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Do gravitational waves distort both space and time? I can understand the warping of space and trying to measure the difference in stretch, but how do you know a time distortion would not affect the LIGO split laser beam?
Submitted by Dave from the USA

You could say that gravitational waves distort both space and time. It's more precise to say they distort spacetime, and whether you see the distortion as purely space, purely time, or some combination depends on how you're moving (what frame of reference you're in).

The usual version of this common question goes something like: "Yesterday you said that LIGO has 4km arms and that a gravitational wave makes the arm length change a little because space is distorted. Today you said that the gravitational wave shows up as a little change in the frequency of the laser light because time is distorted. But if the speed of light is constant and the frequency is changing, the wavelength of the light must be changing too. If the wavelength and the arm length change in the same way, how is that any different from when they don't change?"

Yesterday I gave an answer that was true in one frame of reference, and today I gave an answer that was true in another. You can pick a frame where the arm length changes and the wavelength doesn't. You can pick another frame where the arm length doesn't change, but the wavelength and frequency do. You can pick a frame in between, where wavelength and arm length change by half as much, and so on. You can't pick one where both change in harmony. What is really measured by the instrument is basically the number of wavelengths that fit into an arm or the number of wave periods it takes light to traverse an arm, and that number is the same in all frames.

What's behind the answer is the principle of relativity: Fundamental physical observables are the same in any frame of reference, however it's moving. So you could pick a frame where it's easy to calculate something, or another frame where it's easier to explain what's going on. Or get a result in one frame and use it in another. This switching around is very convenient for physicists to do calculations and becomes second nature to us very quickly, so sometimes we forget it can seem confusing!