Detecting ripples in space-time

Gravity can be expressed as a space-time curvature. A changing mass distribution can create ripples, called gravitational waves, in space-time that propagate away from the source at the speed of light. Albert Einstein posited the existence of these gravitational waves in his 1916 General Theory of Relativity. Now, more than 90 years later, technology is advanced enough to try to detect gravitational waves, which are emitted by the movement of massive bodies such as black holes.

The Laser Interferometer Gravitational Wave Observatory (LIGO) located at Hanford, Washington, and Livingston, Louisiana is operated by Caltech and MIT. The observatory consists of two 1.22 m diameter, 4 km long vacuum tubes joined in the shape of an L. Each of its arms has test masses with mirrors hanging from wires (see Figure 1). The length variation of space created by a gravitational wave corresponds to a displacement of only 10-18 m or 1/1,000 the diameter of the nucleus of an atom. LIGO senses the changes in distance in the mirror’s arms, measuring the interference (phase difference) between the two beams. Laser light bounces back and forth 50 times between the mirrors, so the resolution is increased 50 times to represent a 200 km long arm.

Figure 1: LIGO measures the phase difference between beams using an L-shaped vacuum tube system with hanging mirrors.
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Detecting weak waves requires two installations because regional perturbations like earthquakes, acoustic noise, and laser fluctuations can cause disturbances that simulate a gravitational wave. This may occur locally at one site but is unlikely to happen simultaneously at two sites located 3,000 km apart.

The LIGO detectors were built to operate for 30-plus years with possible future improvements. Because additional sites are required to triangulate the exact location of wave sources, LIGO is part of an international network including GEO600 (Germany), VIRGO (Italy), and TAMA300 (Japan).

Control system

LIGO uses suspension control electronics from dSPACE (Germany) capable of <5 x 10-20 m dynamic control and low-noise operational amplifier circuits with local feedback. While a DS2003 32-channel A/D converter captures the sensor signals, a DS1005 PPC board calculates the set values and feeds them through several DS2102 16-bit resolution D/A converters into the actuators. A DS1005 PPC features a PowerPC microprocessor running many algorithms based on MATLAB/Simulink.

ControlDesk software from dSPACE facilitates the design and operation of complex control algorithms and programs from the control center shown in Figure 2.

Figure 2: The LIGO control center runs complex control algorithms and programs using ControlDesk software.
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Advancements on the way

Although LIGO’s present detectors may identify a few signals, regular gravitational wave detection will only be possible with a tenfold improvement in sensitivity, which is planned for Advanced LIGO, Advanced VIRGO, and the Australian International Gravitational Observatory (AIGO). These detectors will operate from 10 Hz to a few 100 Hz, allowing them to hear neutron stars up to 700 million light-years away.

Detecting gravitational waves at lower frequencies (0.1 mHz to 1 Hz) requires detectors in space. The Laser Interferometer Space Antenna (LISA) is a proposed joint NASA and European Space Agency project aiming to build a gravitational wave detector consisting of three spacecraft in orbit. LISA, which is planned to launch in 2018, will directly detect gravitational waves using an advanced laser interferometry system that might be sensitive enough to observe the cosmic background of gravitational waves, which scientists believe will reveal “echoes” of the Big Bang.

For more information, contact Hermann at