Title: Where is string theory today?

Hard to talk about where we are today without saying where we’ve been. Different eras of string theory: 

1968-74: Dual resonance era, ended with discovery of asymptotic freedom

1975-83: Quantum gravity Era, people pushing this idea were mostly ignored

1984-94: Unification Era, emphasis on CFT techniques

1995-1998: Duality Era, emphasis on spacetime point of view, and role of SUSY

Finally, we might be at the end of AdS/CFT era.

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.

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.

Diagram of correspondence or analogy between QM and GR

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.

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.

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.