Gravitational Wave Detection: Understanding Interferometers

[jeebs]

My understanding of how interferometers work is that they detect gravitational waves because they alter the distance between two mirrors as they pass through. They do this by using the wavelength of a laser beam passing through this space.
However, shouldn't this always fail to detect anything, because the wave would always make the same fractional alteration to the photon wavelength as it does to the distance the light has to travel?
Why/how does this work?
thanks.

 

[Shackleford]

Yeah, we found gravitational waves.

(We need more funding. ) LOL! JK.

 

[bcrowell]

Historically there was a lot of confusion on this issue. At one point, Einstein published a paper claiming that gravitational wave solutions in GR were in fact not observable. GR allows any kind of change of coordinates you like, including changes of coordinates that make the metric look oscillatory when written down on paper, without introducing any change in the actual physical properties of spacetime, such as curvature. This is known as a coordinate wave. However, it turns out that Einstein was wrong, and there really are gravitational wave solutions to GR that are not just coordinate waves. This may be helpful: http://en.wikipedia.org/wiki/Sticky...ents_on_the_properties_of_gravitational_waves

This is similar to the question of why cosmological expansion is observable, and IMO the cosmological expansion version is the easier one to understand. Rulers, solar systems, and galaxies don't expand, but the spaces between galaxies do. Therefore the size of the universe becomes bigger as measured by a ruler. More on this topic here: http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.2 (see subsection 8.2.5).