As the 1960s drew to a close, Rainer Weiss was working as an associate professor at MIT’s Department of Physics.1 Asked to teach an undergraduate course on general relativity by the department chairman, Weiss found himself in the unenviable position of teaching an unfamiliar subject. “I had a terrible time with the mathematics,” Weiss recalled, “[a]nd I tried to do everything by making a Gedankenexperiment out of it.”2
Weiss’s students were curious about the work of physicist Joseph Weber and, in particular, his attempts to detect gravitational waves. Predicted by Albert Einstein’s theory of general relativity, gravitational waves were a much-debated phenomenon for which no experimental evidence had been found. Weber’s efforts were centered on resonant mass detectors of his own design: suspended aluminum cylinders two meters in length and a meter in diameter fitted with a ring of piezoelectric crystals.3 Weber believed that the cylinders would, in effect, act like giant tuning forks; a passing gravitational wave would ring the cylinders at their resonant frequency. In a 1969 paper published by the Physical Review, Weber claimed to have found evidence for gravitational waves.4
Weiss was struggling to teach his course. An attempt to integrate Weber’s work hardly helped: “It was hopeless. I couldn’t understand what Weber was up to.”5 Weiss turned to a particular aspect of general relativity—“[t]he only thing I really understood in the whole damn theory”—and improvised:
And so I gave as a problem, as a Gedanken problem, the idea, “Well, let’s measure gravitational waves by sending light beams between things,” because that was something you could solve. The idea was that here was an object. You’d put another object here and make a right triangle of objects, floating freely in a vacuum. And we’d send light beams between them and then be able to figure out, “What does the gravitational wave do to the time it takes light to go between those things?” It was a very stylized problem, like a haiku, you know? You’d never think that it was of any value.6
Were it not for growing skepticism about Weber’s experiments and Weiss’s own curiosity, the story might have ended there:
I didn’t think much more of it until about a year later, when I began to realize something about Weber’s experiments—nobody was getting the answer he was getting. He had made a huge and powerful claim. And I began to realize, maybe this was wrong, and maybe even his idea of how it works was wrong.7
Further research convinced Weiss that his method for the detection of gravitational waves was feasible. In a paper published in 1972, he outlined a design for an “Electromagnetically Coupled Broadband Gravitational Antenna.”8
A little more than four decades later, gravitational waves were successfully detected using instruments that followed the design proposed by Weiss all those years ago. The Laser Interferometer Gravitational-Wave Observatory (LIGO): a Gedankenexperiment made real.
Amid the jubilation and excitement in February 2016 following LIGO’s official announcement, it was easy to overlook the fact that for Weiss and his collaborators their moment of triumph marked the culmination of years of sustained effort.9 It had been very far from a straightforward process. In truth, the LIGO project had at various stages been subject to stinging criticism of its scientific credentials, goals, experimental approach, and, in particular, the scale of its funding. Even the claim that LIGO was an astronomical observatory had been contested.
Before examining some of these points, it is worth taking a few moments to reflect on the stunning scientific and technological achievement that is the LIGO project.
The Discovery
On September 14, 2015 the LIGO sites at Hanford, WA and Livingston, LA detected a transient gravitational-wave signal.10 The detections were near-simultaneous, with a delay of only 7 milliseconds between them. This tiny delay corresponds to the time for a gravitational wave, which moves at the speed of light, to travel the distance between the two detectors: 3,002 kilometers.
