Marco Giovanelli: Special Relativity as a Theory of Principles.

Oxford Philosophy of Physics Seminars, Hilary term 2023
16 February - Marco Giovanelli (University of Turin)
Title: Special Relativity as a Theory of Principles. On Einstein's Distinction between Constructive and Principle Theories
Abstract: Toward the end of 1919, in a two-column contribution for the Times of London, Einstein famously declared relativity theory to be a 'principle theory,' like thermodynamics, rather than a 'constructive theory,' like the kinetic theory of gases. This distinction has attracted considerable attention in both the historically- and the theoretically-oriented scholarship. As it turns out, its popularity has somewhat hindered the appreciation of its core message. This paper hopes to show that to properly understand Einstein's 'theory of theories,' one has to disentangle the two threads of its fabric, the context of justification, and the context of discovery. The expression ‘theory of principles’ indicates (a) a class of existing theories: principle theories do not entail physical laws, but put constraints on them. Law-like statements do not qualify as physical laws unless they satisfy such constraints (b) a strategy for finding new theories: instead of searching directly for the laws of nature, first one should search for constraints the limit the number of possible candidates. As it is well-known, the current debate about the foundation of spacetime theories has taken the form of the opposition between the geometrical and the dynamical approach. In both cases, relativity theory is ultimately treated as a constructive theory: a constructive theory of the material structure of rods and clocks (Brown) or a constructive theory of the geometrical structure of spacetime (Janssen). The historical material presented will show that the geometrical/dynamical opposition does not capture the critical difference between the Einstein/Minkowski and Lorentz/Poincaré approach. What both the geometrical and dynamical approach fails to grasp is the peculiar modal status of Einstein’s new kinematics, which is not only a factual but also a normative claim. The paper concludes that special relativity is indeed, better characterized as a principle theory.

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In slide 17/43, Marco Giovanelli shows a letter from Einstein to Sommerfeld dated 14-01-1908 with this quote (missing sentence for ellipsis has been reinstated):
“The theory of relativity is not more conclusively and absolutely satisfactory than, for example, classical thermodynamics was before Boltzmann had interpreted entropy as probability. [If the Michelson-Morley experiment had not put us in the worst predicament, no one would have perceived the relativity theory as a (half) salvation.] Besides, I believe that we are still far from satisfactory elementary foundations for electrical and mechanical processes. I have come to this pessimistic view mainly as a result of endless, vain efforts to interpret the second universal constant in Planck's radiation law in an intuitive way.”
I reinstated the missing sentence because there is a strong analogy between Einstein’s motivation for producing the principle theory of special relativity (SR) with its kinematics of the Lorentz transformations, and the current situation in quantum mechanics (QM) with its kinematics of finite-dimensional Hilbert space. Just as physics suffered from a “chaos of possibilities” in an attempt to address the Michelson-Morley experiment constructively, physics today suffers from a “hopeless manifold of possibilities” in an attempt to address quantum entanglement constructively.
As we argue in our book, “Einstein’s Entanglement: Bell Inequalities, Relativity, and the Qubit” (Oxford UP, 2024), Einstein could have used the Stern-Gerlach experiment in analogy with the Michelson-Morley experiment to establish the observer-independence of Planck’s constant h under spatial rotations (also in the Lorentz group) in analogy with the observer-independence of the speed of light c under boosts. The latter empirically discovered fact (with linearity) leads to the Lorentz transformations of SR, while the former empirically discovered fact (with subspace locality) leads to the finite-dimensional Hilbert space of QM. This solves the mystery of quantum entanglement in principle fashion, so it doesn’t violate locality (as in Bohm’s pilot wave), statistical independence (as in superdeterminism or retrocausality), intersubjective agreement (as in QBism), or the uniqueness of experimental outcomes (as in Many Worlds). All this renders the last sentence in the quote quite ironic.
This principle approach to QM is made possible by the axiomatic reconstruction of QM via information-theoretic principles (see my Comment in: Natasha Oughton "Why Quantum Theory? Understanding and Explanation via Reconstruction," Oxford Philosophy of Physics).