On Physics and Philosophyby Bernard d'Espagnat
Among the great ironies of quantum mechanics is not only that its conceptual foundations seem strange even to the physicists who use it, but that philosophers have largely ignored it. Here, Bernard d'Espagnat argues that quantum physicsby casting doubts on once hallowed concepts such as space, material objects, and causality-demands serious reconsideration of
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Among the great ironies of quantum mechanics is not only that its conceptual foundations seem strange even to the physicists who use it, but that philosophers have largely ignored it. Here, Bernard d'Espagnat argues that quantum physicsby casting doubts on once hallowed concepts such as space, material objects, and causality-demands serious reconsideration of most of traditional philosophy.
On Physics and Philosophy is an accessible, mathematics-free reflection on the philosophical meaning of the quantum revolution, by one of the world's leading authorities on the subject. D'Espagnat presents an objective account of the main guiding principles of contemporary physics-in particular, quantum mechanics-followed by a look at just what consequences these should imply for philosophical thinking.
The author begins by describing recent discoveries in quantum physics such as nonseparability, and explicating the significance of contemporary developments such as decoherence. Then he proceeds to set various philosophical theories of knowledgesuch as materialism, realism, Kantism, and neo-Kantismagainst the conceptual problems quantum theory raises. His overall conclusion is that while the physical implications of quantum theory suggest that scientific knowledge will never truly describe mind-independent reality, the notion of such an ultimate realityone we can never access directly or rationally and which he calls "veiled reality"remains conceptually necessary nonetheless.
"In this valuable work, Bernard d'Espagnat brings his considerable expertise in contemporary physics to bear on the difficult philosophical issues arising from the current understanding of the subatomic domain."Thomas Oberdan, Isis
"Written in a very readable style, without an overload of mathematical equations, Of Physics and Philosophy unfolds the exotic features of quantum physics to the accompaniment of philosophical commentary. It is without doubt a work of immense scholarship, and will probably hold its own till the mysteries in the field are adequately understood. D'Espagnat's scholarship is helping understand the bizarre implications of quantum theory in investigating everything from free will and the paranormal to the enigma of consciousness."Sudhirendar Sharma, Caravan
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On Physics and Philosophy
By Bernard d'Espagnat
Princeton University PressCopyright © 2006 Princeton University Press
All right reserved.
Chapter OneBROAD OVERVIEW
1-1 A General Picture
ADMITTEDLY THE new world-view that soared in the early seventeenth century originated from Copernicus' discovery but it is not to be questioned that the change from Aristotelian to Galilean physics played an essential part in its development. Aristotle had kept close to raw data obtained through the senses. He saw that all moving bodies not subjected to any force finally stop, and he raised this observed fact to the level of a basic principle of knowledge. He observed that living beings have all sorts of different shapes, qualitatively differing from one another, and he therefore took the notion of a wide variety of fundamental forms as a guideline for his philosophy. Hence, for him, there were a great abundance and variety of concepts, all of them lying, so to speak, on the same level. Hence also he gave considerable care to detailed qualitative descriptions, counterbalanced by a marked weakness concerning anything physically quantitative. With Galileo, Descartes, etc., on the contrary, the idea that came to the forefront was that of a hierarchy of concepts. Within their approach there are basic and nonbasic ones, and the latter must be accounted for in terms of the former, so that, in the end,the description of the physical world should be entirely expressed in terms of just a few basic notions linked together by quantitative laws. And we know, of course, that within classical physics as well as in all other sciences this is the conception that finally prevailed.
