It took the Large Hadron Collider just three years to find the Higgs boson–but it took nearly 20 years to create the Large Hadron Collider. High energy physics happens at the speed of light, but the underlying practicalities move at the speed of bureaucracy, funding requests, and setting concrete. So to keep things moving forward, the global physics community is constantly envisioning and re-envisioning the next big things in high energy particle physics–things big enough to dwarf even the largest and most expensive science experiment mankind has ever created.

Last month at a meeting in Krakow, Poland, we caught a glimpse of these next big things. CERN’S European Strategy Preparatory Group symposium earlier this month collected particle physicists and science policy makers from around Europe and the globe to consider the current and future needs of the physics community and to discuss its many possible futures. Two things seem certain at this point: The LHC isn’t going anywhere just yet, but eventually we’re going to need a bigger, badder replacement for the LHC.

“The LHC will continue to run, and the LHC will continue to be a very, very big part of the particle physics program for the next 15 or 20 years, primarily in finding out more about the Higgs boson,” says Terry Wyatt, a professor of physics at the University of Manchester and an attendee of the strategy symposium. “One of the main points of consensus that emerged from this meeting was that upgrading the LHC to what we’re calling the High Luminosity LHC will take us through 2030 or so.”

These upgrades would include replacing the current LHC accelerator ring magnets with newer, stronger magnets sometime around 2022, essentially creating a more powerful accelerator in the existing LHC footprint. That would allow researchers to continue performing the world’s most advanced physics beneath the Swiss-French border until the end of the next decade. That also means that if physicists want to undertake another experiment as large and ambitious as the LHC and have it ready to start smashing particles by the time the LHC winds down operations around 2030, they really should already be deep into the design phase and preparing to break ground. They’re not; but they’re certainly talking about it. Also up for discussion at the symposium was the question on everyone’s minds: what’s next?

“If you’re serious about having a new accelerator that might start running in 2035, you have to be moving now.”Wyatt is the former leader of the DZero experiment at Fermilab’s Tevatron collider in Illinois and is currently working on the LHC’s ATLAS experiment. He’s also the former chairman of the LHC experiments committee and was a member of CERN’s research board, and currently sits on CERN’s Scientific Policy Committee. That’s another way of saying Wyatt has spent a fair amount of time thinking about the possibilities for high energy particle physics around the world, and at the Krakow strategy session it was Wyatt that presented a talk titled Next Step Facilities. In it, he touched on everything from a proposed 50-mile accelerator ring that would be three times larger than the existing LHC to linear electron-positron colliders and muon smashers–all with the ability to go to far higher collision energies than the LHC. As with physics itself, the possibilities are many–which isn’t necessarily favorable when you’re trying to secure government funding and break ground on multi-decade construction project.

The lack of a clear path forward for the high-energy physics community isn’t simply the result of a lack of funds or of bureaucratic inertia (at least, it’s not only those things). The LHC has just scratched the surface of the Higgs boson, and it has only reached half of its designed maximum collision energy of 14 terra-electronvolts (TeV). The High Luminosity LHC will produce something like ten times more data than its predecessor, and physicists expect that data to yield many new discoveries that will influence the design and concept for the next high-energy particle collider.

That presents a kind of quandary for physicists. To keep particle physics moving seamlessly forward, we need to start building the next-gen collider today. To make sure we build the right next-gen collider, we need more data from the LHC. So in the interim, physicists and engineers postulate, dream, and get to work at their drawing boards.

“The trouble with this is that the time scales for doing any of this are so long that if you’re serious about having a new accelerator that might start running in 2035, you have to be moving now, already, very, very seriously,” Wyatt says. “The technical and engineering challenges–developing these magnets, for instance–is a very, very high-tech thing that takes many, many years. So people are working very seriously right now on technical and engineering challenges that might not actually be used in an accelerator for another 20 years.”

That’s if they’re even used at all. Wyatt likens this process to medieval architects designing and building cathedrals that they knew they would never live to see completed. There’s no telling exactly which of these cathedrals of collision will end up taking high energy physics to the next level. It could be any of them, or something completely different that we have yet to conceive. Click through the gallery link to take a spin through the many possible futures of high energy particle smashing machines as we envision them today. This list is by no means exhaustive, but representative of some of the leading ideas in physics today (and the ones most fascinating to us). If we’ve missed an accelerator/collider concept that you find particularly intriguing, let us know in the comments.

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What: An upgraded LHC Where: CERN Particles Smashed: Protons Likelihood: Pretty high Why dig a whole new accelerator ring when the Large Hadron Collider already exists in a perfectly good 17-mile tunnel? The High Luminosity LHC (HL-LHC) is a proposed upgrade for the existing LHC facility. Included in the upgrade: new, more powerful beam magnets to replace some of those that are currently deteriorating little by little each time a particle collision releases a burst of radiation and hardware updates to the ATLAS and CMS detectors. The upgrade would not push the LHC’s collision energy all that much higher, but it would increase its luminosity–the number of collisions it can achieve at a given time–tenfold, increasing the amount of data produced by the LHC by the same amount.

