The curious case of spacetime emergence

Abstract

Work in quantum gravity suggests that spacetime is not fundamental. Rather, spacetime emerges from an underlying, non-spatiotemporal reality. After clarifying the type of emergence at issue, I argue that standard conceptions of emergence available in metaphysics won’t work for the emergence of spacetime. I go on to consider spacetime functionalism as a way to make sense of spacetime emergence. I argue that a functionalist approach to spacetime modelled on mental state functionalism is not a viable alternative to the standard conception of emergence in metaphysics. I go on to consider an alternative: ‘partial’ functionalism, whereby certain aspects of spacetime are functionalised, rather than spacetime as a whole.

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Notes

  1. 1.

    For discussion, see Anderson (2012), Bain (2013), Barbour (1999), Butterfield and Isham (1999), Kuchar (1992), Oriti (2014), Rovelli (2004).

  2. 2.

    Wave-function realism interpretations of quantum mechanics also appear to lack spacetime. For discussion see Maudlin (2007), Ney (2015).

  3. 3.

    Causation is usually thought to be a relation between events, which are individuated by their spatiotemporal locations. Baron and Miller (2014), Tallant (2018) argue that causation without time is possible. However, the resulting notion of causation may not be sufficient for observation [see Braddon-Mitchell and Miller (2019)].

  4. 4.

    Braddon-Mitchell and Miller (2019), Healey (2002), Huggett and Wüthrich (2013) and Maudlin (2007), all raise a version of this problem, though aimed at different theories in each case.

  5. 5.

    See, for example, Bain (2013), Butterfield and Isham (1999), Crowther (2016, 2017), Huggett and Wüthrich (2013), Rovelli (2004), Wüthrich (forthcoming).

  6. 6.

    A number of philosophers suggest that there is a non-causal, mathematical style of explanation that plays a substantive role in scientific explanation. Gravitational explanations and geometric explanations are often cited as examples, see Colyvan (2001, pp. 50–52).

  7. 7.

    It may be false for strong emergence, see Barnes (2012).

  8. 8.

    One might argue that some approaches cannot be understood in compositional terms, such as Hu’s (1999) account of emergent gravity, and so we shouldn’t try to capture quantum gravity inside this framework. However, note that Hu appears to characterise his own emergent gravity program in compositional terms (of course we would need to look at the details):

    Something assumed to be ‘elementary’ at a lower energy or with a coarser resolution at one time will later be found to be a composite at a higher energy or under a finer resolution... Constructing the macroscopic structure and dynamics from known microscopic constituents is often easier, such as getting hydrodynamics from molecular dynamics. Going the other way, i.e., deducing molecular features from hydrodynamics, is more difficult. For spacetime structures, the former is the task of emergent gravity, the latter is the goal of quantum gravity. (Hu 2009, p. 3)

    Moreover, even if the compositional approach is a non-starter for Hu’s view, the causal sets approach and loop quantum gravity appear to posit physical entities at high energy scales that could potentially compose entities at low energies.

  9. 9.

    Wüthrich (forthcoming), Lam and Wüthrich (2018) and Ney (2015) object to mereology as a basis for spacetime emergence.

  10. 10.

    I am grateful to an anonymous referee for raising this issue.

  11. 11.

    Braddon-Mitchell and Miller (2006, pp. 224–225) also argue that the concept of a part breaks down when STP is falsified.

  12. 12.

    Le Bihan (2017) makes this suggestion.

  13. 13.

    One option that has been suggested to me by a referee for this journal is to look more carefully at Causal Set Theory. Like LQG, Causal Set Theory, lacks fundamental spacetime (see Huggett and Wüthrich 2013). Nonetheless, the basic elements of Causal Set Theory do seem to be isomorphic to a part-whole structure of some kind. I lack the space to consider this option here, but I agree broadly that causal set theory provides a more natural basis for developing a notion of parthood* and a mereological conception of spacetime emergence.

  14. 14.

    Ney (2015) briefly argues against the use of spacetime functionalism as the basis for emergence. The argument I will present goes beyond the cursory remarks that Ney makes, but is broadly similar in spirit.

  15. 15.

    I am grateful to two anonymous referees for pressing me on this issue.

  16. 16.

    For example, Bilson-Thompson’s (2005) braid model of particles, which has been combined with LQG and spin-foam theories (see Bilson-Thompson et al. 2012, 2009, 2007).

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Acknowledgements

I am grateful to two anonymous referees for their immensely helpful feedback on the paper. Earlier versions of this paper were presented at the CPT/FraMEPhys Workshop on Time and Explanation held at the University of Milan and the First International Workshop on Time held at Osaka City University. I am grateful to audiences at both workshops for their feedback on the paper, and am especially grateful to Alastair Wilson for hosting the Milan workshop and Takeshi Sakon for hosting the Osaka workshop. Work on this paper was funded by an Australian Research Council Discovery Early Career Researcher Award DE180100414 and a Discovery Project DP180100105.

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Baron, S. The curious case of spacetime emergence. Philos Stud 177, 2207–2226 (2020). https://doi.org/10.1007/s11098-019-01306-z

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Keywords

  • Spacetime
  • Emergence
  • Time
  • Composition
  • Functionalism