The difference just sketched between the Aristotelian and the Cartesian-Galilean approaches is quite well known. But what often remains unnoticed is that, notwithstanding this difference, the approaches in question had an important feature in common. In fact, they shared the view that the basic concepts (the nonderived ones) are either obvious ones or, at least, idealizations of obvious ones; that they are familiar notions- "clear and distinct ideas" as Descartes said-whose unquestionable validity is fully guaranteed by commonsense (i.e., by God, according to the same). It is often-and rightly-stressed that Galileo, Descartes, and Newton brought mathematics into physics. But an often overlooked point is that they made use of mathematics primarily for imparting quantitative content to developments exclusively bearing on objects designated by means of familiar concepts. Descartes was bent on describing the whole of the physical world by "figures and motions" and referred to "the pipes and springs that cause the effects of the natural bodies." Newton spoke of material points, that is, (basically) idealized grains or specks. Even Pascal, in his fable of the mite, clearly took it for granted that the domain of validity of the familiar concepts extends to the whole range of conceivable scales, from the infinitely great to the infinitesimally small.
Were they right? Yes of course, in a sense. Pioneers they were and, as such, their most urgent task was to explore the ins and outs of such a natural idea. Moreover, the idea in question proved spectacularly fruitful. Still today, there are many fields of study in which it is possible to describe data and processes by the sole means of basically familiar concepts and in which this is obviously the best way to carry on fruitful research. Consider, for example, molecular biology. Molecular biologists have to do with large molecules whose behavior-for well-known reasons following from the very rules of quantum physics-practically obeys the laws of classical physics. Consequently it is possible and natural to think of them as having rigid atomic structures, fixed shapes (including hooks), and so on; in short, to reason about them as if they were component parts of a clockwork or a machine. This mode of thought opened the way to quite a host of predictions, many of which proved brilliantly successful. No wonder that many biologists are tempted to raise it to the level of an absolute; to view it as yielding the proper canvas for a description of "the Real itself," including thought.
All this naturally leads to a mechanistic world-view styled naïve-to be sure-within philosophical circles but stamped with commonsense and taken therefore by the majority of well-informed people to be by far the most sensible one. Think, for a moment, of an attorney, a senior executive, an engineer, or even a scientist working in some highly specialized field other than theoretical physics. Such people-most of our contemporaries indeed-have to do, all day long, with machines of all kinds, that is, with human-made devices of which the clock is the example par excellence. No wonder that they spontaneously lean toward a generalized mechanistic world-view, in spite of all that the philosophers may write and say! It is therefore fully understandable that such a type of natural philosophy should remain, so to speak, the instinctive one in the minds of both the enlightened laymen and a majority of scientists. The idea that the World- or, at least, its physical part-is of the nature of some gigantic machinery seems infinitely plausible indeed.
And still, the opposite is true! In a move that was slow at first but progressively speeded up, physics taught us, not only that the human mind is able to operate well outside the framework of familiar concepts, but also that it absolutely must do so. Of all sciences, only physics, apparently, yields this message. But it does. And the fact that it does may well be taken to constitute one of its main contributions to the development of thought.
For brevity's sake, let me illustrate this by but one example, that of particle creation in high-energy collisions. This phenomenon is explored and investigated in laboratories where large particle accelerators are available and observed in bubble chambers in which moving particles produce tracks. Two protons are accelerated. Each one has a given motion, a given velocity, and hence a given energy. They collide and they then part from one another. At that time we observe that, even though they are still in existence and did not break up, other particles have appeared, which are "really true" particles-possessing masses, electric charges, and so on-that have been created in the collision, at the expense of the incident protons' total energy. This, at least, is what we see. Admittedly, the phenomenon is quite in agreement with the celebrated E=mc law expressing mass-energy equivalence. But if we insisted on describing it by the sole means of familiar concepts we would have to say that the incident particle motion was changed into particles. Now, motion is a property of objects, so that what we would thereby refer to would be nothing else than the transformation of a property of objects into objects. Such an idea lies entirely outside the realm of our familiar concepts. Within the set of the latter there are, on the one hand, objects and, on the other hand, properties of objects; and no element of either one of these subsets ever transforms into an element of the other one. The very idea of such a transformation looks just as absurd as that of changing the height of the Eiffel tower into another Eiffel tower. Or as the view that, when two taxis collide, they may both emerge undamaged, accompanied by five or six other taxis arising from the initial kinetic energy of the former. All this makes it crystal clear that contemporary physics forces upon us the use of basic concepts lying outside the realm of the familiar ones, with the sole help of which Descartes (and the other "founding fathers" of modern science) originally claimed that physics would describe the World.
