Reviews | The uncertain future of particle physics

The Large Hadron Collider is the largest particle accelerator in the world. It is a 26 kilometer long underground ring, located at CERN in Geneva, in which protons collide at a speed close to light. With a price tag of $5 billion and an annual operating cost of $1 billion, the LHC is the most expensive instrument ever built, even though it reuses the tunnel of an old collider.

The LHC has been collecting data since September 2008. Last month, the second experimental run was completed and the collider will be shut down for the next two years for scheduled upgrades. With the LHC on hiatus, particle physicists are already planning to build an even bigger collider. Last week, CERN unveiled plans to build a bigger and far more powerful accelerator than the LHC, which would cost more than $10 billion.

I was a particle physicist. For my doctorate. thesis, I made LHC predictions, and even though I stopped working in the field, I still believe that the slamming of particles against each other is the most promising way to understand what matter is made of and how she stands. But $10 billion is a high price. And I’m not sure it’s worth it.

In 2012, experiments at the LHC confirmed the discovery of the Higgs boson — a prediction that dates back to the 1960s — and it remains the only discovery made at the LHC Particle physicists are quick to point out that they learned from other things: For example, they now have a better knowledge of the structure of the proton and they have seen new composite particles (although unstable). But let’s be honest: it’s disappointing.

Before the LHC went live, particle physicists had more exciting predictions than this. They thought other new particles would also appear near the energy at which the Higgs boson could be produced. They also believed that the LHC would see evidence of new dimensions of space. They further hoped that this gigantic collider would provide clues to the nature of dark matter (which astrophysicists say constitutes 85% of the matter in the universe) or a unified force.

The stories about new particles, dark matter and extra dimensions have been repeated in countless media from before the launch of the LHC until a few years ago. What happened to these predictions? The simple answer is: those predictions were wrong – it’s now clear.

The problem is that a “prediction” in particle physics today is little more than guesswork. (In case you were wondering, yes, that’s exactly why I left the field.) Over the past 30 years, particle physicists have produced thousands of theories that they can conceive of the mathematics for ” predict” just about anything. For example, in 2015, when a statistical fluctuation in LHC data appeared to be a new particle, physicists produced more than 500 papers in eight months to explain what later turned out to be noise. The same thing happened repeatedly for similar fluctuations, demonstrating how worthless these predictions are.

To date, particle physicists have no reliable prediction that there should be anything new to discover up to about 15 orders of magnitude above currently accessible energies. And the only reliable prediction they had for the LHC was that of the Higgs boson. Unfortunately, particle physicists have not been very forthcoming with this information. Last year Nigel Lockyer, the director of Fermilab, told the BBC: “From a simple calculation of the mass of the Higgs, there must be some new science. This “simple calculation” is what predicted that the LHC should already have seen the new science.

I recently came across a promotional video for the future circular collider that physicists have proposed to build at CERN. This video, hosted on the CERN website, showcases the planned machine as a test for dark matter and as a probe for the origin of the universe. This is extremely misleading: yes, it is possible that a new collider will find a particle that makes up dark matter, but there is no particular reason to think that it will. And such a machine will tell us nothing about the origin of the universe. Paola Catapano, responsible for audiovisual productions at CERN, informed me that this video “is obviously aimed at politicians and not at fellow physicists and uses the same arguments as those used to promote the LHC in the 90s”.

But great science experiments are investments in our future. Decisions on what to fund should be based on facts, not glossy advertising. For that, we need to know when a prediction is just a guess. And if particle physicists only have guesses, maybe we should wait until they have better reasons why a larger collider might find something new.

It is true that certain technological developments, such as powerful magnets, benefit from these particle colliders and that particle physics contributes positively to science education in general. They’re worthwhile investments, but if that’s what you want to spend, you don’t have to dig a tunnel.

And there are other avenues to explore. For example, astrophysical observations pointing to dark matter should be explored further; a better understanding of these observations would help us make more reliable predictions about whether a larger collider can produce the dark matter particle – if even East a particle.

There are also mid-scale experiments that tend to fall on the table because giant projects consume money. An important medium-scale project is the interface between the quantum domain and gravity, which is now accessible for experimental testing. Another place where discoveries could wait is in the foundations of quantum mechanics. These could have major technological impacts.

Now that the LHC is modernized and particle physics experiments at the detector are taking a break, it’s time for particle physicists to step back and reflect on the state of the field. It’s time for them to ask themselves why none of the exciting predictions they promised have led to discoveries. Money will not solve this problem. And a larger particle will also not collide.

Sabine Hossenfelder is a researcher at the Frankfurt Institute for Advanced Study and author of “Lost in Math: How Beauty Leads Physics Astray”.

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