But let vis return to our subject proper. A much more serious and interesting attempt to explain the difficulty away was founded by Bohr and Heisenberg on the idea, mentioned above, that there is an unavoidable and uncontrollable mutual interaction between the observer and the observed physical object. Their ratiocination is briefly as follows. The alleged paradox consists in this, that according to the mechanistic view, by procuring an exact knowledge of the configuration and velocities of all the elementary particles in a man's body, including his brain, one could predict his voluntary actions— which thereby cease to be what he believes them to be, namely voluntary. The fact that we cannot actually procure this detailed knowledge is no great help. Even the theoretical predictability shocks us. To this Bohr answers that the knowledge cannot even be procured in principle, not even in theory, because such accurate observation would involve so strong an interference with “the object” (the man's body) as to dissociate it into single particles—in fact kill him so efficiently that not even a corpse would be left for burial. At any rate, no prediction of behaviour would result, before the “object” is far beyond the state of exhibiting any voluntary behaviour. The emphasis is of course on the phrase “in principle”. That the said knowledge cannot actually be procured, not even for the simplest living organism, let alone a higher animal like man, is clear also without quantum theory and uncertainty relation.
But let vis return to our subject proper. A much more serious and interesting attempt to explain the difficulty away was founded by Bohr and Heisenberg on the idea, mentioned above, that there is an unavoidable and uncontrollable mutual interaction between the observer and the observed physical object. Their ratiocination is briefly as follows. The alleged paradox consists in this, that according to the mechanistic view, by procuring an exact knowledge of the configuration and velocities of all the elementary particles in a man's body, including his brain, one could predict his voluntary actions— which thereby cease to be what he believes them to be, namely voluntary. The fact that we cannot actually procure this detailed knowledge is no great help. Even the theoretical predictability shocks us. To this Bohr answers that the knowledge cannot even be procured in principle, not even in theory, because such accurate observation would involve so strong an interference with “the object” (the man's body) as to dissociate it into single particles—in fact kill him so efficiently that not even a corpse would be left for burial. At any rate, no prediction of behaviour would result, before the “object” is far beyond the state of exhibiting any voluntary behaviour. The emphasis is of course on the phrase “in principle”. That the said knowledge cannot actually be procured, not even for the simplest living organism, let alone a higher animal like man, is clear also without quantum theory and uncertainty relation.
But let us return to our subject proper. A much more serious and interesting attempt to explain the difficulty away was founded by Bohr and Heisenberg on the idea, mentioned above, that there is an unavoidable and uncontrollable mutual interaction between the observer and the observed physical object. Their ratiocination is briefly as follows. The alleged paradox consists in this, that according to the mechanistic view, by procuring an exact knowledge of the configuration and velocities of all the elementary particles in a man's body, including his brain, one could predict his voluntary actions—which thereby cease to be. what he believes them to be, namely voluntary. The fact that we cannot actually procure this detailed knowledge is no great help. Even the theoretical predictability shocks us. To this Bohr answers that the knowledge cannot even be procured in principle, not even in theory, because such accurate observation would involve so strong an interference with ‘the object’ (the man's body) as to dissociate it into single particles—in fact kill him so efficiently that not even a corpse would be left for burial. At any rate, no prediction of behaviour would result, before the ‘object’ is far beyond the state of exhibiting any voluntary behaviour.
“Bohr was primarily a philosopher, not a physicist, but he understood that natural philosophy ... carries weight only if its every detail can be subjected to the ... test of experiment” (Heisenberg in [59], p. 95). As a result his approach differed from that of theschool-philosopherswhom he regarded with a somewhat “sceptical attitude, to say the least” ([59], p. 129) and whose lack of interest in “the important viewpoint which had emerged during the development of atomic physics” he noticed with regret ([59], p. 183).But it also differed, and to a considerable degree, from the spirit of what Professor T. S. Kuhn has called a “normal science.” Looking at Bohr's method of research we see that technical problems, however remote, are always related to a philosophical point of view; they are never treated as “tiny puzzles” whose solution is valuable in itself, even if one has not the faintest idea what it means, and where it leads: “For me” Bohr writes to Sommerfeld in 1922 ([59], p. 71) “[the quantum theory] is not a matter for petty didactic details, but a serious attempt to reach ... an inner coherence.” Emphasis is put on matters of principle ([59], p. 36) and minor discrepancies, or “puzzles” in the sense of Kuhn, instead of being deemphasized, and assimilated to the older paradigm,are turned into fundamental difficultiesby looking at them from a new direction, and by testing their background “in its furthest consequences by exaggeration” ([59], p. 329). A noteworthy example of Bohr's “non-normal” and rather metaphysical approach to physics is his scepticism in the face of the success of his own atomic model.
The founding fathers of quantum mechanics had already perceived the essence of the difficulties of quantum mechanics; today, after almost a century, the discussions are still lively and, if some very interesting new aspects have emerged, at a deeper level the questions have not changed so much. What is more recent, nevertheless, is a general change of attitude among physicists: until about 1970 or 1980, most physicists thought that the essential questions had been settled, and that “Bohr was right and proved his opponents to be wrong”. This was probably a consequence of the famous discussions between Bohr, Einstein, Schrödinger, Heisenberg, Pauli, de Broglie, and others (in particular at the Solvay meetings [1–3], where Bohr's point of view had successfully resisted Einstein's extremely clever attacks). The majority of physicists did not know the details of the arguments. They nevertheless thought that the standard “Copenhagen interpretation” had clearly emerged from the infancy of quantum mechanics as the only sensible attitude for good scientists. This interpretation includes the idea that modern physics must contain indeterminacy as an essential ingredient: it is fundamentally impossible to predict the outcome of single microscopical events; it is impossible to go beyond the formalism of the wave function (or its generalization, the state vector ∣ψ) and complete it.
