Gravity's Shadow: The Search for Gravitational Waves / Edition 2 available in Paperback
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- University of Chicago Press
Gravitational wave detection involves recording the collisions, explosions, and trembling of stars and black holes by evaluating the smallest changes ever measured. Because gravitational waves are so faint, their detection will come not in an exuberant moment of discovery but through a chain of inference; for forty years, scientists have debated whether there is anything to detect and whether it has yet been detected. Sociologist Harry Collins has been tracking the progress of this research since 1972, interviewing key scientists and delineating the social process of the science of gravitational waves.
Engagingly written and authoritatively comprehensive, Gravity's Shadow explores the people, institutions, and government organizations involved in the detection of gravitational waves. This sociological history will prove essential not only to sociologists and historians of science but to scientists themselves.
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The Search for Gravitational Waves
By Harry Collins
The University of Chicago Press
Copyright © 2004 The University of Chicago
All right reserved.
THE START OF A NEW SCIENCE
I began with Tony Tyson's evidence before Congress. Tyson failed to stop or delay for very long the funding of the interferometer project, and the use of huge interferometers to search for gravitational waves is now beginning. These interferometers are the subject of the later sections of the book. Two earlier generations of detectors preceded the interferometers, and there might have been another generation had it not been stillborn. The first generation of detectors, whose heyday was from the late 1960s to the mid-1970s, were solid metal cylinders known as bars. Roughly a couple of meters long and a couple of feet in diameter, and weighing a ton or so, they were encased in vacuum tanks and insulated from every other disturbance as much as possible. Because they were meant to vibrate in response to a passing gravitational wave, they came to be known as room-temperature resonant bars. Since the year 2000 only one room-temperature bar, at most, has been recording data; this is run by an Italian group; the other remaining room-temperature bars died in the year 2000 at the same time as Joe Weber, who pioneered the technology in 1960s.
The second generation of detectors, which dominated the science from roughly the mid-1970s to the mid-1990s, were a development of the room-temperature bars, but they were cooled to the temperature of liquid helium or below; these devices are known as cryogenic bars. In recent years, up to five of them have been operating: one in the United States, one in Australia, and three run by Italian teams, of which one is located at CERN (European Organisation for Nuclear Research) in Geneva. The stillborn generation of devices, which may yet revive, were a further development of the cryogenic technology and used spherical resonators instead of cylinders. They, too, would have been cooled to the lowest temperature possible, but each would have weighed about 30 tons rather than the couple of tons of the bars. The current generation of detectors, increasingly dominant since the mid-1990s, are the interferometers.
This part of the book is about the room-temperature bars, devices that opened up the science of gravitational wave detection at a time when it seemed that such a science belonged only in a world of fantasy.
The person who began the whole business of looking for gravitational radiation was Joseph Weber. Weber, pronounced "Webb-er," is always referred to as Joe Weber. Most of the text of this book, where it discusses Weber, was first written in the present tense; for most of my professional life Weber has been an "is," not a "was." Changing the tense has been painful. From 1948 until his death September 31, 2000, at the age of 81, Weber remained an active physicist and was at various times professor of electrical engineering, professor of physics, or both at once at the University of Maryland, located in the suburbs of Washington, DC. From 1973 to 1989 he was also a visiting professor at the University of California at Irvine. He continued to maintain offices at both institutions until his death.
Weber was born in Paterson, New Jersey, on May 17, 1919, the son of Jewish immigrants from Lithuania. During the Great Depression he worked as a golf caddy for a dollar a day, increasing his earnings tenfold after teaching himself to fix radios. He received the highest score in a public examination and was invited to become a cadet at the US Naval Academy at Annapolis, where he graduated with a bachelor of science degree in 1940. He found himself in active naval service during the Second World War and soon demonstrated his technical expertise, becoming a skilled radar operator and an excellent navigator. One ship on which he served, the aircraft carrier USS Lexington, was sunk during the Battle of the Coral Sea, but Weber survived and became the commander of a submarine chaser, the USS SC-690. As he put it, this was an unusual role for a Jewish boy from a poor background.
