The European Organization for Nuclear Research (CERN), the organization that runs the Large Hadron Collider near Geneva, Switzerland, is looking to build a new particle accelerator — an even bigger one.

The group released its conceptual design report earlier this month; the proposed collider, called the Future Circular Collider, would be more than 60 miles in circumference, cost more than €20 billion ($22 billion), and be completed around 2050.

The Large Hadron Collider lies in a 17-mile-long tunnel beneath the France-Switzerland border. There, beams of protons and heavy ions are collided at high energies, and measuring equipment collects data about our universe.

But do we need another particle collider? When CERN built the Large Hadron Collider in 2008, we had a very strong reason to expect that we’d discover something new in physics — our existing models of how subatomic particles interact weren’t adding up, and they were failing to add up in a way that suggested there was a new particle out there to be discovered in the range of energies the Large Hadron Collider was capable of producing.

But the world of particle physics looks quite different now. We discovered the Higgs Boson, which completed the picture of the standard model of physics, and since then the LHC’s extensive tests have turned up no signs of new discoveries.

CERN argues that a new collider might allow us to peer into remaining mysteries in physics, from dark matter to the abundance of matter over antimatter. But some physicists aren’t so sure. Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute, wrote a response to CERN’s announcement making the case that we shouldn’t bother: “A bigger particle collider is one of the most expensive experiments you can think of, and we do not currently have a reason to think it would discover anything new.”

Particle accelerators have historically been a great way to build a deeper picture of our world, though the work they do has become steadily less likely to have practical applications. Some scientists argue that a new accelerator might be of limited scientific use as well as limited practical use — and that it’s important to make sure the public is aware of this as the debate over the accelerator gets under way.

What are particle accelerators, and what does CERN want to build now?

A particle accelerator is a machine that can propel charged particles at very high speeds and energies, which it does using electromagnetic fields to speed up the particles and to keep them contained. In popular coverage, especially in the 20th century, they were often called “atom smashers,” though most accelerators today smash subatomic particles, not atoms.

These high-energy collisions let us measure features of the universe we have no hope of measuring under more normal conditions, and they have driven forward our understanding of fundamental physics. In 1969, physicist Robert Wilson testified before Congress’s Joint Committee on Atomic Energy about a proposal to build Fermilab’s first particle accelerator. He was quizzed on its implications for defense of the country, and famously answered that it had none, “except to help make it worth defending.”

Technically, not all particle accelerators are colliders — a collider is an accelerator that is equipped to arrange for collisions between high-energy particles. Most useful experiments today involve collisions, though, either to directly study the collisions or because the collisions emit high energy X-rays and gamma rays.

CERN is the organization that runs the Large Hadron Collider, famous for its discovery of the Higgs boson, a particle whose existence had been postulated but that couldn’t be found until the LHC was built. The Higgs Boson completed the so-called standard model of particle physics — the model that explains the fundamental forces, except gravity, and describes all the particles we know of. That doesn’t mean there are definitely no new particles out there, but it is conceivable for the first time that we’ve found them all.

One team at CERN works on planning the future of particle physics, beyond the expected lifespan of the LHC. They started their work in 2014, and this month they released design documents for a new proposed collider. The European nations that collaborate to fund CERN would have to approve the proposal, and the funding would come primarily from their contributions. The new collider would be almost four times the size of the LHC, and would enable colliding particles about 10 times as fast. At minimum, that will let us measure some parameters more precisely. Researchers are hoping it will do more than that.

We have learned some important things from particle accelerators. Early particle accelerators let us discover new isotopes and new elements of the periodic table. Accelerators are used for testing parts and materials for spacecraft. Nearly all of the standard model of physics, which connects all of the forces we know of (except gravity), was built off discoveries from particle colliders.

But as we’ve developed a more complete understanding of physics, the practical applications of new high-energy accelerators have grown sparser and sparser. Fewer scientific questions are unanswered, too. With the discoveries of the LHC, the standard model of particle physics is complete. That’s obviously not to say that we understand everything about the universe — there are still lots of mysteries. But for the first time, it’s plausible that a bigger collider will turn up no new particles.

CERN does not see this as a convincing argument against building a new collider. “If we look back at the history of particle physics,” Arnaud Marsollier, head of Media Relations at CERN, wrote me, “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.”

That’s absolutely the case. But there’s reason to think that this time might be different.

“The scientific case is weaker than it has been for past colliders,” Jared Kaplan, a theoretical physicist at Johns Hopkins University, told me. Historically, it has also sometimes been argued that new physics discoveries will help us develop new technologies. This “humanitarian” case for accelerators hasn’t really been applicable to recent high-energy physics. “The humanitarian case is very weak, and it was weak for the LHC as well.”

Particle accelerators don’t tend to produce concrete technical innovations

Wilson’s testimony in 1969 was unusually direct about something anyone lobbying for additional funding doesn’t always want to acknowledge about their work: humanity is not going to invent new industrial techniques, new energy sources, or new weapons with the discoveries.

As Kaplan and other physicists argue, we’re particularly unlikely to encounter any from the Future Circular Collider. That’s because the areas of particle physics with applications have largely all been explored — and the remaining areas would be exceptionally hard to mine for real-world applications, even if we discovered something unexpected.

