Gravitational waves through outer space: what is LIGO?
Gravitational waves are conceptually defined as spacetime deformations caused by the interaction between supermassive bodies such as planets, stars, galaxies, and black holes. These gravitational waves travel through the vacuum of space, as they do not require any material support to propagate, and they can be measured. Their existence was predicted by A. Einstein in his Theory of General Relativity [1]. Almost a century later, Taylor and Weisberg [2] reported the first indirect evidence of gravitational waves by closely observing the binary pulsar PSR B1913+16.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a huge interferometer designed to measure gravitational waves. Based on the principles of General Relativity, it was the first instrument to record a direct gravitational-wave measurement in history. The research developed around this physical discovery received the Nobel Prize in Physics in 2017 [3].
LIGO consists of two main observatories separated by thousands of kilometres to minimize measurement errors and perturbations caused by local environmental changes. Both observatories are located in the United States: one in Hanford (Washington) and the other in Livingston (Louisiana) [4, 5].
How does LIGO operate? Gravitational light-wave duality
The measurement methodology is based on the physical principle of optical interferometry. Figure 1 shows a visual scheme of LIGO, an L-shaped device where a laser source is located in the corner. Two light beams are directed perpendicularly along the arms of the L until they reach high-precision mirrors placed at the ends. Then, the laser light is reflected by the mirrors and redirected to a detector also located in the corner.
Typically, the distance covered by each light beam in its respective arm is identical, unless a gravitational wave occurs. In that case, the two distances change, causing the beams to arrive at the corner out of phase and form an interference pattern that depends on the intensity of the gravitational wave.
The atmosphere could influence LIGO's measurements due to fluctuations related to various meteorological conditions, such as changes in barometric pressure, temperature, wind speed, and atmospheric turbulence. These fluctuations are usually called "Newtonian noise," which consists of mechanical vibrations mainly caused by atmospheric mass redistribution. Several previous works have discussed the origin of this Newtonian noise, relating it to slight variations in the gravitational field caused by these meteorological conditions [4].
Figure 1. Schematic diagram of the LIGO interferometer
Therefore, quantifying this Newtonian noise, which is directly related to local atmospheric conditions, is an important challenge to address. Future designs for gravitational-wave detectors should be able to implement assessments and estimations to minimize these atmospheric effects on gravitational-wave measurements.
References:
[1] Einstein, A. (1918). Über Gravitationswellen. Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften, 154–167
[2] J. H. Taylor y J. M. Weisberg, «A new test of general relativity - Gravitational radiation and the binary pulsar PSR 1913+16», ApJ, vol. 253, p. 908, feb. 1982, doi: 10.1086/159690.
[3] D. Castelvecchi, «Gravitational wave detection wins physics Nobel», Nature, vol. 550, n.o 7674, pp. 19-19, oct. 2017, doi: 10.1038/nature.2017.22737.
[4] B. P. Abbott et al., «LIGO: the Laser Interferometer Gravitational-Wave Observatory», Rep. Prog. Phys., vol. 72, n.o 7, p. 076901, jul. 2009, doi: 10.1088/0034-4885/72/7/076901.
[5] B. P. Abbott et al., «Properties of the Binary Black Hole Merger GW150914», Phys. Rev. Lett., vol. 116, n.o 24, p. 241102, jun. 2016, doi: 10.1103/PhysRevLett.116.241102.
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