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Quantum physics and language
Physica B 151 (1988) 314-318 North-Holland, Amsterdam
Q U A N T U M P H Y S I C S AND L A N G U A G E Jean-Marc LI~VY-LEBLOND Physique Thdorique, Nice* A novel theory, when it appears, cannot but use old words to label new concepts. In some cases, the extension in meaning thus conferred to standard terminology is natural enough so that the transfer may not lead to too many misunderstandings. Most often, however, and especially when the conceptual gap between the old and the new theory is a wide one, a casual transfer of t~rminology may lead to epistemological and pedagogical difficulties. This situation has been and still is particularly serious in quantum theory. Here, the careless use of words taken from classical physics - such as quantum "mechanics", "uncertainty", e t c . - , is compounded by the uncritical use of interpretative terms linked to a definite, if implicit, philosophical point of view - such as "complementarity", "wave-particle duality", "observables", etc. While these words and the ideas they represent have played a major role in the birth of quantum physics more than half a century ago, they are no longer necessarily the best ones to be used today. It is not argued here that we should start afresh and create from scratch a supposedly adequate vocabulary for quantum physics. Abuse of language certainly is unavoidable in science as it is in any human communication; without it, language would not live and evolve. But, at the very least, let us recognize it for what it is, so that it does not add its troubles to already complicated issues. And in some definite instances, still, a willing effort to replace specially ambiguous words might be worthwhile.
1. Introduction It is a neglected type of interference I w a n t to discuss here: interference b e t w e e n language and physics. Theoretical physics, p e r m e a t e d as it is by m a t h e m a t i c a l formalism, does not consist only of formulas, symbols and letters. It uses words, and c a n n o t but be s p o k e n and written t h r o u g h ordinary language. This is true not only for the c o m m u n i c a t i o n stages (publishing, teaching, popularizing, etc.) but already at the primitive stage of invention. W o r d s are the basic tools of t h o u g h t - even in m a t h e m a t i c a l physics. N o w , words are rarely, if ever, i n v e n t e d ex nihilo. T h e y are b o r r o w e d f r o m o t h e r fields, o t h e r languages: their m e a n i n g is displaced and stretched to fit their new context. A novel t h e o r y , w h e n it appears, c a n n o t but use old words to label new concepts. In some cases, the extension in m e a n ing thus c o n f e r r e d to standard terminology is natural e n o u g h so that the transfer m a y not lead to too m a n y misunderstandings; an example is * Postal address: Laboratoire de Physique Th6orique, Parc Valrose, Universit6 de Nice, 06034-Nice, France
given by the use of " e n e r g y " in Einsteinian mechanics as b o r r o w e d f r o m N e w t o n i a n mechanics (but " m a s s " , in the same case, already gives rise to interesting problems). Most often, h o w e v e r , and especially w h e n the conceptual gap b e t w e e n the old and the new t h e o r y is a wide one, a casual transfer of t e r m i n o l o g y m a y lead to epistemological and pedagogical difficulties (destructive interferences, so to speak...), would it be only by hiding the gap u n d e r the carpet of an apparently continuous theoretical discourse. This situation has b e e n and still is particularly serious in q u a n t u m theory. H e r e , the careless use of words t a k e n f r o m classical p h y s i c s - such as q u a n t u m " m e c h a n i c s " , " u n c e r t a i n t y " , e t c . - , is c o m p o u n d e d by the uncritical use of interpretative terms linked to a definite, if implicit, philosophical point of v i e w - s u c h as " c o m p l e m e n t a r i t y " , " w a v e - p a r t i c l e duality", " o b servables", etc. While these words and the ideas they represent have played a m a j o r role in the birth of q u a n t u m physics m o r e than half a century ago, they are no longer necessarily the best ones to be used to-day; after all, " i m p e t u s " and "vis viva", despite their noble services to the
0378-4363 / 88 / $03.50 (~) Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
J.-M. L~vy-Leblond / Quantum physics and language
cause of classical mechanics, have been allowed to retire in the quiet realm of science history. It is not argued here that we should start afresh and create from scratch a supposedly adequate vocabulary for quantum physics. Abuse of language, I repeat, certainly is unavoidable in science as it is in any human communication; without it, language would not live and evolve. But, at the very least, let us recognize' it for what it is, so that it does not add its troubles to already complicated issues. And in some definite instances, still, a willing effort to replace specially ambiguous words might be worthwhile, helping 1) to clarify the conceptual issues still open and debated, 2) to improve the communication of the fundamental ideas of quantum physics, in teaching and popularizing [1]. Let us consider some examples.
