Who is Chris Mueller?
I am a Ph.D. student in the Department of Physics at the University of Florida. I currently reside in Baton Rouge, LA where I am working on my thesis research at the LIGO Livingston Observatory.
The LIGO project operates a pair of kilometer scale interferometers in the United States. One in Livingston, LA and the other in Hanford, WA. The interferometers are capable of detecting changes in length thousands of times smaller than the width of a proton, which is the sensitivity necessary to detect passing gravitational waves. Gravitational waves are given off by massive objects, such as black holes and neutron stars, moving around in outer space. These waves travel to us unimpeded from far away objects and from times in our distant past. Direct detection of these waves will open up an entirely new method of studying the world beyond our solar system.
My research is in the science of precision measurement, the study of which allows us to detect the tiny length changes caused by a passing gravitational wave. Detection of these minute changes in length is accomplished using monochromatic light (laser light) in a Michelson interferometer to compare the length of the two arms to each other. This scheme has the advantage of rejecting many of the noises which are common to the two arms while being highly sensitive to the differential length changes between the two arms. The sensitivity of the LIGO interferometers is further enhanced by adding resonant optical cavities in the arms which ensure that the light bounces back and forth many times, increasing the effective length.
The difficulty with detection of length changes at the attometer level (that is 10-18 meters!) is that many physical processes which can normally be ignored in lower precision measurements must be mitigated with intricate scientific techniques to keep the mirrors from moving more than this on their own. Some examples of these displacement noises are; seismic motion of the ground caused by human and geologic activity, thermal motion of the molecules which make up the reflecting surface of the mirror, and thermal motion of the molecules in the structure which supports the mirror. In addition to these displacement noises, the process of measuring with light has inherent noises due to the fact that light is made up of discrete packets known as photons. At high frequencies the statistics of counting photons limits the sensitivity while at low frequencies the quantum back action effect of photons transferring momentum to the mirrors is the limiting factor.