Over the course of 0.2 seconds, the signal, designated GW150914, “swept upward” in frequency from 35 to 250Hz, with a combined signal-to-noise ratio 24 times greater than that of the background noise.11 The fractional change measured during the detection, or the peak gravitational wave strain, was 1.0 × 10-21 meters.12
The source of the signal was identified as the inspiral and merger of two black holes of 36 and 29 solar masses, and the subsequent ringdown of a single black hole of 62 solar masses.13 The remaining 3 solar masses were converted into energy and dispersed as gravitational radiation. As theoretical physicist and LIGO co-founder Kip Thorne observed, “It is by far the most powerful explosion humans have ever detected except for the big bang.”14
Just before merging, the two colossal black holes were a mere 350 kilometers apart and had a staggering relative velocity of 1.8 × 108 meters per second, 60% of the speed of light.15 The attribution of the signal to this particular source is, in fact, crucially dependent on the enormous velocities and tiny separation of the two objects. LIGO’s analysis of the signal
impl[ies] that the two components were only a few hundred kilometers apart just before they merged, ie. when the gravitational-wave frequency was about 150Hz. Black holes are the only known objects compact enough to get this close together without merging. Based on our estimated total mass for the two components, a pair of neutron stars would not be massive enough, and a black hole-neutron star pair would have already merged at a lower frequency than 150Hz.16
Following the announcement, LIGO made an audio conversion of GW150914 available on YouTube in two versions.17 The first is a frequency-matched version that sounds curiously like a heartbeat. A second version, with the frequency shifted upwards to make it a little more suited to our ears, has been described as a chirp, but is also somewhat reminiscent of a drip.
Remarkable.
Interferometers
The principles of interferometry were well established when Weiss published his 1972 paper. In 1877, Albert Michelson and his colleague Edward Morley used an interferometer designed by Michelson for an experiment that disproved the existence of the luminiferous aether that, at the time, was thought to pervade the universe.18
The interferometers used by LIGO retain the essential characteristics of Michelson’s device.19 Two arms are positioned at right angles and joined at one corner to form an L-shape. A light source is placed at the corner, aligned with one of the arms, and directed through a partially reflective mirror, which splits the beam in two, reflecting the resulting beams along the arms. Any disparity in the distances followed by the two beams prior to recombination causes a phase shift between them, which in turn creates an interference fringe pattern.
While LIGO’s interferometers embody key aspects of their historical antecedents, there are, of course, notable differences. Two of the most obvious are the increased scale and the use of lasers.
The LIGO interferometers are 360 times larger than Michelson’s 1877 device. Michelson’s 1877 device had arms 11 meters in length, compared to the 4 kilometer arms of LIGO’s interferometers. Over such distances the earth’s curvature becomes an issue, and had construction not taken it into account, the vertical height difference between the opposite ends of each arm would have amounted to 1 meter.20
The Michelson–Morley experiment used sodium light for alignment and white light for measurement. In contrast, LIGO uses a frequency- and power-stabilized laser, amplified to 200W.21 The first laser interferometers appeared during the 1960s.22 Lasers offer two advantages: coherence (the rays are uniform in phase and wavelength), and collimation (the light rays are parallel).
Consider the configuration of an aLIGO (Advanced LIGO, second generation) laser interferometer:23
- An additional mirror in each arm is positioned after the beam splitter. These two mirrors, the input test masses (ITMs, which, as the name suggests, are used to test for changes in interferometer arm length), form a resonant optical cavity, known as a Fabry–Pérot cavity, together with the mirrors at the end of each arm, the end test masses (ETMs).24
- The presence of the Fabry–Pérot cavities results in the beam being effectively trapped and reflected approximately 280 times back and forth along the length of the arms, between the partially-transmitting ITMs, and the highly reflective ETMs. This increases the amount of time available for an interaction with a gravitational wave.25 In effect, the beam in each arm travels 1,120 kilometers before returning to the beam splitter.26
- Another partially-reflective mirror, the power recycling mirror (PRM), was added between the laser source and the beam splitter, forming an optical cavity with the interferometer. Laser light reflected back from the arms towards the laser source is reflected, or recycled, into the interferometer, boosting the laser power by a factor of 3,750—from an initial 200W to a final level of 750kW.27
- A fourth additional mirror, the signal recycling mirror (SRM), is located between the beam splitter and the photodetector, and forms another optical cavity with the interferometer. The SRM can be tuned to enhance the output signal.28
- The interferometer is arranged inside a series of vacuum chambers, which have an air pressure of a trillionth of an atmosphere.29
In sum, it is “a power-recycled and signal-recycled Michelson interferometer with Fabry–Pérot ‘transducers’ in the arms.”30 This is, of course, a simplified description of the aLIGO interferometers, which include many more features such as input and output mode cleaners, a phase modulator, a Faraday isolator, and a seismic isolation system.31
Before aLIGO, there was iLIGO (Initial LIGO, the first generation of LIGO interferometers). In operation between 2001 and 2010, iLIGO failed to detect gravitational waves, but where it “was a resounding success was in the lessons learned about how to operate, maintain, and improve one of the world’s most highly technological measuring devices.”32
Successive generations of interferometers, with constantly increasing sensitivity, were envisioned from the very beginning of the LIGO project.33 aLIGO was initiated in 2008, and became operational in September 2015. The seven years required to redesign and construct the new, advanced interferometers speaks to the complexity of the undertaking.
