I wonder why detectors for gravitational waves have only two perpendicular arms, not three. Having three arms appears to allow for better detection of direction, or may even increase sensitivity (I may be wrong). So far I came up with a few guesses, but I'm by no means an expert.
- two arms are cheaper than three
- a 600m tower is a no-go (stability), a 600m deep hole is a challenge to build and operate
- two arms is all we need, three would not guarantee significantly better results
- photons going down and up would gain and lose energy; those in the surface arms do not or much less so. Signal evaluation would be messy.
What are the true reasons?