These
pieces are primarily commentaries on the 'Ontic Structural Realism
and the Philosophy of Physics' chapter of James Ladyman and Don
Ross's book Every Thing Must Go. There are also a handful of
references to – and quotes from – other parts of that book.
******************************************** James Ladyman and Don Ross's (L & R) philosophy of physics is primarily motivated by the findings of quantum mechanics. Elements of their position can be traced back to - among others - Ernst Cassirer. (Cassirer died in 1945.) Indeed L & R have much to say about Cassirer. For example, they wrote the following:
“OSR
[ontic structural realism] agrees with Cassirer that the field is
nothing but structure. We can’t describe its nature without
recourse to the mathematical structure of field theory.”
What
L & R say about Ernst Cassirer's position on objects is almost
exactly the same as their own. Indeed it was also quantum mechanics
(QM) which provided Cassirer with the motivation to reject
“individual objects”. L and R write:
“Ernst
Cassirer rejected the Aristotelian idea of individual substances on
the basis of physics, and argued that the metaphysical view of the
‘material point’ as an individual object cannot be sustained in
the context of field theory. He offers a structuralist conception of
the field.”
One
can firstly ask whether or not a commitment to the existence of
objects is also automatically a commitment to “individual
substances”; as well as to intrinsic (essential) properties. After all,
bundle theories (among
other theories) reject the notion of substance; though not that of an
individual.
We
can also ask whether or not these positions equally applicable to
objects in the macro-world. Let's put it this way. Ernest Cassirer's
and L & R's positions are far more acceptable when applied the
the quantum world than when applied to the macro-world. More
precisely, all this is far easier to swallow in the “context of
field theory” than it is in relation to human beings or cups.
Essential/Intrinsic
Properties?
It
does seem strange (intuitively strange) that all “quantum
elementary particles of a given type” are deemed to “have the
same mass, size, and shape (if any), charge, and so on” - at least
that's how Ladyman and Ross see things. As it is, mass, size and
charge are seen as intrinsic or essential properties. In fact it's
hard to imagine elementary particles having other (inessential)
properties. After all, we're not talking about the macro-world here.
One can easily imagine a natural kind having its essential properties; though, in a particular instance, also having
contingent properties. Can we also imagine an elementary particle
having contingent properties? If such particles don't have such
contingent properties (due to the nature of the micro-world), then it
may literally be the case that mass, size and charge is all there is
to them. Thus essentialism of some kind may be an easier option to
uphold for particles than it is for macro-objects such a water or
lions.
However,
Ladyman and Ross do cite examples of what can be taken as
“accidental” properties. They cite “velocity or position at a
particular time”. (I'd have thought that physicists would have
taken a particular velocity – or at least mean
velocity – of a particle to be pretty essential to it.)
From
what's just be said, it's therefore no surprise that different particles of a
particular type can be seen as “indistinguishable”. However,
particles may still have properties which aren't intrinsic. That is,
properties which are relational or extrinsic. Ladyman and Ross cite
“spatio-temporal or other state-dependent properties” as
examples.
In
terms of spatio-temporal properties. Does that mean that
particles are even more likelier candidates for being 4-dimensional
objects that macro-objects? As for state-dependent
properties - that must mean that the nature of a particle must
necessarily depend on the parallel (or corresponding) nature of the
“state” (or system) to which it belongs.
Individuals?
L
& R give a very concrete example of the physics which underlies
the problematic nature of seeing elementary particles as single
entities.
Firstly
they put the position of classical physics:
“[C]lassical
physics assumed a principle of impenetrability, according to which no
two particles could occupy the same spatio-temporal location. Hence,
classical particles were thought to be distinguishable in virtue of
each one having a trajectory in spacetime distinct from every other
one.”
Clearly,
in quantum mechanics (QM), many - or all - the assumptions in the
classical picture are rejected. (Or, at the least, on some
readings of QM all these assumptions are rejected.)
Firstly,
the “principle of impenetrability” is either questioned or
rejected. On the classical picture, if particles are impenetrable,
then that means that “no two particles could occupy the same
spatio-temporal location”. However, if they're penetrable (or if
the notion of impenetrability doesn't make sense), then one can
conclude that two particles “could occupy the same spatio-temporal
location”.
Thus
one can immediately ask the following question:
If
two particles occupy the same spatiotemporal location, then is it
correct to talk about two particles in the first place?
