Gravity's Ghost and Big Dog: Scientific Discovery and Social Analysis in the Twenty-First Centuryby Harry Collins
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Gravity’s Ghost and Big Dog brings to life science’s efforts to detect cosmic gravitational waves. These ripples in space-time are predicted by general relativity, and their discovery will not only demonstrate the truth of Einstein’s theories but also transform astronomy. Although no gravitational wave has ever been directly detected, the previous five years have been an especially exciting period in the field. Here sociologist Harry Collins offers readers an unprecedented view of gravitational wave research and explains what it means for an analyst to do work of this kind. Collins was embedded with the gravitational wave physicists as they confronted two possible discoveries—“Big Dog,” fully analyzed in this volume for the first time, and the “Equinox Event,” which was first chronicled by Collins in Gravity’s Ghost. Collins records the agonizing arguments that arose as the scientists worked out what they had seen and how to present it to the world, along the way demonstrating how even the most statistical of sciences rest on social and philosophical choices. Gravity’s Ghost and Big Dog draws on nearly fifty years of fieldwork observing scientists at the American Laser Interferometer Gravitational Wave Observatory and elsewhere around the world to offer an inspired commentary on the place of science in society today.
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Gravity's Ghost and Big Dog
Scientific Discovery and Social Analysis in the Twenty-first Century
By Harry Collins
THE UNIVERSITY OF CHICAGO PRESSCopyright © 2013 The University of Chicago
All rights reserved.
A Brief History of Gravitational-Wave Detection
In 1993 the Nobel Prize for physics was awarded for the observation, over many years, of the slow decay of the orbit of a binary star system and the inference that the decay was consistent with the emission of gravitational-waves. Here, however, we are concerned with the detection of gravitational-waves as a result of their "direct" influence on terrestrial detectors rather than on stars. The smart money says that the first uncontested direct detection will happen six to ten years from now, almost exactly fifty years since Joseph Weber, the field's pioneer, first said he had seen them. Joe Weber's claim was not uncontested. It was one of some half-dozen contested claims to have seen the waves made since the late 1960s. All of these have been consigned, by the large majority of the physics community, to the category of "mistake." The rejection of these results by the balance of the gravitational-wave community was often ferocious, driven by the sense of shame at the field's reputation for unreliability, or flakiness, in the eyes of outsiders. Newcomers to the enterprise also had to justify spending hundreds of millions of dollars on the much larger instruments—the giant "interferometers"—that they felt would finally be able to make a sound detection and atone for past mistakes; if the old cheap technology really could see the waves, then there would be no need for the new, so the credibility of the old cheap technology had to be destroyed.
The proponents of the old technology fiercely resisted the destruction of their project, which caused both sides to dig themselves into polarized positions. The consequence was that for decades the creative energy of most interferometer scientists was directed at finding flaws; the principle activity had become showing how this or that putative signal in either their rivals' or, subsequently, in their own detectors was really just noise. This is the problem of the negative mindset that is a central feature of what is to follow. In the meeting in Arcadia that bitter history stalked the corridors with an almost physical presence.
Weber and the Bars
Joe Weber was a physicist at the University of Maryland. In the 1950s he began to think about how he might detect the gravitational-waves predicted by Einstein's theories. Gravitational-waves are ripples in space-time that are caused by rapid changes in the position of masses, but they are so weak that only cosmic catastrophes such as the explosion or collision of stars or black holes can give rise to enough of the radiation to be even conceivably detectable on the surface of the earth. It would take a great leap of the imagination, a genius for experiment, and a heroic foolhardiness to try it. Weber was equipped with the right qualities, and he built a series of ever more sensitive detectors; by the end of the 1960s, he began to claim he was seeing the waves.
Weber's design was based on the idea that ripples in space-time could be sensed by the vibrations they caused in a mass of metal. He built cylinders of aluminum alloy weighing a couple of tons or so and designed to resonate—to ring like a bell at around the frequency of waves that might plausibly be emitted by a source in the heavens. Every calculation of the energy in such waves and the way they would interact with Weber's detectors implied that he did not have a hope, and when he started he did not think he had a hope either. But he went ahead anyway.
Weber insulated the cylinders from all the forces one could think of, but to see a wave it was necessary to detect changes in the length of the cylinders of the order of 10-15 m, the diameter of an atomic nucleus, or even less. Vibrations of this size, however, are continually present in the metal anyway, no matter how carefully it is insulated. Crucially, Weber built two of the cylindrical devices and separated them by a thousand miles or so. Then he compared the vibrations in the two cylinders. The idea was that, if there was a coincident pulse in both detectors, only something like gravitational-waves, coming from a long way away, could cause it.
