How Do Gravity Wave Detectors and Observatories Work?
The first mention of gravity waves as we know them was by Einstein in a 1916 follow-up to his work on relativity. He predicted that minute changes in mass in space-time would cause a wave of gravity to emanate from the object and travel somewhat like a ripple on a pond (but in three dimensions), not unlike how the movement of electrical charges causes photons to be released. However, Einstein felt that the waves would be too small to detect, according to his original draft for the 1936 Physical Review entitled "Do Gravitational Waves Exist?" Indeed, the only objects currently in existence strong enough to expel lots of energy as well as dense enough to make gravity waves we can detect are black holes, neutron stars, and white dwarfs. Einstein felt his equations generalized too many first-order approximations, which made the non-linear equations he worked with easier to handle. But because of a mistake in his work, he withdrew the paper and later revised it when he noticed that a cylindrical coordinate system resolved many of his qualms with the mathematics, but his viewpoint on the waves being too small remained (Andersen 43, Francis, Krauss 52-3).
The Road to the First Detectors
Many calculations in the 1960s and 1970s indeed pointed to gravity waves being so small that luck itself would play a role in detecting any of them. But Joseph Weber was one of the first to claim detection. Using a 3000 pound, 2 meter long and 1 meter in diameter bar of aluminum, he measured the change in the strain on the end points of the bar as waves would distort it and the time it took in the hope of finding a resonant frequency. Quartz crystals at the the ends of the bar would only complete a circuit if such a frequency was reached. Using this technique Weber claimed to have detected gravity waves in 1969. Peer review however showed flaws in the study (namely that it picks up a lot of noise from the Universe) and the results were discredited. Even after improvements were made to the design (with one even being put on the Moon), nothing was found (Shipman 125-6, Levin 56, 59-63).
Jump now to the 1980s. Scientists thought about the failures of the Weber bar and realized that a similar idea could work: an interferometer (see LIGO for specifications). Ron Drever begins working on a 40-meter prototype version for Caltech based off the ideas of Robert Forward and Weber while Rai Weiss was tasked with doing a noise analysis in an effort to get a clean reading and also set up a 1.5 meter model for MIT. Some things to keep in mind during a noise analysis are tectonics, quantum mechanics, and other astronomical objects potentially hiding the gravity wave signal scientists were hunting for. Drever and Weiss along with Kip Thorne took the lessons from Weber's bar and tried to scale them up. After several years of prototypes and testing, everyone combined their efforts (and therefore funding) and developed Blue Book, a 3-eyar compehensive study that summarized all the findings on gravity wave detection tech. The joint Caltech-MIT effort was branded as C-MIT and presented Blue Book in October, 1983 and the projected cost at the time was $70 million. The NSF decided to give the joint effort their funding, and the project became known as LIGO (more on that below, I promise) (Levin 71-3, 79-86).
How Did Scientists Know Gravity Waves Were Real?
In 1974 Russel Hulse and Joseph Taylor looked at a binary pair of stars with one being a neutron star and the other a pulsar in system PSR 1913+16, located 21,000 light-years away. At an orbital distance of 2.8 solar radii, a mass of 1.4 solar masses each, they completed an orbit around each other ever 7.75 hours and had an average speed of 0.001c. Radio pulses emitted at a rate of 1.7 pulses per second Relativity certainly was in play here. They knew based on calculations that if gravity waves existed then energy would be robbed from both objects as they orbited each other. This loss of energy would cause them to fall closer to their barycenter, or the apparent center of their orbits, and increase their speed (for closer objects orbit faster) as well as decrease their orbital period by 0.001 seconds per year. The scientists came up with the orbital path with gravity waves and without them and observed the binary for years. Sure enough the path predicted by the gravity waves was right and in 1978 their prediction was shown to be true. Hulse and Taylor’s work was so profound it won them the 1993 Nobel Prize in Physics (Andersen 43, Shipman 126-7, Levin 100-1).
