Designing instruments like LIGO's interferometers, which are capable of measuring a change in distance of 10-19 meters, required inventing and refining innovative technology. Most of LIGO’s impressive technology resides in its seismic isolation systems (which remove unwanted, non-gravitational-wave vibrations), vacuum systems (to keep dust out of components and make sure the laser light travels freely), optics components (to preserve and boost laser light and laser power), and computing infrastructure (to handle the scientific data that LIGO collects). These systems are like LIGO's internal organs. If any one fails, the whole instrument suffers.
While each of these components is a feat of engineering in itself, without working together as a single multifaceted instrument, LIGO could never achieve its scientific goals. A basic overview of each of LIGO's critical systems is provided below. For a more detailed discussion of these and other LIGO engineering systems, visit Look Deeper.
LIGO’s greatest strength is also its greatest weakness. Since LIGO is designed to sense the smallest conceivable motions of mirrors caused by the passage of a fleeting gravitational wave, it is also extremely sensitive to all vibrations near and far (from trucks driving on nearby roads to earthquakes on the other side of the world). Without taking extraordinary measures, any number of Earthly vibrations could move LIGO’s “test masses” (the laser-reflecting mirrors at the end of each arm) enough to hide a gravitational wave vibration. Isolating LIGO's test-masses and other sensors from Earthly environmental vibration is the linchpin in LIGO’s quest to detect gravitational waves from space. To that end, LIGO uses two primary means of eliminating vibration: “active” vibration damping and “passive” vibration damping.
Active Vibration Damping
The first line of defense against unwanted vibration is LIGO’s “active” damping system. In this system, called the Internal Seismic Isolation (ISI) system, devices around each detector site (LHO and LLO) sense a wide range of environmental vibration frequencies and ground movements. Each sensor sends its signal to a computer that combines all these motion signals and then generates a net counter-motion to cancel all of the external vibrations simultaneously. It is very similar to how noise-canceling headphones work.
Passive Vibration Damping
The job of LIGO’s passive damping system is to trick the all-important test masses (mirrors) into feeling like they are floating in space by using a 4-stage pendulum called a "quad suspension". In the "quad", the test masses hang at the bottom of this 4-segment pendulum by 0.4 mm thick fused-silica (glass) fibers. There are two sides to the system: The "Main Chain" and the "Reaction Chain". The test mass itself resides on the Main Chain side, which faces the laser beam. The "Reaction Chain" hangs behind the Main Chain, further helping to keep the test mass free of vibrations not caused by a gravitational wave. This pendulum configuration absorbs almost all movement not completely canceled out by the active (ISI) system. In fact, any external vibration that does make it past the active isolation system is further reduced 100-million times by this passive method by the time it reaches the test mass!
Working together, these active and passive vibration damping systems ensure that LIGO's lasers and mirrors are isolated from as much external noise and vibration as is physically possible.
Operating in a Vacuum
LIGO contains one of the largest and purest sustained vacuums on Earth. In volume, it is surpassed only by the Large Hadron Collider in Switzerland. The atmospheric pressure inside LIGO's vacuum tubes is one-trillionth that of air pressure at sea level. LIGO needs to maintain such a good vacuum for two reasons:
1. Air - Even just a few molecules of air can create noise that masks the tiny changes in distance between mirrors we seek to detect. One example of such noise is Brownian motion, which refers to the fact that everything with a temperature above absolute zero is moving with heat energy. Molecules of air hitting the mirrors can cause them to move or vibrate, which changes the distance the laser beam travels. Even the smallest change in distance can mimic or mask a change actually caused by a passing gravitational wave. So keeping air out of the way eliminates Brownian motion noise.
Another way air in the light path can cause problems is similar to the shimmering one sees over a hot road. In such a case, air acts like a lens and can change the path the light takes as it passes through it. In LIGO, if the path of the laser changes, so too does the distance the beam travels, which in turn, would cause an interference pattern that could make LIGO's detectors think a gravitational wave passed when one really didn't.