“The reason for pessimism about practical applications is that we understand nature really, really well in some respects,” Kaplan told me. “You’re getting further removed from relevance to technologies on a human scale.”

The particles we discover in colliders like these exist only under extremely rare conditions, require extraordinary effort to produce, and are incredibly unstable, existing for only fractions of a second. Even if one of them had incredibly useful properties, the physicists I spoke with said, it’s hard to imagine how we’d get to industrial applications.

This highlights an interesting fact about physics: our approximations of the physical world work astoundingly well, allowing us to figure out most industrial applications of physical principles even when our grasp of them is very bare bones. “You don’t need to understand how quarks work to do nuclear physics,” Kaplan told me. “We didn’t know quarks existed when we did the Manhattan project.”

So whatever we discover with the Future Circular Collider, it’s very unlikely to be a new energy source or to produce new products or techniques.

“Can we justify these expenses by the technological breakthroughs that we make along the way? I have mixed feelings about that,” Sean Carroll, a physics professor at Caltech, told me. “The things we would discover have zero chance of leading to technological breakthroughs,” though, he emphasized, “there are absolutely technological breakthroughs from the process we use to build accelerators.”

That said, the case for the Future Circular Collider is that it might teach us new things about the universe, not that it can lead to new techniques because it happens to be a hugely ambitious construction project.

Is there research it’d be better to fund instead?

Theoretical physicists are largely in agreement on all of that. They are nonetheless divided on whether to build CERN’s new accelerator. What divides them is, in significant part, disagreement over where the money could go instead.

“There are a lot of other experiments that are proposed and ongoing that are much cheaper,” Kaplan told me. “There are still huge mysteries in other domains of physics. A lot of the experiments that search for dark matter are $10 million, not $20 billion. It might make more sense to fund a hundred of those experiments than build one collider for 10 times as much money.”

This is at the core of Hossenfelder’s case against the collider, too. “At current, other large-scale experiments would more reliably offer new insights into the foundations of physics,” she argues in her blog post. “Anything that peers back into the early universe, such as big radio telescopes, for example, or anything that probes the properties of dark matter. There are also medium and small-scale experiments that tend to fall off the table if big collaborations eat up the bulk of money and attention.”

Carroll disagrees. He pointed me to the debate in the 1990s about building a particle accelerator in Texas, one large enough to have discovered the Higgs Boson and perhaps even more. Some physicists observed at the time that the money might go farther if it were dedicated to other physics experiments, and the collider was voted down.

But did the money then go to other physics experiments? No. “If you don’t spend the 20 billion on the particle accelerator, they’re not going to give it to other physicists,” he told me. “They never do that.”

From that perspective, the choice is not between funding the accelerator and spending $22 billion on strategic grants to promising scientific research that improves both our understanding of the universe and our daily lives. If you don’t expect funding in any other important areas to increase if we hold back on the accelerator, then doing cost-benefit analyses on the accelerator misses the point a little.

So, should we build it?

Hossenfelder is among those more strongly opposed to the collider, but the striking thing about her series of blog posts opposing it is that she is primarily concerned by what she sees as a dishonest attempt to make the case for the collider sound stronger than it is. She argues that CERN’s press releases overstate how much new science we’ll learn from the new collider, knowing that the public doesn’t know enough to evaluate these claims.

The CERN design reports mention several of the remaining mysteries in physics — for example, dark matter, neutrino masses, and why there’s more matter than there is antimatter. But these are mysteries that the accelerator is unlikely to resolve. Hossenfelder writes:

The accompanying video similarly speaks vaguely of “big questions,” something to do with 95% of the universe (referring to dark matter and dark energy) and raises the impression that a larger collider would tell us something interesting about that. It is correct that the standard model requires extension, but there is no reason that the new physical effects, like particles making up dark matter, must be accessible at the next larger collider. Indeed, the currently most reliable predictions put any new physics at energies 14 orders of magnitude higher, well out of the reach of any collider we’ll be able to build in the coming centuries.

In other words, Hossenfelder’s biggest complaint is not just that the collider likely won’t turn up anything new. It’s that she thinks CERN isn’t being open about that. (Arnaud Marsollier, head of media relations at CERN, responded to her concerns in an email to me, arguing that these are reasonable mysteries to highlight in making the case for the Future Circular Collider: “Clearly, no fundamental project — large or small — can promise a major discovery, but progress is also made of perseverance and if we stop exploring, we may never know what dark matter is, or why there is domination of matter over antimatter.”)

Hossenfelder isn’t the only one with the concern that the particle collider conversation isn’t being driven by a clear picture of the merits. “If you talk to people about how this actually gets decided, it has nothing to do with a serious cost-benefit analysis,” Jess Riedel, a researcher at the Perimeter Institute for Theoretical Physics, said.

I get the sense we need someone like Wilson, who openly told Congress that the accelerator he and other researchers dreamed of would have no defense applications, no security applications, no benefits against the Russians. Then we can decide whether we want a particle accelerator, just because, for its own sake, instead of selling it to the public with an overstatement of what it will permit us to understand.

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