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Quantum Gravity: A Dogma of Unification?

Part of the European Studies in Philosophy of Science book series (ESPS,volume 9)

Abstract

The quest for a theory of quantum gravity is usually understood to be driven by philosophical assumptions external to physics proper. It is suspected that specifically approaches in the context of particle physics are rather based on metaphysical premises than experimental data or physical arguments. I disagree. In this paper, I argue that the quest for a theory of quantum gravity sets an important example of physics’ internal unificatory practice. It is exactly Weinberg’s and others’ particle physics stance that reveals the issue of quantum gravity as a genuine physical problem arising within the framework of quantum field theory.

Keywords

  • Principle of equivalence
  • Unification
  • Quantum field theory
  • Quantum gravity
  • General relativity
  • Graviton/spin-2 particle
  • Lorentz-invariance

I thank Andreas Bartels, Cord Friebe, Stefan Heidl, Niels Linnemann, James Read, Matthias Rolffs, Thorsten Schimannek, and Christian Wüthrich for helpful discussions and remarks. Furthermore, I thank the anonymous referee for pressing me to clarify some paragraphs, especially in the opening sections.

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Fig. 2.1

Notes

  1. 1.

    I do not distinguish between ‘theory’ and ‘model (of a theory)’ here. More accurately, one would refer to the SM as a ‘model (of QFT)’.

  2. 2.

    Furthermore, actually suggestions are put forward for how theory assessment without experimental data could work (Dawid 2013)—a very interesting, but also highly controversial project (cf. Rovelli 2016).

  3. 3.

    Note that in light of the hole argument, the focus has shifted to the metric alone.

  4. 4.

    Note that Carroll’s definition of the SEP is not very precise. Read et al. (2017) carefully distinguish and discuss four versions of the SEP.

  5. 5.

    While Newtonian physics was unable to provide an explanation for why the equivalence principle should hold, the geometrical picture of GR provides an explanation in terms of an elimination (of gravitational potential and gravitational mass). As we will see in a moment, it is also possible to give a reductive account.

  6. 6.

    Here, ‘low energy’ means low energy with respect to the so-called Planck energy. Even the highest presently available energy scales in physics can safely be considered ‘low’ in that sense.

  7. 7.

    Of course, quantum field theory can be thought to be, first and foremost, a theory of fields. The corresponding particles are then derivative of the fields in the sense that they are excitations of the fields. Nevertheless, as the term particle physics stresses, we can also perceive it as a theory of particles. However, by talking about particles instead of fields I do not mean to have claimed anything substantial about the nature of QFT.

  8. 8.

    Here, the charge of a particle is defined as its coupling constant for emission of soft photons (Weinberg 1965b, B989).

  9. 9.

    That means that we demand the polarization vector to transform as 𝜖 μ (p) → ( Λ𝜖) μ (p) + α( Λp) μ .

  10. 10.

    Here we used a slight simplification, but for example Nicolis (2011) carefully proves that the gravitational coupling constants, κ i , are indeed forced to be universal.

  11. 11.

    Still, given that Read et al. (2017) argue that minimal coupling may violate certain versions of the SEP, there definitely remains more to be said. Ultimately, all claims involving the SEP here are in need of further clarification.

  12. 12.

    According to Maudlin (2011), there is another, very general conflict between SR and QM due to Bell’s theorem. Note, however, that this is an entirely different issue closely connected to the debate on the interpretation of QM—a debate which physicists might be safe to ignore as long as the theory is empirically adequate. The high energy conflict mentioned here is not of that kind: While QFT is empirically adequate, consistent and highly predictive at low energies, it becomes non-predictive at high energies.

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Salimkhani, K. (2018). Quantum Gravity: A Dogma of Unification?. In: Christian, A., Hommen, D., Retzlaff, N., Schurz, G. (eds) Philosophy of Science. European Studies in Philosophy of Science, vol 9. Springer, Cham. https://doi.org/10.1007/978-3-319-72577-2_2

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