Gravity Waves1

Is Seeing Believing? Observation in Physics

Allan Franklin

Department of Physics

University of Colorado

In their recent paper, Observation of Gravitational Waves from a Binary Black Hole Merger, the LIGO collaboration stated, This is the first direct detection of gravitational waves. (Abbott, Abbott et al. 2016, p. 061102-1, emphasis added).[endnoteRef:1] This was to distinguish their result from those of Hulse and Taylor (Hulse and Taylor 1975) and of Weisberg and Taylor (Weisberg and Taylor 1981), in which the decrease in the period of a binary pulsar was used to establish, with a high degree of confidence the existence of gravitational radiation as predicted by general relativity (Weisberg and Taylor 1981, p. 1).[endnoteRef:2] The implication by LIGO was that the latter result was not a direct observation. [1: In this case the LIGO collaboration is applying the 5- criterion for an observation. In high energy physics this is a requirement for the use of observation, which is synonymous with discovery. This will be discussed in more detail below. For a more detailed discussion see (Franklin 2013, Prologue).] [2: The LIGO and binary pulsar experiments would count as successful replications. ]

This raises several interesting questions. One might ask how one can distinguish between direct and indirect observation and whether that distinction is seen in the practice of science. One might also ask whether a direct observation is in some way better than an indirect observation and, if so in what way and why. Is a direct observation more credible? Does it have more epistemic or evidential weight? Does it provide more support for a theory or hypothesis?

As Harry Collins (2017) has reported, and as will be discussed below, the use of direct was the subject of considerable discussion within the LIGO collaboration when the discovery paper was being written and even earlier. Philosophers of science have also weighed in of the question of direct observation.[endnoteRef:3] These views range from the privileging of human sense perception, certainly a direct observation, by Bas van Fraasen (1981, pp. 13-19) to what might certainly be considered indirect observation of an entity by Wilfred Sellars, to have good reason for holding a theory is ipso facto to have good reason for holding that the entities postulated by the theory exist (1962, p. 97). Thus, in Sellars view, the correct prediction of the energy spectrum in decay by Enrico Fermis theory of provided grounds for belief in the existence of the neutrino, even though the neutrino itself is not involved in the observation. This is because the neutrino is an essential part of Fermis theory and is needed for its predictions. The argument is that if a group of sentences is each essential to the prediction of an experimental result, then observation of that result supports both the conjunction and each of the statements individually. [3: Typically, although not always, observation is taken to apply to entities.]

In this essay I will begin with the extensive discussion provided by of Dudley Shapere in his essay "The Concept of Observation in Science and Philosophy, (1982). I will also present episodes from the history of physics to examine the roles played by observation and their justification, in an attempt to illustrate and clarify the distinction between direct and indirect observation. As we shall see, the question of direct versus indirect observation did not arise until the 20th century when both experimental apparatuses and analysis procedures became more complex. The earlier discussions concerned unaided human observation as opposed to instrumental observation. In the 20th and 21st century virtually all observations in physic s require instrumentation. As we shall see, the meaning of direct observation has changed.

I. Shapere and Direct Observation

Shapere defined direct observation as follows. x is directly observed (observable) if:

1. information is received (can be received) by an appropriate receptor; and

2. that information is (can be) transmitted directly, i.e., without interference, to the receptor from the entity x (which is the source of the information). (Shapere 1982, p. 492).

He emphasized that what is directly observed, what counts as information, what is an appropriate receptor, and what is direct or undisturbed transmission depends on the current state of physical knowledge. Shapere specifies three elements of the observation situation. They are the theory of the source, the theory of transmission, and the theory of the receptor. In some cases the theory of the phenomenon may be involved in any of these three elements. This raises the issue of the possible theory ladenness of the observation. Can such an observation then be considered a test of that theory? As Shapere notes, and as illustrated below, the fact that in a particular case the theory under test is essential to the design of an experiment, does not guarantee that the experimental result will be in agreement with that theory. He also remarks that an experiment may depend, in an essential way, on background knowledge and information. Thus, in the solar neutrino experiment, discussed below, the calculation of the expected neutrino flux depends on knowledge of various nuclear interaction cross sections and decay rates. It also depends on the structure, temperature, and composition of the Sun.[endnoteRef:4] [4: It is also clear that ultimately the information received must be available in a form perceivable by a human being. ]

The center of Shaperes discussion is the experiment by Ray Davis and his collaborators (1968) that was designed to detect solar neutrinos, those emitted by the sun.[endnoteRef:5], [endnoteRef:6] The experiment was designed to demonstrate that nuclear processes were occurring in the central core of the sun.[endnoteRef:7] [5: Davis won the 2002 Nobel Prize for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos.] [6: The existence of the neutrino had been established earlier in experiments by Frederick Reines, Clyde Cowan, and their collaborators (Cowan, Reines et al. 1956), (Reines, Cowan Jr. et al. 1960). The directness of this observation is discussed below.] [7: There had been earlier doubts as to whether these processes occurred. As Arthur Eddington remarked, I am aware that many critics consider the conditions in stars not sufficiently extreme to bring about the transmutationthe stars are not hot enough.... we tell them to go and find a hotter place (Eddington 1927, p. 102).]

