Apologies for the naive question, hoping someone with a proper physics background can explain this in layman's terms (as opposed to the more detailed answers at LIGO flawed by the identical expansion of laser wavelength and arms in presence of a gravitational wave?)
If gravitational waves affect spacetime itself, how can we detect changes since the instruments themselves are bent?
For example- I imagine spacetime like a piece of celluloid film. It might be bent or rippled, but inside of the frame everything is still relatively the same- it only looks different from the outside. How is this analogy wrong? i.e. how could a bend in the celluloid be detected from "within" the frame (ok that gets a little weird- like as a character in the movie? but hopefully this demonstrates my confusion on the topic well enough to elicit clarification, hehe)
UPDATE: Seems it has to do with my confusion of lightwaves being a measurement of space rather than a measurement of time. Some helpful links are at http://stuver.blogspot.co.il/2012/09/q-if-light-is-stretchedcompressed-by-gw.html and http://www.americanscientist.org/issues/pub/2004/11/wavy-gravy