The Large Hadron Collider, the most powerful particle accelerator ever constructed, is gearing up for another attempt at actually colliding said particles, with the entire 27km ring having been brought down to an operating temperature of 1.9K—colder than outer space itself. This isn't the first time a startup of the massive device has been attempted; a bit over a year ago, particles had circulated within the LHC for over a week before it suffered a catastrophic failure. That failure has led to a series of new safety measures that should avoid similar problems, but other issues may crop up, simply due to the fact that we've never built anything quite like this before.

Of course, that may end up being a good thing. If the LHC can work its way up to its full potential, we may see things that are nothing like anything we've observed before.

The engineering

The LHC is built to accelerate particles to a speed that's so close to that of light that, for most of us, the difference could be considered a rounding error. That's fast enough that particles will run over 11,000 laps around the 27km circumference each second. Obviously, that creates a number of engineering challenges.

For one, you can't afford to have much else in the path of the particle beam, so the entire thing has to be run in a vacuum—the LHC will have an vacuum that's 10 times less dense than the atmosphere of the moon. The other challenge is that these particles will have a very strong tendency to travel in a straight line. To get them to actually run in a curved path, the LHC relies on an enormous collection of exceptionally powerful magnets to curve and focus the beam.

That's where the low temperatures come in, as the magnets require superconductive wiring to generate an appropriately strong magnetic field. That also requires prodigious amounts of electric current, which can only be delivered by wires that are also superconducting. (So much current is involved, in fact, that the LHC's operational schedule is built around the European energy supply.)

All of this means that when something goes wrong—even something small—there's a very real chance that it will go wrong in a big way. That seems to have been what happened in last year's failure, which was triggered by a major leak in the liquid helium coolant.

This is a serious danger because these, in turn, can lead to failures in the superconducting wiring and the magnets it feeds. Since the failure wouldn't spread evenly through the magnet, the carefully-balanced forces between the poles could became unbalanced, and the off-axis force can tear things apart.

Fortunately, there was plenty of opportunity to learn from this problem, and the newly revamped LHC has better monitoring equipment in its superconducting wiring, and a new system for dumping some of the energy out of the magnets once failures are detected. It remains possible that something new might pop up, but a repeat of the same issues seems far less likely.

The science

Once beams start running through the collider again, the idea is to run the LHC over the winter at half its rated power before bringing the collision energy up to its full 14 tera electron volts (TeV) sometime next spring. At that point, we should see... well, we're not entirely sure what. That's not to say that we have no idea what will happen; rather, there are a range of possibilities, some of which may be out of reach of the energies created by the LHC.

That point is worth elaborating on. Based on what we already know about the Universe, it's possible to create models that include things like the Higgs Boson (which conveys mass) and exotic dark matter particles. However, it's possible to have a number of models—in some cases, several entire classes of models—that are consistent with the data we have in hand. Some of these predict that it will take less than 14TeV to produce a Higgs Boson, others don't; a similar thing applies to dark matter.

So it's entirely possible that the Higgs will continue to elude us even after the LHC reaches its full potential. But that will also mean that, by failing to find it at 14TeV, we've eliminated a significant number of potential models, allowing physicists to focus in on the remainder. The flipside to not really knowing what we'll see is that reality may always surprise us with something either considered low probability—quantum black holes, anyone?—or something entirely unexpected.

Of course, all of this assumes that reality won't keep ensuring that the LHC breaks down, a prospect recently considered in The New York Times. Continued failures of the LHC may also constrain various models of reality, but only if we can accurately calculate the probability at which 27 kilometers worth of one-of-a-kind hardware will fail.

You can follow the LHC's progress at the CERN Bulletin.

Listing image by CERN