The physics of the experiment can be schematically summarized as follows. The major source of energy production in stars such as the Sun proceeds by the burning of hydrogen to produce helium (41H1 4He2 + 2 e+ + 2 e), where e+ are positrons and e are electron neutrinos.[endnoteRef:8] This process actually takes place through many nuclear interactions. For the purpose of Daviss experiment, the most important of these results in the production of boron (8B5) which subsequently decayed into beryllium (8B5 8Be4 + e+ + e). These neutrinos had sufficient energy to induce the interaction that Davis would use to detect the neutrinos (e + 37Cl17 37A18 + e-). The argon 37 was radioactive and would subsequently decay (37A18 37Cl17 + e+ + e). Davis proposed to use 100,000 gallons of perchlorethylene (C2Cl4) as a target for the solar neutrinos. The 37A produced would be collected and its decay detected. This would provide evidence for solar neutrinos. (For more details see (Franklin 2001, chapter 8). [8: There are three kinds of neutrinos, those associated with the electron, the muon, and the tau lepton, e, , and .]

Shapere quotes an unnamed philosopher of science who declared that "There is one thing which we can be sure will never be observed directly, and that is the central region of the sun, or, for that matter, of any other star (cited in Shapere 1982, p. 485)." Shapere notes that this seems quite reasonable because the Sun has a radius of 689,000 kilometers and a core temperature of 15 million degrees Centigrade. On the other hand, he quotes two astrophysicists who take a very different view. neutrinos originate in the very hot stellar core, in a volume less than a millionth of the total solar volume. This core region is so well shielded by the surrounding layers that neutrinos present the only way of directly observing it (Weekes 1969, p. 161) and There is no way known other than by neutrinos to see into a stellar interior (Clayton 1968, p. 388).

John Bahcall who did much of the early theoretical work on both the solar neutrino flux and on Daviss experiment gave the following summary.

The principal energy source for main-sequence stars like the sun is believed to be the fusion, in the deep interior of the star, of four protons to from an alpha particle. The fusion reactions are thought to be initiated by the sequence 1H(p,e+v)2H(p,)3He and terminated by the following sequences: (i) 3He( 3He,2p) 4He; (ii) 3He(,)7Be(e-v)7Li(p,)4He; and (iii) 3He(,)7Be(p,)8B() 4He. No direct evidence for the existence of nuclear reactions in the interior of stars has yet been obtained because the mean free path for photons emitted in the center of a star is typically less than 10-10 of the radius of the star. Only neutrinos, with their extremely small interaction cross sections, can enable us to see into the interior of a star and thus verify directly the hypothesis of nuclear energy generation in stars (Bahcall 1964, p. 300).

The last points are crucial. A photon emitted at the center of the sun will take approximately 10 million years to reach the surface of the Sun, but its nature will be considerably changed. Neutrinos, because they interact only weakly with are matter, are unchanged as they travel from the Suns core to the Earth.[endnoteRef:9] They will also travel at the speed of light. [9: This is not strictly true. We now know that neutrinos of one type can transform into other types of neutrino during this journey. ]

At the time Shapere wrote his paper the observed solar neutrino flux was only about one third of that predicted. This led to serious questions concerning the calculation of the solar neutrino flux by Bahcall and others and also questions about the ability of Daviss detector to measure that flux accurately. The solar neutrino problem, as it came to be known, was solved when physicists found evidence for neutrino oscillations,[endnoteRef:10] the fact that some of the neutrinos, in their passage between the core of the sun and the detector on Earth, transformed from one type of neutrino, the electron neutrinos emitted in the energy production processes, can transform into two other types of neutrino, the muon and tau neutrinos (for more details see (Franklin 2016, chapter 14)). This accounts for the measured, lower-than-predicted neutrino flux because Daviss experiment was sensitive only to electron neutrinos. By the time solar neutrinos reached the Earth some of the electron neutrinos produced had transformed into muon or tau neutrinos. Later experiments were able to detect all three types of neutrinos and found that the total neutrino flux was, in fact, in agreement with the theoretical calculation.[endnoteRef:11] [10: For more detailed discussion see (Franklin 2001, chapters 7 and 8).] [11: This process took more than thirty years.]

