Five microns limit how far in the universe we can see!
What limits how far, how many millions of light years away LIGO can detect gravitational waves? You’ll be surprised to learn that it’s a few microns of material on the front face of our mirrors! Read more below
To make our glass test masses highly reflective for our laser, we need to deposit alternating layers of different glasses, amorphous oxides, to create a reflecting coating: the laser beams that are reflected back at each of the interfaces between materials interfere constructively with each other, resulting in a reflection that is higher than 99.99%.
Each layer is made of a different glass and has a different refractive index. Every time the light crosses from one material to another, some part of it is reflected and some is transmitted. You have already seen this effect when you look through a window and you see yourself dimly reflected: that’s because when visible light crosses from air to glass, about 4% is reflected back.
But for LIGO, stacking many layers is necessary to achieve the very high reflectivity. This comes at a price: the atoms in those thin layers, each a few hundreds of nanometers thick, jiggle around continuously because of thermal motion. This is called Brownian motion or noise, and it limits the sensitivity of the LIGO observatories where they are the most sensitive to gravitational waves.
The LIGO Laboratory and the LIGO and Virgo Collaborations have been working for many years to develop new materials with better mechanical and optical properties, to reduce the level of Brownian noise and therefore increase the LIGO reach into the universe. LIGO plans to install new mirrors (test masses) in the coming years, with improved coating materials. This will allow us to reduce the vibrations in the mirror coatings by almost a factor of two, and will contribute to doubling the distance at which LIGO can detect gravitational waves.
I post every week simple explanations of the science and engineering behind the discovery and detection of gravitational waves. Follow and connect!
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