I understand basically what goes on at the LHC: They take protons, run them around the accelerator, they collide in the "experiment centers" such as Atlas and CMS, and then record the results of the collisions from detectors that span out radially from the collision site. That part is clear enough. What I haven't been able to find after much searching, though, is exactly what it means for the LHC to be "turned on" during an experiment. I mean, I've seen documentaries where someone is being interviewed outside the LHC and they're talking about how they can't go in and show the interviewer the guts of the machine because it's "in operation." What I want to know is what does "in operation" mean, and how are these experiments run. They always say they generate an enormous amount of data out of each "run." How long is each "run." One hour, one minute, all day long, all week long? Is this something where they just need to turn the collider on for a few minutes in order to create millions of collisions and generate tons of data? Or is it something where the collisions are relatively rare so they have to leave it running for hours or days at a time to get any meaningful data?