Three key component upgrades between the iLIGO and aLIGO interferometers are worth noting:
- A heavier mirror is less prone to unintended movement and has increased heat absorption. aLIGO’s fused silica test mass mirrors have a mass of 40kg and a thickness of 20cm; they are four times the weight and twice the thickness of those used in LIGO’s first generation interferometers.34
- iLIGO’s test mass mirrors were suspended in a single pendulum design using steel fiber; aLIGO’s suspension is a 360kg quadruple pendulum design using silica fiber.35
- iLIGO relied on passive seismic isolation: the single pendulum design. aLIGO has passive and active isolation: a cascaded arrangement using hydraulics and suspension that provides up to seven stages of seismic isolation.36
These were far from the only improvements. LIGO spokesperson Gabriela González described the scale of the changes in a 2015 interview: “It’s amazing, because if you look at it from the outside, it looks the same. It’s the same observatory, the same vacuum chambers. But on the inside, everything is different.”37
The result is a device with the sensitivity to measure changes in distance equivalent to a thousandth the width of a proton. According to LIGO’s executive director David Reitze, “LIGO is the most precise measuring device ever built.”38
Doubts and Uncertainty
Rhett Allain, writing in Wired shortly after the LIGO announcement, was unequivocal: “LIGO is first and foremost an observatory. It’s a type of telescope that uses gravitational waves instead of electromagnetic waves.”39 Quibbling over nomenclature in the wake of the successful detection seems churlish, but at earlier stages of the project, disagreements as to whether LIGO was, in fact, an observatory, or even anything to do with astronomy, were by no means unusual.
The name for the project was formulated in 1983 by Weiss himself.40 But from the outset, the O in LIGO did not endear the project to astronomers.41 In her recently published book on the search for gravitational waves, Janna Levin recounts:
[Weiss] tells me that the word “observatory” in LIGO’s name caused alarm for philosophical reasons (it’s not an observatory until after you observe something) … and for sociological reasons (the project sounds more like physics than astronomy and hadn’t the right to an astronomical title).42
Harry Collins, a sociologist of science at Cardiff University, is more explicit:
LIGO, by calling itself an observatory, seemed to be vying for a piece of this territory even though the best and soundest predictions of what it would see, at least in its first instantiation, were zero or close to zero. … In reality, [astronomers] believed, LIGO was a high-risk exploratory physics experiment passing itself off as an astronomical instrument without accepting the corresponding responsibility to see things.43
Two points of contention emerge from these accounts. The first is the choice of name. In hindsight, astronomers might be forgiven for raising an eyebrow. LIGO does not precisely observe anything, certainly not in a manner that might be considered even vaguely akin to current methods of astronomical observation, which use optical or radio telescopes. For an experiment without precedent, the inclusion of the O was perhaps a bit of a stretch.
The second objection, that LIGO was a physics project rather than an astronomical one, proved difficult to answer. As recently as 1991, the National Academy of Science (NAS) was quoted in a Science article as affirming that, “the secure scientific goals of LIGO for the 1990s are not astronomical.”44 The distinction made here between LIGO and astronomical projects was probably sharpened by the fact that, throughout LIGO’s history, none of its key figures have been astronomers. A sense that physicists were encroaching on another field was perhaps inevitable.
