Introduction to LIGO & Gravitational Waves
An Interferometer
Diagram of a basic interferometer design. [Image: LIGO]
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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).