In Shaperes terms the theory of transmission, which assumed that there was no interference with the neutrino flux in its passage from the core of the Sun to the detector on Earth, was wrong. Nevertheless, even the reduced flux was sufficient to demonstrate the existence of nuclear reactions in the core of the sun, albeit not at the predicted rate. This episode might be considered a reasonable fit to Shaperes definition of direct observation even though the neutrinos changed. The electron neutrinos, the products of nuclear reactions, were transformed during their transmission to the Daviss detector on Earth.[endnoteRef:12] Later experiments in which the apparatuses was sensitive to all three types of neutrinos, confirmed Bahcalls Standard Solar Model. [12: Although it did not happen, one might speculate whether the measurement of a lower than predicted neutrino flux might be considered as evidence for neutrino oscillations. Oscillations had been suggested before the advent of the solar neutrino problem. I suggest that it shouldnt have because the theory of the source, Bahcalls solar model, was not sufficiently well established at the time. That measurement did, however, encourage the further investigation of neutrino oscillations. My colleague Alysia Marino has remarked that the SNO experiments referred to actually showed only neutrino loss and not neutrino oscillations. Later experiments did, however, show oscillations.]

VIII. Gravity Waves

A. The Binary Pulsar

In 1975 Russell Hulse and Joseph Taylor reported the Discovery of a Pulsar in an Binary System (Hulse and Taylor 1975). They had previously reported the discovery of other pulsars and described both their equipment and their search method (Hulse and Taylor 1974). Their method used electromagnetic radiation detected by the Arecibo Radio Telescope. The earlier paper described the discovery of eleven new pulsars and four previously observed pulsars. Their later paper noted that they had by now found forty pulsars of which thirty two had not been previously observed. The detection of the previously known pulsars gave confidence in both their equipment and in their search method.

Hulse and Taylor inferred that one new pulsar, PSR 1913 + 16, was part of a binary system by noticing that, periodic changes in the pulsation rate indicate that the pulsar is a member of a binary system (1975, p. L51). The importance of the discovery was that for the first time it is possible to observe the gravitational interactions of a pulsar and another massive object (p. L51). They also remarked that the mass of the unseen companion was comparable to the mass of the pulsar. This meant that the object was probably a neutron star or a black hole. In addition to the obvious potential for determining the masses of the pulsar and its companion, this discovery makes feasible a number of studies involving the physics of compact objects, the astrophysics of close binary systems, and special- and general-relativistic effects (p. L51).[endnoteRef:13] [13: The importance of this discovery is shown by the award of the 1993 Nobel Prize in Physics to Hulse and Taylor for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.]

Hulse and Taylor based their results and conclusions on 200 measurements of pulsar arrival times taken over five-minute intervals. They made no explicit comments concerning the use of the binary pulsar system to detect gravitational radiation, or gravity waves. The did, however, set an upper limit for the change in the orbital period of the pulsar, of dPcm/dt < 1 x 10-12, a quantity that would be of importance in the subsequent discovery.

In 1981 Joel Weisberg and Taylor announced that, We describe an experiment which establishes, with a high degree of confidence the existence of gravitational radiation as predicted by general relativity (Weisberg and Taylor 1981, p. 1). More emphatically they stated, The test is new and different from previous tests of relativity because it goes beyond the usual first-order corrections to Newtonian theory. In short, the results provide the first compelling evidence for the existence of gravitational radiation, and the magnitude of the radiation effect is in excellent accord with the prediction of the quadrupole formula in general relativity. We also show that our observations are not in agreement with the predictions of several alternative theories of gravitation (p. 2, emphasis added).

Their data consisted of more than 1500 measurements of pulse arrival times from the binary pulsar. These measurements allowed them to fit a timing model of the binary system. They extracted 20 observable parameters from their data using a least-squares procedure. Only seven of these parameters are of interest for analyzing the orbital dynamics of the system: the projected semimajor axis of the pulsar orbit, apsini, where i is the angle between the plane of the orbit and the plane of the sky; the orbital eccentricity, e, period, P, and longitude of periastron, , the rate of advance of periastron, dot the variable part of the gravitational redshift and transverse Doppler shift, , and the rate of change of the orbital period, Pdot (pp. 2-3). The measured values of these parameters are given in Figure 37. To completely specify the pulsars orbit, as well as the masses of the pulsar and its companion, Weisberg and Taylor used the measured values e, , and apsini, and Keplers third law. The relativistic parameters, and dot provide the two additional equations necessary to solve explicitly for the remaining unknowns. To do so, one must assume that the measured value of dot is entirely the result of relativistic effects, and one must work within the framework of a particular theory of gravity (p. 3).