The level of discontent within the astronomical community regarding LIGO is evident from a 1991 article published in The New York Times that included the following quote from Tony Tyson, an astrophysicist at AT&T Bell Laboratories:
“I perused a list of about 2,000 astronomers and picked 70 who seemed to me likely to have thought about LIGO,” Dr. Tyson said in an interview. “I got 60 replies, and they ran 4 to 1 against LIGO. Most of the astrophysical community seems to feel it would be very difficult to get any important information from a gravity-wave signal, even if one should be detected.”45
In a follow-up article published later that year in The Scientist, Tyson was emphatic: “The astronomers that I have polled feel overwhelmingly that the project is premature.”46 A third objection is evident: the project’s apparently slim chance of success.
Some of the skepticism stemmed from the failure of Weber’s earlier attempts.47 Weber published papers in 1969 and 1970 that presented evidence for gravitational wave detection, but his claims were discredited when attempts to reproduce his results were unsuccessful.48 Some of the fallout can be seen in a 1971 report by the NAS expressing deep ambivalence about the search for gravitational waves.49 Weiss noted that as a result of the Weber controversy, “[t]he whole field was considered very risky.”50 Nonetheless, following LIGO’s successful detection, Thorne has made a point of recognizing Weber’s work, describing him as, “the founding father of this field.”51 Research on resonant mass detectors continues to this day.52
An intriguing side note here is that Einstein himself changed his position on the existence of gravitational waves several times. As author and theoretical physicist Daniel Kennefick notes:
The first mention of gravitational waves that we have from Einstein is of him saying they don’t exist. … When Einstein wrote his paper [predicting gravitational waves] in 1916, he thought he had discovered three different kinds of gravitational waves. Earlier that year, when he thought the waves didn’t exist, he had been using the wrong coordinate system. He changed to a different coordinate system at the suggestion of a colleague, and that allowed him to see more clearly that there were waves.53
In 1936, Einstein made a surprising volte-face in a letter to a friend, the physicist Max Born:
Together with a young collaborator, I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation.54
Einstein then submitted a paper affirming this conclusion to the Physical Review. There followed a plumage-ruffling encounter with peer review that the great man was clearly not anticipating and which led to his withdrawing the paper. An anonymous referee, identified by Kennefick as the mathematician Howard Percy Robertson, had found mistakes in Einstein’s paper. Even Homer nods. In a fit of pique, Einstein resubmitted the paper, unmodified, for publication at the Journal of the Franklin Institute. When it was eventually published, the problems had been corrected and Einstein had returned to his earlier conclusion. Rather than approaching Einstein directly, Robertson instead had discussed the paper with Einstein’s new assistant, Leopold Infeld. “Finding that Einstein had completely ignored his written critique," Kennefick explained, "he took the opportunity of their collegial closeness at Princeton to correct the great man in a less confrontational fashion.”55
Gravitational waves emanating from binary black hole mergers and binary neutron star mergers had long been identified as candidates for a successful LIGO detection.56 Interviewed in 1993, Thorne remarked that “these sources are expected to be the ‘bread and butter’ of the LIGO … diet.”57 The first indirect experimental confirmation for the existence of gravitational waves was provided by Russell Hulse and Joseph Taylor’s discovery of a binary pulsar system in 1974: “Einstein’s 1915 prediction that an accelerating mass should radiate energy in the form of gravitational waves is supported by evidence that a pulsar’s orbit around a companion star is slowly shrinking.”58
LIGO’s successful detection was thus not only in line with earlier predictions, but also provided the first experimental evidence for the existence of binary black holes. The American Astronomical Society described it as a “huge deal for astrophysics” for three reasons: it proved that stellar mass black holes exist; it showed that binary black holes can form in nature; and it confirmed that black hole binaries can form and merge on observable timescales.59
Following LIGO’s successful detection, cosmologist Michael Turner remarked, “They earned their O.”60
Big Science, Big Money
Misgivings about the project’s choice of name and its status within the broader field of astronomy may have been significant, but the most vehement objections to LIGO arose from the size of its funding.61 LIGO has been an enormously expensive undertaking. The National Science Foundation (NSF) noted in a 2008 factsheet that, “LIGO is the largest single enterprise undertaken by NSF, with capital investments of nearly [US]$300 million and operating costs of more than [US]$30 million/year.”62 Eight years later, when LIGO announced the first successful detection, the total cost for the project had grown to US$1.1 billion.63