After the war, Weber continued to study physics, obtaining a Ph.D. from the Catholic University of America in 1951. He also studied physics at the Institute for Advanced Study in Princeton, New Jersey, being influenced by, among others, Robert Oppenheimer and John Wheeler; he was also strongly encouraged by Freeman Dyson. He spent time at the institute in 1955-56, 1962-63, and 1969-70. Among Weber's accomplishments is the development in the 1950s of ideas that prefigured the maser, itself the forerunner of the laser; laser stands for "light amplification by stimulated emission of radiation," whereas maser stands for the same thing, except light is replaced with microwave. Perhaps he should have shared in the Nobel Prize for that work, or at least been given more recognition; he certainly thought so, as did a number of others. A wall of the engineering building at the University of Maryland displays a photograph of Weber with a certificate of appreciation for his pioneering maser work.
When I last saw him, in 1996, Weber, then 78, was still putting in a full workday in his shambles of an office in the University of Maryland's Physics Department. Moreover, he told me that he continued to take a three-mile run at four in the morning to keep fit. Widowed after the death of his first wife in 1971, Weber subsequently married astronomer Virginia Trimble, 24 years his junior, his occasional coauthor, and later very well known in her own right. He told me with a smile that when he married her he was famous and she was not, and now their roles were reversed.
Up to his death, Weber continued to put in applications to the funding agencies, not only for projects on gravity waves but for research on the detection of neutrinos and for a revolutionary type of laser. Up to the early 1990s, some agencies were still responding to him in small ways.
To say that Joe Weber was famous or not famous is to oversimplify. Everyone in the world of physics knows who he was, but his fame largely turned to notoriety between the 1970s and the 1990s. I suspect that now that he is dead, the notoriety will revert to renown, and he will regain his place in history. The peak of Weber's reputation came about 1969 and lasted for five years or so. He became famous when he announced that he had detected gravitational waves. His reputation began to change, however, when others said it was not gravitational waves he was seeing but a problem with the apparatus of or the statistics produced by the experiment. These others-the large majority of physicists in the relevant communities-said that what he claimed to have seen was theoretically impossible. They could calculate the maximum strength of the gravitational radiation that should be passing through Earth and they could calculate the sensitivity of Weber's detector, and the calculations did not match.
To the end of his life, Joe Weber insisted that it was gravitational waves that he saw; he continued to insist that he had seen them 30 years later as he drew to the close of his career. Thus, his fame changed to notoriety because he would not admit he was wrong. Worse, he would not desist from telling people, in the most forthright manner, that he was right and they were wrong. In his later years Weber was very bitter about what he saw as the injustices that had befallen him, and he had a sharp edge to his tongue; as a result he drove some erstwhile colleagues to despair.
A New Force Detected?
If the physics community has a "house journal," it is probably Physics Today, which is published by The American Institute of Physics. The April 1968 issue contained an article by Weber as its lead story. The front cover featured a schematic rendering of two masses joined by a spring-which is how Weber liked to represent the theory of the gravitational wave detector that he had invented. Weber reported an experiment conducted in his Maryland laboratory by his graduate student, Joel Sinsky. It was said to show that vibrations in one aluminum cylinder could be transmitted to another by the changing gravitational attraction between the two bars.
If we were to look very closely at the end of a vibrating bar, we would see it moving in and out-that's what vibrating is. The end might move about one-tenth of a millimeter-a movement almost invisible to the naked eye. But these tiny movements would cause the gravitational field associated with the mass of the bar to change. Any object near the end of the bar would experience slightly less gravitational pull during that part of the cycle of vibration when the end of the bar was one-tenth of a millimeter further away and slightly more gravitational pull when it was one-tenth of a millimeter nearer. Thus, a vibrating bar gives rise to an oscillating gravitational field. Weber was claiming that the tiny changes in gravitational field that resulted from the vibration of the one bar were being sensed by the other bar, causing it to vibrate as well. The vibrations in the second bar would be much smaller than those in the first, however; Weber explained that responses in the second bar on the order of [10.sup.-16] meters were being detected in this experiment. That is to say, Weber was claiming that he could see changes in the length of a two-ton aluminum alloy bar that were somewhat less than the diameter of an atomic nucleus. (See appendix Intro.1 for further elaboration of what it means to say this.)
The second bar detected the minute vibrations via piezotransducers glued to its surface. These are crystals that produce electrical signals when they are squeezed or stretched, thereby linking electricity with physical force. (The spark produced by modern devices for lighting gas stoves is made this way.) Hence the tiny stretchings in the aluminum bar putatively produced by the passage of gravitational waves were to be detected by the electrical signals coming from the tiny stretchings of piezocrystals glued to the surface of the bar and made visible by amplifiers of unprecedented sensitivity.
