How would you or I detect a gravitational wave?
If they were on a magnitude great enough to be detected by the human body (or if we were much closer to a black hole), we would sense our body being stretched followed by compression if a gravitational wave passed through us. The closer you get to a massive object, the stronger the stretching force. As you approach a black hole your body would literally be ripped apart as the difference between the pull at your feet and the pull at your head increased. The problem, however, is that these waves are at such low frequencies that they are incredibly difficult to detect from billions of kilometers away. Sensitivity of instruments is expressed in increments as small as 10-21 meters. This means that as a wave propagates through a massive body it will change the dimensions of that massive body by 0.000,000,000,000,000,000,001 meters! Obviously this is difficult. The main interference, besides the motion of the earth, comes from thermal noise. Even elements in the solid physical state experience some atomic movement and the warmer the body, the greater the motions. Thus these detectors must be as cold as possible. That explains the other name for resonant detectors--"Cryogenic Antennae." Cryogenic merely refers to the production of extremely low temperatures.
Resonant GW detectors draw from various areas of physics including thermodynamics, to cool solids to near-absolute-zero temperatures; electricity and magnetism, to convert vibrations into electrical signals; mechanics, to design filters to remove external vibrations; harmonic motion, for the concept of resonance that is fundamental to these detectors; and many more.
All objects, including human beings, have what is called a "natural frequency." If waves of any sort (electromagnetic, sound, gravitational) pass through an object at its natural frequency, said object will vibrate more strongly at that frequency than any other frequency. For example, the over-thirty set might recall a commercial produced by the cassette tape company Memorex. The famous jazz singer Ella Fitzgerald sings a pitch that is exactly the natural frequency of a wine glass. Ella's voice is recorded on a Memorex tape and the glass shatters when those sound waves are played in its vincinity. The punch line goes, "Is it live or is it Memorex?" The commercial itself is inconsequential but it nicely illustrates the idea of an object's natural frequency. It's obvious that typical singing doesn't shatter glass, otherwise choirs would practice in bomb shelters! Therefore these resonant gravitational wave detectors are designed to vibrate vigorously at specific natural frequencies that are in the predicted range for gravitational waves. In that way the chances of detection are greater because gravitational waves are so miniscule to begin with the amplification effects of resonance are helpful.
In 1963 Joseph Weber built the first resonant GW detector. It was 3100 pounds of aluminum in a cylinder 5 feet long by two feet wide. It could resonate at frequencies down to 1660 Hz.
During the late 1950s and early 1960s when Weber was putting together his ideas and then his detector, not much was known about gravitational waves. The mathematical computations that have since been done to determine the effective range of frequencies at which to detect these waves were not in existence at this time. So Weber essentially guessed. The only knowledge he had at the time was a general idea of the maximum frequency. If an object is orbiting at a velocity equal to the speed of light around the smallest possible object, the frequency of its orbit will be 10,000 Hz (cycles per second). Thus Weber went on the order of a tenth of that value, 103 instead of 104.
Weber developed a technique based on the piezoelectric effect. In 1880, Pierre and Jacques Curie discovered that there happen to be certain materials of the crystal and ceramic families, including simple quartz, that can actually acquire a voltage difference across them if their length is changed by being squeezed or stretched. They all share the common trait of being without a center of symmetry. Weber's first choice would have been to build his entire bar out of such a material. The bigger the object, the greater the voltage drop and so if you just hook it into a circuit it is very easy to detect passing waves. These materials, however, are rather costly. Thus Weber had the idea to attach small crystals all across the surface of his detector and then wire them together. In electronics, if voltages are connected in a series, one after the other, they can simply be added up. That is how Weber increased the potential for him to detect gravitational waves. He was not, however, successful.
There are currently two shapes of resonant GW detectors and they are located all over the globe:
GRAIL--Netherlands
Grail is a 110 ton copper alloy sphere measuring 3 meters in diameter. The sphere is suspended in a vacuum chamber and cooled to temperatures around 10 millikelvin to diminish outside and thermal interference. The vibrations made by gravitational waves are converted to electrical signals through transducers located on the surface. The required sensitivity is 10-21 meters.
TIGA--United States
Tiga operates out of Louisiana State University. Depicted to the right is a prototype aluminum model measuring 33 inches in diameter.
OMEGA--Italy
SFERA--Italy
SFERA weighs 100 tons and should detect at frequencies between 765 and 815 Hz. Its operating temperature is 20 millikelvin.
ALLEGRO--United States, Louisiana State University
Shown in the image at right, aluminum-made Allegro (A Louisiana Low temperature Experiment and Gravitational wave Observatory) weighs in at 2300 kg with a length of 3 meters. Its operating temperature is 4.2 kelvin. The actual resonant cylinder is the lower one. The gold bars on the front of the cylinder are the devices that pick up the signals received by the cylinder.
EXPLORER--Italy
Like Allegro, Explorer is also 2300 kg of aluminum 3 meters long with a diameter of 60 centimeters. It is cooled to the temperature of liquid helium, 4.2 kelvin. When in operation the pressure on the helium reservoir is lowered so the temperature drops to 2 kelvin. The resonance frequencies for Explorer are 906 and 923 Hz.
NAUTILUS--Italy
Another 2300 kg cooled to 0.1 kelvin. Sensitivity is down to 10-18 meters that signifies an energy as little as 10-6 electron volts.
AURIGA--Italy
Ultracryogenic Resonant Antenna for the Gravitational Astronomical Investigation.
NIOBE--Australia, The University of Western Australia
Niobe is made out of 1.5 tons of niobium, which, incidentally, happens to also be one of the trace metal elements found in red supergiant stars. Operating at 5 kelvin, Niobe's resonant frequency of 710 hertz.