Why the 'nightmare scenario' is looming in particle physics — and a proposed collider may be a giant mistake
02 February 2019
Theoretical physicist Sabine Hossenfelder is optimistic about the future of her field, seeing many potential avenues of exploration. Questions about the nature of dark matter, dark energy, zero-point energy, quantum gravity, and the measurement problem in quantum mechanics all present fascinating puzzles for physicists to untangle.
But she also warns that we may have reached the “nightmare scenario” in a particular branch of the discipline: particle physics.
The problem? The best theories imply that there may be no more new elementary particles left to discover.
It’s a potentially devastating conclusion for a particle physicist. There would be no game-changing discoveries on the horizon for the field, no more major eureka moments that transform careers and alter the course of scientific history.
This problem, Hossenfelder argues, also undermines the case for the newly proposed particle collider from CERN, the European Organization for Nuclear Research.
CERN is best known for its construction of the Large Hadron Collider, a massive circular apparatus with a 27-kilometer circumference built underground near Geneva. The collider propels streams of particles and smashes them into each other. It then measures the resulting collisions for evidence about the smallest parts of the universe. Using this method, physicists working on the project were finally able to confirm the existence of the Higgs boson, a particle that has long been predicted by the Standard Model of the universe.
Now, to conduct further research, CERN is hoping to build the Future Circular Collider, an even more massive and more expensive project that would have a 100-kilometer circumference, allowing particles to travel at higher speeds.
It’s an ambitious plan — but it’s not obvious why we should build it.
“The Higgs is the last particle that the Standard Model needs,” Hossenfelder told me. “So now there is nothing more missing.”
In other words, the Large Hadron Collider found, essentially, the last puzzle piece. What is the new collider supposed to find?
To be sure, it’s in the nature of science that our models are often wrong and that new, surprising discoveries can emerge. So just because we appear to have a completed version of the Standard Model, the theory physicists have developed that accounts for all the known elementary particles in the universe, we can’t conclude no more particles will be found.
Arnaud Marsollier, head of media relations for CERN. explained that there are “many cases in science history, where scientific breakthroughs did not come from something predicted or predictable.”
He added: “If we look back at the history of particle physics, we can also see that huge advances in knowledge and technology have been made each time we have reached more precision and more energy, innovating with new larger facilities.”
In Hossenfelder’s view, though, this is largely wishful thinking.
When the Large Hadron Collider was built, physicists could confidently predict that it would uncover evidence of the Higgs boson. Discovering it confirmed the strength and predictive value of the Standard Model, greatly enhancing our scientific understanding of the universe. Had the evidence from the Large Hadron Collider shown that the Higgs did not exist, that would have been perplexing — but it would have told us something was deeply wrong with the Standard Model, which would have been a remarkable discovery in its own right.
The Future Circular Collider doesn’t share the same kind of theoretical justification. It would certainly allow us to fill in some of the gaps in our knowledge, Hossenfelder explained, and allow particle physicists to make increasingly fine-tuned measurements of known phenomena. But there’s no good reason to think it will reshape the way we think about the universe.
For a scientist, there’s nothing bleaker than discovering that there’s nothing left to find. At one point in the experiments at the Large Hadron Collider, physicists were ecstatic when they thought they discovered evidence of a particle not accounted for in the Standard Model — but it turned out, to their disappointment, just to be statistical noise. With this letdown, the nightmare scenario loomed closer. Particle physicists aren’t necessarily there yet, but it’s possible that they just won’t find any new particles hiding in some heretofore unprobed cranny of the universe. Perhaps the Standard Model is, for these purposes, complete.
Over at CERN, though, optimism reigns supreme. Marsollier disagreed with Hossenfelder’s negative assessment of the Future Circular Collider’s potential to answer major puzzles in physics.
