Scientists have been granted permission to launch lasers into space to detect massive disturbances in the fabric of space-time around the Sun.
The European Space Agency (ESA) has approved the first experiment to measure gravitational waves from space. The mission, called the Laser Interferometer Space Antenna (LISA), will use lasers to measure ripples in space-time caused by events like mergers between supermassive black holes. The mission will involve the precise timing of laser beams that will travel across 2.5 million kilometers of the Solar System.
ESA announced on January 25 that construction of the multibillion-euro mission will begin in 2025, with the launch scheduled for 2035. “It’s extremely exciting,” says Valeriya Korol, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany, and a member of the LISA Consortium. “It will open a window to gravitational-wave sources that only LISA can see.”
LISA is a mission with a larger scale compared to the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected gravitational waves in 2015. Its size enables it to observe gravitational waves of a much lower frequency than can be detected on Earth. As a result, LISA will be capable enough to detect phenomena such as black holes orbiting each other that are more massive and further apart than those observed by LIGO.
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The mission has been a long time in the making. “The first time I wrote a proposal for LISA was 31 years ago,” says Karsten Danzmann, an astrophysicist at the Max Planck Institute for Gravitational Physics in Hanover, Germany, who leads the LISA Consortium. The experiment involves measuring the distance that laser light travels to pass between two masses, millions of kilometres apart, with an accuracy of one trillionth of a metre, while nothing other than space-time itself affects the movement of the masses. “People thought it was ridiculous. I said, ‘Just you wait.’”
The LISA (Laser Interferometer Space Antenna) mission will consist of three identical spacecraft orbiting the Sun in an equilateral-triangle formation. Each spacecraft contains a 4.6-centimetre floating cube made of gold and platinum. The mission will utilize lasers to measure the distance between the cubes in each spacecraft with incredible accuracy, enabling the detection of subtle gravitational waves caused by the acceleration of massive bodies. By detecting shifts in the signals, LISA will be able to determine the source of the gravitational waves with great accuracy, even on a scale as small as one picometre (10^-12 metres).
Although making such precise measurements over this distance is challenging, in many ways it is easier to do in space than on Earth, says Danzmann. “In space there’s no shaking, no atmosphere, no vibration, you’re just flying in a vacuum.” The difficult part is making the technology robust enough for all eventualities, he says. “You cannot just send a postdoc there to fix it.”
LISA is a project that aims to detect gravitational waves with wavelengths ranging from 300,000 to 3 billion kilometers. This range is longer than what has been detected on Earth by LIGO and shorter than what has been seen by pulsar timing arrays. These latter studies use stars as ‘beacons’ to observe gravitational waves that span entire galaxies.
Different measurements
All these experiments will observe different phenomena and produce complementary data, similar to how radio telescopes and visible-light instruments do, explains Danzmann. LISA’s massive scale will allow the detection of gravitational waves produced when supermassive black holes merge, as well as signals from systems at earlier stages of collision than LIGO can see. LISA will also be able to capture completely new phenomena, such as the spiraling of colliding white-dwarf stars, which are bigger than black holes, and systems in which two merging black holes are vastly different in mass.
Cosmologists hope that LISA will detect a background hum of gravitational waves created in the early Universe – a phenomenon predicted by theory – and possibly even signals from the first black holes, says Korol. Additionally, since LISA will measure the distance of the sources it detects, scientists hope that its data will help measure changes in the rate of the Universe’s expansion.
China also has plans to launch a space-based gravitational-wave detector in the 2030s. LISA’s development boosts the case for such a mission, according to Yue-Liang Wu, a physicist at the University of the Chinese Academy of Sciences in Beijing and chief scientist for the Taiji project, one of two proposed missions being explored. The Taiji and LISA teams hope that the missions will overlap, so they can complement each other in a “space-based gravitational-wave-detector network”, Wu says.
He adds that ESA’s green light for LISA is “a significant milestone for the scientific community”.
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References:
1. Gibney, E. Successful test drive for space-based gravitational-wave detector. Nature 531, 30 (2016). https://doi.org/10.1038/nature.2016.19452
2. Castelvecchi, D. Gravitational waves: How LIGO forged the path to victory. Nature 530, 261–262 (2016). https://doi.org/10.1038/530261a