After specifying the orbital parameters and the masses of the pulsar and its companion, Weisberg and Taylor could calculate the expected rate of orbital decay expected from gravitational radiation. They used a calculation done by Peters and Mathews (1963). The quoted values of P, e, mp, and mc then yield the calculated value

Pdot = (-2.38 0.02) x 10-12, in excellent agreement with the observed value listed in [Figure 37] (p. 4). That value was (-2.5 0.3) x 10-12.

Weisberg and Taylor also presented the values calculated from both general relativity and from four competing theories (Figure 38). The striking result is that the Rosen, Ni, and Lightman-Lee theories all predict an orbital period increase due to the emission of gravitational radiation, regardless of the magnitude of the dipole term. Thus our measurements are inconsistent with such theories unless one introduces ad hoc effects to explain Pdot. The Brans-Dicke theory is consistent with observation only if the coupling constant BD is very large or if the pulsars companion has mass and internal structure very similar to those of the pulsar so that is small (p. 5).[endnoteRef:14] The authors also noted that some theorists believed that Einsteins quadrupole formula used to calculate the energy loss rate was invalid for the binary pulsar system. Obviously the dispute about what the theory actually predicts must be resolved, but the present experimental situation does not by itself seem to demand any changes. The binary pulsar system containing PSR 1913 + 16 has provided general relativity with one of its most probing tests, and the theory has survived unscathed (p. 5). [14: is the difference in the self-gravitational binding energy per unit mass of the two stars.]

B. The Binary Black Hole Merger

On February 11, 2016 the LIGO-Virgo collaboration announced the Observation of Gravitational Waves from a Binary Black Hole Merger (Abbott, Abbott et al. 2016). The event was labelled GW 150914. The abstract stated, On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0 1021. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 (p. 0611102-1). They went on to state that, This is the first direct detection of gravitational waves and the first observation of a binary black hole merger (p. 0611102-1, emphasis added). This latter statement was the subject of considerable discussion within the collaboration. What exactly should the collaboration claim? The authors did not claim that they had discovered gravitational radiation, but rather that they had made the first direct observation of gravity waves. They attributed the discovery to the work of Hulse, Taylor, and Weisberg. The discovery of the binary pulsar system PSR B1913 + 16 by Hulse and Taylor and subsequent observations of its energy loss by Taylor and Weisberg demonstrated the existence of gravitational waves. This discovery, along with emerging astrophysical understanding, led to the recognition that direct observations of the amplitude and phase of gravitational waves would enable studies of additional relativistic systems and provide new tests of general relativity, especially in the dynamic strong-field regime (p. 0611102-1).

The collaboration began their paper with a brief history of gravitational wave theory. They noted that recent advances together with numerical relativity breakthroughs in the past decade, have enabled modeling of binary black hole mergers and accurate predictions of their gravitational waveforms (p. 061102-1).

I will discuss a few of the details of the experiment because it casts light on the question of direct and indirect observation. A simplified diagram of the experiment is shown in Figure 39. The LIGO detector consists of two very sophisticated and sensitive Michelson interferometers located in Hanford, WA and Livingston, LA, a separation of approximately10 ms for a signal; travelling at the speed of light. A gravitational wave propagating orthogonally to the detector plane and linearly polarized parallel to the 4-km optical cavities will have the effect of lengthening one 4-km arm and shortening the other during one half-cycle of the wave; these length changes are reversed during the other half-cycle. The output photodetector records these differential cavity length variations. (p. 061102-4). This differential length variation alters the phase difference between the two light fields returning to the beam splitter, transmitting an optical signal proportional to the gravitational-wave strain to the output photodetector (p. 061102-3). Servo controls are used to hold the arm cavities on resonance and maintain proper alignment of the optical components (p. 061102-4). The detector was calibrated by measuring its response to test mass motion induced by photon pressure from a calibrated laser.

The collaboration presented several pieces of evidence in support of their observation of both gravity waves and of the merger of two black holes. These included a transient signal observed in both LIGO interferometers with a time delay of 6.9+0.5-0.4 ms between the Livingston and Hanford detectors (the time delay was in agreement with that predicted for a signal to travel at the speed of light between the two detectors); the fact that the signal matched the waveform predicted by General Relativity for the gravitational waves emitted by the merger of two black holes; a matched filter signal to noise ratio of 24; and a false claim rate of less than 1 in 203,000 years.

The first of these is shown in Figure 40. The left hand side shows the signal of strain plotted against time for the Hanford interferometer.[endnoteRef:15] The right hand side shows the Livingston signal superimposed on the Hanford signal (inverted and time shifted to account for the difference in location and the difference in the relative orientation of the detectors). One can see that the signals are extremely similar. In fact, taking into account noise in the detector, it seems fair to say that the signals are the same. [15: The signals were filtered to suppress large fluctuations.]

