Qualla: GEO600

GEO600: Introduction

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Hamelenser, CC BY-SA 3.0. There is nothing to look at. Two shallow trenches run across farmland near Sarstedt, twenty kilometres south of Hannover, each six hundred metres long, meeting at a right angle, each covered by a corrugated steel tube that could pass for a drainage culvert if it were not so implausibly straight. Inside the tubes is a vacuum emptier than the space between planets. Inside the vacuum a laser beam bounces between hanging cylinders of glass, and the machine's whole purpose is to notice if the distance between them changes by a fraction of the width of a proton - which is what happens when a gravitational wave from a pair of colliding black holes passes through Lower Saxony. GEO600 has never caught one. It is still one of the most consequential instruments in the field, because nearly every trick the detectors that did catch them depend on was proved out here first.

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Image: Hamelenser | CC BY-SA 3.0

GEO600: Two Trenches in a Field

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Axel Hindemith, CC BY-SA 3.0. The layout is a Michelson interferometer, scaled up and made monstrously precise. A laser is split in two, sent down two perpendicular arms, bounced back and recombined; change one arm's length relative to the other and the beams interfere differently. GEO600's arms are 600 metre...

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Image: Axel Hindemith | CC BY-SA 3.0

GEO600: Glass Hung From Glass

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Hamelenser, CC BY-SA 3.0. At this sensitivity everything is noise. A mirror hanging on a wire hears the wire; a mirror with a magnet glued to it hears the glue. So GEO600's main mirrors hang from fibres of the same fused silica the mirrors themselves are made of - monolithic suspensions, chosen because si...

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Image: Hamelenser | CC BY-SA 3.0

GEO600: The Small One That Wrote the Playbook

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Hamelenser, CC BY-SA 3.0. Construction began in September 1995, a German-British project run by the Max Planck Institute for Gravitational Physics and Leibniz University Hannover with Glasgow, Birmingham and Cardiff. It has taken data alongside LIGO since 2002 and reached design sensitivity in 2006, with ...

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Image: Hamelenser | CC BY-SA 3.0

GEO600: The Hologram That Wasn't

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Hamelenser, CC BY-SA 3.0. By 2007 the collaboration was reporting, openly and in public talks, that it had unexplained noise in the middle of its frequency band. Craig Hogan, a physicist at Fermilab, proposed something more exciting than a loose cable: that the noise was real - the quantum graininess of s...

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Image: Hamelenser | CC BY-SA 3.0

GEO600: Still On, Until December

Tue, 04 Aug 2026 00:00:00 GMT

Photo credit Bernd Schwabe, CC BY-SA 4.0. On 14 September 2015, when the two Advanced LIGO detectors registered the first gravitational wave ever observed, GEO600 was in engineering mode and would not have been sensitive enough regardless. It joined the observing run four days later. Its contribution has been stubbornnes...

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Image: Bernd Schwabe | CC BY-SA 4.0