2. "Quantum mechanics"? As a first example, consider the usual name we give to quantum theory itself, at least in its elementary form, namely, "quantum mechanics". It is a splendid oxymoron, since the advent of the quantum era precisely marked the end of mechanical physics. Mechanics in effect started as the science of machines, that is, solid and concrete pieces of matter, and generalized to a view of the world as built up of such hard and definite parts. Most tenets of this mechanistic view of the world had to be abandoned under the impact of the quantum ideas. Of course, the new theory had to be built upon the ruins of the old ones and made use of their rubble, relying on the heuristic Correspondence Principle as a guide. Thus were born provisional and partial constructs such as "matrix mechanics" or "wave mechanics", exhibiting in their very name the painful condition of their birth. But, in the same way as grown-ups do not usually show up their navel, or at least do not draw undue attention to it, our present-day quantum theory has no longer any reason to exhibit its mechanical birth scars. I contend that the reluctance to sever this umbili-
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cal chord is partly responsible for some of the difficulties we meet in teaching or popularizing our ideas. Indeed, the wording of "quantum mechanics" carries with it the idea that it consists of mechanics (the usual, classical one) plus some quantum seasoning. It does not put into full light the fundamental fact which is to be stressed, namely, its essential difference with classical mechanics. It is true that some structural analogies between the two theories enable some to try a seemingly painless way to teach quantum mechanics- through the Hamilton-Jacobi route for instance. My own experience is that this is a rather treacherous method, by which the student is led into a foreign country without knowing when and where he has crossed the border; no wonder he becomes lost so easily. I hold it better to warn him loud and clear that he takes foot indeed in a strange and new territory-which deserves a new and specific name. Furthermore, the term "quantum mechanics" unduly stresses but one of the two aspects of classical physics to which quantum theory is related. For, besides the mechanics of point particles, the field theory of waves also played an important historical role in the building of quantum t h e o r y - and both may describe classical approximations to quantum behaviour. How then, should we call the whole quantum theoretical field? I propose a simple and natural solution; in the same way as most fields of physics go by names such as mechanics, thermodynamics, acoustics, etc., why not speak here of "quantics", thus dropping the cumbersome mechanical reference? By the way, this proposal I borrowed from the spontaneous practice of my Students.
3. "Waves/particles" A second important case is the term we use to denote the quantum entities themselves. Here again, for historical reasons, linked mainly to experimental practice this time, we commonly speak of "particles", and feel obliged to emphasize "quantum particles" if we want to explicitly distinguish them from the classical, mass-point, particles. And to stress their quantum nature we
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make lengthy comments upon their double aspect, wavelike and corpuscular, with rather obscure and/or trivial developments upon the "complementarity" of these two pictures. But this so-called "wave-particle duality" either leads to the fallacious view that an electron, for example,/s sometimes a wave, sometimes a partide, or to the weak and insufficient view that it looks sometimes like a wave, sometimes like a particle. While this is true in certain special circumstances, most of the times it looks neither like a wave, nor like a particle - since it is neither one, nor the other [2]. The time has largely come to endow the great number of quantum entities that we know to-day with a common specific name. Here again, there is a simple and natural solution, making use of the accepted standard desinence of most particular names, such as photons, protons, pions, phonons, neutrons, etc. Let us call all of them quantons (as suggested already by M. Bunge). In this wording, instead of the vague and misleading " w a v e particle duality", we may assert an objective statement of the link between quantum and classical physics: "under certain specific circumstances, a quanton may approximately behave either as a classical particle, or as a classical wave".
