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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

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Because the speed of light does not change. So if the length between 2 mirors change, the time of flight will change. In practice this can be mesured at very high precision using interferences between the long-path light and a short-path light.

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  • $\begingroup$ Thanks, I'm still a bit confused though... what am I getting wrong with the following: the length between 2 mirrors changes when looking from outside of spacetime, due to the curvature of it, but not from within. For example with a piece of film- if we bend it then the distance between points on the film changes when measuring in world space, but if we measure it along the plane of the film itself, nothing has changed $\endgroup$ – davidkomer Feb 17 '16 at 8:41
  • $\begingroup$ i.e. isn't the whole thing about gravitational waves that it is a curve in spacetime itself, and therefore the changes can only be noted from outside of spacetime? (again- I'm not a physicist, so I apologize for what might be totally ignorant - but I'm really trying to understand) $\endgroup$ – davidkomer Feb 17 '16 at 8:42
  • $\begingroup$ you really have a variation of length, simply you can't mesure it with a reference length (since it will change as well). So you measure it differently, with time of flight (and interferences). $\endgroup$ – Fabrice NEYRET Feb 17 '16 at 9:11
  • $\begingroup$ I appreciate your help in understanding this. Isn't time "stretched" as well (i.e. the gravity warps spacetime, not just space)? $\endgroup$ – davidkomer Feb 17 '16 at 10:52
  • $\begingroup$ just space. or LIGO experiment would be meaningless ;-) $\endgroup$ – Fabrice NEYRET Feb 17 '16 at 21:27

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