Not all mirrors are created equal. Interesting piece from Quanta Magazine on mirrors, glass and gravitational waves.

They’re nothing like regular mirrors. In your bathroom mirror, light reflects off metal, which has glass in front of it merely for protection. But LIGO’s 100-kilowatt laser would fry any metal. Instead, its mirrors are made entirely of glass.

LIGO’s mirrors are imperfect, however, because of a strange form of noise that is baked into glass, a mysterious substance in general. Glass consists of atoms or molecules that are haphazardly arranged like those in a liquid yet somehow stuck, unable to flow. Physicists believe that the noise inherent in glass comes from small clusters of atoms switching back and forth between two different configurations. These “two-level systems” ever so slightly change the distance laser light travels between LIGO’s mirrors, since the surface of each glassy layer shifts by as much as an atom’s width.

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