Figure 41 shows Gravitational-wave strain projected onto each detector in the 35350 Hz band. Solid lines show a numerical relativity waveform for a system with parameters consistent with those recovered from GW150914 confirmed to 99.9% by an independent calculation. Shaded areas show 90% credible regions for two independent waveform reconstructions. One (dark gray) models the signal using binary black hole template waveforms. The other (light gray) does not use an astrophysical model, but instead calculates the strain signal as a linear combination of sine-Gaussian wavelets. These reconstructions have a 94% overlap. P. 061102-2). The signals from the two interferometers agreed with each other and with the theoretical expectations.

The collaboration searched for gravity waves using two different and independent methods. GW150914 is confidently detected by two different types of searches. One aims to recover signals from the coalescence of compact objects, using optimal matched filtering with waveforms predicted by general relativity. The other search targets a broad range of generic transient signals, with minimal assumptions about waveforms (061102-5). For the generic waveform search the experimenters ranked events using the detection statistic

c = (2Ec/(1 + En/Ec)), where Ec is the dimensionless coherent signal energy obtained by cross-correlating the two reconstructed waveforms, and En is the dimensionless residual noise energy after the reconstructed signal is subtracted from the data. The statistic c thus quantifies

the SNR [signal-to-noise ratio] of the event and the consistency of the data between the two detectors (p. 061102-6). The group considered three mutually exclusive classes of events: events that resembled noise transients (C1), events in which the frequency increases with time, consistent with the expectation for a coalescent gravity wave event (C3), and all other events (C2). The collaboration had to estimate the number background events, which might simulate a gravity wave event. Although the method of estimation was slightly different for the two searches, both used a time-shift technique in which the signal from one detector is shifted by an offset time interval, large in comparison to the intersite transit time. Because the comparison included the gravitational wave signal in one detector with the noise in the other detector, this resulted in an overestimate of the noise background and a conservative estimate of the significance of the candidate events. Using the background estimate the group concluded that the false alarm rate for events that might simulate a real signal was lower than one in 22,500 years. This gives a probability of < 2 x 10-6 of observing one or more events with a signal as strong as that of GW 150914. This corresponds to a statistical significance of 4.6 .[endnoteRef:16] The results of this search are shown ion the left side of Figure 42. The GW150914 event has a value of c = 20.0, which is the strongest event of the entire search. [16: A slightly different method of estimating background gave a significance of 4.4 .]

For the binary coalescence search the collaboration compared the observed signal with 250,000 template forms, calculated on the basis of general relativity. The experimenters calculated (t), the matched-filter signal-to-noise ratio for each template in each detector. Their search maximized the value of (t) with respect to the time of arrival of the signal. For each maximum 2r was calculated to test whether the data were consistent in different frequency ranges. If 2r is near one that indicates that the signal is consistent with coalescence. If 2r was greater than one (t) was reweighted by ^(t) = /{[1 + 2r ]3/2}1/6. The group ranked coincident events based on the quadrature of ^c and for both detectors. The background was computed by time shifting the SNR maxima of detector and calculating a new set of coincident data. The right panel of Fig. 42 shows the background for the search class of GW150914. The GW150914 detection statistic value of c = 23.6 is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in

203 000 years. This translates to a false alarm probability < 2 107, corresponding to 5.1 (p. 061102-7).

Possible sources of noise such as thermal and seismic effects as well as other environmental effects were minimized and monitored. To monitor environmental disturbances and their influence on the detectors, each observatory site is equipped with an array of sensors: seismometers, accelerometers, microphones, magnetometers, radio receivers, weather

sensors, ac-power line monitors, and a cosmic-ray detector (p. 061102-5).

The collaboration concluded, The LIGO detectors have observed gravitational waves

from the merger of two stellar-mass black holes. The detected waveform matches the predictions of general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger (p. 061102-8).

C. Discussion

In both their abstract and in their conclusion the LIGO experimenters stated that their result was the first direct observation of gravitational waves. This had been a topic of both discussion and contention within the collaboration and with others in the gravitational wave community. The discussion concerned both the questions of directness, of whether the term black holes should be used, and whether the group should claim Observation of or Evidence for.[endnoteRef:17] I will concentrate on the question of directness.[endnoteRef:18] [17: At this time the criterion for a discovery in high-energy physics was that the signal have a statistical significance of five standard deviations. For details see (Franklin 2013, Prologue).] [18: In high energy physics the inclusion of Observation of in the title of a paper requires that the observed effect have a statistical significance of at least five standard deviations. Evidence for indicates a significance of leas than five standard deviations.]

