Seismic Isolation

For an instrument that needs to remain as still as possible, it is ironic that LIGO is so sensitive that it can feel the smallest vibrations from near and far. LIGO's interferometers are essentially giant seismometers capable of sensing vibrations from traffic on nearby roads, weather patterns on the other side of the continent, staff driving alongside the detector's arms, the churning waters of the Pacific Ocean (detectable by LIGO Hanford Observatory), and of course nearly every significant earthquake on the planet. Since gravitational waves make themselves known through changing distances between LIGO's mirrors (which can readily be caused by any number of terrestrial vibrations), the only way to make gravitational wave detection possible is to isolate LIGO's components from environmental vibrations to unprecedented levels. The change in distance between LIGO's mirrors (test masses) when a gravitational wave passes is on the order of 10^-19 m, so LIGO must be able to tease out such unimaginably minuscule vibrations from all the larger ones. To achieve this level of sensitivity, LIGO was constructed with multiple levels of active and passive vibration absorption or isolation systems that isolate them from the smallest imaginable terrestrial vibrations that can drown out those arriving from the depths of space. LIGO's vibration isolation systems involve two basic elements: Optics Suspensions and Seismic Isolation.

Optics Suspensions: "Passive" Vibration Isolation

LIGO's mirrors (aka test masses) must be so well shielded from external vibration that only the random motions of the atoms within the mirrors and their housings remain detectable. To achieve this level of shielding, each of LIGO's 40 kg test masses is suspended within a 360 kg quadruple-pendulum system (or 'quad'). This system of four pendulums (diagram below) significantly reduces motion at the last link in the suspension where the test masses reside. The quad suspension system actually employs passive and active vibration isolation strategies.

Two "chains" of suspended masses hang back to back in each quad suspension system. These are called the Main Chain and the Reaction Chain. Each chain contains four masses. (Credit: Caltech/MIT/LIGO Lab)

Main Chain

In the Main Chain, the top two masses are made of steel, and the bottom two are made of pure fused silica. The lower-most cylinder in the Main Chain is the test-mass, measuring 34 cm x 20 cm (13.5 in. x 8 in.), and weighing 40 kg (88 lbs.). This hunk of glass is suspended by glass threads 0.4 mm (400 microns) thick. These threads do not expand or contract in response to temperature variations, thus they isolate the mirrors from such thermal noise. The total weight of the four masses in each chain is 120 kg.

Close-up of fused silica glass fibers attached to one of LIGO's primary optics. The bottom of the photo shows the glass welds binding the fibers to the optic. The fibers taper to a diameter of just 0.4 mm. (Credit: Caltech/MIT/LIGO Lab)

 

Reaction Chain

To operate effectively, the lengths of LIGO's arm cavities (i.e., the distance between the test masses at the ends of each arm) must not vary by more than a fraction of a picometer (one-trillionth of a meter). To hold the masses steady and in place, sometimes we need to push or pull on them (extremely carefully!) This is the role of the Reaction Chain.

In the Reaction Chain, the upper two, steel masses are controlled by simple motors made of permanent magnets and electrical coils that push on the masses. These 'voice coils' work like audio loudspeakers, with the coil producing a magnetic field, which attracts or repels the magnets, which in turn gently moves the masses to counteract vibrations. On the test masses themselves, we use more gentle electrostatic forces, like those that attract balloons rubbed on a sweater to a wall (or hair to a comb on a dry day). The goal is to keep the masses perfectly still without physically touching them.

Seismic Isolation: "Active" Vibration Isolation

LIGO's passive isolation systems are its last line of defense against unwanted vibrations (aka noise). The first line of defense against vibration is LIGO's active damping system. LIGO's quad suspensions are themselves mounted within larger active vibration/seismic isolation systems, giving them the quietest possible environment for operation.

In these systems, internal seismic isolation platforms (ISIs) utilize position- and vibration-sensors (such as seismometers) tuned to different frequencies of environmental vibrations, along with permanent-magnet actuators. Through a feedback mechanism, wherein external signals are fed to actuators that counteract the detected ground motions, the internal components of the interferometer are kept virtually motion-free. This active level of isolation can reduce the magnitude of vibrations introduced to the suspensions (at the point of their attachment to the ISI) to a level of at most 2x10^-13 m. The passive isolation suspensions then take over, reducing this noise level nearly a million times more to help achieve LIGO's desired detection sensitivity of 10^-19 m (the amount which gravitational waves expand and contract spacetime between the test masses).

In these ways, LIGO uses both passive and active vibration isolation systems to facilitate its ability to detect gravitational waves.