2. The second critical reason for operating in a vacuum is to eliminate the chances that dust will drift into the path of the laser, or worse, onto a mirror. A mote of dust passing through the beam could cause some light to scatter (i.e., be reflected in some random direction away from its path). This scattering could be misinterpreted as a flicker of light caused by a gravitational wave. Worse, if a piece of dust were to land on a mirror in line with the laser beam, it would be incinerated by the laser, and could cause irreversible damage to the mirror, rendering it useless. Considering that each of LIGO's test masses costs about $2 Million to produce (the glass and the coatings), avoiding this damage by maintaining one of the cleanest vacuum systems on Earth is critical to LIGO's functionality.
Overall, without operating in a high-quality vacuum, LIGO’s lasers could be absorbed and deflected enough to create signals that make us think we've detected a gravitational wave when we have not, air molecules bouncing off the mirrors could overwhelm an incoming wave, or a mote of dust could simply destroy a critical and expensive component of the instrument.
Creating such a large volume of empty space on Earth was no easy task. Many techniques were used to remove all the air and other molecules from LIGO’s vacuum tubes:
- The tubes were heated to between 150 C and 170 C for 30 days to drive out residual gas molecules that were contained within the metal itself
- Turbo-pump vacuums (little jet engines that create suction instead of thrust) sucked out the bulk of the air contained in the tubes.
- Ion pumps then extracted individual remaining gas molecules by electrically charging them and then attracting them away with opposite charge, like a magnet. In fact, since the metal inside the vacuum chamber is always emitting some gaseous molecules ("outgassing"), these pumps operate continuously in order to maintain the pristine vacuum inside the tubes.
- It took 40 days (1100 hours) to remove all 10,000 m3 (353,000 ft3) of air and other residual gases from each of LIGO’s vacuum tubes to reach an air pressure one-trillionth of an atmosphere.
The pump-down process occurred just one time at each site, in 1998 at LHO, and 1999 at LLO. Remarkably, LIGO has maintained this vacuum in its beamtubes ever since!
LIGO's optics system consists of lasers, a series of mirrors, and a photodetector (a device that measures varying light levels). In order to measure a movement thousands of times smaller than a proton, LIGO's optical components must operate harmoniously and with unprecedented precision. It all begins with the main laser.
We all encounter lasers daily in laser pointers, cat toys, or barcode scanners at the grocery store. Because of their omnipresence most of us tend to take them for granted without really knowing how they work. If you want to know how they work, Cambridge University’s “Naked Science Scrapbook” video, “How do lasers work?” provides a fun, easy-to-understand explanation. Once you grasp the basic principles, understanding LIGO's laser is a snap!
The first thing to understand is that the word, "laser" is an acronym for “Light Amplification by the Stimulated Emission of Radiation”. This means that "laser" refers to a process more than a thing, but most of us now use the term ubiquitously to refer to the device that generates the laser beam, or the beam itself.
The heart of LIGO is its 200 Watt laser beam. But the beam doesn't start out at 200 W. It takes four steps to amplify its power and refine its wavelength to a level of precision never before seen in a laser of this kind.
The very first glimmer of light that ultimately becomes LIGO's powerful laser emerges from a laser diode, which uses electricity to generate an 808 nanometer (nm) near-infrared beam of about 4 Watts. This is the same kind of device that a typical laser pointer uses. While 4 W doesn't seem like a lot, the laser in the average laser pointer shines at less than 5 milliwatts. So LIGO's 4 Watt beam is 800 times more powerful than that laser pointer you use to entertain your cat!
The second step in boosting LIGO's laser up to 200 W occurs when the 4 W beam enters a device called a Non-Planar Ring Oscillator (NPRO). The NPRO consists of a boat-shaped crystal about the size of a fingernail! The 4 W beam bounces around inside this crystal and stimulates the emission of a 2 W beam with a longer wavelength of 1064 nm (in the invisible infrared part of the spectrum).
