A recent proposal to achieve faster-than4ight communication by means of an EPR-type experimental set-up is examined.We demonstrate that such superluminal communication is not possible. The crucial role of the linearity of the quantum mechanical evolution laws in preventing causal anomalies is stressed.

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Achieving understanding of nature is one of the aims of science. In this paper we offer an analysis of the nature of scientific understanding that accords with actual scientific practice and accommodates the historical diversity of conceptions of understanding. Its core idea is a general criterion for the intelligibility of scientific theories that is essentially contextual: which theories conform to this criterion depends on contextual factors, and can change in the course of time. Our analysis provides a general account of how understanding is provided by scientific explanations of diverse types. In this way, it reconciles conflicting views of explanatory understanding, such as the causal-mechanical and the unificationist conceptions. Synthese (2005) 144: 137-170

The symmetrization postulates of quantum mechanics (symmetry for bosons, antisymmetry for fermions) are usually taken to entail that quantum particles of the same kind (e.g., electrons) are all in exactly the same state and therefore indistinguishable in the strongest possible sense. These symmetrization postulates possess a general validity that survives the classical limit, and the conclusion seems therefore unavoidable that even classical particles of the same kind must all be in the same state-in clear conflict with what we know about classical particles. In this article we analyze the origin of this paradox. We shall argue that in the classical limit classical particles emerge, as new entities that do not correspond to the "particle indices" defined in quantum mechanics. Put differently, we show that the quantum mechanical symmetrization postulates do not pertain to particles, as we know them from classical physics, but rather to indices that have a merely formal significance. This conclusion raises the question of whether many discussions in the literature about the status of identical quantum particles have not been misguided.

Holographic' relations between theories have become an important theme in quantum gravity research. These relations entail that a theory without gravity is equivalent to a gravitational theory with an extra spatial dimension. The idea of holography was first proposed in 1993 by Gerard 't Hooft on the basis of his studies of evaporating black holes. Soon afterwards the holographic 'AdS/CFT' duality was introduced, which since has been intensively studied in the string theory community and beyond. Recently, Erik Verlinde has proposed that even Newton's law of gravitation can be related holographically to the 'thermodynamics of information' on screens. We discuss these scenarios, with special attention to the status of the holographic relation in them and to the question of whether they make gravity and spacetime emergent. We conclude that only Verlinde's scheme straightfowardly instantiates emergence. However, assuming a non-standard interpretation of AdS/CFT may create room for the emergence of spacetime and gravity there as well.

Identical classical particles are distinguishable. This distinguishability affects the number of ways W a macrostate can be realized on the micro-level, and from the relation S = k ln W leads to a non-extensive expression for the entropy. This result is usually considered incorrect because of its inconsistency with thermodynamics. It is sometimes concluded from this inconsistency that identical particles are fundamentally indistinguishable after all; and even that quantum mechanics is indispensable for making sense of this. In contrast, we argue that the classical statistics of distinguishable particles and the resulting non-extensive entropy function are perfectly acceptable from both a theoretical and an experimental perspective. The inconsistency with thermodynamics can be removed by taking into account that the entropy concept in statistical mechanics is not completely identical to the thermodynamical one. We observe that even identical quantum particles are in some cases distinguishable, and conclude that quantum mechanics is irrelevant to the Gibbs paradox.

Saunders has recently claimed that "identical quantum particles" with an anti-symmetric state (fermions) are weakly discernible objects, just like irreflexively related ordinary objects in situations with perfect symmetry (Black's spheres, for example). Weakly discernible objects have all their qualitative properties in common but nevertheless differ from each other by virtue of (a generalized version of) Leibniz's principle, since they stand in relations an entity cannot have to itself. This notion of weak discernibility has been criticized as question begging, but we defend and accept it for classical cases likes Black's spheres. We argue, however, that the quantum mechanical case is different. Here the application of the notion of weak discernibility indeed is question begging and in conflict with standard interpretational ideas. We conclude that the introduction of the conceptual resource of weak discernibility does not change the interpretational status quo in quantum mechanics.

A b s t r a c t . We argue that the formalism of quantum theory can be given an empirical interpretation (in the sense of a connection with observable phenomena) in the same way as in any other empirical theory. I n particular, there is no need to supplement the unitary time evolution governed by a Schrodinger-type of equation with a "projection postulate" that applies only to measurements. As a consequence, the quantum formalism has the same status as the formalisms of older physical theories, and is capable of the same philosophical interpretations. The most natural interpretation, we argue, is to conceive of the formalism as an objectivealthough highly abstractdescription of reality.Der Formalismus der Quantentheorie : Eine objektive Beschreibung der Wirklichkeit ? I n h a l t s u b e r s i c ht. Wir erortern, daB der Formalismus der Quantentheorie empirisch interpretiert werden kann (im Sinne einer Verbindung mit wahrnehmbaren Erscheinungen) in gleicher Weise wie bei allen anderen empirischen Theorien. Insbesondere ist es nicht notig der unitlren Zeitentwicklung, beherrscht durch eine Schrodingergleichung, ein ,,Projektionspostulat" beizufugen, das nur fur Messungen zutrifft. Infolgedessen hat der Quantenformalismus den gleichen Status wie die Formalismen von alteren physikalischen Theorien und kann in gleicher Weise philosophisch gedeutet werden. Wir fiihren aus, daB die naturlichste Deutung darin besteht, den Formalismus als eine objektiveobwohl sehr abstrakte -Beschreibung der Wirklichkeit aufzufassen.