Crisis, what crisis? The future of particle physics has been a major talking point of late, with decisions on next-generation high-energy colliders contrasting with skepticism as to whether such monumental (and monumentally expensive) megamachines should be built in the first place. Many physicists say such critiques are unjustified, yet acknowledge the profound uncertainties surrounding plans for future forays deeper into the subatomic realm.
Late last week Japan announced it would delay its decision on whether or not to build a new facility called the International Linear Collider (ILC). Among other goals, this vast machine spanning 20 kilometers and costing an estimated $7.5 billion would enable unprecedented studies of the Higgs boson—the enigmatic particle that imbues others with mass, discovered in 2012 by the purpose-built Large Hadron Collider (LHC) at CERN in Switzerland. Japan’s cautious approach to the ILC is symptomatic of lingering uncertainties over where the field of particle physics itself should go. “It’s an interesting time for particle physics at the moment, because the last big missing piece of the puzzle of the Standard Model was found in the discovery of the Higgs boson,” says Carsten Welsch, head of the Department of Physics at the University of Liverpool in England. “The question is now, what’s coming next?”
Physicists around the world were closely watching Japan’s debate about the ILC, because a “yes” or “no” on that project could trigger a domino effect causing similar plans in other countries to be scrapped or approved. Europe and China are each considering new colliders of their own, but their final decisions will be heavily dictated by what occurs elsewhere. “If the Japanese government had said they really wanted to build the ILC, that would have had a huge impact on the European strategy for sure,” says Jon Butterworth from University College London (U.C.L.), the U.K.’s delegate for the CERN Council’s European Strategy for Particle Physics. “Given they’ve effectively said ‘no’ for now, that will also have an impact.”
The major debates in particle physics at the moment concern which questions researchers should attempt to answer next. The discovery of the Higgs boson essentially completed the Standard Model of physics, the theory that governs our understanding of the subatomic world and dictates how all but one of the known fundamental forces should collectively behave (the force of gravity is the glaring omission). Nevertheless, scientists are now wondering whether we should explore this area further, creating so called “Higgs machines” to churn out countless Higgs bosons, or whether we should instead seek to dive even deeper into physical frontiers by smashing particles together at ever-greater energies. “We’re right on the cusp of a revolution but we don’t really know where that revolution is going to be coming from,” says James Beacham, a particle physicist on the LHC and postdoc at Duke University. “It’s so exciting and enticing. I would argue there’s never been a better time to be a particle physicist.”
At present, the LHC remains the most powerful particle collider on Earth—and is presently receiving upgrades to maintain its front-runner status into the 2020s. But an even larger circular collider would reach energies higher than those possible at the LHC, allowing physicists to probe new parts of the subatomic realm. Earlier this year CERN unveiled a proposal for such a machine, called the Future Circular Collider (FCC), which would use a 100-kilometer ringed tunnel to surpass the LHC’s power by a factor of 10, reaching energies of 100 tera–electron volts (TeV). Such a machine, however, would likely cost in excess of $20 billion and only begin operations in the 2050s, leading some critics to question whether it is the right route to take.
Further complicating matters is China’s own plan for a similar large collider, called the Circular Electron Positron Collider (CEPC). Most experts agree there would be little need to construct both the CEPC and the FCC, so discussions are taking place in Europe on whether to work with China on such a project, unilaterally build the FCC or let China pursue the CEPC alone. These questions will be formally addressed in the European Strategy for Particle Physics, set to be drafted in January 2020. “I wouldn’t think there would be room for two machines of that scale,” Welsch says.
CERN also has a proposal for a linear collider akin to Japan’s ILC, called the Compact Linear Collider (CLIC). Again, there is little need for both the ILC and the CLIC, so Japan’s final decision on whether to proceed could effectively determine Europe’s decision, too. At the same time two next-generation non-collider experiments are being developed, one in the U.S. called the Deep Underground Neutrino Experiment (DUNE) and another in Japan called Hyper-Kamiokande (HK). Both of these projects intend to perform breakthrough studies of neutrinos, nearly massless particles that exhibit subtle hints of physics beyond the Standard Model.
This cavalcade of detectors and experiments demonstrates that although particle physics is hardly facing a crisis, it is certainly at a crossroads. Additionally, floating above all the proposals is the notion that the wisest approach would be delaying new machines entirely until potential breakthrough techniques become available. One such technique is plasma wakefield acceleration, a cheaper, more efficient method of accelerating particles using plasmas in comparison with the sprawling and expensive electromagnets employed in present-day colliders. “Everyone's keeping a lazy eye on that, and some people are doing full-time research on it,” Beacham says. “But it’s really not going to be possible to use [in] a gigantic collider for probably decades, if not longer.”
Some scientists even suggest it is premature to consider expensive, multidecadal projects like the FCC at all, given the current listlessness of particle physics. They argue that in the case of circular colliders, we would be looking for physics that we aren’t even sure exists. The nightmare scenario would be a project with energies beyond that achievable by the LHC that would only reveal what some theorists call “the desert,” a barren region otherwise devoid of new discoveries. “This next larger collider will be ridiculously expensive and it has no clear discovery potential,” says Sabine Hossenfelder, a theoretical physicist from the Frankfurt Institute for Advanced Studies in Germany. “If the LHC does not see anything in the upcoming run and at the high luminosity phase, then I think it’s not a good investment to build a larger collider at this point.”
Sir David King, the U.K.’s former chief scientific advisor, even goes as far to suggest it might be time to wrap up particle physics as we know it, not only because of what might be diminishing returns in terms of new discoveries but also due to the opportunity cost next-generation machines would bear for dealing with more pressing concerns. “I’m happy to draw a line at the FCC, congratulate all the particle physicists on the amazing work they’ve done, but suggest they move on to other extraordinarily challenging aspects of fundamental science,” he says. “I'm saying this at a time when humanity is faced with the biggest potential crisis it has ever had to face up to, which is climate change. I believe our intellectual resources should be focused on that.”
Most leading physicists, understandably, disagree with this viewpoint. “Only people who have no knowledge about science can believe that we are at the end of particle physics,” says Gian Giudice, the head of CERN’s Department of Theoretical Physics. “There are still lots of open questions that need to be answered.”
Those questions include searching for weakly interacting massive particles (WIMPs), the leading candidates for dark matter, which could—but not definitely—spring up in machines such as the FCC. Scientists are also keen to test supersymmetry, the idea each particle in the Standard Model has a “partner particle” of sorts. And, of course, there is the troubling matter–antimatter problem—namely, if matter and antimatter were created in equal amounts in the big bang and destroyed each other, how did a tiny amount of matter manage to survive?
It is perhaps unsurprising physicists can offer no guarantees for future multibillion-dollar colliders answering such questions—otherwise, the reasoning goes, what would be the point of asking? But even if those hoped-for future facilities arise and fail to bear that fullest fruit, the knowledge gained along the way and perhaps even the chance of non-discoveries at higher energy levels present enticing prospects of their own. “We’re faced with some enormous questions, things that we don’t understand about the universe,” says Ritchie Patterson, director of the Cornell Laboratory for Accelerator-based Sciences and Education. “If there’s a chance of finding the answers, then we need to pursue that.”
Discussions over the next year will be crucial in deciding what direction is taken, if one is taken at all. “In an ideal world we’ll have something like the FCC that goes up to 100 TeV, and we’ll also have one of those ‘tweezer’ machines that really understands the Higgs boson,” Beacham says. But whether that will be the case remains to be seen. “It’s more exciting and more uncertain now than I think it’s ever been in my career,” Butterworth says.