1. Big Dog

On September 16, 2010 a very large signal was detected by LIGO and nicknamed Big Dog. This generated considerable excitement within the group because it might very well have been the detection of the first gravity wave signal. One member of the collaboration stated, I think this is the golden event we were all hoping for (quoted in (Collins 2013, p. 242)). One source of worry, however, was the possibility that the event was a blind injection, a false simulated gravity wave signal injected into the data streams of both interferometers. These were designed to test the ability of the analysis procedures to detect real gravity wave signals. It was known, at the time, that there might be zero, one, two, or three such injections. The number was known only to the two members of the collaboration responsible for the injections. The answer to the question of whether Big Dog was a blind injection would be answered at a collaboration meeting scheduled for March 2011.

The collaboration proceeded on the assumption that Big Dog was a real signal. This included the writing of a detection paper and it was in this process that the internal discussion of the language to be used began. The changing answers to the question of directness are shown in the changing titles of the proposed discovery paper. The first title proposed, by the end of September 2010, was, The First Detection of (or First Evidence for) Gravitational Radiation from Black Hole Coalescence (p. 190).[endnoteRef:19] The proposer also stated that, I agree that Direct should be used. On January 18, 2017 the title of the paper was, Direct Detection of Gravitational Waves from Compact Binary Coalescence, but by January 19 the draft title had been changed to Observation of Gravitational Waves from Compact Binary Coalescence. One member of the collaboration nevertheless stated, This is a direct detection, as opposed to the indirect detection of Hulse and Taylor. We dropped Direct from the title and abstract to make it shorter, and we hope it is nonetheless clear. The word direct is used in the introduction (p. 254). [19: This discussion was far more extensive, complex, and interesting than the brief account I have given, It has been documented and discussed in detail in Collins (2013, chapter 8 12).]

On February 25 a senior member of the collaboration[endnoteRef:20] proposed, Evidence for the Direct Detection of Gravitational Waves from a Black Hole Binary Coalescence. His argument was, I argue for Evidence for as an acknowledgement that a single event is far from an ideal way to make as important a claim as this is. I dont believe that we do ourselves any harm by adding those words of cautionI predict that almost everyone in the community will forget those two words almost as soon as they see our case, but it is better to let them decide on the strength of our claim than to possibly over-sell it. We have a very good case, about as good as I think one can do with a single event. However, if we really want to hold ourselves to the 5-sigma standard, we fall short (Collins 2013, p. 264).[endnoteRef:21] The Big Dog did, however, have a significance greater than five sigma. This was the title presented to the collaboration at the March meeting. [20: Collins does not identify the scientists by name.] [21: With apologies to Jacqueline Susann, once may be enough.]

In discussing the question of how much support can be provided be a single event, one member of the collaboration mentioned the work of Blas Cabrera (1982). Cabrera was searching for magnetic monopoles. In 1982 he reported a single event which was not only much larger than any other signal he had detected, but was also fit the signal magnitude that was theoretically expected for a monopole extremely well. Cabrera, however, made no discovery claim because he could not eliminate all of the other possible causes or explanations of the event. The title of Cabreras paper was "First Results from a Superconductive Detector. Later work with larger and more sensitive detectors failed to find a similar event. The consensus is that Cabreras event was not a monopole. (For more details see (Franklin 2016, Chapter 18). Another senior scientist in the Virgo group circulated a reading list of earlier discovery papers, including Cabreras paper. He recommended looking at the titles of those papers in urging caution in making claims.

Some of the scientists involved in the work on the binary pulsar had a quite different view. They were insistent that their result was the first direct observation of gravity waves. Thibault Damour, who performed the first complete calculation of gravity-wave interactions argued that the binary pulsar result demonstrated that the signal between the two bodies travelled at the speed of light and that this constitutes direct evidence for the reality of gravitational radiation (quoted in Collins 2013, p. 198). Joseph Taylor agreed and offered a longer argument.

In the binary pulsar experiment, and also in a LIGO-like experiment, one infers the presence of gravitational radiation based on its effects it induces in a detector. If a ruler could be used to measure the displacement of LIGOs test masses, I would grant that detection to seem rather more direct than one based on timing measurements of an orbiting pulsar halfway across our Galaxy. However, LIGO cant use a ruler; instead they use servomechanisms, very sensitive electronics, and finally long sequences of calculations to infer that a gravitational wave has passed by. Such a detection, like the binary pulsar timing experiment, is arguably many stages removed from being what most people would call direct.

There is one significant difference. The detector in the binary pulsar experiment is the pair of orbiting neutron starsthe same thing as the transmitter. Detection involves measuring the back-reaction of the emitted gravitational waves on the transmitter itself. The time and place of detection are the same as the time and place of emission.

These things will not be true of gravitational waves detected by LIGO. In that case, the waves will necessarily have traveled a very large distance from transmitter to receiver.

These thoughts suggest that a better distinguishing characteristic of the two experiments would be something like in place and remote rather than indirect and direct. (quoted in Collins 2013, pp. 198-99).

