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Unusual spike

Interferometer detects earthquake by accident

On Sunday 11 February, an ultra-precise heterodyne interferometer was undergoing a performance test in Airbus Defence and Space's Laboratory for Enabling Technologies (LET) in Friedrichshafen when it produced an unusual measurement spike. This spike was much larger than the background noise of the instrument and occurred at 23:45. At this exact time in Zurich, Switzerland, an earthquake of magnitude 4.2 was recorded.

Airbus Defence and Space's heterodyne interferometer

The small earthquake occurred 30km below the Earth’s surface, and the shock waves travelled 85km to Friedrichshafen. The instrument was supported on a floating optical table but the very low noise level of the interferometer meant that it clearly picked up the vibrations. “It is the first time the instrument has recorded a traceable external event,” says Martin Gohlke, who took the measurement.

The interferometer is an instrument that is being designed and tested for the future LISA mission, which aims to detect gravitational waves. Einstein predicted the existence of gravitational waves, minute distortions in space-time typically on the scale of about the diameter of a hydrogen atom (40 picometers) over a distance of 5 million kilometres (or about 10 times the distance between Earth and the moon), but this very small variation means that no gravitational wave has been directly observed to date.

Artist’s impression of LISA Pathfinder (© ESA)

Artist’s impression of LISA Pathfinder (© ESA)

An interferometer uses an arrangement of mirrors and beam splitters, in which two laser beams are generated by a single laser. Any movement in an obstacle in the path of one of the beams can be recorded by recombining the two beams and measuring the phase difference in relation between them.

LISA (the Laser Interferometer Space Antenna) is one of three ‘large’ mission concepts currently undergoing down selection in ESA’s Cosmic Vision programme. Crucial elements of the novel technologies required for realisation of LISA are set to be demonstrated by a precursor mission, LISA Pathfinder, which is primed by Airbus Defence and Space and is in its test phase.

The Laboratory for Enabling Technologies was founded at Airbus Defence and Space Friedrichshafen in 2007, with the goal of performing research and development of novel technologies crucial to the success of future space missions and systems. It is operated by Airbus Defence and Space’ Earth Observation, Navigation & Science team (Science Programmes) in close collaboration with the University of Applied Sciences, Constance, Germany. Among its key competences established over the past years is in particular know-how and hands-on-experience for quantum-metrological instruments and systems for application in space contexts.

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