Invisible gravitational waves constantly cross our Solar System. These waves travel in the fabric of spacetime and thus conventional detectors like our eyes, telescopes and particle detectors cannot observe them. However, recent advances in technology have made measurements possible. Two types of observatories are active on Earth, and ESA is preparing the first mission to study these waves from space.
Powerful events like colliding black holes shake the fabric of spacetime, creating ripples within. These gravitational waves travel outwards at the speed of light, contracting and expanding spacetime as they pass along. Trying to measure them with rulers or other conventional instruments is fruitless because they too, contract and expand.
Einstein proposed in his theory of special relativity that the speed of light is constant. This physical principle has allowed engineers and scientists to create methods of measuring the otherwise invisible gravitational waves.
Two long arms
The first direct measurement of gravitational waves was performed by the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. LIGO has two locations in the USA, and similar observatories exist in Europe (VIRGO) and Japan (KAGRA).
These observatories rely on an experimental set-up developed by Michelson and Morley in 1887. In this set-up, a beam of light is split into two beams travelling perpendicular to each other. Using mirrors, the beams are reflected towards each other and are combined creating an interference pattern. When the two beams travelled the exact same distance, the signal is zero and there is no pattern. Only when one beam travelled longer or shorter than the other, an interference pattern appears, announcing a detection of gravitational waves.
In ground-based observatories, the light beams travel along corridors that are three to four kilometres long. But if light only travelled the corridor once, the signal would be way too small to measure. That is why mirrors reflect the beams about 300 times before the beams are combined. For LIGO that changes the effective travel distance from 4 km to 1200 km. On these distance-scales it becomes possible to measure gravitational waves passing by Earth, particularly those coming from black holes colliding.
Pulsar arrays
More recently, in June 2023, another type of observatory has discovered gravitational waves by looking at objects in space called pulsars. When massive stars die, they explode in a supernova and leave behind either a black hole, or a compact “dead” stellar core called a neutron star. Some neutron stars are called pulsars because they rotate and send out beams of light from their magnetic poles. These beams are only seen when the pulsar’s poles are pointed towards Earth, similar to how you can only see the light of a lighthouse when the beam is turned towards you.
Because the rotation of pulsars is very regular, radio telescopes on Earth receive the signal of their pulses at regular intervals. While the light from these pulsars travels through space, it can encounter gravitational waves that stretch spacetime and lengthen or shorten the path of the pulses. By monitoring the time between the arrival of pulses, observatories can detect gravitational waves encountered on their journey.
There are four active pulsar time arrays: the Parker Pulsar Timing Array, the European Pulsar Timing Array, the North American Nanohertz Observatory for Gravitation Waves and the Indian Pulsar Timing Array. Each of these arrays study signals coming from several pulsars to discover tiny variations in their arrival time. Their studies are sensitive to the combined signal of merging supermassive black holes at the centres of galaxies, each black hole weighing more than a hundred times the Sun.
LISA
Observatories on the ground can only detect signals from certain cosmic objects. ESA is working on the first designated space mission to study gravitational waves that cannot be measured by ground-based interferometers and pulsar timing arrays, called the Laser Interferometer Space Antenna (LISA).
The mission will comprise three spacecraft to study gravitational waves coming from binary systems of white dwarf stars, neutrons stars, and black holes. LISA can detect their signals when these objects spiral around each other, long before their eventual collision. Additionally, the system is also sensitive to signals coming from supermassive black holes at the centres of galaxies.
Each of LISA’s three spacecraft contains two golden cubes, slightly smaller than Rubik’s cubes, which lie at the heart of the LISA measurement concept. The golden cubes are in free fall and are protected against external forces by the spacecraft housing. Within each spacecraft the cubes are free to move. By constantly monitoring the distance between the cubes in different spacecraft, the mission will detect gravitational waves and locate their sources in the Universe.
Together with interferometers and pulsar arrays on Earth, LISA will cover a large range of signals from gravitational sources and enable a new way of studying our Universe.
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