So why should we care for such an elusive but known target? Gravity waves may be the key to seeing beyond the cosmic microwave background, the moment light was able to pass through matter (about 380,000 years after the Big Bang). This was because of the high energies that the Universe was in which prevented light from being transmitted freely. But the movement of matter through space-time should have created gravity waves, which can indeed be detected regardless of the conditions prior to the cosmic microwave background. They could be the key to proving that inflation happened and thus the possibility of a multiverse (Andersen 42)! However, the smallest waves would be kilometers in length so we would need such a sized detector if we hope to see some waves (Francis).
That being said, one team has hunted for primordial gravity waves in a different way. Known as the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP 2), the project worked for over 3 years in the cold of Amundsen-Scott South Pole Station in the Antarctica, which lies over 2,800 feet above sea level. This makes for optimal viewing conditions as air is thinner and less light is absorbed. The team was led by John Kovac as the Harvard Center for Astrophysics, the University of Minnesota, Stanford University, the California Institute of Technology, and JPL combined their resources as BICEP 2 scanned about 2% of the sky looking for B-modes. This is a phase of light which has had one of its two main components (the magnetic or B and the electric or E) stretched while the other is compressed, which leaves a pattern in the CMB by curling that light. If directly measured then such a curl would be indicative of the elusive gravity waves (Ritter, Castelvecchi, Moskowitz).
In March of 2014, the BICEP 2 team announced that they had found the signal they had been hunting for. But the science community was skeptical, especially when it was found that the dust from our galaxy was not taken into account. It too could have made those B-mode patterns and contaminated the data. After reviewing data of the CMB from the Planck spacecraft it was found that the BICEP 2 results had indeed been contaminated by the dust. Fortunately, the Planck data does give the team new and better places to look where the dust is not as prevalent (Cowen, Timmer 22 Sept. 2014).
The Laser Interferometry Gravitational Wave Observatory (LIGO) started its hunt in 2002, but only after years of development and design.Costing $570 million to build, the detector utilizes a ground-based method for finding gravity waves that relies on laser interferometry. What exactly does that entail? Two beams are sent parallel to each other, one going away and the other reflected back towards the detector. The return beam will be purposefully out of phase based on how it is reflected back so that the detector will have two waves of opposite amplitude meet and cancel each other out for a net signal of zero. But if a gravity wave passes by then the return beam will not be perfectly out of phase and so the detector should then record a signal (Andersen 43, 45; Wolchover).
The two LIGO observatories are separate interferometers, one in Washington State while the other resides in Louisiana. Each arm of a detector has a laser that is send down the entire length (4 kilometers). The detector’s job is to see if changes in distance less than one part in 1020 between the laser beams occur by a gravity wave warping space-time. Unfortunately, the only events that generates waves strong enough for LIGO are when binary pulsars fall close to each other and neutron star collisions. Amazingly, these events would only produce a maximum signal height “less than the diameter of a proton.” In short, miniscule (Andersen 43, 45; Faesi, Francis, Haynes "A Wrinkle" 24-5).
LIGO ended its initial run in 2010 with no measurements of gravity waves, but a $200 million upgrade began shortly thereafter and ended in 2015. Now rebranded Advanced LIGO, it should be able to measure gravity waves from as far away as 500 million light-years away, increasing its possible detections by a factor of 100. And the upgrade definitely paid off, for on February 11, 2016, a hundred years after Einstein predicted their existence, LIGO captured evidence of gravity waves. According to Dr. David Reitze, the executive director at LIGO, a black hole merger 1.3 billion light-years away was detected on Sept. 14, 2015 and produced enough of a disturbance for the detector to register. The delay in announcing the result came from the verification process as 25 other telescopes looked at the spot, with Fermi detecting a small gamma ray burst in the area (which is to be expected from such a merger). Scientists wanted to make sure it was not a glitch, a hack, or something else. But the 5-sigma result held through, meaning that scientists are 99.9999% confident in the result. A new age of astronomy has begun (Andersen 43, Francis, Freeman, Wolchover, Haynes "A Wrinkle" 23, Haynes "Gravitational").
