Overview

• Three examples of multi-level explanation in physics

• Some general morals for the structure of science

• The unification hypothesis• The quantum measurement problem

The “peg problem” (after Putnam)

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

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 ...

Overall story

Structure of Science

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

Unity hypothesis

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

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

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

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