Rather than the kilometre-length observatories of today, future gravitational-wave detectors could be just a few metres long. That is the goal of physicists in the UK and the Netherlands, who have put forward a design for a matter-wave interferometer that would rely on the superposition of tiny objects such as diamond crystals rather than laser beams. They say that the device would be sensitive to low- and mid-frequency gravitational waves.
Gravitational waves were first observed directly in 2015, when the LIGO observatory in the US picked up the emission from a pair of merging black holes. These black holes broadcast a series of ripples through space-time that caused the pairs of perpendicular arms making up LIGO’s interferometers to undergo a series of miniscule expansions and contractions. Those tiny changes were registered as variations in the interference between laser beams sent along the arms.
Such laser-based observatories, however, are very large. A passing gravitational wave will typically induce fractional length changes on the order of 10-19 or less, meaning that the detector’s arms must be several kilometres long if the facility is to yield a reasonable signal above the many sources of background noise. In the case of LIGO, each arm extends for 4 km.
The latest work proposes a far smaller type of observatory based on interfering beams of matter rather than light. The particles in question would have a mass of about 10-17 kg, corresponding to a de Broglie wavelength of 10-17 m. This is about 100 billion times smaller than the wavelength of laser light used in existing observatories and could be exploited in an interferometer measuring as little as 1 m in length.
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