new clues about to where to look next.

In other words, the theorists had done too good a job.

    "We are struggling to find clear indications that can point us in the right direction. Some people see in this state of crisis a source of frustration. I see a source of excitement because new ideas have always thrived in moments of crisis." - Gian Giudice, head of the Theory Department at CERN.

Before these discoveries, physicists were standing on the edge of a metaphorical flat Earth, suspecting it was round but not knowing for sure.  Finding both the Higgs boson, and evidence of gravitational waves has brought scientists closer than ever to understanding two of the great theories of our time – the Standard Model and the theory of relativity.

Now the future of theoretical physics is at a critical point – they proved their own theories, so what is there to do now?

So what next?

    "Taking unexplained data, trying to fit it to the ideas of the universe […] – that’s the spirit of theoretical physics" – Gian Giudice

In an earlier article in this series, we spoke about how experimental physicists and theoretical physicists must work together. Their symbiotic relationship – with theorists telling experimentalists where to look, and experimentalists asking theorists for explanations of unusual findings – is necessary, if we are to keep making discoveries.

Just four years ago, in 2012, physicists still held a genuine uncertainty about whether the lynchpin of the Standard Model, the Higgs boson existed at all. Now, there’s much less uncertainty.

“We are still in an uncertain period, previously we were uncertain as to how the Standard Model could be completed. Now we know it is pretty much complete so we can focus on the questions beyond it, dark matter, the future of the universe, the beginning of the universe, little things like that,” says John Ellis, a theoretical physicist from Kings College, London who began working at CERN since 1973.

Image above: Michelangelo Mangano moved to the US to work at Princeton just as String Theory was made popular. "After the first big explosion of interest, there’s always a period of slowing down, because all the easier stuff has been done. And you’re struggling with more complex issues," he explains. "This is something that today’s young theorists are finding as they struggle to make waves in fields like the Standard Model. Unexpected findings from the LHC could reignite their enthusiasm and help younger researchers to feel like they can have an impact." (Image: Maximillien Brice/CERN).

With the discovery of the Higgs, there’s been a shift in this relationship, with theoreticians not necessarily leading the way. Instead, experiments look for data to try and give more evidence to the already proposed theories, and if something new is thrown up theorists scramble to explain and make sense of it.

"It’s like when you go mushroom hunting," says Michelangelo Mangano, a theoretical physicist who works closely with experimental physicists. "You spend all your energy looking, and at the end of the day you may not find anything. Here it’s the same, there is a lots of wasted energy because it doesn’t lead to much, but by exploring all corners of the field occasionally you find a little gold nugget, a perfect mushroom."

At the end of last year, both the ATLAS and CMS experiments at CERN found their mushroom, an intriguing, albeit very small, bump in the data.

This little, unexpected bump could be the door to a whole host of new physics, because it could be a new particle. After the discovery of the Higgs most of the holes in the Standard Model had been sewn up, but many physicists were optimistic about finding new anomalies.

Image above: John Ellis' office. (Image: Maximillien Brice/ CERN).

"What happens in the future largely depends on what the LHC finds in its second run," Ellis explains. "So if it turns out that there’s no other new physics and we’re focusing on understanding the Higgs boson better, that’s a different possible future for physics than if LHC Run 2 finds a new particle we need to understand."

While the bump is too small for physicists to announce it conclusively, there’s been hundreds of papers published by theoretical physicists as they leap to say what it might be.

“Taking unexplained data, trying to fit it to your ideas about the universe, revising your ideas once you get more data, and on and on until you have unravelled the story of the universe – that’s the spirit of theoretical physics,” expresses Giudice.

Image above: John Ellis classifies himself as a 'scientific optimist', who is happy to pick up whatever tools are available to him to help solve the problems that he has thought up. 'By nature I’m an optimist so anything can happen, yes, we might not see anything beyond the Higgs boson, but lets just wait and see.' Here he is interviewed by Harriet Jarlett (left) in his office at CERN. (Image: Sophia Bennett/CERN).

But we’ll only know whether it’s something worthwhile with the start of the LHC this month, May 2016, when experimental physicists can start to take even more data and conclude what it is.

Next generation of theory

This unusual period of quiet in the world of theoretical physics means students studying physics might be more likely to go into experimental physics, where the major discoveries are seen as happening more often, and where young physicists have a chance to be the first to a discovery.

Speaking to the Summer Students at CERN, some of whom hope to become theoretical physicists, there is the feeling that this period  of uncertainty makes following theory a luxury, one  that young physicists, who need to have original ideas and publish lots of papers to get ahead, can’t afford.

Image above: Camille Bonvin is working as a fellow in the Theory Department on cosmology to try and understand why the universe is accelerating. If gravity is described by Einstein’s theory of general relativity the expansion should be slowing, not accelerating, which means there’s something we don’t understand. Bonvin is trying to find out what that is. Bonvin thinks the best theories are simple, consistent and make sense, like general relativity. "Einstein is completely logical, and his theory makes sense. Sometimes you have the impression of taking a theory which already exists and adding one element, then another, then another, to try and make the data fit it better, but its not a fundamental theory, so for me its not extremely beautiful." (Image: Sophia Bennett/CERN).

Camille Bonvin, a young theoretical physicist at CERN hopes that the data bump is the key to new physics, because without new discoveries it’s hard to keep a younger generation interested: “If both the LHC and the upcoming cosmological surveys find no new physics, it will be difficult to motivate new theorists. If you don't know where to go or what to look for, it's hard to see in which direction your research should go and which ideas you should explore.”

The future's bright

Richard Feynman, one of the most famous theoretical physicists once joked, "Physics is like sex. Sure, it may give some practical results, but that's not why we do it."

And Gian Giudice agrees –while the field’s current uncertainty makes it more difficult for young people to make breakthroughs, it’s not the promise of glory that encourages people to follow the theory path, but just a simple passion in why our universe is the way it is.

“It must be difficult for the new generations of young researchers to enter theoretical physics now when it is not clear where different directions are leading to,” he says. “But it's much more interesting to play when you don't know what's going to happen, rather than when the rules of the game have already been settled.”

Image above: “It's much more interesting to play when you don't know what's going to happen, rather than when the rules of the game have already been settled,” says Giudice, who took on the role of leading the department in 2016 (Image: Sophia Bennett/ CERN) (Image: Sophia Bennett/CERN).

Giudice, who took on the role of leading the theory department in January 2016 is optimistic that the turbulence the field currently faces makes it one of the most exciting times to become a theoretical physicist.

“It has often been said that it is difficult to make predictions; especially about the future. It couldn't be more true today in particle physics. This is what makes the present so exciting. Looking back in the history of physics you'll see that moments of crisis and confusion were invariably followed by great revolutionary ideas. I hope it's about to happen again,” smiles Giudice.

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. You can read the previous articles in the series here:

CERN - In Theory: Which came first…?

CERN - In Theory: Why are theoreticians filled with wanderlust?

CERN - In Theory: Are theoreticians just football fanatics?

CERN - In theory: Welcome to the Theory corridor:

CERN - In Theory: why bother with theoretical physics?:

CERN congratulates the discoverers of gravitational waves:


CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.

Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 22 Member States.

Related links:

The Standard Model:

ATLAS experiment:

CMS experiment:

For more information about the European Organization for Nuclear Research (CERN), visit:

Images (mentioned), Text, Credits: CERN/Harriet Jarlett.

Best regards,

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