It is shown that it is impossible for an observer to distinguish all present states of a system in which he or she is contained, irrespective of whether this system is a classical or a quantum mechanical one and irrespective of whether the time evolution is deterministic or stochastic. As a corollary, this implies that it is impossible for an observer to measure the EPR-correlations between himself or herself and an outside system. Implications of the main result are discussed for (...) how we have to conceive of universally valid theories. (shrink)

Uncertainty about the actual orientation of the measurement device has been claimed to open a loophole for hidden variable models of quantum mechanics. In this paper I describe the statistics of inaccurate spin measurements by unsharp spin observables. A no‐go theorem for hidden variable models of the inaccurate measurement statistics follows: There is a finite set of directions for which not all results of inaccurate spin measurements can be predetermined in a non‐contextual way. In contrast to an earlier theorem (Breuer (...) 2002) this result does not rely on the assigment of approximate truth values, and it holds under weaker assumptions on the measurement inaccuracy. (shrink)

Uncertainty about the actual orientation of the measurement device has been claimed to open a loophole for hidden variable models of quantum mechanics. In this paper I describe the statistics of inaccurate spin measurements by unsharp spin observables. A no-go theorem for hidden variable models of the inaccurate measurement statistics follows: There is a finite set of directions for which not all results of inaccurate spin measurements can be predetermined in a non-contextual way. In contrast to an earlier theorem [Breuer, (...) Phys. Rev. Lett. 88(2002), 240402] this result does not rely on the assigment of approximate truth values, and it holds under weaker assumptions on the measurement inaccuracy. (shrink)

It is shown that for an inside observer it is impossible to distinguish all states in which a system was at some past time. This holds for classical and quantum systems, but an assumption of determinism is essential in the proof.

General results about restrictions on measurements from inside are applied to quantum mechanics. They imply subjective decoherence: For an apparatus it is not possible to determine whether the joint system consisting of itself and the observed system is in a statistical state with or without interference terms; it is possible that the apparatus systematically mistakes the real pure state of the joint system for the decohered state. We discuss the relevance of subjective decoherence for quantum measurements and for the problem (...) of Wigner's friend. (shrink)

John von Neumann was among the first to learn about Kurt Gödel’s results on the incompleteness of formal systems. Did this shape his views on the completeness of quantum mechanics? I will investigate this question from two viewpoints: von Neumann’s no-hidden-variables proof and his treatment of the quantum measurement problem.

Philosophy of modern physics seemed not to be among the strongholds of post-war German philosophy. As this book testifies, Martin Carrier is among the few exceptions. He makes valuable and original contributions to some important problems in philosophy of science: the theory-ladenness of observation, the possibility of universally valid theories, and the status of physical geometry. I will report first on his main theses and then point to some open problems.