Consciousness and the

Collapse of the Wave Function

David Chalmers

[and Kelvin McQueen]
 Two Questions

• What is the place of consciousness in

nature?

• What is the reality behind quantum
mechanics?
 Consciousness

• If consciousness can’t be explained in

physical terms, then it is nonphysical and
fundamental.

• But if the physical domain is closed,
consciousness can’t play a causal role.
 Quantum Mechanics

• Quantum mechanics postulates a wavelike

reality where things don’t have definite
properties, but we experience a world with
definite properties.

• How can this be explained?
 The Wave Function
• In classical physics, systems are described
by definite values

• A particle’s position is specified by a
definite location.

• In quantum mechanics, systems are
described by wave functions.

• A particle’s position is specified by a wave
function, with different amplitudes for
different locations.
 Superposition
• Sometimes a wave function will specify a
definite position (all the amplitude at one
position).

• But often it will specify multiple positions
(nonzero amplitude at many positions).

• Then the particle is in a superposition of
different positions.
 The Schrödinger
Equation

• The wave function usually evolves

according to the Schrödinger equation

• Systems that start in definite states tend to
evolve into superpositions.
 Measurement

• When one measures a quantity (such as

position), one always observes a definite
result.

• When a system is in a superposition of
values, the measurement might reveal any
of these values, probabilistically.
 The Born Rule

• If one measures position, the probability of

finding that the particle is at that position is
given by the Born rule.

• The probability depends on the wave
function’s amplitude at that position.
 Collapse
• After measurement, the wave function
enters a new state corresponding to the
measurement result.

• Initially: a superposition of position.

• After: a definite position (an eigenstate).

• This process is often called collapse.
Formalism and Reality

• Something like this story is standard

formalism for predicting measurement
results in quantum mechanics.

• But what is really going on in reality?
 The Measurement
Problem
• The formalism says collapse takes place on
measurement; but measurement is an
imprecise notion.

• What is measurement?

• And how can it play a fundamental role in
physical dynamics?
Schrödinger’s Cat
 Alternative
Interpretations
• Hidden-variables (Bohm):

• Particles have definite positions all along

• Many worlds (Everett):

• Even macro systems are in superpositions

• Spontaneous collapse (GRW):

• Collapses happen randomly
 Face-Value
Interpretations
• Collapses happen in reality, triggered by
measurement events.

• One needs to precisify the notion of
measurement and clarify the basic
principles.
 Two Options

• Measurement = observation by
consciousness.

• Consciousness triggers collapse

• Measurement = a physical process

• A physical process triggers collapse
 M-properties

• Hypothesis: There are special properties, m-

properties (m-quantities or m-observables).

• Fundamental principles: m-properties can
never be superposed.

• A system’s wave function is always in an
eigenstate of the m-operator.
 Superposition

• Whenever an m-property enters a

superposition, it collapses to definiteness.

• Whenever it is about to enter a
superposition, it collapses to definiteness.

• Probabilities are given by Born rule for the
associated m-operator.
What are M-Properties

• One could in principle take any property to

be an m-property.

• Different choices of m-properties yield
different interpretations.
 M-Particles

• Illustrative idea: m-properties = position of

special particles, m-particles.

• Fundamental or not (e.g. molecules)

• Law: M-particles always have definite
positions
 Dynamics

• Dynamics given by mathematics of

continuous strong measurement of m-
quantities.

• As if: someone external to the system was
constantly measuring m-quantities.
 Entanglement
• Whenever a superposed property becomes
(potentially) entangled with an m-property,
that property collapses.

• E.g. a photon with superposed position
interacts with an m-particle

• The m-particle probabilistically collapses
to definite position, so does the photon.
 Superposition
Dynamics
• Initially: Photon is in superposition P1 + P2,
M-particle is in location M.

• Photon interacts with M-particle in a way
that would produce P1.M1 + P2.M2

• M-particle collapses onto M1 or M2

• Result: P1.M1 (or P2.M2). Photon collapses
too!
 M-Particles as
Measurers
• The M-particle in effect acts as a measuring
instrument.

• If an M-particle is in a slit of the double-slit
experiment, it collapses the position of a
superposed photon.

