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Tumbleweeds Interrupt the Discovery of Black Holes LIGO detectors are sensitive enough to detect air and ground noise.

“How moving!” is what the Earth would say when it comes into contact with gravitational waves. Albert Einstein’s general theory of relativity predicted these gravitational waves. These waves are disturbances in space caused by very massive objects and events. LIGO, (Laser Interferometer Gravitational-Wave Observatory) located in Livingston, Louisiana, has a press release from last summer stating their detection of gravitational waves. These detected gravitational waves were created deep in space by the merging of two black holes.

Example interference pattern from two sources. Source

How do scientists know this? LIGO shoots laser beams down two 2.5-mile-long tubes. These lasers create an interference pattern when the two beams are combined. When gravitational waves move through the Earth, the waves cause a slight disturbance in the laser beams, which dramatically changes the interference pattern detected. This is how LIGO observes gravitational waves.

Gravitational waves, while they might sound large, are actually small and slight to observe. LIGO is sensitive enough to detect a disturbance in the laser beams at 1/1000 the diameter of a proton. That is 1/10,000,000,000,000,000 the width of a hair. In fact, LIGO is so sensitive to disturbances that it is often difficult to determine when a gravitational wave has actually been detected. Due to the extreme sensitivity of this device, avoiding any exterior noises and vibrations are crucial to being able to detect these elusive gravitational waves.

Which brings us to today’s paper. Here, the scientist Dr. Teviet Creighton noted that many airborne and ground effects will be detected by LIGO, and be difficult distinguish from a real gravitational wave.

Here, Dr. Creighton enumerates other objects that could be accidently picked up by LIGO, such as wind, and sounds. Wind can whistle past the observatory and rattle the equipment. Furthermore, sounds from planes, trains, or loud noises can cause vibrations in the structure, also slightly moving the laser beams of LIGO. These situations would make things difficult for detecting a gravitational wave.

Most notably, the effects of tumbleweeds can be observed by LIGO! Tumbleweeds are quite common near Livingston, Louisiana, and would bump into the detector. These tumbleweeds would roll across the ground and ram into the interferometer, jostling the lasers and mirrors.

As a result of this analysis, a fence has been built around the observatory, at a far enough distance that the tumbleweeds will not affect LIGO. Furthermore, as a result of this analysis, external noise detection has been added to LIGO to cancel out the effects of exterior noise on the detector, similar to noise cancelling headphones. These methods were crucial to allow LIGO to make the discoveries last year and in the future. Without this, it would be impossible to determine a black hole ramming into another from a tumbleweed bumping into a building.

Article Information

Edited By: Gina Riggio
Source: Tumbleweeds and airborne gravitational noise sources for LIGO
Publication Date: Feb 7 2008 - IOPScience - Classical and Quantum Gravity

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