LIGO’s funding began far more modestly. In 1975, Weiss received a grant for US$53,000 from the NSF for interferometer research, just enough to support Weiss, and to pay for one postdoc and some equipment.64 By 1979, there were two teams working on the project, one at MIT led by Weiss, and another at Caltech, led by Thorne. The NSF provided funding for an initial round of research and development to study a large interferometer at MIT, and to construct a 40 meter prototype at Caltech.65 Funding for both initiatives amounted to less than US$1 million per year.66
In the years that followed, LIGO’s progress was slower than anticipated. The groups at Caltech and MIT were merged in 1983 to save money. A project development plan for LIGO was approved by the NSF in 1984, but funding was not forthcoming. In 1986, the NSF commissioned a rigorous review of LIGO’s activities. While the review endorsed the project’s scientific feasibility, at the NSF’s insistence the project team was restructured. By the end of 1987, LIGO’s research and development funding had already amounted to US$11 million.67 In 1990, the NSF approved LIGO’s construction proposal for iLIGO. Funding was initially rejected by the US Congress, but in 1991, the Congress approved a US$30 million first year budget. In 1992, the NSF approved US$272 million for the construction of iLIGO. Reviews, restructuring, management changes, and the approval process for a final budget of US$395 million, meant that construction at the Hanford and Livingston sites did not begin until late 1994.
Throughout this period, LIGO often proved controversial. Writing in 1990, M. Mitchell Waldrop grumbled, “LIGO is an elegant little experiment that could make gravity-wave astronomy a reality—if only it weren’t so expensive.”68 Articles published during 1991 in Science (May 3: “LIGO in Limbo”) and The Scientist (November 25: “Funding of Two Science Labs Revives Pork Barrel vs. Peer Review Debate”) aired doubts about the project and drew attention to LIGO’s use of a public relations firm to lobby for government funding.69 A letter written to the NSF by an astronomer in May 1991 illustrates a certain level of discontent:
The project seems to me to embody some of the worst excesses of big science. The knowledge that we can hope to gain is disproportionately small compared to the expense we are certain to incur. The most likely outcome will be an increasingly hostile environment for smaller science projects, whose contribution to our expanding knowledge of the universe is, by any standard, much larger.70
Fears that LIGO’s funding would lead to a reduction in the budgets of smaller NSF-funded projects fueled increasing opposition to the project. At the same time, the NSF’s overall budget was facing cuts. Writing in The New York Times the following year, William Broad summed up the concerns of LIGO’s opponents:
The nation’s top Federal agency for the support of basic scientific research is battling to reconcile the growing financial appetite of its biggest project with an overall budget crunch. The conflict is threatening to pull the plug on more than a hundred small-scale science projects across the country.71
A Science article published in 1993, “LIGO: A $250 Million Gamble,” described conflicts and personality clashes at the highest levels of the project.72 Dire warnings were issued by two scientists quoted in the piece: “I think LIGO could come back to greatly haunt the scientific community if we spend $250 million and see nothing”; and “There’s been so much unhappiness out there about all this that I don’t think we will be able to easily forget it.”73 The results of the 1994 US mid-term congressional elections only served to heighten fears about reductions to the NSF budget.74
Despite these concerns, support for the project at the NSF remained solid. When objections were raised around the time of the NSF’s 1992 funding request, the counter-argument offered by the NSF and LIGO was that it would “define a new budget line and therefore ensure in the long term more money for science.”75 Neal Lane, who served as the director of the NSF between 1993 and 1998, recalls, “[t]he thing about the project that excited me was that it’s just the kind of thing NSF ought to do. It was really basic, fundamental research.”76
Another concern raised by LIGO’s opponents was the particular role of the NSF in infrastructure projects of this scale. Illustrating just how long the ill-feeling towards LIGO persisted, Collins recounts a 1999 conversation with the astrophysicist Jeremiah Ostriker who claimed that, “money was being taken from physicists and put into the pockets of bulldozer drivers, and the like…”77 In 1994, a new NSF capital budget was approved, the Major Research Equipment (MRE) account. Funding LIGO via this route allowed the NSF to increase LIGO’s budget and would, it was claimed, “allow Congress to support multiyear projects without having to shrink the pot for bread-and-butter research grants.”78
