Abstract
There is a significant body of literature, which includes Itamar Pitowksy’s “Betting on the outcomes of measurements,” that sheds light on the structure of quantum mechanics, and the ways in which it differs from classical mechanics, by casting the theory in terms of agents’ bets on the outcomes of experiments. Though this approach, by itself, is neutral as to the ontological status of quantum observables and quantum states, some, notably those who adopt the label “QBism” for their views, take this approach as providing incentive to conclude that quantum states represent nothing in physical reality, but, rather, merely encode an agent’s beliefs. In this chapter, I will argue that the arguments for realism about quantum states go through when the probabilities involved are taken to be subjective, if the conclusion is about the agent’s beliefs: an agent whose credences conform to quantum probabilities should believe that preparation procedures with which she associates distinct pure quantum states produce distinct states of reality. The conclusion can be avoided only by stipulation of limitations on the agent’s theorizing about the world, limitations that are not warranted by the empirical success of quantum mechanics or any other empirical considerations. Subjectivists about quantum probabilities should be realists about quantum states.
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Notes
- 1.
One might also consider the following situation. You are reading a text of which you are not the author. Presumably, you take what you are reading as informative about what choices of keystrokes the author made while composing it. If not, then why are you still reading?
- 2.
The framework in this section is based on the ontological models framework of Harrigan and Spekkens (2010).
- 3.
This says that any system composed of N subsystems of the same type can be regarded as a part of a larger system consisting of a greater number of systems of the same type.
- 4.
Of course, we could reasonably expect something stronger, that this holds, not just for projections, but also for any positive-operator valued measure (POVM). But the weaker condition is all that we need.
- 5.
It is worth re-emphasizing: I am not claiming that anything about the laptop or the kumquat follows from assertions about the agent’s credences about these systems. The agent may prefer the laptop to the kumquat as a note-taking device because of a false belief that the laptop is more suitable to play that role in a causal chain between the agent’s actions and future experiences. The point is merely that the agent’s belief, true or false, is a belief about the physical workings of the laptop.
- 6.
It might be claimed, in response, that Alice, in sending a signal to Bob, is concerned about the impact on Bob only insofar as it will later impinge on her. This, as everyone knows, is false. Some of the intended results of a communication are ones that the sender will never be aware of. As a particularly vivid example, consider a spy whose cover has been blown, who sends out one last encrypted message before taking a cyanide pill to avoid capture. A QBist must regard this as a misuse of quantum mechanics if the encryption scheme is a quantum scheme. It is not! Nor is it a misuse of quantum mechanics for an engineer designing a nuclear waste storage facility to use quantum mechanics to compute half-lives with the aim of constructing a facility that will be safe for generations to come.
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Myrvold, W.C. (2020). Subjectivists About Quantum Probabilities Should Be Realists About Quantum States. In: Hemmo, M., Shenker, O. (eds) Quantum, Probability, Logic. Jerusalem Studies in Philosophy and History of Science. Springer, Cham. https://doi.org/10.1007/978-3-030-34316-3_20
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