Causal modelling provides a powerful set of tools for identifying causal structure from observed correlations. It is well known that such techniques fail for quantum systems, unless one introduces 'spooky' hidden mechanisms. Whether one can produce a genuinely quantum framework in order to discover causal structure remains an open question. Here we introduce a new framework for quantum causal modelling that allows for the discovery of causal structure. We define quantum analogues for core features of classical causal modelling techniques, including (...) the causal Markov condition and faithfulness. Based on the process matrix formalism, this framework naturally extends to generalised structures with indefinite causal order. (shrink)

Realist interpretations of quantum mechanics presuppose the existence of elements of reality that are independent of the actions used to reveal them. Such a view is challenged by several no-go theorems that show quantum correlations cannot be explained by non-contextual ontological models, where physical properties are assumed to exist prior to and independently of the act of measurement. However, all such contextuality proofs assume a traditional notion of causal structure, where causal influence flows from past to future according to ordinary (...) dynamical laws. This leaves open the question of whether the apparent contextuality of quantum mechanics is simply the signature of some exotic causal structure, where the future might affect the past or distant systems might get correlated due to non-local constraints. Here we show that quantum predictions require a deeper form of contextuality: even allowing for arbitrary causal structure, no model can explain quantum correlations from non-contextual ontological properties of the world, be they initial states, dynamical laws, or global constraints. (shrink)

Despite the tremendous empirical success of quantum theory there is still widespread disagreement about what it can tell us about the nature of the world. A central question is whether the theory is about our knowledge of reality, or a direct statement about reality itself. Current interpretations of quantum theory, regardless of their stance on this question, regard the Born rule as fundamental and add an independent state update (or ‘collapse’) rule to describe how quantum states change upon measurement. In (...) this paper we present an alternative perspective and derive a unified probability rule that subsumes both the Born rule and the collapse rule. We show that this more fundamental probability rule can provide a rigorous foundation for informational, or ‘knowledge-based’, interpretations of quantum theory. Our result requires an assumption of instrument non- contextuality, a key notion that generalises previous approaches to non-contextuality. Therefore, the framework also permits one to consider non-contextuality in scenarios with arbitrary causal structure. (shrink)

O objectivo primário do presente artigo é estudar algumas das implicações, sobretudo epistemológicas, associadas com o auto-intitulado movimento da Física Ariana (Deutsche Physikj, movimento esse que aqui se considera como tendo sido iniciado pelos físicos, laureados com o Prémio Nobel, Philipp Lenard e Johannes Stark. Assim, em primeiro lugar, procura-se analisar questões como a da ligação entre ciência e raça, a da função do método experimental e do método dedutivo nas descobertas das ciências naturais, bem como a da relação entre (...) matemática e natureza, já para não mencionar a dimensão histórica da ciência. Em vez de recorrer a conceitos supostamente puros do que sejam ciência ou racionalidade em ordem a instaurar um tribunal da história, cujo veredicto seria a condenação da Física Ariana como irrisória anomalia epistemológica no corpo histórico da ciência, os autores do presente artigo procuram, antes de mais, traçar a origem do movimento a partir das biografias de Lenard e Stark, acreditando que o carácter profundamente conturbado de ambos, coadunado com a conflituosa situação política que antecedeu a ascensão de Hitler ao poder na Alemanha em 1933, seriam razões suficientes para a formação das doutrinas que atacavam a Física Teórica. Nesse sentido, depois de expor os princípios e as ideias defendidas pelos físicos arianos, os autores procuram evidenciar até que ponto uma análise restrita ao campo da física e dos seus documentos não é suficiente para uma compreensão cabal de tal movimento. Ao mesmo tempo, procuram mostrar também por que razão é necessário considerar que algumas objecções levantadas contra a física teórica se constituíam como problemas verdadeiramente relevantes, sendo que alguns deles foram também formulados por autores que não pactuavam com as convicções políticas de Lenard e Stark. Finalmente, o artigo comporta uma análise das posições defendidas por Werner Heisenberg em ordem a mostrar a seriedade com a qual as críticas dos físicos arianos foram recebidas por este autor, nelas reconhecendo algo digno de ser respondido, e isso não meramente por razões políticas, mas antes porque constituíam dilemas reais instalados na prática das ciências naturais. Ou seja, recorrendo aos argumentos esgrimidos por Werner Heisenberg, os autores do artigo pretendem demonstrar que a Física Ariana não merece atenção apenas na sua qualidade de aberração histórica, mas também pelo facto de ela ter levantado questões ainda hoje epistemologicamente relevantes. /// The primary aim of the present article is to study some of the implications, particularly the epistemological ones, associated with the movement of the so called German Physics (Deutsche PhysikJ, a movement that in this article is mainly associated with the work of the physicists, and Nobel laureates, Phillip Lenard and Johannes Stark. The article begins by analyzing questions related to the connection established between science and race, as well as to the function of the experimental and deductive methods in the discoveries of the natural sciences, not to mention the problems associated with the relation between mathematics and nature as well as the historical dimension of science. The goal, therefore, is not to take supposedly pure concepts about what science and rationality might be and establish them as judges in the tribunal of history, whose verdict would be the condemnation of the German Physics as an insignificant epistemological anomaly within the historical body of science. Rather, the authors of the article want to trace the origin of the movement of the German Physics by reading the biographies of Lenard and Stark, whereby it is assumed that the problematic character of these physicists together with the political situation that anteceded Hitlers ascension to power in Germany in 1933 would be reason enough for the formation of doctrines directed against theoretical physics. In this sense, after expounding the principles and the doctrines defended by the Arian physicists, the authors of the article show that an analysis circumscribed to the domain of physics would not be sufficient for a global understanding of this movement. At the same time, however, the authors also want to show the reason why it is necessary to consider some of the objections raised against theoretical physics as serious problems, even though some of them were also formulated by authors that did not share the political convictions of Lenard and Stark. Finally, the article contains an analysis of the positions defended by Werner Heisenberg in order to demonstrate the seriousness with which the criticisms of the Arian physicists were received by this most important author, so much so that he recognized in them something worth a response, and this not only for political reasons, but also because he recognized the problems raised as a dilemma situated at the very heart of the practice of the natural sciences. In other words, by a close reading of the arguments presented by Werner Heisenberg, the authors of the article want to demonstrate that the movement of German Physics is worth attention not only in its quality as an historical aberration, but also due to the fact that it raised questions that are still epistemologically relevant. (shrink)