A paradox, according to dictionaries, is a surprising but perhaps true statement. Copernicus's heliocentrism was once a paradox. The problem with a true paradox is not in reducing it but in expressing it--as Einstein did with his relativity theory expressing the physical content of the Lorentz-Poincare' formulas of electrodynamics. Today again we have good formulas, those of quantum mechanics, but have not yet understood all their meaning. It is the paradox uncovered by Einstein at the 1927 Solvay Conference (and better known as the Einstein-Podolsky-Rosen [E.P.R.) paradox) which forces us to a dramatic revision of traditional ways of thinking about time. The paradox emerges with special clarity from a conceptual experiment in which two spatially separated systems on the quantum scale are made to interact, but the only causal link between them is by way of an event in their common past. The paradox can be expressed in terms of the formal (de jure) symmetry between retarded and advanced interactions in physical theory, interactions which affect the future and the past respectively, independently of the de facta preponderance in nature of retarded interactions. The possibility that advanced interactions may occur de facta in large-scale systems is briefly discussed. * Lecture delivered at the Third Conference of the International Society for the Study of Time, Alpbach, July 1-10 (1976).
A paradox, according to dictionaries, is a surprising but perhaps true statement. Copernicus's heliocentrism was once a paradox. The problem with a true paradox is not in reducing it but in expressing it--as Einstein did with his relativity theory expressing the physical content of the Lorentz-Poincare' formulas of electrodynamics. Today again we have good formulas, those of quantum mechanics, but have not yet understood all their meaning. It is the paradox uncovered by Einstein at the 1927 Solvay Conference (and better known as the Einstein-Podolsky-Rosen [E.P.R.) paradox) which forces us to a dramatic revision of traditional ways of thinking about time. The paradox emerges with special clarity from a conceptual experiment in which two spatially separated systems on the quantum scale are made to interact, but the only causal link between them is by way of an event in their common past. The paradox can be expressed in terms of the formal (de jure) symmetry between retarded and advanced interactions in physical theory, interactions which affect the future and the past respectively, independently of the de facta preponderance in nature of retarded interactions. The possibility that advanced interactions may occur de facta in large-scale systems is briefly discussed. * Lecture delivered at the Third Conference of the International Society for the Study of Time, Alpbach, July 1-10 (1976).
The difficulties of the old quantum theory were largely dispelled by the development of quantum mechanics in the 1920s. First Heisenberg (1925) developed the rather abstract matrix mechanics and soon after Schro¨dinger (1926; see also Eckart, 1926) developed the physically more appealing wave mechanics. Subsequently he showed them to be mathematically equivalent. The physicists were greatly relieved; now they had once again a mathematical formalism that enabled the results of experiments to be calculated. It was possible to move forward. It is certainly a very elegant and indeed beautiful formalism, and does enable the results of many physical measurements to be calculated with great accuracy. It is used daily by atomic and nuclear physicists; there is no other way to make calculations. From the earliest days there has been intense debate about the interpretation of quantum mechanics, and it shows no signs of being settled; if anything it is becoming more intense over the years. As Penrose has remarked, no one understands quantum mechanics. Feynman (1967; see also Sykes, 1994) also admitted that ‘nobody understands quantum mechanics’, and Einstein once remarked: ‘I have thought a hundred times as much about the quantum problems as I have about general relativity theory’ (Pais, 1994, p. 57). It should therefore be evident that the most that can be done here is to try to show where the real difficulties lie, and how the failure to recognize that quantum mechanics is an essentially statistical theory has led to a series of confusing quantum paradoxes. It is just these paradoxes that have often been used to support theological speculations. It is useful to ask what ‘understanding quantum mechanics’ actually means. Certainly it means being able to calculate the results of many measurements. The same applies to Newton’s theory of gravitation, which we understand in the sense that we can use it as a basis of calculation, although we do not really understand the gravitational force itself. Very often, especially in modern physics, the mathematical formalism does not give us the sort of intuitive physical feeling for what is going on that we often obtain from classical mechanics. Perhaps we can never achieve more than this for quantum mechanics. In classical mechanics we can assume that a property that is measured objectively exists prior to the interaction of the measuring apparatus with the observed system. Quantum mechanics, however, ‘is incompatible with the proposition