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Traveling at the Speed of Thought
Einstein and the Quest for Gravitational Waves
By Daniel Kennefick PRINCETON UNIVERSITY PRESS
Copyright © 2007 Princeton University Press
All rights reserved.
ISBN: 978-1-4008-8274-8
CHAPTER 1
The Gravitational Wave Analogy
In the early years of the twenty-first century, several large detectors designed to be the first "observatories" of gravitational waves went, one by one, into operation. These detectors trace their ancestry to around 1960, when Joseph Weber, an American physicist working at the University of Maryland, first began the experimental effort to detect gravitational waves. Until 1969 the "field" of gravitational wave detection consisted of Weber and his students, but when he claimed to have detected gravitational waves (Weber 1969), others (some of whom had previously considered working on this subject) began to build their own devices. It proved to be a false dawn. In a highly controversial episode lasting several years, the new detector groups all failed to replicate Weber's results with their instruments (Collins 2004). Nevertheless, despite this controversial and turbulent start, most of these groups continued in the field, persisting through decades of hard effort and many different instruments. Today it is widely expected that the first direct detection of gravitational waves will take place within the decade.
As an illustration of how likely the detection of gravitational waves was thought to be in earlier years, one of the most dedicated boosters of the effort, Kip Thorne, had no difficulty in 1981 in finding a taker for a wager that gravitational waves would be detected by the end of the last century. The wager was made with the astronomer Jeremiah Ostriker, one of the better-known critics of the large detectors then being proposed. Thorne was one of the chief movers behind the largest of the new detector projects, the half-billion- dollar Laser Interferometer Gravitational Wave Observatory, or LIGO. He lost the bet, of course. One can see the record of it posted in the west corridor of the Bridge Building at the California Institute of Technology (Caltech), outside Thorne's office. It stands beside more than half a dozen other such wagers between Thorne and his colleagues (the most famous of Thorne's wagering friends is Stephen Hawking), most of which Thorne has won. On it is written his note of concession, "I underestimated the time required to make LIGO a reality." It is actually more remarkable that he and others managed to make LIGO a reality at all, especially when one considers the controversial history of the field, for that controversy was not only on the experimental side. The theory of gravitational waves has an even longer history of disputes, false dawns, and setbacks. At one time or another, many theorists doubted whether such waves existed at all. Albert Einstein himself, who founded the theory of gravitational waves in 1916, numbered himself among the doubters on at least two occasions. How that controversy came to be replaced by the certainty and conviction necessary to motivate the great projects of today is the principal story of this book.
The ambivalent status of gravitational waves within physics at the time of the Weber controversy (the early 1970s) is expressed in the following comments relating to Weber's claims from the National Academy of Sciences report Astronomy and Astrophysics for the 1970s. This report was written in 1971 as a guide to U.S. government funding agencies, in particular the National Science Foundation, for the decade ahead:
The detection of gravitational waves bears directly on the question of whether there is any such thing as a "gravitational field," which can act as an independent entity. All actively pursued gravity theories deal with the concept of a gravitational field, so the mere existence of gravitational waves does not exclude any of these theories. Thus this fundamental field hypothesis has been generally accepted without observational support. Such credulity among scientists occurs only in relation to the deepest and most fundamental hypotheses for which they lack the facility to think differently in a comparably detailed and consistent way. In the nineteenth century a similar attitude led to a general acceptance of the ether and atoms decades before the experiments that abolished the ether and confirmed the atom.
The basic style of all physics so far in the twentieth century has been set by the field concept, which arose in electromagnetic theory to replace the vanished ether. This idea has been so overwhelmingly convincing, when tested in experimental and industrial applications, that scientists have tried to package every other known fundamental domain of physics in the same mold. Field theory is incontrovertibly successful in the case of the electromagnetic field. Application of field concepts to particle physics has been successful in many respects, but there are still many unresolved problems. Confirmation of the gravitational wave experiments would show that this concept is suitable for at least one of the other areas — that of gravitational phenomena — in which it is customarily employed. (pp. 282–283)
Thus belief in the existence of gravitational waves, while unsupported by any physical evidence as of the early 1970s, nevertheless prevailed among most physicists because they could see no alternative to modeling the gravitational force other than by analogy with the field theory that described electromagnetic theory. In 1916, shortly after his discovery of the field equations of general relativity, Einstein became the first to describe gravitational waves within a complete field theory of gravity. Beginning with his approach, relativity theorists looked to various analogies with the electromagnetic field as they attempted to construct a theory of gravitational waves in the absence of experimental evidence. Along the way, disputes arose over the theoretical description of this phenomenon and even over whether the theory predicted its existence at all. Given this skepticism, it is not surprising to find that some relativists regarded the analogy underpinning faith in the orthodox picture of gravitational waves as inadequate. It is interesting to examine the views of those whose skepticism made them defy the consensus understanding of a hypothesis without which modern physicists lacked "the facility to think ... in a ... detailed and consistent way."