4. "Observables"? My third example is more specific, and consists not in advocating a neologism, but in indicting a standard terminology. The accused is the term "observable", used in quantum t h e o r y - p a r d o n me, in quantics - as a generic term for the physical properties of a quanton, namely, its position, momentum, energy, etc. The origin of this terminology, as is well known, lies in the positivistic spirit of the Copenhagen school with its tenet of introducing in a physical theory only directly observable quantities. Now, if this requirement played a major heuristic role as a scaffolding to build the new quantum theory, it cannot be held as a solid epistemological principle. Indeed, the very essence of any theory is to define and relate conceptual constructs which are abstracted from
empirical reality and thus cannot be "directly observed". An elementary example of this statement is given by the common use in physics (whether it be classical or quantum) of the (badly named!) "real (non-rational) n u m b e r s " - w h i c h , of course, no experimentalist will ever obtain as a numerical result. A little thinking may convince oneself that position, momentum, etc. of a quanton are, as such, not less observable than those of a classical particle. By the way, it is a nice little irony of history that Heisenberg, in the very same paper where he introduced matrix mechanics by stressing the role of observable quantities, did it by arguing about the unobservability of the electron's p o s i t i o n - a bona fide observable to-day [3] .... Conversely, let us recall that to name "observable" any Hermitian operator is quite preposterous in view of the small number of properties actually open to experimental observation. The specific quantum nature of the physical properties lies not in their observability, but in their spectrum properties, their possible incompatibility (that is, noncommutativity), etc. It does not seem worthwhile, then, to use other words than "physical properties", or "physical m a g n i t u d e s " - w i t h the epithet " q u a n t u m " , if necessary. We may perhaps regret that common terminology has followed Heisenberg and Bohr rather than Dirac, whose "c-numbers" and "q-numbers" offered a simple and clear alternative.
5. "Many-universes"? My fourth and last example consists also in a critical analysis of a more recent terminology, introduced in the context of an heterodox point of view on quantum theory. I want to speak of the so-called "many-worlds", or "manyuniverses interpretation". First, to set clear the historical record, let me emphasize that it is not to be identified with the Everett interpretation, since Everett himself never used such a terminology, neither in his celebrated paper, nor in his less known but most interesting thesis [4]. In fact, the term was, to my knowledge, introduced by B. De Witt in his efforts to promote Everett's
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views [5]. Indeed, it constituted a brilliant advertising idea and played an important role to spread these views, within the physics community, but also outside, in the press, science-fiction literature and scientifico-mystical sects. Unfortunately, this wording contributed to the confusion as well as to the diffusion of the message... Here is not the place to develop an assessment of Everett's views. Let me only say that his main point is to stress that the so-called "reduction of the wave-packet" (by the way, this also is an example of poor terminology) is not a necessity. One reaches a consistent point of view by refusing this "collapse of the state vector" (better !) after a measurement and keeping all its components - suitably coupled to states of the measuring apparatus. By interpreting these coupled components as separated individual state vectors of the universe, one may pretend to view the global state vector as describing a collection of universes which the initial state has branched in. But, precisely, there are no independent branches! The confusion, here, comes from interpreting the quantum superposition (vector addition) as a classical collection (ensemble). Everett, on the contrary, stresses the unicity of the world's vector state. Far from there being many universes, there is but a single o n e - b u t it is a quantum world [6]. The flimsy "many worlds" terminology has been most unfair to Everett's ideas, by giving them a simplistic, unconvincing and provocative look. This example shows clearly that terminological questions are not super-
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ficial and formal questions, second to deep conceptual problems, but partake of their very nature.
6. Conclusion
The use of language leads to and requires abuse of language; let us try not to misuse it. Or, in other words, since there is necessarily interference between language and physics, let us make it as constructive as possible.
References [1] As a matter of fact, most of the present ideas have emerged during the writing of a textbook in quantum physics, which aim to some originality from the linguistic and conceptual points of view (and from the experimental one as well, introducing in this book many of the modern and beautiful experiments described in this workshop), J.-M. L6vy-Leblond and F. Balibar, Quantique (Rudiments) (Inter6ditions-CNRS, Paris, 1984) [English translation to be published by North-Holland]. [2] See J.-M. L6vy-Leblond, Int. J. Qu. Chem. 12, suppl. 1, (1977) 415; Eur. J. Phys. 2 (1981) 44. [3] W. Heisenberg, Z. Phys. 33 (1925) 879. [4] H. Everett, Rev. Mod. Phys. 29 (1957) 454 and Ph. D. thesis, both reprinted in ref. [5]. [5] The Many-Worlds Interpretation of Quantum Mechanics, B.S. De Witt and N. Graham, eds. (Princeton Univ. Press, Princeton, 1973). [6] See, for instance, E.J. Squires, Eur. J. Phys. 8 (1987) 171.