Although unconnected to LIGO, and with a vastly larger budget, another Big Science project of the era and its fate are worth recalling. The Superconducting Super Collider (SSC) was a particle accelerator project that, if completed, would have been three times the size of Europe’s Large Hadron Collider.79 Construction on the project began in 1991 in Texas. Initially approved with a budget of US$4.4 billion, by 1993 estimated costs had risen to US$11 billion. Amid cost overruns and mismanagement, Congress abruptly cancelled the project in October 1993. US$2 billion had already been spent.80 The SSC project created a tremendous amount of discord within the field of physics. As with LIGO, researchers were fearful that the SSC would swallow funding from other projects. In hindsight, there is a lesson here according to Nobel Prize winning physicist Burton Richter: “Once a project is approved, shut up.”81 As disastrous and wasteful as the SSC project became, there was an up-side for LIGO. “We were able to attract some very talented individuals who are used to the rhythm of designing and constructing a project that takes years,” recalled Gary Sanders, a physicist who worked on the SSC and was recruited to LIGO.82
Interviewed in 1990, when the LIGO project was already considered expensive and its final cost would have seemed inconceivable, Weiss lamented, “It’s a most unfortunate situation that this field needs facilities of this size. Most of us in this field hate Big Science.”83
A New Era
When announcing the first successful detection, LIGO’s González described the discovery as “the beginning of a new era: The field of gravitational wave astronomy is now a reality.”84 Of this, there can be little doubt. The detection of GW150914 just two days after aLIGO began its first observational period was an emphatic vindication of the project and for an emerging field of research. Having achieved this milestone, what might be reasonable expectations for the future?
The first four-month-long observational run for aLIGO’s detectors ended in January 2016; a second, six-month observational period is scheduled to begin in July 2016. It should be noted that aLIGO’s detectors are still some years away from achieving full design sensitivity; this is not expected until 2019. Although the detection of GW150914 was a triumph, the ultimate capabilities of aLIGO’s detectors have not yet been fully explored.
The sensitivity of a gravitational-wave detector is often described using a sensitivity curve: a graph that indicates detector sensitivity at a particular frequency.85 If a gravitational wave source appears above the curve, it can be detected. A February 2016 paper by the LIGO Collaboration provides a graph which plots target sensitivity as a function of frequency (Hz), with overlaid curves for aLIGO as it builds toward full sensitivity.86 The projected step-by-step increases between 2015 and 2019 can be seen in a series of progressively lower curves. Each step means that gravitational waves emanating from particular events can be detected from farther and farther away. The paper notes:
A standard figure of merit for the sensitivity of an interferometer is the BNS range RBNS: the volume- and orientation-averaged distance at which a compact binary coalescence consisting of two 1.4 [solar mass] neutron stars gives a matched filter signal-to-noise ratio (SNR) of 8 in a single detector.87
This distances are provided in megaparsecs (Mpc: a million parsecs, 1 parsec = 3.26 light-years), and the projected improvements are considerable. Operating in its first run from 2015 to 2016, the BNS range for aLIGO was 40–80 Mpc, increasing to 80–120 Mpc for the upcoming 2016–17 run, to 120–170 Mpc for another observational run in 2017–18, and to 200 Mpc at full design sensitivity.88
In its first run, aLIGO was already four times more sensitive than iLIGO; full design sensitivity will amount to a ten-fold increase, greatly extending the possibilities for gravitational wave detection:
[aLIGO] can see a volume of space more than a thousand times greater than initial LIGO, and extends the range of compact masses that can be observed at a fixed signal strength by a factor of four or more.89
In a 2010 paper, Gregory Harry offered the following estimates for what aLIGO, operating at full design sensitivity, would expect to detect in a year: 40 neutron star mergers, 30 binary black hole (of at least 10 solar masses) mergers, and 10 neutron star/black hole mergers.90
Whatever the sensitivity of the detector, there remains considerable uncertainty about how often these events actually occur. Working backwards from GW150914 yields clues, but more detections will be needed to obtain a clearer picture.91
It should also be noted that GW150914 resulted from the merger of two black holes of unexpectedly large masses. LIGO scientist Vicky Kalogera:
The 29 and 30-plus solar masses come as an unusual surprise. If you look at most binary stars in [the Milky Way] galaxy, given the composition of the stars, we don’t expect black holes of this mass.92
Analysis of GW150914 determined that this event occurred approximately 420 Mpc, or 1.3 billion light years distant from the earth.
