Quantum mechanics, with its revolutionary implications, has posed innumerable problems to philosophers of science. In particular, it has suggested reconsidering basic concepts such as the existence of a world that is, at least to some extent, independent of the observer, the possibility of getting reliable and objective knowledge about it, and the possibility of taking (under appropriate circumstances) at least some properties to be objectively possessed by physical systems. It has also raised many others questions which are well known to those involved in the debate on the interpretation of this pillar of modern science. One can argue that most of the problems are not only due to the intrinsic revolutionary nature of the phenomena which have led to the development of the theory. They are also related to the fact that, in its standard formulation and interpretation, quantum mechanics is a theory which is excellent (in fact it has an unprecedented success in the history of science) in telling us everything about what we observe, but it meets with serious difficulties in telling us what there is. We are making here specific reference to the central problem of the theory, usually referred to as the measurement problem, which is accompanying quantum theory since its birth. It is just one of the many attempts to overcome the difficulties posed by this problem that has led to the development of Collapse Theories, i.e., to the Dynamical Reduction Program (DRP). As we shall see, this approach consists in accepting that the dynamical equation of the standard theory should be modified by the addition of stochastic and nonlinear terms. The nice fact is that the resulting theory is capable, on the basis of a single dynamics which is assumed to govern all natural processes, to account at the same time for all wellestablished facts about microscopic systems as described by the standard theory, as well as for the socalled postulate of wave packet reduction (WPR), which accompanies the interaction of a microscopic system with a measuring device. As is well known, such a postulate is assumed in the standard scheme just in order to guarantee that measurements have outcomes but, as we shall discuss below, it meets with insurmountable difficulties if one tries to derive it by assuming the measurement itself to be a process governed by the linear laws of the theory. Finally, the collapse theories account in a completely satisfactory way for the classical behavior of macroscopic systems. Two specifications are necessary in order to make clear from the beginning what the limitations and the merits of the program are. The only satisfactory explicit models of this type (the model proposed by Ghirardi, Rimini, and Weber (1986), usually referred to as the GRW theory, as well as all subsequent developments) are phenomenological attempts to solve a foundational problem. At present, they involve phenomenological parameters which, if the theory is taken seriously, acquire the status of new constants of nature. Moreover, the problem of building satisfactory relativistic generalizations of collapse models is very difficult, though some improvements have been made, which have elucidated some crucial points. In spite of their phenomenological character, Collapse Theories are assuming a growing relevance, since they provide a clear resolution for the difficulties of the formalism, to close the circle in the precise sense defined by Abner Shimony (1989). Moreover, they have allowed a clear identification of the formal features which should characterize any unified theory of micro and macro processes. Last but not least, Collapse Theories qualify themselves as rival theories of quantum mechanics and one can easily identify some of their physical implications which, in principle, would allow crucial tests discriminating between the two. Getting stringent indications from such tests requires experiments, whose technology has been developed only very recently. Actually, it is just due to remarkable improvements in the field of optomechanics and cold atoms, as well as nuclear physics, that specific bounds have already been obtained for the parameters characterizing the theories under investigation; more important, precise families of physical processes in which a violation of the linear nature of the standard formalism might emerge have been clearly identified and are the subject of systematic investigations which might lead, in the end, to relevant discoveries.
Collapse Theories
Angelo Bassi^{ Membro del Collaboration Group};Giancarlo Ghirardi^{ Membro del Collaboration Group}
20200101
Abstract
Quantum mechanics, with its revolutionary implications, has posed innumerable problems to philosophers of science. In particular, it has suggested reconsidering basic concepts such as the existence of a world that is, at least to some extent, independent of the observer, the possibility of getting reliable and objective knowledge about it, and the possibility of taking (under appropriate circumstances) at least some properties to be objectively possessed by physical systems. It has also raised many others questions which are well known to those involved in the debate on the interpretation of this pillar of modern science. One can argue that most of the problems are not only due to the intrinsic revolutionary nature of the phenomena which have led to the development of the theory. They are also related to the fact that, in its standard formulation and interpretation, quantum mechanics is a theory which is excellent (in fact it has an unprecedented success in the history of science) in telling us everything about what we observe, but it meets with serious difficulties in telling us what there is. We are making here specific reference to the central problem of the theory, usually referred to as the measurement problem, which is accompanying quantum theory since its birth. It is just one of the many attempts to overcome the difficulties posed by this problem that has led to the development of Collapse Theories, i.e., to the Dynamical Reduction Program (DRP). As we shall see, this approach consists in accepting that the dynamical equation of the standard theory should be modified by the addition of stochastic and nonlinear terms. The nice fact is that the resulting theory is capable, on the basis of a single dynamics which is assumed to govern all natural processes, to account at the same time for all wellestablished facts about microscopic systems as described by the standard theory, as well as for the socalled postulate of wave packet reduction (WPR), which accompanies the interaction of a microscopic system with a measuring device. As is well known, such a postulate is assumed in the standard scheme just in order to guarantee that measurements have outcomes but, as we shall discuss below, it meets with insurmountable difficulties if one tries to derive it by assuming the measurement itself to be a process governed by the linear laws of the theory. Finally, the collapse theories account in a completely satisfactory way for the classical behavior of macroscopic systems. Two specifications are necessary in order to make clear from the beginning what the limitations and the merits of the program are. The only satisfactory explicit models of this type (the model proposed by Ghirardi, Rimini, and Weber (1986), usually referred to as the GRW theory, as well as all subsequent developments) are phenomenological attempts to solve a foundational problem. At present, they involve phenomenological parameters which, if the theory is taken seriously, acquire the status of new constants of nature. Moreover, the problem of building satisfactory relativistic generalizations of collapse models is very difficult, though some improvements have been made, which have elucidated some crucial points. In spite of their phenomenological character, Collapse Theories are assuming a growing relevance, since they provide a clear resolution for the difficulties of the formalism, to close the circle in the precise sense defined by Abner Shimony (1989). Moreover, they have allowed a clear identification of the formal features which should characterize any unified theory of micro and macro processes. Last but not least, Collapse Theories qualify themselves as rival theories of quantum mechanics and one can easily identify some of their physical implications which, in principle, would allow crucial tests discriminating between the two. Getting stringent indications from such tests requires experiments, whose technology has been developed only very recently. Actually, it is just due to remarkable improvements in the field of optomechanics and cold atoms, as well as nuclear physics, that specific bounds have already been obtained for the parameters characterizing the theories under investigation; more important, precise families of physical processes in which a violation of the linear nature of the standard formalism might emerge have been clearly identified and are the subject of systematic investigations which might lead, in the end, to relevant discoveries.File  Dimensione  Formato  

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