Are we then faced with an enigma exceeding the powers of understanding? Quite on the contrary, theoretical physicists not only know how to deal with this creation phenomenon but also had predicted it, on the basis of their equations. Which shows that, when applied to physics, mathematics makes it possible to really reach beyond all familiar concepts; to actually coin new concepts. This reminds us of Pythagoras' famous saying: "Numbers are the essence of things." A sentence to be understood, of course, as "mathematics are the essence-the very essence-of things."
This, to be sure, is not the proper place for entering into a detailed account of how physics manages to describe the creation phenomenon. Already at this stage let us note, however, that it offers several ways of doing so, grounded on quite different basic concepts. The existence of this diversity should not disconcert us, but it is important that we should be aware of it, since it casts quite a serious doubt on the very possibility of univocally determining, by means of physics, a list of the truly basic notions. Which, in turn, makes it unlikely that Pythagorism is the "last word" of our story. Indeed the diversity in question is a good example of a philosophically important phenomenon-called "incomplete determination of theory by experience"-that we shall frequently have to take into consideration. With respect to the case under study, it so happens that the mathematical formalism yields not one but three distinct theories, all of them grounded on the general quantum rules, yielding essentially the same observational predictions, but widely differing concerning the ideas they call forth. They are called the "theory of the Dirac sea," "Feynman graph theory," and "quantum field theory."
We shall soon get acquainted with the two first named ones (sections 2-6 and 2-7), and shall then have the opportunity of observing how widely both depart from the Cartesian ideal of a description using only familiar concepts. But, for the time being, let us focus our attention on quantum field theory.
Unquestionably more general than the first one-Dirac's-this theory is, in a way more basic than the second one-Feynman's-which primarily appears as a powerful method of calculating, grounded, as its author himself stressed, precisely on the very rules of quantum field theory. To form a broad idea of the general guiding lines of the latter let us begin by observing that the notion of creation is not a scientific one: We do not know how to capture it, and even less quantify it. It is therefore appropriate to try and reduce it to something we can master. Now we do master the notions of a system state and changes thereof. We know how to calculate transition rates from one state to another. And the brilliant idea, the breakthrough, just came from this. It consisted in considering that the existence of a particle is a state of a certain "Something," that the existence of two particles is another state of this same "Something," and so on. Of course, the absence of a particle is also a state of this "Something." Then, the creation of a particle is nothing else than a transition from one state of this "Something" to another, and therefore we may hope to be able to treat it quantitatively. It is just as simple as that! In practice- believe it-the matter is appreciably more complex. Quantum field theory textbooks are big, fat objects, full of formulas, many of which are in no way beautiful. But by plodding through the latter it proved possible to account for observed phenomena with a precision that, in some cases, extends to the seventh decimal. Which, really, is "not too bad"!
The reader will be spared the calculations. Instead, let him or her reflect on the just described basic idea of the quantum field theory. True, the problem of the "real nature" of the "Something" that it brings-at least implicitly-into play is, as we shall see, an inordinately delicate one. However, concerning it, one point at least seems rather clear. It is that the cornerstone role tacitly attributed to it somehow suggests the presence, within the core of present-day physics, of a wholeness of some sort, radically foreign to classical physics. The point is that classical science was very much in favor of what may be adequately said to be a multitudinist world-view. In other words, it favored a conception of Nature in which basic Reality-matter, as it was called-was constituted of a myriad simple elements-essentially localized "atoms" or "particles"-embedded in fields, and hence interacting by means of forces decreasing when distance increased. The first two of the above mentioned theoretical approaches still are more or less compatible with such a view, although, as we shall see, they considerably weaken it by attributing to the particles behaviors that the mind cannot imagine. On the other hand, the-more general- quantum field theory is radically at variance with it. Not only is it true that, in it, the particles no longer play the role of the constitutive material of the Universe. What is more, the only "entity" that, in it, might conceivably be thought to constitute basic Reality is the "Something," of which we saw that it is fundamentally the only one of its species.