DISCUSSION (Q) H. Bernstein: Bohr often used to say we can only use classical terms to discuss physics. Your position seems to deny this basic tenet (or dogma) of the Copenhagen interpretation. Do you follow up, beyond the introductory remarks here, to propose a whole new interpretation of quantum theory? (A) J.M. L~vy-Leblond: I do agree with Bohr at least on one point, namely, the importance of language - more precisely, of a reflective attitude towards the language we use in "talking physics". Now, of course, Bohr and his fellows could not use another language than that of classical physics, since no other physics yet existed! But I feel he was wrong to erect this idea as an absolute statement. After all, even his own classical terms did not exist one century before him (think of
"electromagnetic field", for instance...), while they had by then become natural for him. In the same way, many quantum ideas have become natural to us - despite the remaining obscurities - and we have certainly developed a sort of (educated) "quantum common sense". It suffices to listen to experimentalists in this workshop to be convinced that they "feel" quantum behavior. Is it not then natural to choose the better words for speaking up this inner understanding? Would Bohr have accepted to talk about field theory exclusively in mechanical terms? By drawing attention to this question of language, 1 do not wish to introduce a new "interpretation". Quite on the contrary, 1 do believe that we must accept quantum theory as it is without extraneous assumptions; in this endeavour, we may be helped by using a more adequate vocabulary.
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(C) J.F. Clauser: I would like to add to your excellent list my favorite misused word - m e a s u r e m e n t . It is really the ego of the experimenter which allows him to believe he has measured what he claims to have. He does an experiment and gets a result. T h e rest usually involves m u c h preconception to fully convince one's self that something in particular was measured. (A) J.M. Ldvy-Leblond: I agree with you. A s a matter of fact, it has always struck me that we pretend to have a "theory of m e a s u r e m e n t " in q u a n t u m physics, which, in fact, boils down to an (admittedly) efficient empirical recipe (the so-called "collapse of the wave vector") - it is the ego of the theorist which allows him to believe he understands what the experimenter does. Anyway, why is there no classical theory of m e a s u r e m e n t ? A little thinking suffices to convince oneself that, even in classical physics, the connection between the theoretical formalism and the experimental results is by no m e a n s a trivial o n e - which does not seem to bother us too much. (C) S. lida: I think that there are so m a n y inadequate concepts, or incorrect concepts in physics. For instance, F e y n m a n said that electromagnetism has m a n y inconsistencies. In the present electromagnetism, there are. But, as presented in my paper on the new frames in physics, we have m a d e m a n y efforts to reorganize them. As a result, the new frames have been established and there is no inconsistency, e.g., in the new frame of electromagnetism. Further, to separate the q u a n t u m physics into two, c-number physics and q - n u m b e r physics, eliminates several of the ambiguous concepts.
(A) J.M. L~vy-Leblond: No comment. (Q) E.H. Walker: I am concerned about this idea of introducing new terms. I am afraid we have already seen the introduction of beaucoup new terms that always seem to turn out to be a new way to hide a n t h r o p o m o r p h i s m s - introduce the observer in a privileged role without having to m a k e it explicit what has been done. At present all of the interpretations that I know about do t h i s - m a k e this same error. T h e term " m e a s u r e m e n t " , for example, is never introduced as a definite, distinct physical interaction, but only as a m e a n s of hiding an a n t h r o p o m o r p h i s m . W h a t we need is not new terms, but a clearer and simpler use of terms. If we need to speak of " m e a s u r e m e n t " or of " t h e observer", let us make c l e a r - clear in our e q u a t i o n s - what it is that distinguishes m e a s u r e m e n t or the observer from other kinds of interactions. T h a t is what needs to be done to resolve the measurement problem. (A) J.M. Ldvy-Leblond: I am definitely not advocating a novel terminology as such and my examples show that I would rather refuse or criticize admitted terms than to invent new ones. " Q u a n t i c s " and " q u a n t o n s " I propose because they seem to simplify our way of speaking and make our thinking more consistent. But I do not view them as fancy and arbitrary t e r m s - r a t h e r as natural modifications or generalizations of c o m m o n parlance. Concerning specifically the m e a s u r e m e n t problem, I do agree that what we need are specific and concrete models of q u a n t u m m e a s u r e m e n t processes; fortunately, there have been some successful attempts in that direction (Hepp, Bell, Cini, etc.).
