Presents German physicist Werner Heisenberg's 1958 text in which he discusses the philosophical implications and social consequences of quantum mechanics and other physical theories.

This article summarizes the Quantum Bayesian point of view of quantum mechanics, with special emphasis on the view's outer edges---dubbed QBism. QBism has its roots in personalist Bayesian probability theory, is crucially dependent upon the tools of quantum information theory, and most recently, has set out to investigate whether the physical world might be of a type sketched by some false-started philosophies of 100 years ago (pragmatism, pluralism, nonreductionism, and meliorism). Beyond conceptual issues, work at Perimeter Institute is focused on (...) the hard technical problem of finding a good representation of quantum mechanics purely in terms of probabilities, without amplitudes or Hilbert-space operators. The best candidate representation involves a mysterious entity called a symmetric informationally complete quantum measurement. Contemplation of it gives a way of thinking of the Born Rule as an addition to the rules of probability theory, applicable when an agent considers gambling on the consequences of his interactions with a newly recognized universal capacity: dimension (formerly Hilbert-space dimension). (The word "capacity" should conjure up an image of something like gravitational mass---a body's mass measures its capacity to attract other bodies. With hindsight one can say that the founders of quantum mechanics discovered another universal capacity, "dimension.") The article ends by showing that the egocentric elements in QBism represent no impediment to pursuing quantum cosmology and outlining some directions for future work. (shrink)

Introductory survey -- Atomic theory and mechanics -- The quantum postulate and the recent development of atomic theory -- The quantum of action and the description of nature -- The atomic theory and the fundamental principles underlying the description of nature.

The development of quantum information theory has renewed interest in the idea that the state vector does not represent the state of a quantum system, but rather the knowledge or information that we may have on the system. I argue that this epistemic view of states appears to solve foundational problems of quantum mechanics only at the price of being essentially incomplete.

A clear and accessible presentation of quantum theory, suitable for researchers yet accessible to graduates.

The paper presents a revised view of quantum mechanics centered on the notion (“genuine fortuitousness”) that the click in a counter is a totally lawless event, which comes by itself. A crucial point is the distinction between events on the spacetime scene and the content of the symbolic algorism. A revised conception of matrix variables emerges, by which such a variable, as part of a whole, does not have a value, under any circumstance. This conception is at variance with that (...) of indeterminate variables. A matrix variable not having a value does not enter spacetime and is not a measurable quantity, but manifests itself by a click in a counter with the remarkable property of having an onset, a beginning, from which the click develops. The individual click with its immense complexity is unique and lawless, even beyond probability. The notion of probability only applies to clicks in low resolution, and the completeness of a probabilistic theory is thereby seen in a new perspective. The genuinely fortuitous click is not produced by the impact of a particle, nor caused by an event in the source prior to the click in the counter. Indeed, there are no particles on the spacetime scene. The theory is thereby liberated from notions, which go with particles having indeterminate variables, and which have given to quantum mechanics the image of an unfathomable theory. With no particle as intermediary, the connection between source and detector is non-local, as is the entire theory, which deals exclusively with distributions of clicks. The locality permeating quantum mechanics is a symbolic one. The wave function enters in the sole role of encoding the probability distributions of clicks. The quest to understand the occurrence of the click in terms of the evolution of the wave function loses its meaning. However, click distributions in low resolution can be analyzed in terms of the connection between click distributions for different sets of counters, as given by the wave function. With increasing resolution, the probabilities, and the wave function, gradually lose significance, whereby the onset remains beyond reach. Thus, the downward path from events on the spacetime scene does not extend beyond the onset. The notion that quantum mechanics deals with particles (or fields) is rooted in the historical evolution but appears unable to accomodate genuine fortuitousness. The latter concept is given its due place by fully accepting the abstract nature of the matrix variables. (shrink)

Entanglement was initially thought by some to be an oddity restricted to the realm of thought experiments. However, Bell’s inequality delimiting local - behavior and the experimental demonstration of its violation more than 25 years ago made it entirely clear that non-local properties of pure quantum states are more than an intellectual curiosity. Entanglement and non-locality are now understood to figure prominently in the microphysical world, a realm into which technology is rapidly hurtling. Information theory is also increasingly recognized by (...) physicists and philosophers as intimately related to the foundations of mechanics. The clearest indicator of this relationship is that between quantum information and entanglement. To some degree, a deep relationship between information and mechanics in the quantum context was already there to be seen upon the introduction by Max Born and Wolfgang Pauli of the idea that the essence of pure quantum states lies in their provision of probabilities regarding the behavior of quantum systems, via what has come to be known as the Born rule. The significance of the relationship between mechanics and information became even clearer with Leo Szilard’s analysis of James Clerk Maxwell’s infamous demon thought experiment. Here, in addition to examining both entanglement and quantum information and their relationship, I endeavor to critically assess the influence of the study of these subjects on the interpretation of quantum theory. (shrink)