![]() In other words, the more laser photons merge from each arm, the sharper the fringes that are measured by the photodetctor.īasic Michelson with Fabry Perot cavities and Power Recycling mirror. While increasing length amplifies tiny changes in arm length, increasing laser power results in increasing the interferometer's resolving power. ![]() Just as increasing length increases the interferometer's sensitivity, increasing laser power also enhances its performance. Length isn't the only limiting factor in LIGO's sensitivity. LIGO, on the other hand, was designed to feel them, and and with arms effectively 1200km long, LIGO's interferometers can amplify the smallest conceivable vibrations enough that they are detectable and measurable. In a telescope, these vibrations are unwanted. Increasing the focal length of a telescope (also how far the light travels between mirrors or lenses before reaching your eye) not only increases the magnification achievable by any given eyepiece, but it also amplifies the smallest vibrations in the telescope. There is an analogous effect in optical telescopes. And thanks to Fabry Perot cavities, LIGO can achieve this sensitivity with arms just 4km long. Since we know that the longer the arms of an interferometer, the more sensitive the instrument is to vibration, this design significantly increases LIGO's sensitivity and enables it to detect changes in arm length much smaller than a proton-the size of changes expected to be caused by a gravitational wave. It increases the distance traveled by each laser from 4km to 1200km thereby solving our length problem! (The light in Michelson's original interferometer only traveled 11 meters.) It builds up the laser light within the interferometer, which increases LIGO's sensitivity (more photons also makes LIGO more sensitive)Ģ. After entering the instrument via the beam splitter, the laser in each arm bounces between its two mirrors about 300 times before being merged with the beam from the other arm. This 4km-long space comprises the Fabry Perot cavity. An additional mirror is placed in each arm near the beam splitter (the box on the 45-degree angle), 4km from the mirror at the end of that arm. The figure at left shows a basic Michelson design modified to include such cavities. The paradox was solved by altering the design of the Michelson to include something called "Fabry Perot cavities". This greatly increases LIGO's sensitivity to the smallest changes in arm length. Additional mirrors are inserted near the beam splitter to facilitate multiple reflections of the laser, containing it within the interferometer and increasing the distance traveled by the beams. So how can LIGO possibly make the measurements it does?īasic Michelson interferometer with Fabry Perot cavities. And of course there are practical limitations to building such a precision instrument much larger than 4km. While 4km-long arms seems pretty huge, if LIGO's interferometers were simple Michelsons, they would still be too short to enable the detection of gravitational waves. And having to measure a change in distance 10,000 times smaller than a proton means that LIGO has to be larger and more sensitive than any interferometer ever before constructed. The longer the arms of an interferometer, the smaller the meaurements they can make. The scale of LIGO's instruments is crucial to its search for gravitational waves. (By contrast, the interferometer Michelson and Morley used in their famous experiment to study the "aether" had arms about 1.3m long). With arms 4km (2.5 mi.) long, LIGO's interferometers are by far the largest ever built. The first most obvious difference between a typical Michelson interferometer and LIGO's interferometers is its scale. The size and added complexity of LIGO's interferometers are far beyond anything the world's first interferometers could have achieved.
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