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Astronomy and space

Astronomy and space

Researchers in China propose novel gravitational-wave observatory

04 Feb 2025 Michael Banks
Artist’s impression of the Tetrahedron Constellation Gravitational Wave Observatory
Taking shape: an artist’s impression of the Tetrahedron Constellation Gravitational Wave Observatory (courtesy: Hong-Bi Jin)

Researchers in China have proposed a novel gravitational-wave observatory to search for cracks in Einstein’s general theory of relativity. The Tetrahedron Constellation Gravitational Wave Observatory (TEGO) would detect gravitational waves via four satellites that form a tetrahedral structure in space. Backers of the conceptual plan say TEGO offers significant advantages over designs consisting of a triangular configuration of three satellites.

Gravitational waves are distortions of space–time that occur when massive bodies, such as black holes, are accelerated. They were first detected in 2016 by researchers working on the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) located in Hanford, Washington and Livingston, Louisiana.

The current leading design for a space-based gravitational-wave detector is the Laser Interferometer Space Antenna (LISA). Led by the European Space Agency it is expected to launch in 2035 and operate for at least four years with an estimated to cost €1.5bn.

LISA comprises three identical satellites in an equilateral triangle in space, with each side of the triangle being 2.5 million kilometres – more than six times the distance between the Earth and the Moon.

While ground-based instruments detect gravitational waves with a frequency from a few Hz to a KHz, a space-based mission could pick up gravitational waves with frequencies between 10–4–10–1 Hz.

China has two proposals for a space-based gravitational-wave mission. Dubbed TAIJI and TianQin, they would be launched in the 2030s and, like LISA, consists of three spacecraft in a triangular formation each separated by 2.5 million km.

According to Hong-Bo Jin from the National Astronomical Observatories, Chinese Academy of Sciences, in Beijing, one disadvantage of a triangular array is that when the direction of gravitational-wave propagation as a transverse wave is parallel to the plane of the triangle, it is more difficult to detect the source of the gravitational wave.

A tetrahedral configuration could get around this problem while Jin says that an additional advantage is the extra combinations of optical paths possible with six arms. This means it could be sensitive to six polarization modes of gravitational waves. Einstein’s general theory of relativity predicts that gravitational waves have only two tensor polarization modes, so any detection of so-called vector or scalar polarization modes could signal new physics.

“Detecting gravitational waves based on the TEGO configuration will possibly reveal more polarization modes of gravitational waves, which is conducive to deepening our understanding of general relativity and revealing the essence of gravity and spacetime,” says Jin.

Yet such a design will come with costs. Given that the equipment for TEGO, including the telescopes and optical benches, is twice that of a triangular configuration, estimates for a tetrahedral set-up could also be double.

While TEGO has a separate technical route than TAIJI, Jin says it can “refer” to some of its mature technologies. Given that many technologies still need to be demonstrated and developed, however, TEGO has no specific timeline for when it could be launched.

Italian gravitational-wave physicist Stefano Vitale, a former principal investigator of the LISA Pathfinder mission, told Physics World that “polyhedric” configurations of gravitational-wave detectors are “not new” and are much more difficult to implement than LISA. He adds that even aligning a three-satellite configuration such as LISA is “extremely challenging” and is something the aerospace community has never tried before.

“Going off-plane, like the TEGO colleagues want to do, with telescope add-ons, opens a completely new chapter [and] cannot be considered as incremental relative to LISA,” adds Vitale.

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