that measurement discovers some unknown but pre-existing property’ (Peres, 1997, p. 14). There is an important distinction between mysteries and absurdities. We may be forced to live with mysteries, but there is no reason why we should put up with absurdities such as the wave-particle duality, tunnelling through potential barriers, acausal events and the collapse of the wave function. These are the quantum paradoxes that will be discussed later on. They arise within the Copenhagen interpretation of quantum mechanics and are solved by other interpretations. These alternative interpretations enable us to make conjectures about what is going on, even though we cannot understand it in detail. This is acceptable and a stimulus to further research. It is certainly difficult enough to understand quantum mechanics, but there is no need to try to visualize logical absurdities in the effort to do so. Quantum mechanics is a mathematical formalism that makes it possible to calculate the results of some experiments. Anything more than this, such as questions about the interpretation of what is measured and whether it tells us what is actually going on in the system being studied, belongs to philosophy. Thus we are concerned here with what is essentially a philosophical debate; if it were just a matter of physics the debate would have been settled long ago. One of the difficulties is that it invokes the results of experiments and uses the language of quantum mechanics. Inevitably this language uses philosophical terms and then it is easy to give the impression that the philosophical propositions are in some way entailed by the physical results. These in turn can influence theological thinking. In order to be quite clear which arguments are justified and which are not, it is essential to separate the three levels of discourse: physical, philosophical and theological. Our task is to relate these together, and this immediately raises the traditional problems of the philosophy of science, including the relation of experiment to theory, and of theory to explanation. We begin by discussing experiments, distinguishing between those that are real and those that are imaginary.
ASPECTS OF THE DEBATE The debate between Niels Bohr and Albert Einstein concerning the interpretation of quantum mechanics extended from the fifth Solvay Conference in 1927 until the end of Einstein's life. The most dramatic exchange occurred in 1935, when Einstein, in collaboration with B. Podolsky and N. Rosen, published a paper in Physical Review entitled “Can quantum-mechanical description of physical reality be considered complete?”, concluding with a negative answer, and Bohr replied in the same journal, with a paper of the same title but giving a positive answer. Their arguments were restated, in some respects with greater clarity, in the Library of Living Philosophers volume on Einstein in 1949. The disagreements between Bohr and Einstein concerned not only the physical question expressed in the common title of their papers, but also philosophical questions about physical reality and human knowledge. Much light was thrown upon the Bohr–Einstein debate by the theorem of J. S. Bell (1964) and the experiments which it inspired. Bell showed that any physical theory which applies to a pair of spatially separated systems (as considered in the thought experiment of Einstein, Podolsky, and Rosen) and satisfies a certain locality condition will, in certain circumstances, disagree statistically with quantum mechanics. Consequently, the supplementation of the quantum mechanical description envisaged by Einstein and his collaborators (usually referred to as a “hidden variables theory”) must either violate the locality condition or clash with quantum mechanical predictions.
As Quine wrote in 1961, what strikes us in a paradox is its initial air of absurdity, which develops into a sort of psychological discomfort when we compare the conclusions of the reasoning with the apparently irrefutable arguments on which it is based. However, as he went on to observe, ‘More than once in history the discovery of paradox has been the occasion for major reconstruction at the foundations of thought’. Catastrophe may therefore lurk even in the most innocent-seeming of paradoxes and force us to recognize the arbitrary nature ‘of a buried premise or of some preconception previously reckoned as central to physical theory, to mathematics or to the thinking process’. More recently, ideas of this kind have been used to assert in general that the role of paradox in the growth of knowledge is to generate category switches and to construct new universes of discourse. It could also be argued that paradoxes constitute elements of the logic of discovery capable of amplifying and thus making intelligible the still obscure phases accompanying paradigm shifts. Paradoxes would thus represent points of accumulation for the tension created between a given system of conceptual representation and a theory or empirical discovery that violates it. Historians of science have tried different interpretative approaches in their attempts to unravel the thought process that was to lead Einstein in 1905 to the formulation of special relativity and the simultaneous demolition of the classical conception of space and time.
