quantum theory and the many- worlds interpretation · 2014. 10. 27. · quantum theory and the...
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Quantum Theory and the Many-
Worlds Interpretation
David Wallace (Balliol College, Oxford)
LSE, October 2014
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Interpreting superpositions
|live cat> - represents system with a living cat in
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Interpreting superpositions
|live cat> - represents system with a living cat in
|dead cat> - represents same system where the cat is dead
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Interpreting superpositions
|live cat> - represents system with a living cat in
|dead cat> - represents same system where the cat is dead
a|live cat> + b|dead cat> - represents ??????????????
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Probabilities and amplitudes
Born rule:
When superpositions are measured, the mod-squared amplitude
of a term in the superposition is the probability that the
measurement outcome corresponds to that term
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Probabilities and amplitudes
Born rule:
When superpositions are measured, the mod-squared amplitude
of a term in the superposition is the probability that the
measurement outcome corresponds to that term
Probability interpretation:
Superpositions represent systems in an unknown but definite state
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Problems for probabilistic
interpretation
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Problems for probabilistic
interpretation
Interference
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Problems for probabilistic
interpretation
Interference
Kochen-Specker Theorem
Gleason’s Theorem
Pusey-Barrett-Rudolph theorem
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The Measurement Problem
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The Measurement Problem
Microscopic quantum states cannot be interpreted
probabilistically because of interference
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The Measurement Problem
Microscopic quantum states cannot be interpreted
probabilistically because of interference
Macroscopic quantum states cannot be interpreted physically
because of Schrodinger cat states
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The Measurement Problem
Microscopic quantum states cannot be interpreted
probabilistically because of interference
Macroscopic quantum states cannot be interpreted physically
because of Schrodinger cat states
Actual physical practice shifts inchoately between these
interpretations
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Change the philosophy?
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Change the philosophy?
Operationalism?
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Change the philosophy?
Operationalism?
Complementarity?
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Change the philosophy?
Operationalism?
Complementarity?
Quantum logic?
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Change the physics?
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Change the physics?
Collapse of the wavefunction
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Change the physics?
Collapse of the wavefunction?
Hidden variables?
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Change the physics?
Collapse of the wavefunction?
Hidden variables?
Retrocausation?
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The paradox of electromagnetism
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The paradox of electromagnetism
A(x,y,z,t)- represents a pulse of radio waves going from Earth
to Moon
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The paradox of electromagnetism
A(x,y,z,t)- represents a pulse of radio waves going from Earth
to Moon
B(x,y,z,t)- represents a pulse of radio waves going from Mars to
Venus
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The paradox of electromagnetism
A(x,y,z,t)- represents a pulse of radio waves going from Earth
to Moon
B(x,y,z,t)- represents a pulse of radio waves going from Mars to
Venus
a A(x,y,z,t) + b B(x,y,z,t) – represents ??????????
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The Emergent Multiverse?
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The Emergent Multiverse?
Physics (decoherence) tells us that the quantum state, at large
scales, has the structure of a branching multiverse with the
branches obeying quasiclassical dynamics
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The Emergent Multiverse?
Physics (decoherence) tells us that the quantum state, at large
scales, has the structure of a branching multiverse with the
branches obeying quasiclassical dynamics
Philosophy tells us (should tell us!) that higher-order ontology
is a matter of autonomous higher-order structure and
dynamics
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Two Problems of Probability
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Two Problems of Probability
(1) What, if anything, is the categorical basis for probabilities?
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Two Problems of Probability
(1) What, if anything, is the categorical basis for probabilities?
(2) Why does that categorical basis play the probability role?
Lewis: Principal Principle?
Papineau: Inferential & Decision-Theoretic Links
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The “what” problem
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The “what” problem
Frequentism?
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The “what” problem
Frequentism?
Best-systems analysis?
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The “what” problem
Frequentism?
Best-systems analysis?
Bare postulate?
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The “what” problem
Frequentism?
Best-systems analysis?
Bare postulate?
Everett: probabilities are mod-squared amplitudes
in regimes where decoherence guarantees they
obey the probability calculus
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The “Why” problem
“[I]s there any way that any Humean magnitude could fill the
chance-role? Is there any way that an unHumean magnitude
could? What I fear is that the answer is “no” both times! Yet how
can I reject the very idea of chance, when I know full well that
each tritium atom has a certain chance of decaying at any
moment?”
(Lewis)
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The “Why” problem, Everett-style
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The “Why” problem, Everett-style
Probability from locality
(Zurek, Carroll/Sebens)
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The “Why” problem, Everett-style
Probability from locality
(Zurek, Carroll/Sebens)
Probability from decision theory
(Deutsch, Greaves, Myrvold, DW)
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The “Why” problem, Everett-style
Probability from locality
(Zurek, Carroll/Sebens)
Probability from decision theory
(Deutsch, Greaves, Myrvold, DW)
The Everettian Epistemic Theorem (EM 218-223)
(roughly) “An agent who obeys normal decision-theoretic axioms,
and who considers Everettian QM as a live epistemic probability,
will treat mod-squared amplitudes in that theory as probabilities”
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