How is it that an idea can be universally, or at least "generally," held in science "without observational support?" To most ears, whether they belong to scientists or lay people, this does not sound terribly scientific. However, such a state of affairs is actually not uncommon in science, because it does happen that an extremely useful concept (e.g., that of a "field" of force), which is designed to provide an underlying explanation for observed phenomena, may arise without itself necessarily being detected or detectable. The case of gravitational waves is admittedly a little more unusual. In this case, we have a phenomenon that is suggested by the field concept as something whose effects on matter should be detectable. However, the phenomenon has never been detected and has been without even indirect evidence for decades, yet widespread belief in its existence persists. Some rather powerful motivation must lie behind this belief, and if we look for it, we find that the force behind this extraordinarily tenacious scientific belief is that of analogy. Specifically, the analogy in question is between the gravitational field, which underlies our modern theory of gravity (general relativity), and the electromagnetic field, which originated in the work of Michael Faraday and James Clerk Maxwell in the nineteenth century and which is the centerpiece of modern physics. The most dramatic prediction and confirmation of Maxwell's theory was the existence of radio waves, which are part of a spectrum of electromagnetic radiation that actually includes light itself. The basic analogy here is that if gravity is described by a field theory, should it not also have waves which play the fundamental role in that theory as electromagnetic waves do in the theory of Maxwell and his successors?
The history of the field concept is itself a controversial one, dominated by arguments from analogy. Once the idea of electromagnetic radiation became widely accepted, nineteenth-century physicists naturally looked to analogies with other wave phenomena, like sound, which suggested that waves require a medium (such as air in the case of sound) to propagate in. No medium means no wave. The field concept became associated in the nineteenth century with the idea of the luminiferous ether, which was an invisible substance with bizarre properties that pervaded all of space and was the medium or carrier for the electromagnetic field. The old ether theory was discarded completely early in the twentieth century, and nowadays, insofar as we say that electromagnetic waves have a medium at all, we say that that medium is the electromagnetic field, an entity which is not even a part of the material world, although it is, of course, generated by particles that make up the material world. The electromagnetic field is generated only by particles which carry electric charge, but since energy has mass, all particles that exist (i.e., have energy) in the material world generate a gravitational field. Keep in mind, of course, that particles are themselves idealizations designed to help physicists model matter in their equations. In some sense we do not directly experience fields as real entities at all but instead observe their influence on the matter that surrounds us. Thus an electromagnetic wave exists for us only in so far as we have some device at hand that can absorb energy from it as it passes by.
Now how is it that such highly abstract ideas as fields of force have come to play so important a role in physics? It happened in stages, with the level of abstraction increasing at each step. This development went hand in hand with the increasingly dominant role of mathematics in physics. At one time, for instance before Newton, it used to be thought that physics, which was primarily involved with explaining the qualities of physical things, was not a very suitable discipline for the use of mathematics. One characteristic of modern physics since Newton has been an escalating mathematization of the subject. Indeed, Einstein's introduction of general relativity played a major role in this process, as did Newton's introduction of his gravitational theory in the Principia. A very important agent of this increasing abstraction has been the creative use of analogies. For instance, in ancient times Greek philosophers and Roman engineers proposed that sound was a kind of wave traveling through the air, drawing an analogy with the motion of waves on the surface of bodies of water. Via the physics of Aristotle, this concept passed into the physics of the Middle Ages. At the time of Newton, through the work of his contemporary, Christiaan Huygens, and again in the nineteenth century, it was proposed that light was also a wave phenomenon, based on an analogy with the propagation of sound. At this stage the analogy was already becoming further removed from the immediate physical source of the metaphor, because advocates of the wave theory of light were more apt to make a comparison with sound rather than directly with water waves. The disconnection from direct physical experience increased greatly in the second half of the nineteenth century with the development of the Maxwellian theory of electromagnetic radiation, owing to the work of Maxwell, Heinrich Hertz, and others. A further stage of abstraction was added by the early relativity theorists in the first decades of the twentieth century, when they hypothesized that gravitational waves might exist in a field theory of gravitation. They based their analogy on the case of electromagnetic waves, already several stages removed from the kind of wave that we can actually see operating on the surface of water. Also, in this case, the extension of the analogy was being used to predict rather than to explain the existence of a new phenomenon. Some of the great drivers of change in twentieth -century physics were the discoveries of new forms of radiation and new particles, yet gravitational waves were to remain in a kind of limbo, predicted but not observed, for most of the century.