The Nobel Prize in Physics 2022 was awarded to three experiment physicists: Alain Aspect, John F. Clauser and Anton Zeilinger, for their contributions to “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this paper, the history, basics, and applications related with the Bell inequalities are reviewed. The discussion of fundamental issues on quantum mechanics dated back to the year 1927 when the fifth Solvay conference was held, highlighting intense arguments between Albert Einstein and Niels Bohr. At the time, Einstein expressed, publicly for the first time, his critical attitude towards quantum mechanics. While the arguments did not reach a decisive conclusion, Bohr’s probabilistic interpretation of quantum mechanics, the Copenhagen interpretation, had since been well circulated among the community. Einstein’s arguments with Bohr lasted many years after the conference. The arguably most famous one was the Einstein-Podolsky-Rosen (EPR) paradox, proposed in a 1935 paper written by the three authors. They stated, by flawless reasoning, that quantum mechanics is ‘incomplete’. Bohr replied immediately. So did Erwin Schrödinger, who, in particular, coined the term ‘entangled’ for the first time to describe the bizarre quantum mechanical property. Nevertheless, the EPR paper received few citations at its publication, until 1990’s when quantum information science started blossoming. From today’s perspective, the paper resembles the ‘sleeping beauty’ who slumbered for decades before fully awakening. It has been widely accepted that it was John S. Bell who eventually resolved the EPR paradox. But many important figures before him should not go unnoticed, including John A. Wheeler, Chien-Shiung Wu, David Bohm, to name a few. In 1964, Bell wrote down a mathematical inequality which satisfies local hidden variable theory but which can be violated in quantum mechanics. The significance is that Bell’s inequality brings the rather philosophic debates on the EPR paradox down to be experimentally testable. Hence, an experiment is here appealing, as is the case in all fields of physics. To the end, John F. Clauser, alongside Stuart J. Freedman, conducted an experiment in 1972 to test a Bell inequality, the Bell-Clauser-Horne-Shimony-Holt (CHSH) inequality, proposed by him and his collaborators in 1969. The results, however, leaves loopholes open. One of the loopholes was closed in a later experiment led by Alain Aspect in 1982. The Bell-CHSH inequality was later tested under a much stricter condition by Anton Zeilinger’s team. Two most important loopholes were both closed by three teams in 2015 independently. Zeilinger’s team was among them. Almost all experiments to date have confirmed quantum mechanical predictions. Zeilinger’s team also performed the first teleportation of unknown quantum states, the first entanglement swapping, along with other contributions pioneering the quantum information science. In retrospect, the foundational study of quantum theory has been developed over a century. It once drew much of quantum physics founders’ attention, went virtually unnoticed while other physical fields advanced, had for quite a while been considered to have very weak relationship with major physics, and finally revived in recent decades leading to rapid progress in quantum information science and quantum technology. We believe that the Nobel Prize in Physics this year is not only an award and recognition to past achievements, but also heralds a new era of science in future.
In previous work, the authors have shown that collapse in quantum mechanics can be made consistent with relativity and that at least abstract quantum systems can support paradox-free backwards-in-time causality. This paper considers the challenges in implementing Negative Time Reconnaissance (NTR). The holy grail of reconnaissance has been real time reconnaissance, reducing the time between an event and our acquisition of it to zero. NTR is the technical capability to witness an adversary’s actions literally before they do them. The conceptual design of a machine with the capability to send short digital messages backwards in time by perhaps as much as six hours is presented and analyzed. In classical physics, the paradigm is that the present is an infinitely thin interface between the past and the future. In the quantum systems studied that can support backwards-in-time causality, this paradigm is modified. The present has a temporal “thickness”, the future transitions into the past in a malleable way. While not time travel in the science fiction sense, it does appear that a window can be set up in the immediate future in which the present consists of unresolved pasts and futures. The width of the window is a function of technical maturity, not of physics. Put colloquially, in a classical reality the future hasn’t happened yet while in a quantum reality neither has the present. The bottom line is that if this property of abstract quantum systems is also true of quantum mechanics, then it should be possible to construct a device that could be used to observe and possibly prevent acts of aggression before they occur.