No, it's not wavelength change, it's the number of wavelengths that fit in each "arm" of the detector. With 4 km arms and 1000 nanometer laser wavelength (actua

No, it's not wavelength change, it's the number of wavelengths that fit in each "arm" of the detector. With 4 km arms and 1000 nanometer laser wavelength (actual numbers), you will have 4 000 000 000.00 wavelengths that fit in each arm. When a gravitational wave passes, one arm will have 4 000 000 000.10 wavelengths and the other is unchanges. Since we're working with interference, this parts-per-billion change is converted into a large change in light. Mechanical analogy: there are two very long very fine pitch helical gears that are both suspended from one end and mesh perfectly at the other end. When the gravitational wave makes one gear undetectably longer, we can easily see that the gears no longer mesh.