Spacetime emergence in quantum gravity: functionalism and the hard problem

Abstract

Spacetime functionalism is the view that spacetime is a functional structure implemented by a more fundamental ontology. Lam and Wüthrich have recently argued that spacetime functionalism helps to solve the epistemological problem of empirical coherence in quantum gravity and suggested that it also (dis)solves the hard problem of spacetime, namely the problem of offering a picture consistent with the emergence of spacetime from a non-spatio-temporal structure. First, I will deny that spacetime functionalism solves the hard problem by showing that it comes in various species, each entailing a different attitude towards, or answer to, the hard problem. Second, I will argue that the existence of an explanatory gap, which grounds the hard problem, has not been correctly taken into account in the literature.

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Notes

  1. 1.

    For a review, see e.g. Huggett and Wüthrich (2013), Crowther (2016) and Le Bihan and Linnemann (2019).

  2. 2.

    See e.g. Monton (2002, 2006), Maudlin (2007) and Ney and Albert (2013).

  3. 3.

    Note that the expression “emergence”, in this context, should be understood as a neutral expression, a placeholder for a problem, which does not entail any particular interpretation of the nature of the relation. This point deserves our attention since the term has a different meaning in philosophy and general philosophy of science on the one hand, and in philosophy of physics and physics on the other hand—the separation may be drawn differently, but what matters here is that there exists two different terminological traditions. In the field of general philosophy, the relation of emergence is a very specific notion associated with highly specific features: emergent entities are regarded as not owned by the system from which they emerge, being both novel in some sense, and ontologically dependent upon the entities they are emerging from, thereby going against reductionism. In contrast, in physics and philosophy of physics proper, the relation is generally regarded as a generic one that still has to be interpreted further and is even consistent with reductionism (see Butterfield 2011 and Crowther 2018).

  4. 4.

    Spacetime functionalism also designates an approach defended by Knox (2013, 2014, 2019) according to which the concept of spacetime within a physical theory, and in particular general relativity, should be analysed as a particular functional role in the theory. I will not say anything about this view and how it relates to Lam’s and Wüthrich’s account.

  5. 5.

    Other candidate relations for this spacetime “emergence” are philosophical emergence, brute constitution, grounding and mereological composition (see Le Bihan 2018b).

  6. 6.

    Yates (forthcoming) already made interesting distinctions between various sorts of spacetime functionalisms. However, I will not use his classification since it is not primarily designed to address the hard problem of spacetime. Rather, I will start with the classification of functionalisms about the mind that I find the most promising to address the hard problem and, then only, I will apply it to spacetime functionalism.

  7. 7.

    For a more general review, cf. Huggett and Wüthrich (2013) and Le Bihan and Linnemann (2019).

  8. 8.

    See e.g. Rickles (2011, 2013, 2017); Teh (2013); Matsubara (2013); Read and Møller-Nielsen (2018); Le Bihan and Read (2018); Weatherall (2019) and Butterfield (forthcoming).

  9. 9.

    Other philosophical interpretations of duality might avoid saying goodbye to spacetime, for instance by adopting the nihilist view that none of the dual theories offer accurate enough descriptions of the world—thereby entailing that duality has no metaphysical significance (see Le Bihan and Read 2018). This is not the place to examine the consequence of each philosophical interpretation of duality on the problem of spacetime emergence. What matters for my purpose is that at least some, prima facie plausible, interpretations of duality entail that space is missing.

  10. 10.

    Barrett (1996).

  11. 11.

    See e.g. Monton (2002, 2006), Maudlin (2007) and Albert (2015).

  12. 12.

    It is tempting to use the word “effective” in this context. However, the term is ambiguous in that it is sometimes associated in the literature with a particular domain of energy, and sometimes with the concept of approximation.

  13. 13.

    Note that this expression might also be used to refer to another related problem, namely the problem of understanding the ontological status of non-spatio-temporal entities (see Lam and Wüthrich 2018).

  14. 14.

    Mental states are sometimes said to have a third defining feature in being owned by a particular entity, namely a self. The problem of subjectivity is more specific since not everyone agrees that there actually is a sense of the self, a qualitative self with a phenomenological specificity similar to qualia, that should be explained. See for instance Benovsky (2018).

  15. 15.

    This is not to say that the two scientific problems are identical. In the context of quantum gravity, the problem is to find a derivation of one theory from the other; in the context of the philosophy of mind, the aim is to map mental states upon physical states.

  16. 16.

    It might be that the quantum theory of gravity is not fundamental.

  17. 17.

    For a discussion of this kind of worry in the context of canonical quantum gravity, see Lam and Esfeld (2013).

  18. 18.

    See e.g. Pooley (2013) for a presentation of those approaches.

  19. 19.

    The relationist/substantivalist opposition relies on several differences, the relevance of the container metaphor is just one of them, see e.g. Le Bihan (2016).

  20. 20.

    I thank an anonymous reviewer for raising this point.

  21. 21.

    And again, note that even if there is no hard problem of spacetime after careful consideration of the topic, it remains important to acknowledge the prima facie existence of a problem in order to address intuitions of scholars who claim that spacetime emergence is an intrinsically inconsistent idea that should not be seriously considered.

  22. 22.

    In Le Bihan (2018b), I defend that the best interpretation of spacetime emergence is that GR spacetime exists and is constituted by a non-spatio-temporal structure, by analysing spacetime emergence as a form of spacetime composition. This approach allows making sense of both the predictive success of GR and of its non-fundamentality, without committing to a stratified ontology or a metaphysical relation of emergence.

  23. 23.

    It might be that the a priori/empirical distinction might play an interesting and important role in solving the problem of empirical coherence; it would be interesting to explore this in future works.

  24. 24.

    See e.g. Heil (2003).

  25. 25.

    One might argue that such a form of physicalism should rather be understood as a form of neutral monism, since the very difference between the mental and the physical is at the level of the language, and not at the ontological level. See e.g. Stubenberg (2018).

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Acknowledgements

For helpful comments on an earlier draft of this essay and for discussion of its ideas, I would like to thank Jiri Benovsky, Filipe Drapeau Contim, Vincent Lam, Pierre Joray, Christian Wüthrich and two anonymous reviewers. Special thanks to Niels Linnemann for his thoughtful comments. This work was supported by the research Grant “To and Fro: Scientific Metaphysics at Physics’s Frontiers” (169313) from the Swiss National Science Foundation.

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Le Bihan, B. Spacetime emergence in quantum gravity: functionalism and the hard problem. Synthese (2019). https://doi.org/10.1007/s11229-019-02449-6

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Keywords

  • Spacetime functionalism
  • Hard problem
  • Qualia
  • Spacetime emergence
  • Empirical coherence
  • Quantum gravity