This experiment is worth describing again to make clear just how ambitious it was. The relationship between force and electricity exhibited by piezocrystals can be reversed. If you apply an electrical potential to them, they will compress or expand according to whether the potential is positive or negative. If you apply an alternating potential to them, the crystals will vibrate. In the Sinsky experiment, two bars with piezocrystals glued to their surfaces were placed near each other. The "driving bar," eight inches in diameter and five feet long, was caused to vibrate by energizing the piezocrystals on its surface with an alternating electrical potential. These vibrations were sensed by the second bar-the detector-which was 22 inches in diameter and five feet in length, and weighed approximately 1.5 tons. The means of transmission of the vibrations from one bar to the other was gravitational attraction.
This experiment was extraordinary, for the gravitational field associated with an object that is less than the size of a planet is hard to sense. It was a triumph of experimental science when, in 1798, Cavendish measured the gravitational attraction between two massive lead balls, because the attractive force between them comprised only one 500-millionth of their weight. Now what Sinsky and Weber were doing was detecting the minute changes in the gravitational field caused by the vibration of such a mass-that is, the minute changes in the field caused by the changes in the location of one end of a massive aluminum bar as it "rang." It is not as though the bar was either there or not there, as in the case of the lead balls used by Cavendish; the bar stayed in place and just changed its shape slightly, and that phenomenon was recognizable because it meant that the part of the bar nearest to the detector moved fractionally nearer to and further from the detector with each cycle of vibration.
Incidentally, this force is a change in a gravitational field having the same frequency as the gravitational radiation that Weber's bar was designed to detect-around 1660 Hertz (cycles per second). The force will change with time in the same way as a wave, though this changing gravitational force is not a gravitational wave; gravitational waves cause changes that are transverse to their direction of travel, but this kind of wave, consisting of changes in the force of gravitational attraction, causes changes which are parallel to the direction from which they are coming. Thus, we are not talking about gravitational radiation in this experiment but changes in ordinary gravitational attraction. This force is no different in principle from the force that causes the tides-the changing attraction of the Moon for movable objects on Earth as the Moon changes its position in the sky. Gravitational radiation is, as I will explain in chapter 3, something far more subtle. Nevertheless, Sinsky's experiment was meant to show the sensitivity of Weber's apparatus to any potential gravitational waves.
Compared with the strains induced in the second bar, the driven bar's vibrations were very large. The second bar, it was calculated, was seeing strains on the order of [10.sup.-16] in response to strains in the first bar of [10.sup.-4] or so (which is about one-tenth of a millimeter). In other words, the secondary vibrations were about 100,000 million times smaller than the first (the first strain x [10.sup.-11]). But that result, of course, is exactly what was wanted if this experiment were to demonstrate that the second bar could indeed detect the tiny effect of gravitational waves.
Even this calibration experiment was an exciting business. The first bar had to be caused to vibrate so hard that the piezocrystals often fell off, and it heated itself though its own vibrations. The vibrating bar was also enclosed in a vacuum chamber to try to get rid of any nongravitational sources of disturbance transmitting themselves to the first bar. If vibrations were transmitted through the air, the floor of the laboratory, the housings of the bars, or some kind of coupled electrical or magnetic forces, then nothing would be proved. Below, I will discuss how Weber and Sinsky tried to show that their experiment was sound and that none of these other couplings were taking place.
Taking this experiment at face value, Joe Weber had proved that his detector was very sensitive to tiny oscillating gravitational forces. Still more important, however, in the Physics Today article he reported that after this "calibration" experiment was finished, the driving cylinder was converted to a second detector and mounted about a mile (1.5 kilometers) from the first. He claimed to have seen about ten coincident energy pulses between the two detectors which were not correlated with vibrations in the ground or with electromagnetic disturbances. These appeared to be interesting signals from an outside source.
What did Weber's claims mean, and why were they important enough to feature on the cover of Physics Today? The first half of the claim-the calibration experiment-represents a triumph of engineering, and if it is correct, it makes the second claim interesting. The second claim intimates that the coincident energy pulses could have been caused by something very small indeed-something that, given a generous interpretation, could just conceivably have been gravitational radiation.