Q U A N T U M P H Y S I C S AND L A N G U A G E Jean-Marc LI~VY-LEBLOND Physique Thdorique, Nice* A novel theory, when it appears, cannot but use old words to label new concepts. In some cases, the extension in meaning thus conferred to standard terminology is natural enough so that the transfer may not lead to too many misunderstandings. Most often, however, and especially when the conceptual gap between the old and the new theory is a wide one, a casual transfer of t~rminology may lead to epistemological and pedagogical difficulties. This situation has been and still is particularly serious in quantum theory. Here, the careless use of words taken from classical physics - such as quantum "mechanics", "uncertainty", e t c . - , is compounded by the uncritical use of interpretative terms linked to a definite, if implicit, philosophical point of view - such as "complementarity", "wave-particle duality", "observables", etc. While these words and the ideas they represent have played a major role in the birth of quantum physics more than half a century ago, they are no longer necessarily the best ones to be used today. It is not argued here that we should start afresh and create from scratch a supposedly adequate vocabulary for quantum physics. Abuse of language certainly is unavoidable in science as it is in any human communication; without it, language would not live and evolve. But, at the very least, let us recognize it for what it is, so that it does not add its troubles to already complicated issues. And in some definite instances, still, a willing effort to replace specially ambiguous words might be worthwhile.
1. Introduction It is a neglected type of interference I w a n t to discuss here: interference b e t w e e n language and physics. Theoretical physics, p e r m e a t e d as it is by m a t h e m a t i c a l formalism, does not consist only of formulas, symbols and letters. It uses words, and c a n n o t but be s p o k e n and written t h r o u g h ordinary language. This is true not only for the c o m m u n i c a t i o n stages (publishing, teaching, popularizing, etc.) but already at the primitive stage of invention. W o r d s are the basic tools of t h o u g h t - even in m a t h e m a t i c a l physics. N o w , words are rarely, if ever, i n v e n t e d ex nihilo. T h e y are b o r r o w e d f r o m o t h e r fields, o t h e r languages: their m e a n i n g is displaced and stretched to fit their new context. A novel t h e o r y , w h e n it appears, c a n n o t but use old words to label new concepts. In some cases, the extension in m e a n ing thus c o n f e r r e d to standard terminology is natural e n o u g h so that the transfer m a y not lead to too m a n y misunderstandings; an example is * Postal address: Laboratoire de Physique Th6orique, Parc Valrose, Universit6 de Nice, 06034-Nice, France
given by the use of " e n e r g y " in Einsteinian mechanics as b o r r o w e d f r o m N e w t o n i a n mechanics (but " m a s s " , in the same case, already gives rise to interesting problems). Most often, h o w e v e r , and especially w h e n the conceptual gap b e t w e e n the old and the new t h e o r y is a wide one, a casual transfer of t e r m i n o l o g y m a y lead to epistemological and pedagogical difficulties (destructive interferences, so to speak...), would it be only by hiding the gap u n d e r the carpet of an apparently continuous theoretical discourse. This situation has b e e n and still is particularly serious in q u a n t u m theory. H e r e , the careless use of words t a k e n f r o m classical p h y s i c s - such as q u a n t u m " m e c h a n i c s " , " u n c e r t a i n t y " , e t c . - , is c o m p o u n d e d by the uncritical use of interpretative terms linked to a definite, if implicit, philosophical point of v i e w - s u c h as " c o m p l e m e n t a r i t y " , " w a v e - p a r t i c l e duality", " o b servables", etc. While these words and the ideas they represent have played a m a j o r role in the birth of q u a n t u m physics m o r e than half a century ago, they are no longer necessarily the best ones to be used to-day; after all, " i m p e t u s " and "vis viva", despite their noble services to the
0378-4363 / 88 / $03.50 (~) Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
J.-M. L~vy-Leblond / Quantum physics and language
cause of classical mechanics, have been allowed to retire in the quiet realm of science history. It is not argued here that we should start afresh and create from scratch a supposedly adequate vocabulary for quantum physics. Abuse of language, I repeat, certainly is unavoidable in science as it is in any human communication; without it, language would not live and evolve. But, at the very least, let us recognize' it for what it is, so that it does not add its troubles to already complicated issues. And in some definite instances, still, a willing effort to replace specially ambiguous words might be worthwhile, helping 1) to clarify the conceptual issues still open and debated, 2) to improve the communication of the fundamental ideas of quantum physics, in teaching and popularizing [1]. Let us consider some examples.