It is worth mentioning that the analogy with electromagnetism was a powerful tool for Einstein in his discovery of the field equations of general relativity. The route Einstein took to create this theory was a long and difficult one and has been extensively studied in recent years.
Much has been written about analogies and their use in science (see especially Hesse 1966), but it is clear that there is no straightforward definition of what a scientific analogy is. Most analogies, like the one between gravitational and electromagnetic waves, could also be described as models. It is obvious that when physicists talk about gravitational waves being analogous to electromagnetic waves, they are thinking of the latter as a model for the former. The use of models is highly characteristic of physics, and in preferring the term analogy in this case, I am doing so because that is the term most often used by the physicists who work in this field. In addition I think it helps to clarify exactly what kind of model we are talking about. There are models that physicists hold to be actually true depictions of the thing being modeled, for instance, the kinetic model of gases, which visualizes them as consisting of many tiny molecules. Nowadays physicists believe that this is exactly what gases consist of. A model may also be a kind of construction of many parts, each of them imaginary, but whose whole forms a functionally equivalent depiction of the thing modeled (what we have in mind when we speak of "model-building"). An example of this kind would include Maxwell's attempts to model the luminiferous ether as a mechanical system of gears and cogs. In our case physicists do not believe that gravitational waves really are electromagnetic waves (though when the idea first emerged this seemed a real possibility), nor are they constructing a model from simple building blocks. They are saying that gravitational waves behave like electromagnetic waves and that there is often a point-for point comparison to be made between the equations which govern each. This makes the word analogy a very apt term to describe what is going on, since when we talk about analogy we often understand by the term a set of correspondences between two systems, such that features of one system correspond to certain features of the other system. The analogy that physicists studying gravitational wave refer to is a rather formal, mathematically based analogy, which establishes that there are correspondences between the equations governing gravitational waves — and the quantities appearing in them — and the equations governing electromagnetic radiation. However, there are other analogies of interest to us that are more descriptive and informal in nature, such as claims that gravitational waves can be thought of as "ripples in the curvature of spacetime," evoking water waves on the surface of a pond. To be clear I will use the term metaphor to describe this descriptive kind of analogy and try to reserve the term analogy for the more formal one that lies at the heart of my argument.
For our purposes we will focus on two main uses of analogy, the first of which is as a heuristic, or finding, device. In this case, one is not necessarily committed to making every point of the analogy correspond, because one is not supposing that the two entities are really the same thing. Thomas Kuhn has discussed the importance of this kind of analogy in the practice of physics, emphasizing what a critical role analogic thinking plays in enabling physicists to make the widest possible use of the tools available to them. Although physicists come to a new problem with a large repertoire of mathematical techniques for solving problems, it will not be immediately clear how best these techniques can be applied to the problem at hand (Kuhn 1977, pp. 306–7). Kuhn argued convincingly that looking for analogies between the new problem and those solved successfully in the past is the chief way in which a physicist will try to deal with an unfamiliar topic: "Once that likeness or analogy [between the new problem and the old] has been seen, only manipulative difficulties remain" (Kuhn 1977, p. 305). The beauty of finding such an analogy is that it enables the physicists to unleash their arsenal of techniques, hard won through experience in other subjects, onto a new problem. But since, as Kuhn emphasized, the sort of analogy that is being exploited is by no means a strict set of correspondences, there is plenty of room for argument about the validity of the analogy. As another philosopher of science has said, "Arguments from analogy may be fruitful, but they are always invalid" (Mario Bunge; quoted in North 1981, p. 135). There is plenty of scope for being wrong in this business. In cases where the dependence on analogy persists for a (perhaps unusually) long period, we will not be surprised to find that it is fertile ground not only for discovery, but also for controversy.
(Continues...)
Excerpted from Traveling at the Speed of Thought by Daniel Kennefick. Copyright © 2007 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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