Hello all, I was thinking about the speed of light and why it's constant and it brought me to the principle of the LIGO experiment for which I have an assumption that I want to verify. I'm a novice at this so please bear with me. From what I know, the LIGO experiment splits an emitting light beam into 2 beams that travel along their 2 paths, let's call those paths A and B, which are then reflected to meet each other again at the spot they got split. Furthermore, if I understand correctly, gravitational waves are waves that temporarily stretch the spacetime fabric. If there's no stretch during the experiment (ruling out all other influences of course) the waves of the 2 light beams cancel each other out when they meet each other. I have 3 questions about why the 2 light beams wouldn't cancel each other out when a gravitational wave passes by. 1. If a gravitational wave passes through and stretches the spacetime fabric of 1 of the two paths, let's say path A, does the light beam that travels along that path "interpret" this stretch as an increase in length that it has to travel along path A? 2. If so, would that increase in length for the light beam also mean an increase in the physical length that we would measure of path A or is it only an increase in length from the view of the light beam, i.e. merely an increase in stretch of the spacetime fabric? In other words, when I myself run along that path instead of the light beam with a constant v, would I also take a bit longer when a stretch in the spacetime fabric passes by that path (even if the increase in length is infinitesimally small)? 3. If it's merely an increase in length from the view of the light beam and we wouldn't measure any difference in the physical length of that path A (not even an infinitesimally small increase), wouldn't that mean that the light beam would take a longer time to travel along path A while we don't measure any difference in the physical path length and thus measure the light having a lower speed than the speed of light? I have a hunch that my conclusion in point 3 is wrong, but if the first 2 are correct, i.e. there's a stretch in length only with respect to the light beam, then I'd really want to know how the speed of light would still stay the same. EDIT: Just noticed I posted this in the wrong forum. Sorry about that. Could someone please move this to General Relativity?