2. "Quantum mechanics"? As a first example, consider the usual name we give to quantum theory itself, at least in its elementary form, namely, "quantum mechanics". It is a splendid oxymoron, since the advent of the quantum era precisely marked the end of mechanical physics. Mechanics in effect started as the science of machines, that is, solid and concrete pieces of matter, and generalized to a view of the world as built up of such hard and definite parts. Most tenets of this mechanistic view of the world had to be abandoned under the impact of the quantum ideas. Of course, the new theory had to be built upon the ruins of the old ones and made use of their rubble, relying on the heuristic Correspondence Principle as a guide. Thus were born provisional and partial constructs such as "matrix mechanics" or "wave mechanics", exhibiting in their very name the painful condition of their birth. But, in the same way as grown-ups do not usually show up their navel, or at least do not draw undue attention to it, our present-day quantum theory has no longer any reason to exhibit its mechanical birth scars. I contend that the reluctance to sever this umbili-
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cal chord is partly responsible for some of the difficulties we meet in teaching or popularizing our ideas. Indeed, the wording of "quantum mechanics" carries with it the idea that it consists of mechanics (the usual, classical one) plus some quantum seasoning. It does not put into full light the fundamental fact which is to be stressed, namely, its essential difference with classical mechanics. It is true that some structural analogies between the two theories enable some to try a seemingly painless way to teach quantum mechanics- through the Hamilton-Jacobi route for instance. My own experience is that this is a rather treacherous method, by which the student is led into a foreign country without knowing when and where he has crossed the border; no wonder he becomes lost so easily. I hold it better to warn him loud and clear that he takes foot indeed in a strange and new territory-which deserves a new and specific name. Furthermore, the term "quantum mechanics" unduly stresses but one of the two aspects of classical physics to which quantum theory is related. For, besides the mechanics of point particles, the field theory of waves also played an important historical role in the building of quantum t h e o r y - and both may describe classical approximations to quantum behaviour. How then, should we call the whole quantum theoretical field? I propose a simple and natural solution; in the same way as most fields of physics go by names such as mechanics, thermodynamics, acoustics, etc., why not speak here of "quantics", thus dropping the cumbersome mechanical reference? By the way, this proposal I borrowed from the spontaneous practice of my Students.
3. "Waves/particles" A second important case is the term we use to denote the quantum entities themselves. Here again, for historical reasons, linked mainly to experimental practice this time, we commonly speak of "particles", and feel obliged to emphasize "quantum particles" if we want to explicitly distinguish them from the classical, mass-point, particles. And to stress their quantum nature we
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make lengthy comments upon their double aspect, wavelike and corpuscular, with rather obscure and/or trivial developments upon the "complementarity" of these two pictures. But this so-called "wave-particle duality" either leads to the fallacious view that an electron, for example,/s sometimes a wave, sometimes a partide, or to the weak and insufficient view that it looks sometimes like a wave, sometimes like a particle. While this is true in certain special circumstances, most of the times it looks neither like a wave, nor like a particle - since it is neither one, nor the other [2]. The time has largely come to endow the great number of quantum entities that we know to-day with a common specific name. Here again, there is a simple and natural solution, making use of the accepted standard desinence of most particular names, such as photons, protons, pions, phonons, neutrons, etc. Let us call all of them quantons (as suggested already by M. Bunge). In this wording, instead of the vague and misleading " w a v e particle duality", we may assert an objective statement of the link between quantum and classical physics: "under certain specific circumstances, a quanton may approximately behave either as a classical particle, or as a classical wave".