Members of LIGO did not agree at the time, and, as we have seen, the 2016 LIGO observation paper stated that it was the first direct detection of gravitational waves. I disagree that the Hulse/Taylor pulsar was the first direct detection. It was most definitely an indirect detection, as the waves themselves were not observed. In fact, I think this is exactly why the paper title should use the term direct detection. Thats kind of the whole point of this enterprise, and we should make that clear in our first detection paper (Collins, 2013, p. 199).The discussion became moot when on March 14, 2012 Jay Marx, the LIGO project director announced, The first slide will tell you the answer: the Big Dog is a blind injection (p. 284).

2. The Event

On September 14, 2015 another very large signal was detected by the LIGO interferometers. The led to actions by the collaboration similar to those following Big Dog, but there were some interesting differences. Although the possibility of a blind injection was again raised, this was discounted by many of the scientists because the signal was detected during an engineering run, designed to test the apparatus before the data taking run began. It was thought that it was unlikely that an injection would be inserted in an engineering run. The suspense was resolved three weeks later when it was revealed that there were no blind injections. The possibility of a malicious injection was also discussed. This was rejected because it would have taken a conspiracy of a large number of the members of the collaboration to create it. We cant say that faking GW150914 was impossible, but we can say that faking it would have required an internal conspiracy of our most knowledgeable people (Collins 2017, p. 101).

Once again caution was urged in stating the results of the experiment. The Cabrera monopole was mentioned along with the more recent claim to have observed gravity waves by the BICEP2 collaboration. That collaboration claimed to have seen a 5 effect by looking at the polarization of the electromagnetic radiation in the Cosmic Microwave Background (Ade, Aikin et al. 2014). That claim was later withdrawn, or at least the size and significance of the effect greatly reduced because the collaboration had not adequately included the effects of cosmic dust (Mortonson and Seljak 2014). Members of the LIGO collaboration did, however, cite the positive result of the discovery of the - particle, discussed earlier. In that episode a single event was sufficient to support a discovery claim.[endnoteRef:22] [22: No mention was made of the discovery of the positron.]

Some of the uncertainty caused by having only a single event was removed when a second gravity wave event was detected on October 12, 2015. This was a weaker signal than The Event, it had only a 3 significance, and would certainly not have been sufficient to merit a discovery claim, but in the presence of The Event, it did provide additional support. Peter Saulson wrote to Harry Collins and stated, A genuinely marginal event in the latest PyCBC box opening that happened one hour ago. At 3 , it is going to be tougher, but this is what we expected[endnoteRef:23] this is quite a weak event by cWB standards, however it has a beautiful chirp shape both in time and in TF domains (quoted in Collins 2017, pp. 118-119). [23: The Event was a very large signal. The collaboration expected to find more and smaller signals.]

The question of direct versus indirect observation was again raised. Collins reports that there were 2500 e-mails concerning the draft of the detection paper. They spanned a wide range of opinion from caution to strong advocacy of making a strong statement about a direct observation by LIGO. One experimenter cited ten websites in which the binary pulsar experiment was considered to be an indirect observation of gravity waves. In addition, when Hulse and Taylor were awarded the 1993 Nobel Prize in Physics for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation, the press release issued by the Nobel Prize Institute stated, The good agreement between the observed value and the theoretically calculated value of the orbital path can be seen as an indirect proof of the existence of gravitational waves. We will probably have to wait until next century for a direct demonstration of their existence http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html emphasis added.

A moderate comment was, I would also like to suggest that we avoid the phrases "direct observation" and "direct detection", at least in the title, abstract and introduction. While I know that we use those terms as a way of distinguishing what we do from other methods of GW detection I have come to understand that there are people in our broader community who think that this terminology is meant to diminish the importance of their work. In keeping with the modest tone I advocate, I see no benefit in using these particular terms. Indeed, without further definition, they do not convey any clear meaning We have a great result, and it will not be any less great without this characterization(Collins, 2017, pp. 147-48). Another commentator did not think the issue of direct was very important, but did believe that there was a significant difference between the binary pulsar and LIGO experiments. The difference is the Taylor crowd observed a distant GW transmitter and figured out how it worked! We figured out how to build a sufficiently sensitive GW receiver and since we built it, we know exactly how it works. If anybody misses the impact of those italicized words, check out BICEP-2's and Planck's experience to date. Those words in italics represent a huge advance for GW physics and astronomy (Collins, 2017, p. 148).

As is seen in these quotes there was a general view that one should avoid giving offense to others in the gravity wave community. As Collins remarks, Taylor and Damour are very much liked among more senior members of the gravitational wave community and their work is hugely admired so there are many who do not want to offend them (Collins, 2017, p. 147).[endnoteRef:24] [24: A similar view was expressed to me by Peter Saulson (private conversation), a former spokesperson for LIGO.]

