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Explaining unification in physics internally


In this paper I challenge two widespread convictions about unification in physics: (1) unification is an aim of physics and (2) unification is driven by metaphysical or metatheoretical presuppositions. I call these external explanations of why there is unification in physics. Against this, I claim that (a) unification is a by-product of physical research and (b) unification is driven by basic methodological strategies of physics alone (without any appeal to metaphysical or metatheoretical presuppositions). I call this an internal (or methodological) explanation of why there is unification in physics. To support my claims, I will investigate the actual practice undertaken in physics in paradigmatic examples of unification.

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  1. The aim of theoretical consistency is straightforwardly absorbed in the stronger aim of unification (see also Sect. 2), but empirical adequacy is not.

  2. See Crowther and Linnemann (2018).

  3. See Steinberger (2017) with respect to general issues of rationality: “The thought is that any instance of my violating (NC) [the requirement to not both believe A and \(\lnot A\)] is eo ipso an instance in which my beliefs are out of whack with the evidence. For when I hold contradictory beliefs, at least one of the beliefs must be unsupported by the evidence.” I thank Stefan Heidl for pointing this out to me.

  4. In Salimkhani (2018), I provide an analysis of unification in the context of quantum gravity research that is in line with this paper (see also Sect. 3.4), but do not generalise the findings or clarify key notions such as ‘internal’ and ‘external’ unification.

  5. This ambiguity is usually not acknowledged in the literature and stems from the fact that such positions typically remain silent on how (aiming at) unification is obtained methodologically.

  6. This understanding trivialises the claim that physics aims at unification: After all, unification is reduced to the status of being the result of other aims—so why should we consider it an aim? Why should we hold on to this ambiguous way of speaking?

  7. My translation of “Was Gott getrennt hat, soll der Mensch nicht vereinen” (Treder 1983).

  8. I do not argue against Kao’s proposal, as I take her reading of unification playing some role in heuristics to be weak enough to be compatible with what I try to put forward in this paper. In fact, I agree with Kao on what physicists are actually concerned with (essentially also diminishing the alleged heuristic role of unification): “[...] indeed, it does not even seem to be the case that finding an overarching theory was the primary motivation to work in the various domains. [...] it is worth noting that this is the opposite of what we might think would be the case if the goal were, for instance, to find unifying theories because unifying explanations are a crucial characteristic of theories. Instead, it seems that the primary motivation was to solve existing problems in each scientific domain” (Kao 2017). My project is that of taking this insight seriously.

  9. As a position which refrains from metaphysical presuppositions in explaining the tendency towards unification, the internal perspective starts neither from a unitist’s (e.g., von Weizsäcker (1980)) nor from a disunitist’s (e.g., Cartwright (1999)) view—insisting on either ‘unity’ or ‘disunity’ in nature. Still, ultimately, we may favour one view over the other by help of inference to the best explanation (see also Sect. 4).

  10. I shall assume that Maudlin’s general picture is accepted albeit it with minor qualifications (see, for example, Morrison (2000), and Rickles and French (2006)).

  11. For a more detailed discussion see also Sect. 3.1.

  12. One might object as follows: Why not say that ontological unification is a theoretical desideratum in physics? Since physical theories should be about the world, they should be about ontology. Ontological considerations would then, pace my own argument, be internal to physics: Often, genuine physical arguments just are arguments for ontological unification—I thank Isaac Wilhelm for raising this point. I think there are three issues here. First, I do not disagree that genuine physical arguments can result in ontological unification. I am arguing that this does not imply that ontological considerations are themselves part of the genuine methodology of physics. Second, I take it that most physicists (and philosophers thinking that physics is about empirical adequacy, for example) would not be on board with saying that ontological considerations are part of physics. Physics certainly is about the world, but it is specifically about the empirical (observable) aspects of the world. Third, whether ontological considerations are internal to physics or not is irrelevant with respect to the question of how to explain the unificatory practice: Claiming that ontological considerations are internal to physics is not sufficient to account for ontological unification. Being about ontology is not synonymous with being about a unified ontology. The real job (to answer why ontological considerations lead to a unified ontology) still remains to be done. For this we can again either appeal to external, i.e., philosophical reasons, or try to spell out internally why unification is the result of genuine physical arguments. This is precisely what I put forward from the outset: Often, genuine physical arguments turn out to promote ontological unification (as a by-product).