4. "Observables"? My third example is more specific, and consists not in advocating a neologism, but in indicting a standard terminology. The accused is the term "observable", used in quantum t h e o r y - p a r d o n me, in quantics - as a generic term for the physical properties of a quanton, namely, its position, momentum, energy, etc. The origin of this terminology, as is well known, lies in the positivistic spirit of the Copenhagen school with its tenet of introducing in a physical theory only directly observable quantities. Now, if this requirement played a major heuristic role as a scaffolding to build the new quantum theory, it cannot be held as a solid epistemological principle. Indeed, the very essence of any theory is to define and relate conceptual constructs which are abstracted from
empirical reality and thus cannot be "directly observed". An elementary example of this statement is given by the common use in physics (whether it be classical or quantum) of the (badly named!) "real (non-rational) n u m b e r s " - w h i c h , of course, no experimentalist will ever obtain as a numerical result. A little thinking may convince oneself that position, momentum, etc. of a quanton are, as such, not less observable than those of a classical particle. By the way, it is a nice little irony of history that Heisenberg, in the very same paper where he introduced matrix mechanics by stressing the role of observable quantities, did it by arguing about the unobservability of the electron's p o s i t i o n - a bona fide observable to-day [3] .... Conversely, let us recall that to name "observable" any Hermitian operator is quite preposterous in view of the small number of properties actually open to experimental observation. The specific quantum nature of the physical properties lies not in their observability, but in their spectrum properties, their possible incompatibility (that is, noncommutativity), etc. It does not seem worthwhile, then, to use other words than "physical properties", or "physical m a g n i t u d e s " - w i t h the epithet " q u a n t u m " , if necessary. We may perhaps regret that common terminology has followed Heisenberg and Bohr rather than Dirac, whose "c-numbers" and "q-numbers" offered a simple and clear alternative.
5. "Many-universes"? My fourth and last example consists also in a critical analysis of a more recent terminology, introduced in the context of an heterodox point of view on quantum theory. I want to speak of the so-called "many-worlds", or "manyuniverses interpretation". First, to set clear the historical record, let me emphasize that it is not to be identified with the Everett interpretation, since Everett himself never used such a terminology, neither in his celebrated paper, nor in his less known but most interesting thesis [4]. In fact, the term was, to my knowledge, introduced by B. De Witt in his efforts to promote Everett's
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views [5]. Indeed, it constituted a brilliant advertising idea and played an important role to spread these views, within the physics community, but also outside, in the press, science-fiction literature and scientifico-mystical sects. Unfortunately, this wording contributed to the confusion as well as to the diffusion of the message... Here is not the place to develop an assessment of Everett's views. Let me only say that his main point is to stress that the so-called "reduction of the wave-packet" (by the way, this also is an example of poor terminology) is not a necessity. One reaches a consistent point of view by refusing this "collapse of the state vector" (better !) after a measurement and keeping all its components - suitably coupled to states of the measuring apparatus. By interpreting these coupled components as separated individual state vectors of the universe, one may pretend to view the global state vector as describing a collection of universes which the initial state has branched in. But, precisely, there are no independent branches! The confusion, here, comes from interpreting the quantum superposition (vector addition) as a classical collection (ensemble). Everett, on the contrary, stresses the unicity of the world's vector state. Far from there being many universes, there is but a single o n e - b u t it is a quantum world [6]. The flimsy "many worlds" terminology has been most unfair to Everett's ideas, by giving them a simplistic, unconvincing and provocative look. This example shows clearly that terminological questions are not super-
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ficial and formal questions, second to deep conceptual problems, but partake of their very nature.
6. Conclusion
The use of language leads to and requires abuse of language; let us try not to misuse it. Or, in other words, since there is necessarily interference between language and physics, let us make it as constructive as possible.
References [1] As a matter of fact, most of the present ideas have emerged during the writing of a textbook in quantum physics, which aim to some originality from the linguistic and conceptual points of view (and from the experimental one as well, introducing in this book many of the modern and beautiful experiments described in this workshop), J.-M. L6vy-Leblond and F. Balibar, Quantique (Rudiments) (Inter6ditions-CNRS, Paris, 1984) [English translation to be published by North-Holland]. [2] See J.-M. L6vy-Leblond, Int. J. Qu. Chem. 12, suppl. 1, (1977) 415; Eur. J. Phys. 2 (1981) 44. [3] W. Heisenberg, Z. Phys. 33 (1925) 879. [4] H. Everett, Rev. Mod. Phys. 29 (1957) 454 and Ph. D. thesis, both reprinted in ref. [5]. [5] The Many-Worlds Interpretation of Quantum Mechanics, B.S. De Witt and N. Graham, eds. (Princeton Univ. Press, Princeton, 1973). [6] See, for instance, E.J. Squires, Eur. J. Phys. 8 (1987) 171.