We’re talking about an event that took place 1.3 billion years ago.
LIGO will soon be joined by a number of other observatories, including upgrades to existing facilities as well as new initiatives.
Currently there are four laser interferometer observatories. Aside from the Hanford and Livingston LIGO detectors in the USA, there are two in Europe. One is the Virgo detector near Pisa, Italy, part of a consortium called the European Gravitational Observatory (EGO), operated by France’s Centre national de la recherche scientifique and Italy’s Istituto Nazionale di Fisica Nucleare. The second is the GEO600 detector near Hanover, Germany, operated by the Max Planck Institute.93
The Virgo and GEO600 detectors differ from the two LIGO detectors in both scale and sensitivity. The arms of the GEO600 and Virgo detectors, for example, are 600 meters and 3 kilometers in length, respectively, compared with the 4 kilometer arms of the LIGO detectors. At the time GW150914 was detected, the Virgo detector had been offline since 2011 and was in the process of being upgraded, while the GEO600 detector lacked the sensitivity to detect the signal.94 The upgraded Advanced Virgo detector will be online in late 2016, boasting a ten-fold increase in sensitivity and a comparable detection capability to aLIGO.95
A number of new laser interferometer projects are in various stages of planning and development.
Within a week of LIGO’s announcement of the first successful detection, India’s government approved a proposal for the LIGO-India, or IndIGO, project.96 The components for an additional aLIGO detector have already been fabricated in the US; they may be shipped to India for assembly and operation by a joint US–Indian team.97 One of the benefits from the LIGO-India project will be improved source localization. Accounting for the 7 millisecond delay between the detection of GW150914 at the Livingston and Hanford interferometers enabled researchers to localize the source to an area of the sky of approximately 600 square degrees near the Large Magellanic Cloud in the Southern Hemisphere. This is about 1.5% of the total sky.98 Still, this is a vast area to search and contains an enormous number of galaxies. When operating in conjunction with the two U.S.-based aLIGO detectors and Italy’s Advanced Virgo detector, it is anticipated the LIGO-India detector will mean a five-to ten-fold increase in accuracy.99
Blurring conventional notions of what might constitute an observatory or telescope still further, two new interferometer projects will see interferometers constructed underground. Key considerations in both designs are improved seismic noise isolation and reduced thermal noise.
The successor to an earlier 300m interferometer, called TAMA300, Japan’s Kamioka Gravitational Wave Detector (KAGRA) project was launched in 2010.100 Located around 250 kilometers northwest of Tokyo, the Mozumi Mine at Kamioka is the site of a number of research facilities, including a neutrino physics laboratory.101 The KAGRA interferometer, with an arm length of 3 kilometers, is being built at a depth of 200 meters below the surface, in tunnels with a combined length of over 7 kilometers. The underground location will deliver a hundred-fold improvement in seismic noise isolation.102 KAGRA also aims at reducing thermal noise through the use of cryogenics, at an operating temperature of only 20 degrees Kelvin.103 This, in turn, has necessitated the adoption of sapphire mirrors.104 Considered at one time for aLIGO, sapphire is preferable to fused silica because of its higher density and thermal conductivity, which reduce thermal noise.105
In addition to the Virgo detector currently operated by the EGO, a new proposal has been advanced for a 10 kilometer triangular underground interferometer based in Europe, the Einstein Telescope (ET). The designs of laser interferometers built to date have all followed the L-shaped configuration of the Michelson interferometer. The ET is a clear departure, not only in scale—the arms will be more than double the length of the aLIGO observatories—but also in terms of the quantity of detectors—two at each corner—and overall layout. The triangular design comprises three nested detectors composed of two interferometers each, one of which “will detect low-frequency gravitational wave signals (2 to 40Hz), while the other will detect the high-frequency components.”106 The ET will also use similar cryogenic technologies to the KAGRA project to reduce thermal noise. As a result, the Einstein Telescope will be a hundred times more sensitive than current interferometers.107
Earth-based detectors will always be confronted with the issue of seismic noise, no matter how adept and ingenious engineers become at attenuating the effects of “passing traffic, logging in distant forests and the constant, almost unnoticeable seismic grumblings of the earth itself.”108 Space-based detectors would avoid this issue altogether.