The odd concepts of quantum physics are nearly a century old. They may seem difficult and forbidding at first sight, but the barrier to understanding is not their difficulty but their differentness . Unlike classical models, quantum theory does not attempt to simulate Nature. It gives us information about observations of Nature. But it does contain a “theoretical entity” that claims to contain all information, observation notwithstanding, namely the quantum state vector in Hilbert space. Scientists routinely use the language of Hilbert space, and nonscientists do not use it at all, which is understandable but unfortunate. A sort of baby-talk has become the lingua franca for much of the popular journalism of quantum physics, an awkward patois that combines some of the early groping language of Bohr, de Broglie, Schrödinger, and Heisenberg with more modern words about symmetries and states. Earnest amateurs still ask me about the mystery of Bohr orbits. There is no mystery because there are no orbits, except as historical curiosities. Same for “wave–particle duality.” Same for “quantum jumps.” A friend once asked me to address a philosophy seminar about the “paradoxes of quantum mechanics.” He said he had approached C. N. Yang first, who replied “What paradoxes?” There are some mathematical and logical rough spots in our current best theory of matter, and perhaps some of them can be framed as paradoxes. But the theory is actually rather straightforward, and it has a perfectly clear interpretation linked to experiments that, in their simplest form, anyone can perform (and students do routinely in college laboratories). No empirical observations are known that are inconsistent with the quantum framework.
Bohr is said to have remarked that anybody who is not shocked by quantum theory has not understood it. No physicist, however, challenges its practical usefulness in predicting experimental results and providing an explanatory framework for an enormous range of study. Nevertheless, more thoughtful physicists have been astonished and disturbed by what is implied when attempts are made to interpret its meaning. As a result a variety of ingenious, and sometimes bizarre, schemes have been devised since its inception in order to make these implications more acceptable to the scientific mind. Some of these schemes have resulted in proposals which appear stranger than the problems they have been designed to resolve. After some sixty years there is no consensus of opinion upon the solution of the epistemological problems posed. Indeed the debate keeps on resurfacing in the scientific literature and continues to generate library shelves of books. Some progress, however, has been made and the late John Bell himself and the experimental work of Alain Aspect have demonstrated that some epistemological positions cannot be sustained.
MAN AND THE INDETERMINACIES* JOHN R. PLATTf Understanding the external world is the objective of science, and accurate prediction is the test ofit. Ifwe really understand genetics, we can breed better hybrid corn. Ifwe really understand geology, we can locate minerals and oil. If we really understand the motion of the atmosphere and its water vapor, we can predict the weather. But some ofthe most thought-provoking problems ofscience are those involving "indeterminacy," where accurate prediction for some reason becomes impossible. It is as though a sign said, "Nonsense!—Your prying or your prediction at this point interferes with itself, so your supposed observations orpredictions can no longer be made objectively or meaningfully ." Many ofthese problems are a little bit like my answer to your question whether I will wear my red tie tomorrow morning. I may tell you that I will, but this is an intention, not an objective prediction like predicting rain—and you will immediately show that you know this by laughing at me ifI try to bet with you on it. It is obvious that my knowledge ofthe prediction and the bet would change my performance, and I am likely to wear the red tie or not, as the case may be,just so I will win—or to do the converse to prove my unpredictability. Everyone knows, really, that predictions of one's own conscious decisions are not objective predictions in any scientific sense. The most famous case ofindeterminacy, where getting data for a prediction interacts with the outcome in somewhat the same way, is that of the Heisenberg "uncertainty principle" in atomic physics, which upset the whole philosophical world forty years ago. But indeterminacy and * A chapter from the book The Step to Man, published byJohn Wiley & Sons, Inc.,June, 1966; reprinted by permission. t Acting Director, Mental Health Research Institute, University ofMichigan, Ann Arbor. 67 related questions of prediction also touch many other problems, for example , in areas such as biology and psychology. It is interesting to look at some ofthese problems to seejust what goes wrong with deterministic predictions, and why. This gives us a much clearer view ofwhat questions objective science can meaningfully ask and what it cannot, and what parts oflife must be left out ofthe scientific equation. It appears that several of the non-classical conclusions may be ofcentral importance in understanding the relations between manandman and between man and the universe. The Areas ofDeterminism The concept ofdeterminismis closely bound up with the concept ofthe "isolated system." An isolated system, in astronomy or physics or chemistry , is a body or collection ofbodies—for example, the sun and planets, or the chemicals in a flask—which are supposed to interact in some respect only with each other. It is a system which an observer can either start off or can observe accurately at some initial time, and which can then continue to be observed indefinitely without any appreciable disturbance to its motions or behavior from interactions with the observer himself or with the rest ofthe universe. These conditions are evidently essential in order for determinism to hold, that is, in order for our specification of the initial state ofthe system to suffice for determining its final state accurately at any later time. Ofcourse, isolation from the surroundings is never perfect. The chemicals in the flask boil offinto the air. The earth's tides slow down the moon by amounts that can be measured. Nevertheless, the "isolated system" is a useful and accurate approximation for most ofthe problems we know how to solve in physics and chemistry and engineering; and in fact, that is why we know how to solve them. Ifwe correct for the small disturbances , Newton's laws will predict eclipses and the motions ofplanets or man-made satellites for years or centuries ahead. The planets are heavy bodies, not much affected by the dust in their paths or the sunbeams by which we observe them. Few other predictions in science are as certain as those of the planets, but in most cases the uncertainties are not believed to be due to any breakdown of determinism. Forecasting the weather, for example, is difficult apparently because of the great complexity of...