The notion of the macroscopic in fundamental quantum theory is analyzed. After a brief summary of use of the term macroscopic, its use in quantum theory is compared with its previous use elsewhere. Next, the connections specifically to the Copenhagen interpretation of quantum mechanics and quantum measurement theory more generally, where this term first began to deviate from previous uses, are explained and exhibited through a number of examples. Then, recent attempts to define accurately and quantitatively the extent of being macroscopic, that is, macroscopicity are discussed and their implications considered. This is done most particularly in the realm of quantum optics, where it differs most from previous uses and has recently been of considerable interest. Finally, with the benefit of this analysis, recommendations are made regarding future use of the notion of the macroscopic in fundamental physics.
The notion of the macroscopic in fundamental quantum theory is analyzed. After a brief summary of use of the term macroscopic, its use in quantum theory is compared with its previous use elsewhere. Next, the connections specifically to the Copenhagen interpretation of quantum mechanics and quantum measurement theory more generally, where this term first began to deviate from previous uses, are explained and exhibited through a number of examples. Then, recent attempts to define accurately and quantitatively the extent of being macroscopic, that is, macroscopicity are discussed and their implications considered. This is done most particularly in the realm of quantum optics, where it differs most from previous uses and has recently been of considerable interest. Finally, with the benefit of this analysis, recommendations are made regarding future use of the notion of the macroscopic in fundamental physics.
The notion of macroscopic realism has been used in attempts to achieve consistency between physics and everyday experience and to locate some boundary between the realms of classical mechanics and quantum meachanics. Its ostensibly underlying conceptual components, realism and macroscopicity, have most often appeared in the foundations of physics in relation to quantum measurement: reality became a prominent topic of discussion in quantum physics after the notion of element of reality was defined and used by Einstein, Podolsky and Rosen in that context, and macroscopicity is often explicitly assumed to be an essential property of any measuring apparatus. However, macroscopicity turns out to be a rather vaguer and less consistently understood notion than typically assumed by physicists who have not explicitly explored the notion themselves. For this reason, it behooves those investigating the foundations of quantum mechanics from a realist perspective to look for alternative notions for grounding quantum measurement. Here, the merits of treating the measuring instrument as a ‘natural kind’ as a means of avoiding anthropocentrism in the foundations of quantum measurement are pointed out as a means of advancing quantum measurement theory.
This paper deals with the problem of the interpretation of quantum mechanics, the nature of physical theory, and the future of physics. First, the question of why there are so many different interpretations of quantum mechanics proposed and accepted by outstanding quantum experts is treated. This requires a discussion of the relation between physics and philosophy and drawing a distinction between a physical formalism and a physical explanation. The Copenhagen interpretation is considered, criticized, and found to be unsatisfactory in some respects. Finally, an interpretation that contains ideas from previous interpretations but which may itself be new, is proposed. Hopefully, this is superior to the previous interpretations. It has, I believe, the advantage of logical clarity. In the process of doing this, I introduce a theory of levels of physical theory, according to which microscopic physical theory, as it develops in successive stages, becomes more and more adaptable to explaining—or at least taking cognizance of—fundamental biological or ultimately psychological phenomena.
Quantum physics, also known as quantum mechanics, is one of the most profound and successful theories in modern science, fundamentally transforming our understanding of the microscopic world. It governs the behavior of matter and energy at atomic and subatomic scales, where classical physics fails to provide accurate explanations. This review paper presents a comprehensive examination of the fundamental concepts of quantum physics, including wave–particle duality, quantum states, superposition, uncertainty principle, quantum measurement, entanglement, and quantum dynamics. The paper traces the historical development of quantum theory, reviews seminal contributions from key physicists, and analyzes major theoretical frameworks such as Schrödinger’s wave mechanics, Heisenberg’s matrix mechanics, and Dirac’s formalism. A systematic literature review of 25 scholarly sources is conducted to highlight the evolution, interpretations, and applications of quantum concepts. A comparative analysis of classical and quantum physics is provided to emphasize conceptual differences and paradigm shifts. The discussion explores philosophical implications, experimental validations, and emerging applications in quantum computing, cryptography, and quantum technologies. Finally, the paper concludes by summarizing key insights and outlining future directions for research and education in quantum physics.
The Copenhagen interpretation of quantum theory is investigated from a philosophical point of view. It is justified the opinion that the philosophical attitude the Copenhagen interpretation is based on is in principle inevitable for a real comprehension of quantum theory. This attitude is mainly related to epistemological arguments. However, the measurement problem often seems not to be treated clearly enough within the interpretation. By referring to the property of the necessity to use macroscopic measurement instruments obeying classical concepts it is made the attempt to solve the measurement problem. According to this consideration the indeterministic character of quantum theory seems to have its origin in a lack of knowledge and thus it appears in a similar but more principle way than in statistical mechanics. It is emphasized the ontological character of the uncertainty relation and the related non locality of quantum theory suggesting that the existence of a position space is not as fundamental as the assumptions of general quantum theory.
One of the main unsolved question in Quantum Mechanics (QM) is its compatibility with classical mechanics. The laws of QM, which describe the microscopic world, must merge into the classical ones for large enough physical systems, but it is still unknown at which point, if any, the transition occurs and if the transition is smooth or sudden as the size increases. Furthermore, a strict extension of QM to the macroscopic world leads to well known 'paradoxes', which is not straightforward to solve. This question is tightly connected with the measurement problem, since any measurement apparatus must give a response which can be described at classical level. Many experiments have been performed to answer to this question. They mainly try to find to which extent the number of particles can be increased in order to observe clear evidence of violation of standard QM. In this paper we argue that the macroscopicity parameter is not the number of particles but the number of independent degrees of freedom, as proposed in a recent model for the completion of QM. This introduces a sort of change of paradigm. To support this claim a representative set of well known experiments are analyzed from this point of view. This brings to a new interpretation of the experiments. A general scheme for the quantum-classical transition is discussed in some details.
One of the fundamental features that separates quantum physics from classical physics is the idea of quantum superposition, also known as coherence. This thesis concentrates on understanding quantum coherence in the mathematical framework of resource theories, viewing it both as a resource to be harnessed and as a way to quantitatively characterise quantum states in contrast to classical states. We first examine the type of coherence resource theory which has emerged recently to cope with general settings where the physical nature of the medium encoding information is not crucial, such as computation. We identify the set of quantum processes in which coherence is neither created nor used, and use these to provide a physically motivated resource theory pictured in terms of interferometry. Using the same concepts, we then find connections between coherence and discord, a type of quantum correlation. In particular, we show how coherence can be used to generate discord, and explore basis-dependent discord as an intermediate quantity. The second part of the thesis applies the resource theory framework to quantify quantum macroscopicity, taken here to mean the extent to which coherence exists in a system on a macroscopic scale. We find the appropriate type of resource theory for this purpose, giving criteria for good measures of macroscopic coherence. We use these criteria to evaluate some previously proposed measures and highlight the role of the quantum Fisher information. Next, we build up measures based on the concept of macroscopic distinguishability and use them to show that macroscopic quantum states are fragile to noise induced by interaction with an environment. Finally, we apply measures based on the Fisher information to a range of experiments involving mechanical degrees of freedom, in order to compare their macroscopicity.