Interpreting Heisenberg Interpreting Quantum States

Abstract: The paper investigates possible readings of the later Heisenberg's remarks on the nature of quantum states. It discusses, in particular, whether Heisenberg should be seen as a proponent of the epistemic conception of states---the view that quantum states are not descriptions of quantum systems but rather reflect the state assigning observers' epistemic relations to these systems. On the one hand, it seems plausible that Heisenberg subscribes to that view, given how he defends the notorious “collapse of the wave function ” by relating it to a sudden change in the epistemic situation of the observer registering a measured result. On the other hand, his remarks on quantum probabilities as “potentia ” or “objective tendencies ” are difficult to reconcile with such a reading. The accounts that are attributed to Heisenberg by the different possible readings considered are subjected to closer scrutiny; at the same time, their respective virtues and problems are discussed. 1. Heisenberg and the epistemic conception of quantum states The epistemic conception of quantum states is the idea that quantum states are not descriptions of the properties of quantum systems but rather reflect the state assignin