• M-particle = Medusa particle (everything it
looks at turns to stone).
 Medium Rare
M-Particles
• M-Particles would need to be rare enough

• So that superpositions could persist,
yielding the interference effects we see

• But they can’t be too rare

• E.g. found in macro systems or brains, so
that measurements always yield results
 Constraints on M-
Properties
• Same constraints on m-properties

• Rare enough that observed interference
effects don’t involve m-properties

• Rules out position, mass, buckyballs

• Common enough that measurements
always involve m-properties

• At least present in brains
 Some Candidates

• Configurational properties of complex

systems (e.g. molecular shape)

• Molecular energy (above a threshold)

• Tononi’s phi (above a threshold)

• Mental properties (e.g. consciousness).
 Different Predictions
• Different hypotheses yield different
empirical predictions

• Interferometer: try to prepare a system
in a superposition of m-properties, see if
interference effects result.

• Very hard to test! (So far: buckyballs?)

• But in principle makes all this testable.
 Objections

• Is energy conserved?

• Is this compatible with relativity?

• Are there infinite long tails?

• What about the quantum Zeno effect?

• Are m-properties fundamental?
 Consciousness and
Collapse

• Consciousness collapses the wave function?

• von Neumann (1932), London and Bauer
(1939), Wigner (1961), Stapp (1993)

• Never made rigorous.
 Consciousness as an
M-Property

• Hypothesis: consciousness is an m-property

• I.e. consciousness can never be superposed

• Whenever consciousness is about to enter
a superposition, the wave function collapses
 Entanglement with
Consciousness
• Take a superposed electron: S1 + S2

• We consciously perceive it, potentially
yielding S1.C(S1) + S2.C(S2)

• Consciousness collapses probabilistically to
C(S1) [say], electron collapses to S1

• Result: definite state S1.C(S1).
Virtues of Consciousness
as M-Property
• Conceptual: clarifies measurement

• Epistemological: saves observation data

• Explanatory: explains nonsuperposability

• Metaphysical: fundamental property in law

• Causal: physical role for consciousness
 Physicalism and
Dualism
• This is consistent with physicalism

• Consciousness is complex/physical

• Also consistent with dualism

• Consciousness is fundamental/nonphysical

• Not consistent with panpsychism!
 Causal Closure

• Philosophers often reject dualism because

physics is causally closed, leaving no role for
consciousness.

• In fact, physics leaves a giant causal opening
in the collapse process.

• Perfectly suited for consciousness to fill!
Physics and Philosophy

• Physicists often reject consciousness-

collapse because of dualism

• Philosophers often reject dualism because
of incompatibility with physics

• Independent reasons for rejection needed!
 Property Dualism

• Consciousness is a fundamental property,

involved in fundamental psychophysical laws

• Epiphenomenalism: unidirectional laws,
physics to consciousness

• Interactionism: bidirectional laws
 Bidirectional Laws
• Physics-to-consciousness law:

• Physical quantity P (e.g. Tononi: high-phi)
yields consciousness

• Consciousness-to-physics law

• Consciousness is never superposed

• C-collapse yields P-collapse
 Worry: Macro
Superpositions

• Worry: Unobserved macroscopic systems

will be in superpositions

• Response: This depends on the complexity
of property P; but if so, so be it.
 Worry:
Indistinguishability
• C-Collapse is empirically equivalent to P-
collapse: P (e.g. high-phi) is an M-property

• Quantum zombie worlds?

• Response: C-collapse has extra explanatory,
metaphysical, and causal virtues.
Test for Consciousness
• An empirical criterion for consciousness?

• Say we find empirically that property P is
associated with collapse

• This will give us (perhaps nonconclusive)
reason to accept that P is the physical
correlate of consciousness

• Especially if P is independently plausible
as a correlate, e.g. high-phi.
Worry: Causal Role in
Action
• What about a causal role in action?

• Consciousness collapses brain states that
lead to action (.red causes ‘I’m seeing red’)

• Collapses of agentive experience yield an
especially direct role
 Worry: Dice-Rolling
Role
• Consciousness is just rolling quantum dice

• Yielding probabilistic outcomes the same
as in quantum zombies

• Doesn’t make us more likely to behave
intelligently or say ‘I’m conscious’

• But: at least it’s playing/explaining the role
Worry: Quantum Zeno
Effect
• Quantum Zeno Effect: Frequent quantum
measurement makes it hard for measured
quantities to change

• Worry: continuous collapse of
consciousness will make it hard (probability
zero) for consciousness to evolve
Quantum Zeno Effect

• How to handle Quantum Zeno Effect?

• Intra-consciousness dynamics?

• Intermittent collapse?

• Realist representationalism: collapse
represented properties.
 Conclusion
• C-collapse interpretations promise
simultaneously

• an attractive, empirically testable
interpretation of QM

• an attractive approach to the mind-body
problem.

• A place for the mind in nature?