Faced with this wide variation in opinion the writing team conducted a poll among the LIGO group members. One question listed eight possible titles for the discovery paper and asked for an ordered preference.

1. Observation of Gravitational Waves from a Binary Black Hole Merger.

2. Direct Observation of Gravitational Waves from a Binary Black Hole Merger.

3. Detection of Gravitational Waves from a Binary Black Hole Merger.

4. Direct Detection of Gravitational Waves from a Binary Black Hole Merger.

5. LIGO Observation of Gravitational Waves from a Binary Black Hole Merger.

6. LIGO Detection of Gravitational Waves from a Binary Black Hole Merger.

7. Observation by LIGO of Gravitational Waves from a Binary Black Hole Merger.

8. Direct Observation by LIGO of Gravitational Waves from a Binary Black Hole Merger. (from Collins 2017, p. 154-55)

Another question asked for an opinion on whether direct (detection and/or observation should be used in the body of the text.

1. No problem to use direct in the paper.

2. Use direct once in the introduction and abstract only.

3. Use direct once in the conclusion only.

4. Dont use direct. (Collins 2017, p. 155)

The results were

Poll 1. No direct and no LIGO in the title. Preferences are Observation or Detection of GW from BBH merger.

Poll 3. It is OK to use direct (detection/observation) in the body of the paper. (Collins 2017, p. 156)

As we have seen, the initial LIGO discovery paper followed the results of the poll.

A further argument for the difference between the binary pulsar and LIGO experiments was presented in the Outlook section of the LIGO paper. This was that the observation combined with the future observations expected when other detectors came online would open a new era in gravity wave astronomy. Efforts are under way to enhance significantly the global gravitational-wave detector network. These include further commissioning of the Advanced LIGO detectors to reach design sensitivity, which will allow detection of binaries like GW150914 with 3 times higher SNR [signal to noise ratio]. Additionally, Advanced Virgo, KAGRA, and a possible third LIGO detector in India will extend the network and significantly improve the position reconstruction and parameter estimation of sources (Abbott, Abbott et al. 2016, p. 061102-8).

In terms of Shaperes discussion of direct observation we see that the LIGO-VIRGO collaboration did make a direct observation. The gravity waves interacted with the two interferometers. The fact that there was complex instrumentation and analysis should not change that conclusion. It seems fair to say that the binary pulsar observation was indirect. The existence of gravity waves was inferred from the decrease in period of the pulsar, which was transmitted to the detector by electromagnetic radiation. As discussed below, I do not believe that the direct-indirect distinction had any epistemological significance. Recall that LIGO attributed the discovery of gravity waves to binary pulsar work of Hulse, Taylor, and Weisberg

Figure Captions

Figure 37. Observed orbital parameters for the binary pulsar. From Weisberg and Taylor (1981).

Figure 38. Predictions for the rate of orbital period change of the binary pulsar for various theories (normalized to general relativity).

Figure 39. Simplified diagram of an Advanced LIGO detector. Inset (a): Location and orientation of the LIGO detectors at Hanford, WA (H1) and Livingston, LA (L1). Inset (b): The instrument noise for each detector near the time of the signal detection. From Abbott et al. (2016).

Figure 40. The gravitational-wave event GW150914 observed by the LIGO Hanford and Livingston detectors. The left column: H1 strain; right: L1 strain. GW150914 arrived first at L1 and 6.9 +0.5 0.4 ms later at H1; for a visual comparison, the H1 data are also shown, shifted in time by this amount and inverted (to account for the detectors relative orientations). From Abbott et al. (2016).

Figure 41. Gravitational-wave strain projected onto each detector in the 35350 Hz band. Solid lines show a numerical relativity waveform for a system with parameters consistent with those recovered from GW150914 confirmed to 99.9% by an independent calculation. Shaded areas show 90% credible regions for two independent waveform reconstructions. One (dark gray) models the signal using binary black hole template waveforms. The other (light gray) does not use an astrophysical model, but instead calculates the strain signal as a linear combination of sine-Gaussian wavelets. These reconstructions have a 94% overlap. From Abbott et al. (2016).

Figure 42. Search results from the generic transient search (left) and the binary coalescence search (right). These histograms show the number of candidate events (orange markers) and the mean number of background events (black lines) in the search class where GW150914 was found as a function of the search detection statistic and with a bin width of 0.2. The scales on the top give the significance of an event in Gaussian standard deviations based on the corresponding noise background. The significance of GW150914 is greater than 5.1 and 4.6 for the binary coalescence and the generic transient searches, respectively. From Abbott et al. (2016).

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