Introduction to LIGO & Gravitational Waves

An Interferometer


Diagram of a basic interferometer design. [Image: LIGO]
Basic Design of the LIGO Interferometer
To measure the relative lengths of the arms, a single laser beam is split at the intersection of the two arms.  Half of the laser light is transmitted into one arm while the other half is reflected into the second arm.  Mirrors are suspended as pendula at the end of each arm and near the beam splitter. Laser light in each arm bounces back and forth between these mirrors, and finally returns to the intersection, where it interferes with light from the other arm.  If the lengths of both arms have remained unchanged, then the two combining light waves should completely subtract each other (destructively interfere) and there will be no light observed at the output of the detector.  However, if a gravitational wave were to slightly (about 1/1000 the diameter of a proton) stretch one arm and compress the other, the two light beams would no longer completely subtract each other, yielding light patterns at the detector output.  Encoded in these light patterns is the information about the relative length change between the two arms, which in turn tells us about what produced the gravitational waves.

Many things on Earth are constantly causing very small relative length changes in the arms of LIGO.  These every-present terrestrial signals are regarded as noise (and would sound very much like static when the signal is sent through a speaker).  In science, noise is defined to be anything that is measured that is not what was intended to be measured.  Here, LIGO is trying to measure the change in length of its arms due to a gravitational wave and not the incessant little motions of LIGO’s components caused by the environment.  To help minimize local effects on the detector, LIGO has made many enhancements to the basic interferometer design (besides requiring both detectors to detect the same signal within the light travel time between detectors). 

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