And it continues on. On June 15, 2016, LIGO scientists released the findings on the second observed measurement of gravity waves. These came from GW151226 (that is, Gravity Waves from Dec. 26, 2015), another black hole merger, and observations of it were made just 70 seconds after the gravity wave detection. The reason for the delay in the finding was so that scientists could achieve the 5 sigma confidence in their results, knowing that the observation is incredibly unlikely to be a fluke. The masses of the black holes (one about 8 solar masses and the other about 14) were much smaller than those in the first merger and thus made accurate measurements more challenging. The new black hole is likely 1.4 billion light-years away, is about 21 solar masses and radiated about a sun's worth of mass in gravitational energy. That size played a big role, for it allowed a longer signal for scientists to analyze. Based off the signal and modeling, scientists feel one of the black holes was spinning, a result of the star it came from prior to its death via a supernova (Timmer 15 Jun. 2016, Betz "LIGO", Wenz "LIGO Finds," Klesman).
On January 4, 2017, LIGO detected a third gravitational wave event, GW 170104. Once again, a binary set of black holes from 3 billion light-years away were the culprit as they merged long ago. One was 31.2 solar masses while the other was 19.4 solar masses, with the resulting black hole being 48.7 solar masses. Where did the nearly 2 solar masses disappear to? They were converted into energy during the merging process at a rate of 1034 megatons per second. Another thing noticed was that the length of the merger was fast, implying that the spins of both black holes were well aligned and not in disjunction with each other. This hints at a common origin for both of them, likely in the same cluster of stars. Another big discovery was a lack of dispersion in the gravitational wave signal that would result in different frequencies. That is huge because it reinforces relativity and also reduces the possible size of the graviton to something less than 7.7 * 10-23 eV, a 30% reduction from the previous limit (Betz "3rd," Timmer "LIGO Spots," Wenz "LIGO Detects," Naeye "Gravitational).
On August 14, 2017, a fourth gravity wave was detected by LIGO and with the help of other detectors such as Fermi and INTEGRAL were able to provide a triangulation for the source of the waves. Now they know where GW170814 happened to within 60 square degrees as opposed to the standard 1,000 square degrees. Two black holes located 130 mega light years with masses of 25 and 31 solar masses merged long ago to create a 53 solar mass resultant (Betz "Scientists," Timmer "New," Betz et al.).
But the biggest news yet came on October 16, 2017 when scientists announced GW 170817 was detected by LIGO and Virgo (a new detector in Pisa Italy, started up on August 1 of 2016). Why was it such a big deal? It was the first gravity wave event of a kilonova, associated with the merger of neutron stars, opening the doors for much scientific insight into several disciplines. Many telescopes turned to the source in NGC 4993, located 130 million light-years away, and began to record data like crazy. A gamma ray burst was seen coming from the region just 1.7 seconds after the waves and lasted for 2 seconds, providing evidence for a long believed theory on their generation as well and the speed of gravity waves (which is near c), hinting at the possibility for gravitons. Data pointed to stars with 1.1 and 1.5 solar masses and the resultant object (which is not known) could be 1-2 percent smaller in mass because of energy dissipation. Over 70 visual telescopes turned their eyes to the event to record the other EM happenings for 10-11 hours after the event. All together, the gravity wave event lasted 100 seconds, just slightly less than theory be that could be because of an off-axis merger relative to us. Rarely has such a massive event been given so much attention, and especially such a weak event as a neutron star merger. Who knows what surprised await us as the data is analyzed for some time to come (Naeye "LIGO," Naeye "Gravitational").
While not yet in existence, the ongoing drama of the ESA’s Laser Interferometer Space Antenna (LISA) will surely be worth it once realized. It has been in various stages of development for the past 30 years and likely will not be built and launched for 20 more years. The basic approach is simple enough: three spacecraft located at the L1 Lagrange point make up the points of an equilateral triangle with sides of 5 million kilometers. A laser is sent back and forth to each probe so that the constant distance is known. If a gravity wave passes by, the distance will be altered and detection possible (Andersen 43-4, Haynes "LISA Tests").
