1. Overview Three examples of multi-level explanation in physics Some general morals for the structure of science The unification hypothesis The quantum measurement problem

2. The “peg problem” (after Putnam)

3. The peg problem (2) We don’t understand the problem fully unless we understand that the system instantiates rigid-body geometry – even if we know all the micro-physics We also want to know why the system instantiates rigid-body geometry Knowing the answer to the second question also helps us know when the instantiation fails

4. The solar system Each planet’s motion, individually, well described by 2-body Newtonian dynamics This is instantiated (approximately) by a 9- body Newtonian system This in turn is instantiated (approximately) by a certain general-relativistic system together with a phenomenological theory of matter The matter theory, in turn, is...

5. Overall story

6. Structure of Science

7. Properties of instantiation relation A bit noisy – and noise can’t be understood from the instantiated perspective Not usually guessable without a lot of help from the phenomena Only applies to certain states of instantiating system One system can instantiate lots of systems

8. Unity hypothesis

9. Types of unity hypothesis Strong version: there is some theory such that the full structure of the Universe is isomorphic to the structure of one of its systems Weaker versions: – Any region in which self-gravity can be neglected is structured like a system described by the Standard Model – Any region in which self-gravity and relativity can be neglected is structured like a system described by QED + nuclei + gravitational potential

10. Reasons to accept unity hypotheses Metaphysical reasons (?) – Compositionality story? (hard to sustain) – Are alternatives coherent? Consistency – Once we say that a system has the structure of the Standard Model, there’s not much space for other structural claims Success of the hypothesis

11. Roadblocks for unity hypothesis Living tissue? (thoroughly refuted) Consciousness? (silly, in my view; not all agree) Statistical mechanics? (probably just a matter of understanding better the constraints on the instantiating system’s state) The quantum measurement problem

12. Decoherence and Everett Decoherence theory tells us that classical systems are, in a wide variety of situations and conditions, instantiated by quantum systems BUT the instantation is multiple – a given quantum system generally instantiates a multitude of distinct classical and semiclassical processes So a quantum world is structured like a multiplicity of classical worlds