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Measuring To Microns One Million Mile Distances

Gravitational waves are miniscule ripples in space-time, thought to be triggered by massive events in space like exploding stars and the merging of black holes at the centre of galaxies.

Einstein predicted the existence of gravitational waves a century ago, but the first conclusive detection was not until 2015, from ground-based observatories.

This discovery offers scientists an exciting prospect – an entirely new way of observing the Universe, even for large scale events billions of light years away and objects that emit no light such as black holes. However, ground-based observations are limited by background gravitational noise and by the curvature of the Earth, because measurements must be made over vast distances to detect these tiny perturbations. Ground-based observatories can only be used to detect the highest frequency gravitational waves, which means that the ability to observe cataclysmic events in space is still limited.

To capture the lower frequency waves and study them in more detail a space-based observatory is needed. This will require three spacecraft flying in a triangular formation with a laser metrology system linking them that is capable of measuring with unprecedented precision, to a few millionths of a millionth of a metre, over distances of 1.5 million km. It also requires test masses to be housed inside the spacecraft in gravitational free fall, isolated entirely from all other gravitational effects from their surroundings (including those of the spacecraft itself). This means that any changes in distance between the test masses measured by the lasers can only be due to the passage of gravitational waves, causing the fabric of space and time to stretch and contract.

Because the technology requirements are so demanding, ESA launched Pathfinder to prove it could be done before embarking on the much larger LISA mission. Pathfinder was an outstanding success, exceeding all expectations and demonstrating conclusively that the measurement principles needed for LISA could be achieved. A team from UK’s Glasgow University developed the optical bench for Pathfinder, a core element of the laser metrology system, with funding from the UK Space Agency.

Building on this success, the Glasgow team is now working in partnership with UKRI-STFC’s Astronomy Technology Centre in Edinburgh to design the optical benches for LISA. The UK Space Agency is also funding development of a LISA Data centre in the UK, led by Edinburgh and Birmingham Universities, to ensure that UK scientists will be at the forefront of gravitational wave research when LISA is launched.

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