2011 was a critical year for the development of LISA as a big test towards the full-scale version was successfully accomplished. The Optical Meteorology Subsystem was put under space-like conditions by being in a vacuum and under cold conditions yet managed to beat the required precision by 3 times the expected value. The test was conducted at Ottobrunn by the Astrium Ltd.. On June 25, 2016, a space-based test looked at the laser interferometer technology and it worked great (Plotner, Haynes "LISA Pathfinder").
This is another future gravity wave detector that is in development. The Kamioka Gravitational Wave Detector will be located in Japan below the surface (to minimize seismic surface fluctuations) and will operate under the interferometer principle with a beam splitter. The L-shape detector will travel into Mt. Ikenoyama (which is home to the Super Kamiokande Neutrino Observatory) and have two arms at a length of 3 kilometers each. The beam splitters will be made of sapphire (because of their structural integrity at low temperatures) and cooled to -424 degrees Fahrenheit (to lower heat noise in the signal). The goal is to detect gravity waves in the 100 Hz range, a result of a black hole merger (Hornyak).
The team at Advanced Concepts over at NASA is led by Babak Saif (the interferometer engineer for the James Webb Space Telescope) and Mark Kasevich (professor of applied physics at Stanford University) is working on an alternate method. While nothing is set in stone with it yet, an atom interferometer could be a potential detector. Unlike the traditional interferometer, which uses mirrors and lenses, an atom interferometer makes use of quantum mechanics. Clouds of atoms are placed in superposition by lasers, allowing them to occupy the same places in space-time. In this case the lasers cause the atoms to achieve a state of nearly perfect rest, aka absolute zero, then another set of lasers causes the superposition to occur. 10 seconds after being put in this state the clouds are released from the lasers, supposedly back in their original state minus the changes in momentum they may have gone under. But if a gravity wave passed by while the clouds were in superposition then the atoms will have their state changed. Unlike a laser interferometer, this version will have a high level of accuracy down to the nanoscale and it does not need large distances (5000 times less, actually) for it to work unlike its counterparts (Andersen 44, Keesey).
Pulsar Timing Array
Gravity waves should be a result of a natural process, so why not use another one as a detection tool? Millisecond pulsars fit the bill. This subclass of the spinning neutron stars pulses thousands of times a second courtesy of an angular momentum transfer from a binary companion. They pulsate at such a regular interval that any deviation would potentially be caused by depressions in the fabric of space-time caused by gravity. But one would not be enough to show this. We would need a network of them to show specifically what was causing it and to see if others are impacted. NANOGrav (short for nanoscale gravitational waves) and The European Pulsar Timing Array has found some possible upper limits as to what the amplitude of a gravitational wave could be but nothing concrete has been found yet. Even over a decade of observations by the Parkes Pulsar Timing Array didn't find anything. This was despite finding 24 pulsars with low activity being observed, which would allow for any significant changes like gravitational waves to be easily seen. Such seemingly null findings are in fact able to eliminate some models for gravitational wave production, so not all is lost (Keesey, Timmer 27 Sept. 2015, Murray).
Surprisingly, ordinary stars may be all we need to find evidence of gravity waves. Barry McKeman from the American Museum of Natural History in New York and his team have shown that it is possible for a star to absorb some of the gravitational energy the wave would supply so long as it was in tune with the right frequency of the wave, aka resonance. This would cause an output increase in terms of brightness that we can measure. Of course many things can cause a star to brighten up, so the study authors suggest that if a group of stars could be found then it would be possible for them all to be hit by the resonant wave and thus increase as a group. The best-case scenario for this would be around a black hole, for the stars would be close together and near a potentially large gravity wave maker (Choi, American Museum of Natural History).
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© 2016 Leonard Kelley