overview three examples of multi-level explanation in physics some general morals for the structure...
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Overview
• Three examples of multi-level explanation in physics
• Some general morals for the structure of science
• The unification hypothesis• The quantum measurement problem
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The “peg problem” (after Putnam)
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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
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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 ...
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Overall story
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Structure of Science
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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
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Unity hypothesis
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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
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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
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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
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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