Step three in LIGO's laser amplification occurs when the now 2 W, 1064 nm beam enters another amplifying device that boosts it to 35 W. This 35 W beam is then sent through a device called a High Powered Oscillator (HPO), which further amplifies and refines the beam. Ultimately, the 35 W laser is boosted to 200 W of "lased" light. This is the beam that enters LIGO's interferometer.
This multi-stage amplified laser is required for LIGO because it needs to continually produce a pristine single wavelength of light. In fact, LIGO's laser is the most stable ever made to produce light at this wavelength. This stability is one of several factors critical for LIGO's ability to detect gravitational waves.
Maintaining the stability and purity of LIGO's laser is one of LIGO's biggest challenges, but it is not enough just to generate a clean, stable laser. Since the laser reflects hundreds of times within each arm, the mirrors and test masses that the beam encounters are equally as important to LIGO's operation and success as the laser itself.
LIGO’s mirrors are the highest quality available, both in material and shape. Made of ultra-pure materials laid down in nano-meter thick layers, the reflective surface absorbs just one in 5 million photons. This means that most of the laser light is reflected rather than absorbed, reducing how much the mirrors heat up. This is critical because too much heat from the laser would alter the shape of the mirror enough to distort the reflected light, causing it to be lost before it returns to the photodetctor. Any degradation hampers LIGO's ability to distinguish a gravitational wave from environmental noise. Lastly, the highly reflective surface means that more photons reflect back into the arms, thus preserving laser power; the more power, the better LIGO's ability to sense gravitational waves.
The mirrors also refocus the laser, keeping the beam traveling coherently, meaning that it doesn't spread out as it travels back and forth through the arms before finally reaching the photodetector. If the beam continually spread out, little light would be left to strike the photodetector!
Finally, the mirrors were shaped and polished so precisely that the difference between the theoretical design (the perfect mirror shape as designed on a computer) and the actual polished mirror surface is measured in atoms! This is critical because, with all the reflections it goes through, each laser in each arm travels about 1120 km before being merged with its partner before finally reaching the photodetector. If the beam continually spread out, little light would be left to strike the photodetector!
Without a clean, stable laser reflecting off of nearly-perfect reflecting surfaces (the mirrors and test masses), LIGO would find it virtually impossible to detect gravitational waves.
Computation and Data Collection
Powerful, sophisticated computers are required to control LIGO's interferometers and store and process the data that they collect.
When it is in 'observing' mode, LIGO generates terabytes (1000's of gigabytes) of data every day. All of this information must be transferred to a network of supercomputers for storage and archiving. Such supercomputers are located at each of the observatories, at Caltech, at MIT, and at various other institutions. Once the data is secured, scientists can use customized computer programs to scour the data for gravitational waves.
The amount of data LIGO collects is as incomprehensively large as gravitational wave signals are small. LIGO's archive already holds the equivalent over 1-million DVDs of data and will add the equivalent of about 178-thousand DVDs each year to its archive. In actual numerical terms, the data archive at Caltech holds over 4.5 Petabytes (Pb) of data, and will grow at a rate of about 0.8 Pb (800 terabytes) per year. What's a petabyte? If you wanted to count up to a petabyte by counting one byte per second, it would take you 35.7 million years to reach one petabyte!
Storing information is one thing; processing it is another. Processing and analyzing all of LIGO's data requires a vast computing infrastructure. For LIGO's first observing run in 2015, the LIGO Lab will provide 35 MSU (million service units) worth of computing cycles/time. This is equivalent to running a modern 4-core laptop computer for 1,000 years! The amount of computing time is expected to grow by a factor of 10 to around 400 MSU by the time LIGO has completed its third observing run.
If you'd like to learn even more about all of LIGO's remarkable technology and engineering, visit Look Deeper.