But what is gravitational radiation? Are we sure it exists and that shaking, spiraling, and exploding stars really do emit it? Scientists took a long time to reach a consensus about gravitational waves; the .avor of the theoretical debate which preceded Weber's work is indicated in chapter 3. Einstein, who originally proposed their existence, changed his mind in the 1930s and then changed it back again. Though by the 1960s the consensus was broad, as late as 1962 Richard Feynman wrote to his wife that the continuing argument about whether detectable waves were emitted that he encountered at a conference was bad for his blood pressure. As it turned out, published arguments about whether the waves could have an effect on gravitational wave detectors continued into the 1990s. For example, in 1996, the abstract of an electronically circulated paper by Luis Bel read as follows:
We present a new approach to the theory of static deformations of elastic test bodies in general relativity [resonant detectors].... We argue on the basis of this new approach that weak gravitational plane waves do not couple to [affect] elastic bodies and therefore the latter, whatever their shape, are not suitable antennas to detect them.
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Table of ContentsPreface
Common Acronyms in Gravitational Wave Research
Introduction Two Kinds of Space-Time
PART I à LA RECHERCHE DES ONDES PERDUES
Chapter 1 The Start of a New Science
Chapter 2 From Idea to Experiment
Chapter 3 What Are Gravitational Waves?
Chapter 4 The First Published
Chapter 5 The Reservoir of Doubt
Chapter 6 The First Experiments by Others
Chapter 7 Joe Weber's Findings Begin to Be Rejected in the Constitutive Forum
Chapter 8 Joe Weber Fights Back
Chapter 9 The Consensus Is Formed
Chapter 10 An Attempt to Break the Regress: The Calibration of Experiments
Chapter 11 Forgotten Waves
Chapter 12 How Waves Spread
PART II TWO NEW TECHNOLOGIES
Chapter 13 The Start of Cryogenics
Chapter 14 NAUTILUS
Chapter 15 NAUTILUS, November 1996 to June 1998
Chapter 16 The Spheres
Chapter 17 The Start of Interferometry
Chapter 18 Caltech Enters the Game
PART III BAR WARS
Chapter 19 The Science of the Life after Death of Room-Temperature Bars
Chapter 20 Scientific Institutions and Life after Death
Chapter 21 Room-Temperature Bars and the Policy Regress
Chapter 22 Scientific Cultures
Chapter 23 Resonant Technology and the National Science Foundation Review
Chapter 24 Ripples and Conferences
Chapter 25 Three More Conferences and a Funeral
Chapter 26 The Downtrodden Masses
Chapter 27 The Funding of LIGO and Its Consequences
PART IV THE INTERFEROMETERS AND THE INTERFEROMETEERS—FROM SMALL SCIENCE TO BIG SCIENCE
Chapter 28 Moving Technology: What Is in a Large Interferometer?
Chapter 29 Moving Earth: The Sites
Chapter 30 Moving People: From Small Science to Big Science
Chapter 31 The Beginning of Coordinated Science
Chapter 32 The Drever Affair
Chapter 33 The End of the Skunk Works
Chapter 34 Regime 3: The Coordinators
Chapter 35 Mechanism versus Magic
Chapter 36 The 40-Meter Team versus the New Management, Continued
Chapter 37 Regime 4 (and 5): The Collaboration
PART V BECOMING A NEW SCIENCE
Chapter 38 Pooling Data: Prospects and Problems
Chapter 39 International Collaboration among the Interferometer Groups
Chapter 40 When Is Science? The Meaning of Upper Limits
PART VI SCIENCE, SCIENTISTS, AND SOCIOLOGY
Chapter 41 Coming On Air: The Study and Science
Chapter 42 Methodology as the Meeting of Two Cultures: The Study, Scientists, and the Public
Chapter 43 Final Reflections: The Study and Sociology
Chapter 44 Joe Weber: A Personal and Methodological Note
Coda: January 2004
Appendix Intro.1 What Is Small?
Appendix Intro.2 Gravitational Waves, Gravitational Radiation, and Gravity Waves: A Note on Terminology
Appendix Intro.3 Roger Babson's Essay, "Gravity—Our Enemy Number One"
Appendix III.1 Colonial Cringe
Appendix V.1 The Method