Since both of the cylinders would suffer from random vibrations, there were bound to be coincident pulses every now and again just as a result of chance. But Weber used a very clever method of analysis. He used something called the "delay histogram," which is nowadays referred to as the method of "time slides" or "time shifts"—a method that is still at the heart of gravitational-wave detection forty years on and that will be at the heart of the method for the foreseeable future. Imagine the output of the detector drawn on a steadily unwinding strip of paper, as in those machines that record the changing temperature over the course of a day, but, in this case, sensing vibrations microsecond by microsecond; it will be a wiggly line with various larger pulses impressed upon it. One takes the strip from one detector and lays it alongside the strip from the other. Then one can look at the two wiggly lines and note when the large pulses are in coincidence. Those coincidences might be caused by a common outside disturbance such as a gravitational-wave, or they might be just a random concurrence of noise in the two detectors. Here comes the clever bit: one slides one of the strips along a bit and makes a second comparison of the large pulses. Since the two strips no longer correspond in time, any coincidences found can only be due to chance. By repeating this process a number of times, with a series of different time slides one can build up a good idea of how many coincidences are going to be there as the result of chance alone—one can build up a picture of the "background." A true signal will show itself as an excess in the number of genuinely coincident pulses above the background estimates generated from the time slides.
A time slide can also be called a "delay." The signal will appear, in the language of Weber, as a "zero-delay" excess. Nowadays scientists look not for a zero-delay "excess" but at isolated coincidences between signals from different detectors. Nevertheless, the calculation of the likelihood that these coincidences could be real rather than some random concatenation of noise is based on an estimate of the background done in a way that is close to the method that Weber pioneered.
As the 1960s turned into the 1970s Weber published a number of papers claiming he had detected the waves, while other groups tried to repeat his observations without success. By about 1975 Weber's claims had largely lost their credibility and the field moved on. Weber's design of detector continued to be the basis of most of the newer experimental work, but the more advanced experiments increased the sensitivity and decreased the background noise in the "bars" by cooling them with liquid helium. Most of the experiments were cooled to between 2 degrees and 4 degrees of absolute zero, with one or two teams trying to cool to within a few millidegrees of absolute zero. Collectively, such "cryogenic bars" were to be the dominant technology in the field until the start of the 2000s. Just two groups, one based in Frascati and sometimes known as the "Rome Group" or "the Italians," and an Australian group, kept faith with Weber's claims, promulgating results that most gravitational-wave scientists believed were false—the latter view being one which would now be almost impossible to overturn.
Nearly everyone outside the maverick supporters of Weber came to believe that Weber had either consciously or unconsciously manipulated his data in a post hoc way to make it appear that there were signals when really he was really seeing nothing but noise. This can happen easily unless great care is taken. Weber did not help his case when he made some terrible mistakes. In the early days he claimed to have a periodicity in the strength of his signals of twenty-four hours when proper consideration of the transparency of the earth to gravitational-waves suggested that the right period should have been twelve hours. Somehow, shortly after this was pointed out, the period mysteriously became twelve hours in Weber's discussions and papers, and this led some people to be concerned about the integrity of his analysis. He also found a positive result that should have been ruled out because it was caused by a computer error, and, most damningly, he claimed to have found an excess of zero-delay coincident signals between his bar and that of another group when it turned out that a mistake about time standards meant that the signal streams being compared were actually about four hours apart, so that no coincidences should have been seen.
Those who had faith in Weber's experimental genius were ready to accept that these were the kind of mistakes that anyone could make, but those who were less charitable used the events to destroy his credibility. Weber did his case further harm by the way he handled these stumbles. Instead of quickly and gracefully accepting the blame, he tended to try to turn it aside in ways that damaged his credibility. Weber's reputation fell very low, and the community tried to convince him that he should admit that he was wrong from start to finish, allowing them to give him more credit for his adventurous spirit and his many inventions and innovations, but he never gave in.
Weber died in the year 2000, insisting to the end that his results were valid and even publishing a confirmatory paper in 1996—a paper which nobody read. Weber was a colorful and determined character without whom there would almost certainly be no modern billion-dollar science of gravitational-wave detection. I have heard Joe Weber described as hero, fool, and charlatan. I sense his reputation is growing again, as it has become easier to give credit to his pioneering efforts now that he is no longer around to argue with everyone who doesn't believe his initial findings. I believe he was a true scientific hero and that his heroism was partly expressed in his refusal to admit he was wrong; believing what he did, a surrender for the sake of short-term professional recognition would not have been an "authentic" scientific act. That the published results that indicate that he detected the waves are almost certain to remain in the waste bin of physics is another matter.
