The waves do travel at the speed of light, since that's the upper speed limit in GR.
The light is affected only because the distance it has to travel changes when spacetime compresses and expands. So the time it takes from A to B changes, but also the wavelength of the light.
What is used in Ligo etc. is interference. They shoot two perpendicular laser beams that collide in a point. Ordinarily, the lasers interfere at this point and everything is aligned so they cancel each other out almost perfectly. But when a gravitational wave changes the length in one of the arms, the interference isn't perfect anymore and you can detect the laser signal.
To a very very good approximation, a gravitational wave front hitting Earth is a flat plane. This means the detectors cannot see waves that hit the arms at close to 45°, as well as waves that hit the Earth's surface close to vertically at the detector location.
Jesus, I can't imagine the type of resolution necessary to make those detectors work!
So the indication that "gravity wave happened" is wavelength change? Freaky stuff, I really have a hard time wrapping my head around all this, even after reading layman intros.
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.
How do they distinguish between 4 000 000 000.10 and 4 000 000 001.10 wavelengths? Do they simply count how often they cross a full-phase change and keep track of the direction of that change?
Your statement is true, but it is important to emphasise that they travel at the speed of light in vacuum, this is important because the universe is only almost empty and electro-magnetic radiation actually travels slower than the speed of light in vacuum in this medium. This means that one would expect for light to arrive a few seconds after the detection of the gravitational wave.
The statement is also only true for perturbations with respect to a fixed background metric. In principle it is possible for space-time to expand much faster than the speed of light (this is believed to have happened just after the big-bang).
The light is affected only because the distance it has to travel changes when spacetime compresses and expands. So the time it takes from A to B changes, but also the wavelength of the light.
What is used in Ligo etc. is interference. They shoot two perpendicular laser beams that collide in a point. Ordinarily, the lasers interfere at this point and everything is aligned so they cancel each other out almost perfectly. But when a gravitational wave changes the length in one of the arms, the interference isn't perfect anymore and you can detect the laser signal.
To a very very good approximation, a gravitational wave front hitting Earth is a flat plane. This means the detectors cannot see waves that hit the arms at close to 45°, as well as waves that hit the Earth's surface close to vertically at the detector location.