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New directions in the foundations of physics,
Washington DC, 24 April 15
The measurement problem and the
primitive ontology of
quantum physics
Michael Esfeld
Université de Lausanne
[email protected]
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The methodology
physis: the domain of what exists in itself (Aristotle)
physics: the science of that domain
natural philosophy: physics and philosophy coming in one in the
quest for understanding the natural world: physics on the basis of
fundamental concepts about nature
Newton: Philosophiae naturalis principia mathematica
against both a priori metaphysics &
operationalism in physics
The paper
1)
2)
art
The quantum measurement problem:
Primitive (= fundamental) ontology:
points: spatial structure
change: dynamical structure
 atomism & holism / structural realism
state of the
matter
The measurement problem
(Tim Maudlin 1995)
A The wave-function of a system is complete, i.e. the wave-function specifies
(directly or indirectly) all of the physical properties of a system.
B The wave-function always evolves in accord with a linear dynamical
equation (e.g. the Schrödinger equation).
C Measurements of, e.g., the spin of an electron always (or at least usually)
have determinate outcomes, i.e., at the end of the measurement the
measuring device is either in a state which indicates spin up (and not down)
or spin down (and not up).
A ∧ B  not C
Problem touches any quantum theory: concerns link between theory and data
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The ontology of quantum physics:
state of the art
EITHER not C: quantum state realism
formalism refers to quantum state = physical object: e.g. field in highdimensional space, represented by Y
 problem how to account for empirical data
OR not A / not B: primitive ontology
physical objects in 3d space or 4d space-time as referent of quantum
formalism
 primitive = not derived from formalism, has to be put in as referent of
formalism; provides the link between theory and data
dualism implausible
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Primitive ontology
If C endorsed, then not important whether not A or not B. QM formalism in any case
incomplete in the sense that it does not specify what the distribution of matter in spacetime is; specifies only quantum state = dynamics of that distribution
de Broglie-Bohm: not A: particles & guiding equation
GRW: not B: Schrödinger equation + collapse parameters
How does the GRW equation link up with the distribution of matter in space-time?
GRWm (Ghirardi): matter density field described by Y
GRWf (Bell): collapse of Y (spontaneous localization in configu-ration space) describes
single events (flashes) in physical space
In any case, no radical change from classical to quantum physics: objects the same,
dynamics changes: non-local dynamics by Y
Primitive ontology as solution to measurement
problem
distribution of matter in 3d space (or 4d space-time) (no
superpositions) necessary condition
& dynamics of that distribution (that includes
entanglement) sufficient condition
task: develop primitive ontology that is most
parsimonious and most general
Democritus (about 460-370 before J.C.)
“There is an infinite number of
impenetrable atoms, without
qualities and indestructible, which
move in the void where they are
distributed. But when they come
close to each other or collide, their
aggregation results in water, in
fire, in a plant, or in a human
being.”
Newton, Opticks (1704)
“… it seems probable to me, that
God in the Beginning form'd
Matter in solid, massy, hard,
impenetrable, moveable Particles
… the Changes of corporeal Things
are to be placed only in the various
Separations and new Associations
and motions of these permanent
Particles."
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First approximation
discrete (particles)
inserted into absolute background space; 3d, Euclidean
 particle = what occupies a point of space
 variation: points of space occupied or empty
change in which points of space are empty and which ones are occupied as time passes
If change such that continuous lines of occupation of points of space, then worldlines =
continuous sequences of events = particles (QM: Bohmian mechanics)
If not, then only single events (QM: GRW flash theory)
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Problem
What makes up the difference between a point of space being
occupied and a point of space being empty?
no intrinsic properties such as mass or charge (dynamical parameters
in CM; situated on level of Y in QM)
no primitive thisness (haecceity)
bare substrata with primitive stuff-essence mysterious
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Contra gunk
continuous fundamental ontology (gunk) instead of discrete objects (particles) (QM: GRW
matter density theory)
Allori et al. (2014): “Moreover, the matter that we postulate in GRWm and whose density is
given by the m function does not ipso facto have any such properties as mass or charge; it
can only assume various levels of density.”