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What: A bigger, stronger LHC Where: CERN Size: 50 miles circumference Particles Smashed: Protons (or maybe electrons-positrons) Likelihood: Low, for the time being This concept is basically the Large Hadron Collider injected with performance enhancing drugs. Like the LHC, the proposed super-ring would likely house a proton-proton collider, and like the LHC it would accelerate particles by hurling them around in a circle via tightly-controlled beams. The most striking difference is the size: this new collider would be 50 miles in circumference, dwarfing the LHC’s 17-mile ring. The larger the ring and the more powerful the magnets and accelerator apparatus, the higher collision energies you can produce. This produces more massive new particles, that scientists could then identify, study, and hopefully fit into the Standard Model. Moreover, we could do this without reinventing the wheel–or the ring, as it were. The proposed 50-mile tunnel is basically just the LHC made bigger and stronger–able to theoretically approach a maximum collision energy of 80 Tev (compared to the LHC’s current maximum design energy of 14 TeV). It would require some advances in magnet technology and the like, but the fundamentals would be largely the same as the ones governing the existing LHC. But, Wyatt cautions, it’s seeming simplicity (relatively speaking) doesn’t mean this collider is any less speculative than other potential accelerators, at least for the time being. a€œWe’re not really in a position yet to make a decision about whether such a ring would be the most sensible thing to do,a€ he says. a€œFor many of these future facilities we still need to see what kind of physics come out of the LHC. It would be crazy to start digging that tunnel now.a€

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What: A long, straight accelerator for smashing lighter-mass particles Where: Undetermined (possibly in Japan) Size: 19 miles long Particles Smashed: Electrons and positrons Likelihood: Fair Protons aren’t the only particles worth smashing, but they have their advantages, the most critical being mass. The more massive a particle, the less it radiates when it’s being accelerated. For a large ring collider like the LHC, it’s very difficult to accelerate lower-mass particles like electrons effectively because each time around the ring they radiate roughly four percent of their energy. Given that particle acceleration and collision happens at extremely high velocities, it’s easy enough to see how an electron could very quickly lose most of its energy while circling the loop–even a 17-mile one. If you want to smash electrons, then, you build a linear collider–a straight pipe with a particle accelerator at either end, through which you send electrons and positrons hurtling toward each other (and toward mutual destruction). This way the particles don’t lose most of their energy while looping the accelerator ring. But there are other disadvantages to take into account. First, linear colliders have to accelerate these particles to very high speeds over relatively short distances–the proposed International Linear Collider is roughly 19 miles from end to end, so from end to collision point in the middle is half that–and that requires a lot of energy up front. Then, when only a fraction of the electron-positron pairs collide, the unused accelerated particles can’t go around the loop again and try for a second collision. They are wasted, discarded, and the whole energy-intensive process starts over again. All said, it’s less efficient than a proton collider. Yet the ILC is one of the clear favorites within the physics community right now and it or something like it could very well be world’s the next big science experiment. a€œTechnically, such an accelerator is likely feasible,a€ Wyatt says. a€œThe problem is, it will cost many billions of whatever currency unit you like to think in.a€

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What: A more powerful linear collider Where: Undetermined, possibly at CERN Size: Undetermined Particles Smashed: Electrons and positrons Likelihood: Not likely in near term (technology still unproven) The Compact Linear Collider (CLIC) concept works on most of the same principles as the International Linear Collider (and other linear colliders in general). The key difference is in how CLIC provides the electromagnetic fields that accelerate the electrons and positrons it collides. In theory, these fields can be much stronger than those produced by a standard linear collider, but in practice there’s still a lot of proving out to be done. To generate these stronger accelerating fields, the CLIC begins with two beams in each accelerator–a main beam and a drive beam. The drive beam would contain many, many particles (making it high-intensity) but would be at a relatively low energy. A specially designed exchanger (and the magic of particle physics) then siphons particles and energy from the drive beam and injects them into the main beam, which is accelerated via this transfer. These main beams go on to collide at the center of the CLIC. a€œIt’s an idea where people have started to demonstrate that this idea works, but there are still many more stages in the R&D to really demonstrate that this technique could work on a massive scale when you have a 30- or 40-kilometer long accelerator,a€ Wyatt says. a€œWhere the proponents of the ILC say we could virtually start building it tomorrow, the CLIC needs to go through several more stages of demonstrating. It’s further away in terms of technological readiness.a€

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What: A ring-accelerator/collider for muons Where: Undetermined (Fermilab has drawn up a purely speculative proposal for its site in Illinois) Size: Undetermined Particles Smashed: Muons Likelihood: Not likely in 2030s, maybe by mid-century With masses between those of protons and electrons, muons are ideal for colliding–they are much heavier than electrons so they don’t radiate all their energy away, but not quite as heavy as protons. Protons are made of constituent particles, so when they collide not all of their energy is available to create new particles–some washes away in the dissolution of the proton itself into more elementary particles. Muons, on the other hand, are elementary. If we could smash them, nearly all of their energy would go toward creating new particles. But muons are inherently unstable. Produce them in the lab, and they decay almost immediately. However, when you’re talking about accelerating things to velocities approaching the speed of light, relativity says you can cheat a little. Time dilation–the idea in relativity that tells us that the life cycle of a particle increases with its speed–means that if we could get muons moving fast enough before they begin decaying, they could live for much longer, at least long enough to make a few laps around an accelerator ring and smash into each other. Muon colliders (the one pictured is a theoretical setup envisioned by Fermilab) are a cool idea that some physicists are putting a lot of thought into, Wyatt says, but they are unlikely to be the next big thing in particle physics. Like the 50-mile proton-proton ring, we’re just not at the point in the larger study of particle physics where we can–in good conscience–make a multi-billion commitment to smashing them. File under a€œinherently awesome, but unlikely before mid-century.a€