Also why LIGO works but Michelson-Morley failed: you can use the ruler distortion as a signal, with some caveats.

In LIGO, we detect the far away distortions propagating through our rulers and the differential effect of those waves.

But Michelson-Morley failed because the ruler distorted synchronously with the signal we tried to measure — since we mistakenly believed EM and matter were different fields.