What does constitute the various levels of density of matter at points of space, if there are
no properties such as mass or charge available?
 primitive stuff-essence that can assume various levels of density at points of space
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Fundamental ontology
no cogent answer to the question of what distinguishes matter from space available
 abandon dualism of matter and space: no absolute space into which matter is
inserted
relationalism about space (Leibniz): matter points connected by spatial relations (nonvanishing distance and direction)
only matter points and spatial relations, no points of space: spatial structure
background independence
Cartesianism: matter points, because connected by spatial relations (res extensa); standing
in spatial relations distinguishes matter points from (hypothetical) mind points.
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Matter points
fundamental: matter points not composed of anything, compose everything
else
primitive objects: no intrinsic physical properties, but not bare substrata;
spatial relations their essence.
factual: configuration of matter points simply there
 most parsimonious and most general way to conceive fundamental
ontology of the natural world that is able to account for familiar
macroscopic domain: matter points connected by spatial relations
All experimental evidence is evidence of particles.
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Dynamics
change in the spatial relations among the matter points
 time from change as suitable parametrization
relationalism: motion = change of spatial relations among the matter points: interaction
 velocity has to be specified for each transition from one configuration of matter points to another
one
task: fix velocity such that specifying initial conditions at an arbitrary time and plugging them into
velocity law is sufficient to determine the motion of the matter points at any time
further dynamical parameters necessary: mass, charge, energy, spin, wave function, etc. 
dynamical structure
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Newtonian mechanics
Newton‘s gravitational constant G and masses of matter points as
determining the potential V (given the spatial relations) & the initial
velocities: dynamical structure of Newtonian classical mechanics
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Bohmian quantum mechanics
Newton’s gravitational constant G and and masses of matter points as
determining the potential V, Planck’s constant & the initial wave function
Ψ0: dynamical structure of Bohmian quantum mechanics
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Dynamical structure
spatial structure: permutation invariant
dynamical structure: distinguishes matter points; sorts them into various particle species
 particle species through dynamics, not intrinsic
dynamical structure defined only for particle configuration as a whole: to solve the
equations, initial data for entire configuration required
dynamical relations that couple motion of particles to one another interaction = correlated
change of velocities
no properties needed, structures sufficient: spatial structure individuating material objects,
dynamical structure fixing change of spatial relations
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Dynamical structure
spatial structure: factual
dynamical structure: modal: fixes for any configuration of matter points given as initial
condition how the world would evolve if that configuration were the actual one
Humeanism: nothing modal in the world
 dynamical structure only structure of theory that describes change in spatial relations in
most simple & informative way (best system); only spatial relations and change that
happens to occur in their configuration in the world
modal realism: dynamical structure real physical relations like spatial structure
 power that literally determines change in spatial relations
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Dynamical structure:
classical & quantum
in any case non-local correlations
stronger in quantum physics than in classical physics: one wave-function Y
for whole particle configuration  correlates in principle the velocity of all
matter points independently of their distance
small deviations in initial conditions will lead to widely divergent trajectories ( no sense
to calculate real trajectories)
 more prominent role for probabilistic descriptions
only difference between classical and quantum
 objects the same through theory change, dynamical structure varies
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Conclusion
1) primitive ontology: link between theory and data; solves measurement problem
2) spatial configuration of matter points; persisting, substances; structurally individuated by
spatial relations
3) change persisting: dynamical structure to capture change
4) spatial structure: permutation invariant; dynamical structure: sorts matter points into
different kinds of particles
5) spatial structure: factual; dynamical structure: modal, power that determines correlated
change in spatial relations (interaction)
6) dynamical structure applies in any case to the configuration of matter points as a whole;
correlations between motions of matter points stronger in quantum physics than in
classical physics
7) objects the same through theory change; dynamical structure varies
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