The idea that, here, is seen to come to light (though dimly as yet) is, to repeat, the notion of a wholeness of some sort. Within elementary particle physics wholeness, admittedly, remains ambiguous since while, say, manifest in one formulation, it is evanescent in the other two-in spite of the fact that all three are equivalent and proceed from the same-quantum- formalism. This perhaps explains why, when quantum mechanics appeared, the notion in question was clearly apprehended by neither the epistemologists nor even the physicists, with the perhaps unique exception of Schrödinger. But theoretical as well as experimental advances gradually made people realize that it constitutes an inherent part of the very quantum formalism and has quite specific experimental consequences.
Nowadays the common name nonseparability serves to designate both the just mentioned mathematical features of the formalism and the corresponding observable effects. A most important point is to be noted concerning it. It is the fact that the range of validity of the notion it designates is even wider than that of the presently accepted theory. Indeed, it has been established that in one at least of its main aspects-nonlocality-it will certainly remain true, even if the quantum formalism must, one day, be replaced by some other, more general, one. As will be seen (chapter 3), this follows from the Bell theorem and the experiments-such as Aspect's-associated with it, for the results of the latter are incompatible with some consequences of the inverse hypothesis-locality, and this quite independently of any theory whatsoever.
To be sure, scientists and even physicists go on expressing themselves in terms of particles, molecules, and so on, all words calling forth the idea of individual, localized objects depending less on one another as the distances between them grow greater. In short, they go on making use of a multitudinist language. And from their angle they are right for, as we saw, this amounts to referring to a model that is, by far, the most convenient one in an enormous variety of cases. But, by now, it appears more and more clearly that it is merely a model. With due reservations a comparison could be ventured here with Ptolemy's geocentric model, which also works quite well on specific problems. In both cases, to raise the model to the level of a description of "what really is" is scientifically illegitimate.
In this respect, let it be noted that the question "reality or just model?" never comes to light in the articles physicists write. The latter wisely remain on "secure ground," which means that their theoretical constructions, elaborate as they may be at the level of equations and methods, are left by them very much "open" regarding concepts. In fact, when they work on such constructions the condition they impose on them is just that they should be highly general models, correctly accounting for what we observe in a great variety of experiments. Consequently it is without qualms that they ground them-tacitly at least-on the basic principles of "standard" quantum mechanics, without being in the least worried by the fact that, as we shall see, some of these principles impart a fundamental role to such notions as "measurement" and "preparation of system states." Now, this fact-the occurrence of a reference to human action within the very axioms of physics-is sometimes explicitly stated. Often it is kept implicit. But in any case it implies that the theories built up in this way markedly depart from a principle that was one of the main guidelines of all classical ones. I mean the rule that basic scientific statements should be expressed in a radically objectivist language, making no reference whatsoever, be it explicit or implicit, to us ("operators" or "measurers").
Excerpted from On Physics and Philosophy by Bernard d'Espagnat Copyright © 2006 by Princeton University Press. Excerpted by permission.
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What People are saying about this
Jeffrey Bub, University of Maryland, author of "Interpreting the Quantum World"
Andrew Z. Wayne, University of Guelph, coeditor of "Ontological Aspects of Quantum Field Theory"
Abner Shimony, Professor Emeritus of Philosophy and Physics, Boston University
Arthur G. Zajonc, Amherst College, coauthor of "The Quantum Challenge"
Roland Omnes, Professor Emeritus of Theoretical Physics, Universite de Paris-Sud
Tian Yu Cao, Boston University
Meet the Author
Bernard d’Espagnat is professor emeritus of physics at the University of Paris-Orsay, where he was director of the Laboratory of Theoretical Physics and Elementary Particles from 1970 to 1987, and winner of the 2009 Templeton Prize. His books include the classic Conceptual Foundations of Quantum Mechanics and Veiled Reality.
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