Recently, the European Space Agency (ESA) has officially approved the first experimental project to measure gravitational waves in space. The project, called Laser Interferometric Space Antenna (LISA), will detect the massive ripples in space-time triggered by cosmic events such as the merger of supermassive black holes by analyzing the precise timing of the propagation of a laser beam over 2.5 million kilometers within the solar system.
The European Space Agency announced on Jan. 25 that construction of the multi-billion euro mission will begin in 2025, with a 2035 launch planned. "The mission is highly innovative and will open a window on the sources of gravitational waves that only LISA can detect," said Valeriya Korol, an astrophysicist at the Max Planck Institute for Astrophysics in Garsing, Germany, who is a member of the LISA collaboration team.
What makes LISA unique compared to ground-based detectors is its ability to detect gravitational waves at lower frequencies, which means it can shed light on much larger and more distant cosmic phenomena, such as black holes orbiting each other, that are much larger in scope and distance than those first detected by the ground-based Laser Interferometric Gravitational Wave Observatory (LIGO) in 2015.
The LISA project has been a long time in the making, recalls Karsten Danzmann, director of the Max Planck Institute for Gravitational Physics in Hannover, Germany, and head of the LISA consortium, who originally wrote the proposal for LISA 31 years ago. The experiment aims to detect gravitational waves by measuring the distance (to the nearest trillionth of a meter) at which a laser travels between two points of mass millions of kilometers away, with such precision that virtually nothing other than spacetime itself can affect the motion of the mass points.Karsten Danzmann is confident that the project will come to fruition, stating that "At first people thought the idea was ridiculous, but I told them to just be patient."
LISA Mission Golden Triangle: Gravitational wave detection in space
The LISA program will consist of three uniformly configured spacecraft that will orbit the Sun in an equilateral triangle formation (Figure 1). Inside each spacecraft is a 4.6 cm long floating cube made of gold and platinum. By using lasers to accurately measure the relative distances between these cubes, LISA is able to detect subtle spatial and temporal variations induced by gravitational waves with very high precision (to the order of picometers, or one billionth of a millimeter). Such changes are produced by the accelerated motion of massive objects. These fine measurements allow LISA to pinpoint the source of gravitational waves, a device Valeriya Korol describes as "an instrument from science fiction".
Figure 1 Conceptual diagram of the LISA mission spacecraft. 3 satellites will form a triangle in orbit around the Sun.
Karsten Danzmann points out that despite the challenges of making high-precision measurements at such distances, the space environment is much more conducive to such experiments than the ground. The absence of vibrations, atmospheric disturbances and other oscillations in space provides an almost perfect vacuum for experiments. However, the complexity of the technology requires that the equipment be extremely reliable, as it cannot simply be repaired on-site once launched.
The LISA project is designed to detect gravitational waves at wavelengths between 300,000 kilometers and 3 billion kilometers, a range that fills the gap between the detection capabilities of the ground-based Laser Interferometric Gravitational Wave Observatory (LIGO) and the Pulsar Timing Array. It can detect gravitational waves at longer wavelengths compared to LIGO and at relatively shorter wavelengths compared to the Pulsar Timing Array. Currently, the Pulsar Timing Array is carrying out research to observe and study gravitational waves throughout the galaxy by using some stars as "beacons", which emit periodic signals that help scientists detect longer wavelengths of gravitational waves.
Prospects for LISA and the Global Collaboration on Gravitational Wave Detection
Karsten Danzmann points out that different experiments observe their own unique phenomena and provide complementary data, just as radio telescopes and visible-light devices do. Thanks to its sheer size, LISA will be able to detect gravitational waves from supermassive black hole mergers, as well as signals that were emitted earlier in the collision phase detected by LIGO.LISA is also expected to reveal never-before-seen cosmic events, such as white dwarf collisions beyond the size of black holes and binary black hole mergers of unequal mass.
Astronomers expect the experiment to detect gravitational wave background noise generated in the early universe, which is predicted by theory, and potentially capture signals emitted by the earliest black holes, Valeriya Korol said.LISA is expected to help scientists understand more accurately how the rate of expansion of the universe has changed by measuring the distances to the sources of the gravitational waves it detects.
China is also planning to launch its own space gravitational wave detectors in the 2030s, which are referred to as the Space Gravitational Wave Detection "Taiji Program" and the "Tianqin Program". China's space gravitational wave detection program has been assisted by the development of the LISA project, which enhances the possibility of its implementation. The program of the "Tianqin Program" is known as the Chinese program, and unlike the European LISA program, its goal is to build a space-based gravitational wave observatory around 2035 by deploying three all-synchronous satellites in geocentric orbits at a height of about 100,000 kilometers, constituting an equilateral triangular constellation with a side length of about 170,000 kilometers "Tianqin", to carry out space detection of gravitational waves. The "Taiji Plan" is basically the same as the European LISA plan, in the heliocentric orbit about 50 million kilometers away from the Earth, launching three all-similar satellites, the three-star formation orbit with the sun as the center, the design of the interfering arm arm length that is, the satellite spacing of 3 million kilometers. Tianqin, LISA, Taiji three plans, the former is a geocentric orbit program, the latter two are heliocentric orbit program, need the same core technology, also have their own different technical difficulties, but the space gravitational wave detection has complementary. The approval of the LISA program by ESA marks an important milestone in the scientific community.
