Abstract: The research in quantum gravity has jauntily grown in the recent years, intersecting with conceptual and philosophical issues that have a long history. In this paper I analyze the conceptual basis on which Loop Quantum Gravity has grown, the way it deals with some classical problems of philosophy of science and the main methodological and philosophical assumptions on which it is based. In particular, I emphasize the importance that atomism (in the broadest sense) and relationalism have had in the construction of the theory.
2013-09-28 13:15:17: Judging from the discussion paper (here) and the slides, Vidotto’s talk will argue that in LQG, the ancient dispute between substantivalists and relationalists gets resolved by being dissolved: space(time) is a real quantum field, but quantum theory is inherently relational, and moreover the classical gravitational field (= spacetime) emerges from a discrete, i.e. atomistic, basis. Her talk will represent a Rovellian philosophy and approach to quantum gravity.
Historical evolution of notions of space, matter and relations
2013-09-28 13:28:15: Vidotto distinguishes two senses of discreteness in LQG: a real physical discreteness that follows just from the existence of a minimum length scale, and the discreteness that comes with starting from spin networks, graphs etc.
2013-09-28 13:34:33: Quantum mechanics + gravity (GR) entails minimum length in nature; as Bronstein argued in 1933, the fact that finer and finer localization requires higher and higher energy, and energy gravitates in GR, puts a limit on how finely localized something can be without the energy of localization being such as to hide the localized system inside its own Schwarzschild limit.
2013-09-28 13:41:45: Vidotto argues that both QM and GR are relational in an important sense. In both theories, understood properly, everything observable and measurable amounts to the relationship between one system and another.
In GR and QG, this leads to a kind of locality: all interactions are between contiguous systems.
Audience: how does this square with EPR non-locality? Vidotto: Relational QM resolves the EPR ‘paradox’ by allowing a way of understanding what’s going on without anything non-local happening.
2013-09-28 13:50:56: The reconciliation: LQG gives us a spacetime that is both relationalist and substantial. Spacetime is made up of spacetime quanta and their relations with their neighbors. (Blogger notes: this makes it congenial to Decartes’ relational view of extension!)
Now Vidotto heads into a more technical phase of her presentation, putting up some of the apparatus of the LQG approach, but the point is to argue that geometry (e.g. distance) emerges naturally from the structures, with no need to presuppose any background manifold.
2013-09-28 13:57:46: Now Vidotto describes some of the dynamics of spinfoam amplitudes, and claims that some beautiful work (Barrett, Dowall, Fairbain, Gomes, Hellmann, Alesci…’09) has shown that in the semi-classical limit, one can get some nice classical structures to fall out of the dynamics, e.g. FRW-like geometry.
Conclusion: Atomism is fundamental, and so is relationalism. To exist is to interact, to stand in relations. Fundamentally what exists are processes; spacetime itself is a process.