For a time Weber was one of the world's most famous scientists, thought to have discovered gravitational-waves with an experiment that was an astonishing tour de force. Many scientists now see the Weber claims as having brought shame to the physical sciences. Much of the subsequent history of gravitational-wave detection has to be understood in the light of what happened.
Long after most scientists considered Weber to have been discredited, a group based near Rome, which will be referred to frequently in this book, published or promulgated several papers claiming to see the waves. The claims were based on coincidences between a cryogenic bar in Rome and one in Geneva, on coincidences with the cryogenic bar in Australia, and on coincidences between one of their original room-temperature bars and Weber's room-temperature bar. These claims were sometimes ignored by the rest of the gravitational-wave community and sometimes greeted with outrage.
The outrage, I believe, and have attempted to show in my more complete history of the field, can be to some extent correlated with the need to get funds to build a new and much more expensive generation of detectors. These are the interferometers which today dominate the field. An experiment like Weber's could be built for a hundred thousand dollars, whereas the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) started out at around a couple of hundred million. If gravitational-waves could be detected for a fraction of the price, and Weber once wrote to his Congressional representative to argue just this, funding for big devices would be hard to justify. Therefore, it became a political as well as a scientific necessity to stress that the bars could not do the job that Weber and the Rome Group were claiming for them. On the basis of almost every theory of how these instruments worked, the interferometers were going to be orders of magnitude more sensitive than the bar detectors, and on the basis of almost every theory of the distribution and strength of gravitational-wave sources in the heavens, only the interferometers had any chance of seeing the waves. Furthermore, even the first generation of these more expensive devices could not be expected to see more than one or two events at best. The consensual view among astrophysicists was that the sky was black when it came to gravitational radiation of a strength that could be seen by the bars, including the cryogenic bars, and that it might emit a faint twinkle, perhaps once year, as far as the first generation of interferometers was concerned. The promised age of gravitational-wave astronomy, involving observation of many different sources with different strengths and waveforms, helping to increase astrophysical understanding, would not be here until a second or third generation of interferometers were on the air. It was only the promise of gravitational astronomy, not first discovery, which could justify the huge cost of the interferometers.
Thus the scene was set for the unfolding of a battle between the cryogenic bars and the interferometers, with the bar side led by the Rome Group. Some bar teams, such as those based in Louisiana and in Legnaro, near Padua, accepted the view of the interferometer teams and agreed to strict data analysis protocols based on a model of the sky in which signals would be rare and strong. This ruled out any chance of detecting weak signals near the noise that might otherwise have been used as a basis for tuning the detection protocols. This was the bars' last chance—it was probably the only way they could work toward an understanding of any weak signals good enough to survive the more severe statistical tests needed for a claim. But the Rome Group was not prepared to accept the dismal astrophysical forecasts and exerted the experimentalist's right to look at the world without theoretical prejudice. If the Rome Group could work themselves into a position that enabled them to find some coincidences that could not be instantly accounted for by noise, and which flew in the face of theory, then they were determined to say so—in the spirit of Joe Weber. They were not willing to put all their effort into explaining away every putative signal just because it was supposed to be theoretically impossible. Thus were they to give rise to a continuing history of "failed" detection claims right into the twenty-first century, and thus did they give teeth and muscle to the history-monster stalking Arcadia's corridors.
Five working interferometers play a part in this story. The size of an interferometer is measured by the length of its arms. The smallest, with arms 600 m long, is the German-British GEO 600, located near Hannover, Germany. Virgo, a 3 km French-Italian device, is located near Pisa, in Tuscany. The largest are the two 4 km LIGO interferometers, known as L1 and H1, located respectively in Livingston, Louisiana, close to Baton Rouge, and on the Hanford Nuclear Reservation in Washington State. There is also a 2 km LIGO device, H2, located in the same housing as H1.
An interferometer has two arms at right angles. Beams of laser light are fired down the arms and bounced back by mirrors. The beams may bounce backward and forward a hundred times or so before the light in the two arms is recombined at the center station. If everything works out just so, the changing appearance of the recombined beam indicates changes of the lengths in the arms relative to each other — a change that could be caused by a passing gravitational-wave. It should thus be possible to see the "waveform," of a passing gravitational-wave in the changing pattern of light that results from the recombination of the beams.
Excerpted from Gravity's Ghost and Big Dog by Harry Collins. Copyright © 2013 The University of Chicago. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS.
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Meet the Author
Harry Collins is the Distinguished Research Professor of Sociology and director of the Centre for the Study of Knowledge, Expertise, and Science at Cardiff University, and a fellow of the British Academy. Among his numerous books are Gravity’s Shadow, Rethinking Expertise, and Tacit and Explicit Knowledge, all published by the University of Chicago Press. He lives in Cardiff, UK.
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