“[Many] experimentalists and theoreticians think a next collider would be really useful to address these questions in the future,” he told me. “For instance, amongst all of the possibilities for the microscopic nature of dark matter, there are well-motivated classes of dark matter particles, to which a future collider will have excellent sensitivity.”
“I cannot exclude the possibility,” Hossenfelder said of the chance of the new collider finding a brand new particle. “The only thing that I can tell you is that there is no good reason to think it should happen.”
She argued that physics has entered something of a crisis as the nightmare scenario draws closer, and many unscientific, speculative and ad hoc theories have been published. Researchers have incentives to concoct new hypotheses and models that posit the existence of particles that could, in theory, be detected at attainable collider speeds. But there’s no credible basis for most of these theories, she said — they’re essentially fantasies dressed up in math. Anyone can create farfetched and falsifiable claims, but unless they’re somehow grounded in reality, there’s little value in disproving them.
Adam Falkowski, a particle physicist, agreed that the promulgation of unscientific theories in the discipline is a problem, though he suggested that there have been serious efforts to curtail the issue.
And while he echoed some of Hossenfelder’s reservations about the proposed collider, observing that these types of arguments have roiled the physics community at large, he said he would “love” to see the data from the project.
“Definitely, we would learn a lot from this collider,” he said. But he admitted that there’s little reason to hope we’ll learn about a new particle.
“From what we know, there would be no inconsistency in our theories if there are no particles in this energy range that would be explored in this new collider,” he said.
He also said he cringed when he heard advocates invoke claims about potential new particles, dark matter, or the relationship between matter and anti-matter to justify the collider.
The chances that the collider would shed light on these topics “are pretty slim,” he said.
That brings us to one of Hossenfelder’s biggest complaints about the collider. While its advocates are making dubious arguments in its favor — she argued that a promotional video for the project was “full of lies” — they’re asking for a massive amount of money for it: $10 billion, at the low end of the estimate, ranging to over $20 billion.
“Is it worth $10 billion? I don’t think so,” she said.
Marsollier, the CERN spokesperson, warned that abandoning the project may force us to miss out on priceless discoveries.
“I even remember a time when some said the prediction of the Higgs should have been wrong. If we had just given up and not built the LHC, the Higgs would still be in question today,” he said.
But given the apparently low likelihood that the new collider will deliver any groundbreaking findings, is the cost worth it? Perhaps not — especially if we recognize that there are many other projects in the study of physics that might be worthy of funding.
“It’s not that it’s a bad investment per se,” said Hossenfelder, “I just think that there are better investments.”
Falkowski noted, on the other hand, that the financial question is complex. It’s not the case that, if the money isn’t spent on the collider, it will go to other worthwhile scientific studies. Maybe it will, but governments that fund such projects don’t always have the best priorities — and in light of the difficulty of imagining plausible counterfactual spending scenarios, Falkowski was agnostic on the question of whether the price tag was defensible.
As a particle physicist, of course, he is more interested than most in what the collider is likely to uncover. One question the Future Circular Collider would be well-suited to probe, Falkowski said, is how the Higgs boson interacts with itself. This would be the most concrete and important test the collider could perform, he said.
More broadly, Falkowski isn’t too concerned about the “nightmare scenario” for particle physics that Hossenfelder discussed.
“We know that we haven’t reached the end of the line yet. We know that our theories are not complete,” he said.
Outstanding questions about dark matter, anti-matter, and the masses of neutrinos continue to bewilder particle physicists and suggest fascinating new discoveries are still waiting.
What’s “nightmarish,” Falkowski said, is that physicists don’t have a plan to solve these mysteries. For decades, there had always been a clear path forward. Starting in the 1950s, they started building particle colliders and making discoveries. And there was always a good reason to build the next, bigger collider.
Now, while the justifications for building another collider are getting thinner, the costs are getting higher. And the costs don’t only come in dollars but in time. The next collider, if built, might not come online until 2040, and it won’t be fully operational until the 2050s — pushing the bounds of many working physicists’ lifetimes.