LIGO’s Lasers Can See Gravitational Waves, Even Though The Waves Stretch The Light Itself
“But this is where the puzzle comes in: if space itself is what’s expanding or compressing, then shouldn’t the light moving through the detectors be expanding or compressing too? And if that’s the case, shouldn’t the light travel the same number of wavelengths through the detector as it would have if the gravitational wave had never existed?
This seems like a real problem. Light is a wave, and what defines any individual photon is its frequency, which in turn defines both its wavelength (in a vacuum) and its energy. Light redshifts or blueshifts as the space it’s occupying stretches (for red) or contracts (for blue), but once the wave has finished passing through, the light returns to the same wavelength it was back when space was restored to its original state.
It seems as though light should produce the same interference pattern, regardless of gravitational waves.”
Have you ever thought about how gravitational wave detectors work? By passing light down two mutually perpendicular arms, reflecting them back and reconstructing an interference pattern, we can detect a passing wave by how it changes the arm-lengths of the light. But the light itself also gets compressed and expanded, and shouldn’t those effects cancel out?