Space-based detectors would also offer the opportunity for greatly increased scales, far in excess of anything that could be constructed on earth. Increasing arm lengths by orders of magnitude would allow for the detection of very low-frequency gravitational waves, otherwise undetectable on earth. It would become possible to detect gravitational waves generated by super-massive black holes and by events such as the formation of galaxies.109
Led by the European Space Agency, a proof-of-concept satellite for this approach, the Laser Interferometer Space Antenna (LISA) Pathfinder was launched on December 3, 2015. If successful, the initial mission will be a precursor to eLISA (evolved LISA) planned for launch in 2034. eLISA in its final configuration will comprise three spacecraft traveling in an equilateral triangular formation a million kilometers apart. Needless to say, this project presents some extraordinary engineering challenges. The Pathfinder satellite, currently orbiting 1.5 million km from earth, has two vacuum chambers 38cm apart, with an interferometer between them. Suspended inside each of the vacuum chambers is a gold-platinum cube-shaped test mass, 4.6cm in size and weighing 4kg. LISA team member Stefano Vitale notes:
As they fall freely through space, the two test masses should be extraordinarily still, since no other force is perturbing their gravitational motion—only a gravitational wave could jiggle them around.110
The Pathfinder phase is focused on providing experimental verification for eLISA:
[E]ven in space, there are forces capable of disturbing the cubes, including the radiation and wind from the Sun, and they need be isolated from all of these non-gravitational influences. To do so, LISA Pathfinder continually measures their positions and manoeuvres around them with microthrusters to avoid ever touching them. … [T]he scientists will now spend the next six months running experiments, ‘poking’ the masses to verify how still they really are.111
As LISA project scientist Paul McNamara notes, “The main goal of the mission is not so much to measure how well we’re doing, but to understand how well we’re doing.”112
The LISA project is not alone in exploring the potential of space-based detectors. A Chinese project, TianQin, and a Japanese effort, the DECI-hertz Interferometer Gravitational wave Observatory (DECIGO), are also currently in development, with launch dates tentatively scheduled between 2025 and 2030.113 The three space-based interferometer projects will be searching for gravitational waves in differing frequency ranges: TianQin, 0.1–100mHz; DECIGO, 0.1–10Hz; eLISA, 0.1mHz–1Hz.114 By comparison, aLIGO (from 2016 onwards) and Virgo (Advanced) will operate in the 10Hz–10kHz range, with the proposed Einstein Telescope in the 1Hz–10kHz range.115
On March 8, 2016, a press release from the ESA confirmed the beginning of LISA Pathfinder’s science mission.116 Space-based laser interferometers of a previously unimaginable scale may be only a few decades away.
Closure
Rainer Weiss was present at the press conference held at the National Press Club in Washington DC on February 11, 2016, when the first successful detection of gravitational waves was announced. Beside him on the dais was his long-time collaborator Kip Thorne, whose involvement in the project dated back to the mid-1970s. A third key figure, the physicist Ronald Drever, was sadly too ill to attend. This trio of physicists, often referred to as the troika, were the founders of LIGO. To them must go immense credit for the most remarkable scientific discovery of recent decades.
To me, it’s a closure to something which has had a very complicated history. The field equations and the whole history of general relativity have been complicated. Here suddenly we have something we can grab onto and say, “Einstein was right. What a marvelous insight and intuition he had.”
—Rainer Weiss117
The author would like to acknowledge the contribution of Shawn Witkowski to an initial version of this essay.
