This morning at 9:30 local time, proton-proton collisions returned to the Large Hadron Collider for the first time in more than two years. While the collisions were nowhere close to the full energy that the LHC is now rated for, they represent an important step in the process that will return the machine to readiness for experimental physics.

The LHC is at the end of a long chain of smaller accelerators that feed it protons at an energy of 450 Giga-electronVolts (GeV). With its upgraded hardware, the LHC should be capable of accelerating these to 6.5 Tera-electronVolts (TeV), leading to collisions at 13TeV. For testing purposes, however, this morning's collisions were simply run at the injection energies, leading to 900GeV collisions. CERN's announcement indicates that these collisions were used in part to check out the detectors, which have undergone significant upgrades as well.

As previous accelerator tests have accelerated protons to 6.5TeV but not collided them, all the pieces appear to be in place. 13TeV collisions will also be part of the checkout process that will ultimately return the machine to a full program of physics experiments. (Assuming the first collisions don't result in the collapse of the Universe or something similar.)