  13. At first sight, Maudlin (1996) seems to endorse an internal view when he argues that it is possible to identify empirical or theoretical evidence suggesting unification in many cases. However, Maudlin’s conviction that physics is commited to unification as “the aim of physical theory” (Maudlin 1996, p. 129f), his presentation of the physicist’s tale, and his interpretation of certain case studies (see Sect. 3.2, for example) are incompatible with the internal perspective I advocate in this paper and rather point to a position that views physics as constrained and guided externally.

  14. In this paper I shall remain neutral on the issue of whether there is any conceptual link between unification and explanation. In general, I conceive my position on unification as compatible with views on which physics aims at explanation or truth.

  15. See also the discussion of Einstein’s dissatisfaction with Maxwell’s theory of electromagnetism in the next section.

  16. I thank Isaac Wilhelm for pressing me on this.

  17. Note that the ‘Moon-test’ (Principia, Book III, Proposition IV) provides a more precise, quantitative argument based on empirical data for the fact that “the force by which the moon is retained in its orbit is that very same force which we commonly call gravity; for, were gravity another force different from that, then bodies descending to the earth with the joint impulse of both forces would fall with a double velocity [...]; altogether against experience” (Newton 1846).

  18. Let me briefly comment on a worry that was similarly raised to me by Andreas Hüttemann and Isaac Wilhelm. One could ask: what is so bad about an imperfect representation? What is so bad about having symmetric explananda, but asymmetric explanantia? Implicit in the question is the suggestion that ‘aiming at unification’ is the answer. I do not see how aiming at a correct representation of phenomena involves ideas on unification. In some cases it may imply unification as a result; but this is not what I am arguing against, but arguing for.

  19. Let me stress that, strictly speaking, this presentation adopted from Maudlin (1996) is somewhat imprecise: we did know not only some phenomenological details about the weak interaction, but also already had some effective theoretical modelling; I thank Sébastien Rivat for reminding me of this. Still, it is correct to say that electroweak unification was not about unifying two equally accepted and well-behaved theories. A sensible theory of the weak interaction was not worked out yet.

  20. In fact, there is no consensus on what exactly the quest is.

  21. Alternative approaches to QG usually have a rather different and less comprehensive objective. Loop quantum gravity (LQG), for example, is merely concerned with a quantum theory of (empty) spacetime.

  22. Let me add that I am not suggesting that a theory is ‘correct’ if and only if it is UV-complete. Crowther and Linnemann (2018) nicely evaluate this issue.

  23. Note that the internal–external distinction may also be used to classify approaches as more or less promising. Considering the amount of disagreement between the different approaches to QG it should come as no surprise if certain approaches eventually become viewed as poorly justified.

  24. This is not only characteristic of physics, but of mathematics as well. Consider the example of number theory where exploiting the notion of ‘number’ further and further (‘pushed to its limits’) ultimately provided substantial links to other areas of mathematics. I thank Andreas Bartels for this.

  25. To be clear, this means that if there were two perfectly fine, non-unifyable theories with no hints towards an encompassing framework, the same reasoning would suggest a disunitist position.


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I especially thank Andreas Bartels, Karen Crowther, Andreas Hüttemann, Rasmus Jaksland, Niels Linnemann, Isaac Wilhelm, and two anonymous reviewers for very helpful feedback on earlier versions of this paper. I am also grateful to Claus Beisbart, Stefan Heidl, Niels Martens, Sébastien Rivat, Matthias Rolffs, and Ben Young for useful discussions and remarks. Furthermore, I would like to thank the audiences at the Frühjahrstagung der Deutschen Physikalischen Gesellschaft in Hamburg in February 2016, the 9th European Congress of Analytic Philosophy in Munich in August 2017, the XXIV. Kongress der Deutschen Gesellschaft für Philosophie in Berlin in September 2017, and the 3rd Annual Conference of the Society for the Metaphysics of Science in New York City in October 2017 for valuable comments.

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Correspondence to Kian Salimkhani.

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Grants: This work was supported by the German Research Foundation (DFG), Grant Number: FOR 2495 (research unit Inductive Metaphysics).

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Salimkhani, K. Explaining unification in physics internally. Synthese 198, 5861–5882 (2021).

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  • Unification
  • Methodology
  • Physics
  • Metaphysics