DISCUSSION (Q) H. Bernstein: Bohr often used to say we can only use classical terms to discuss physics. Your position seems to deny this basic tenet (or dogma) of the Copenhagen interpretation. Do you follow up, beyond the introductory remarks here, to propose a whole new interpretation of quantum theory? (A) J.M. L~vy-Leblond: I do agree with Bohr at least on one point, namely, the importance of language - more precisely, of a reflective attitude towards the language we use in "talking physics". Now, of course, Bohr and his fellows could not use another language than that of classical physics, since no other physics yet existed! But I feel he was wrong to erect this idea as an absolute statement. After all, even his own classical terms did not exist one century before him (think of
"electromagnetic field", for instance...), while they had by then become natural for him. In the same way, many quantum ideas have become natural to us - despite the remaining obscurities - and we have certainly developed a sort of (educated) "quantum common sense". It suffices to listen to experimentalists in this workshop to be convinced that they "feel" quantum behavior. Is it not then natural to choose the better words for speaking up this inner understanding? Would Bohr have accepted to talk about field theory exclusively in mechanical terms? By drawing attention to this question of language, 1 do not wish to introduce a new "interpretation". Quite on the contrary, 1 do believe that we must accept quantum theory as it is without extraneous assumptions; in this endeavour, we may be helped by using a more adequate vocabulary.
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(C) J.F. Clauser: I would like to add to your excellent list my favorite misused word - m e a s u r e m e n t . It is really the ego of the experimenter which allows him to believe he has measured what he claims to have. He does an experiment and gets a result. T h e rest usually involves m u c h preconception to fully convince one's self that something in particular was measured. (A) J.M. Ldvy-Leblond: I agree with you. A s a matter of fact, it has always struck me that we pretend to have a "theory of m e a s u r e m e n t " in q u a n t u m physics, which, in fact, boils down to an (admittedly) efficient empirical recipe (the so-called "collapse of the wave vector") - it is the ego of the theorist which allows him to believe he understands what the experimenter does. Anyway, why is there no classical theory of m e a s u r e m e n t ? A little thinking suffices to convince oneself that, even in classical physics, the connection between the theoretical formalism and the experimental results is by no m e a n s a trivial o n e - which does not seem to bother us too much. (C) S. lida: I think that there are so m a n y inadequate concepts, or incorrect concepts in physics. For instance, F e y n m a n said that electromagnetism has m a n y inconsistencies. In the present electromagnetism, there are. But, as presented in my paper on the new frames in physics, we have m a d e m a n y efforts to reorganize them. As a result, the new frames have been established and there is no inconsistency, e.g., in the new frame of electromagnetism. Further, to separate the q u a n t u m physics into two, c-number physics and q - n u m b e r physics, eliminates several of the ambiguous concepts.
(A) J.M. L~vy-Leblond: No comment. (Q) E.H. Walker: I am concerned about this idea of introducing new terms. I am afraid we have already seen the introduction of beaucoup new terms that always seem to turn out to be a new way to hide a n t h r o p o m o r p h i s m s - introduce the observer in a privileged role without having to m a k e it explicit what has been done. At present all of the interpretations that I know about do t h i s - m a k e this same error. T h e term " m e a s u r e m e n t " , for example, is never introduced as a definite, distinct physical interaction, but only as a m e a n s of hiding an a n t h r o p o m o r p h i s m . W h a t we need is not new terms, but a clearer and simpler use of terms. If we need to speak of " m e a s u r e m e n t " or of " t h e observer", let us make c l e a r - clear in our e q u a t i o n s - what it is that distinguishes m e a s u r e m e n t or the observer from other kinds of interactions. T h a t is what needs to be done to resolve the measurement problem. (A) J.M. Ldvy-Leblond: I am definitely not advocating a novel terminology as such and my examples show that I would rather refuse or criticize admitted terms than to invent new ones. " Q u a n t i c s " and " q u a n t o n s " I propose because they seem to simplify our way of speaking and make our thinking more consistent. But I do not view them as fancy and arbitrary t e r m s - r a t h e r as natural modifications or generalizations of c o m m o n parlance. Concerning specifically the m e a s u r e m e n t problem, I do agree that what we need are specific and concrete models of q u a n t u m m e a s u r e m e n t processes; fortunately, there have been some successful attempts in that direction (Hepp, Bell, Cini, etc.).