# Quantum Theory and the Limits of Objectivity

## Abstract

Three recent arguments seek to show that the universal applicability of unitary quantum theory is inconsistent with the assumption that a well-conducted measurement always has a definite physical outcome. In this paper I restate and analyze these arguments. The import of the first two is diminished by their dependence on assumptions about the outcomes of counterfactual measurements. But the third argument establishes its intended conclusion. Even if every well-conducted quantum measurement we ever make will have a definite physical outcome, this argument should make us reconsider the objectivity of that outcome.

## Keywords

Quantum theory Objectivity Wigner’s friend Brukner Frauchiger and Renner## Notes

### Acknowledgements

Thanks to Jeff Bub for a helpful correspondence on Frauchiger and Renner’s argument, to Časlav Brukner for conversations and correspondence over several years, and to a reviewer for good strategic advice. None of them should be taken to endorse the analysis or conclusions of this paper.

## References

- 1.Fuchs, C.: QBism, the perimeter of quantum bayesianism (2010). arXiv:1003.5209
- 2.Fuchs, C., Mermin, N.D., Schack, R.: An introduction to QBism with an application to the locality of quantum mechanics. Am. J. Phys.
**82**, 749–54 (2014)ADSCrossRefGoogle Scholar - 3.Brukner, Č.: On the quantum measurement problem. In: Bertlmann, R., Zeilinger, A. (eds.) Quantum Unspeakables II, pp. 95–117. Springer International, Cham (2017)CrossRefGoogle Scholar
- 4.Brukner, Č.: A no-go theorem for observer-independent facts. Entropy
**20**, 350 (2018)ADSCrossRefGoogle Scholar - 5.Rovelli, C.: Relational quantum mechanics. Int. J. Theor. Phys.
**35**, 1637–78 (1996)MathSciNetCrossRefGoogle Scholar - 6.Popper, K.R.: The Logic of Scientific Discovery. Hutchinson, London (1959)zbMATHGoogle Scholar
- 7.Deutsch, D.: Quantum theory as a universal physical theory. Int. J. Theor. Phys.
**24**, 1–41 (1985)MathSciNetCrossRefGoogle Scholar - 8.Wallace, D.: The Emergent Multiverse. Oxford University Press, Oxford (2012)CrossRefGoogle Scholar
- 9.Frauchiger, D., Renner, R.: Single-world interpretations of quantum theory cannot be self-consistent” (2016). https://arxiv.org/abs/1604.07422
- 10.Bell, J.S.: Speakable and Unspeakable in Quantum Mechanics, Revised edn. Cambridge University Press, Cambridge (2004)CrossRefGoogle Scholar
- 11.Wigner, E.: Remarks on the mind-body question. In: Good, I. (ed.) The Scientist Speculates. Heinemann, London (1961)Google Scholar
- 12.Frauchiger, D., Renner, R.: Quantum theory cannot consistently describe the use of itself. Nat Commun.
**9**, 3711 (2018)ADSCrossRefGoogle Scholar - 13.Maudlin, T.: Quantum Non-locality and Relativity, 3rd edn. Wiley-Blackwell, Chichester (2011)CrossRefGoogle Scholar
- 14.Healey, R.A.: Locality, probability and causality. In: Bell, M., Shan, G. (eds.) Quantum Nonlocality and Reality, pp. 172–94. Cambridge University Press, Cambridge (2016)CrossRefGoogle Scholar
- 15.Pusey, M.:“Is QBism 80% complete, or 20%”, talk given at the
*Information-Theoretic Interpretations of Quantum Mechanics*workshop, Western University, Canada. Available at https://grfilms.net/v-matthew-pusey-is-qbism-80-complete-or-20-_9Rs61l8MyY.html) (2016) - 16.Fine, A.: Joint distributions, quantum correlations, and commuting observables. J. Math. Phys.
**23**, 1306–10 (1982)ADSMathSciNetCrossRefGoogle Scholar - 17.Lee, D., Chang-Young, E.: Quantum entanglement under Lorentz boost. New J. Phys.
**6**, 67 (2004)ADSCrossRefGoogle Scholar - 18.Kochen, S., Specker, E.: The problem of hidden variables in quantum mechanics. J. Math. Mech.
**17**, 59–87 (1967)MathSciNetzbMATHGoogle Scholar - 19.Einstein, A.: Autobiographical notes. In: Schilpp, P. (ed.) Albert Einstein: Philosopher-Scientist. Open Court Press, La Salle (1949)Google Scholar
- 20.Wigner, E.: The problem of measurement. Am. J. Phys.
**31**, 6–15 (1963)ADSMathSciNetCrossRefGoogle Scholar