Gravitational wave detectors and particle accelerators have at least one thing in common -- they require long vacuum tubes through which a narrow beam is fired (a laser in the gravitational wave case, a particle beam in the accelerator case). In both cases, the vacuum tube is many orders of magnitude wider than the beam itself. But interestingly, while the LHC's vacuum tubes are 6.3 cm in diameter, LIGO's are about 20 times wider at 1.2 m in diameter.
So my question is: why are LIGO's vacuum tubes so wide? This must have been a conscious design consideration, since it means that a much larger volume of vacuum must be maintained, and more material must be used to construct the tube. The main consideration for tube width that I can think of is that you have to be able to aim your beam within the width allotted, but surely on these grounds LIGO could have gotten away with a much narrower tube. (Actually, I have no idea -- is this even the deciding factor for the tube width at the LHC?)