In
consequence, the second part of the classical picture is either
rejected or questioned. That second part (which follows from the
first) is that
“classical
particles were thought to be distinguishable in virtue of each one
having a trajectory in spacetime distinct from every other one”.
Clearly, if the penetrability argument is true and two particles may occupy the same location, then each particle can't be seen to have its own trajectory in spacetime. In other words, it will share that trajectory with other particles.
This
has the result that the Leibnizian picture breaks down (at least
according to L and R) in the case of QM. Or as L and R put it:
“Thus
for everyday objects and for classical particles, the principle of
the Identity of Indiscernibles is true...”
Particles as Package-Deals
It
can be seen that the QM notion of fields plays an important
part in L & R's philosophy. The central argument is that fields
and particles are intimately connected. Indeed they're so strongly
connected that a distinction between the two hardly seems warranted.
Firstly,
there's the problem of distinguishing particles from the states they
'belong' to. Thus, in an example given by L & Ross, we can
interpret a given field/particles situation in two ways:
i)
A two-particle state.
ii)
A single state in which two “two particles [are] interchanged”.
Since it's difficult to decipher whether it's a two-particle state or a single state in which two particles are interchanged, L and R adopt the “alternative metaphysical picture” which “abandons the idea that quantum particles are individuals”. Thus all we have are states. That means that the “positing individuals plus states that are forever inaccessible to them” is deemed (by L & R) to be “ontologically profligate”.
L
and R back-up the idea that states are more important than
individuals (or, what's more, that there are no individuals) by
referring to David Bohm's
theory. In that theory we have the following:
“The
dynamics of the theory are such that the properties, like mass,
charge, and so on, normally associated with particles are in fact
inherent in the quantum field and not in the particles.”
In
other words, mass, charge, etc. are properties of states, not of
individual particles. However, doesn't this position (or reality)
have the consequence that a field takes over the role of an
individual (or of a collection of individuals) in any metaphysics of
the quantum world? Thus does that also mean that everything that's said
about particles can now be said about fields?
On
Bohm's picture ( if not L and R's) “[i] t seems that the particles
only have position”. Yes; surely it must be a particle (not a
field) which has a position. Indeed particles also have trajectories
which account for their different positions.
To
Bohm (at least according to L and R), “trajectories are enough to
individuate particles”. It's prima facie strange how
trajectories can individuate. Unless that means that each type of
particle has a specific type of trajectory. Thus the type trajectory
tells you the type of particle involved in that trajectory.
L
& R spot a problem with Bohm's position. That problem is summed
up in this way:
If
all we have is trajectory (as with structure), then why not dispense
with particles (as individuals at least) altogether?
This
is how L & R explain their stance on Bohm's theory:
“We
may be happy that trajectories are enough to individuate particles in
Bohm theory, but what will distinguish an ‘empty’ trajectory from
an ‘occupied’ one?”
Here
again L & R are basically saying that if all we've got are
trajectories (part of the “structure”?), then let's stick with
them and eliminate particles (as individuals) altogether.
L
& R go into more detail on this by saying that
“[s]ince
none of the physical properties ascribed to the particle will
actually inhere in points of the trajectory, giving content to the
claim that there is actually a ‘particle’ there would seem to
require some notion of the raw stuff of the particle; in other words
haecceities seem to be needed for the individuality of particles of
Bohm theory too”.
If
L & R's physics is correct, then what they say makes sense.
Positing particles seems to run free of Occam's razor. That is, Bohm
is filling the universe's
already-existing ontological slums with yet more superfluous
entities (to mix two metaphors).
One
way of interpreting this position is by citing two different
positions. Thus:
i)
The positing of particles as individuals which exist in and of
themselves.
ii)
The positing of particles as part of a package-deal which includes
fields, states, trajectories, etc.
Then
there's L & R's position.
iii)
If there are never particles in splendid isolation (apart from
fields, etc.), then why see particles as individuals in the first
place?
L
& R are a little more precise as to why they endorse iii) above.
They make the metaphysical point that “haecceities seem to be
needed for the individuality of particles of Bohm's theory too”. In
other words, in order for particles to exist as individuals (as well
as to be taken as existing as individuals), they'll require
“individual essences” in order to be individuated. However, if
the nature of a particle necessarily involves fields, states, other
particles, trajectories, etc., then it's very hard (or impossible) to
make sense of the idea that it could have an individual (or any)
essence.
In
simple terms, a specific particle - and indeed all particles - is
part of a package-deal. Particles simply can't be individuated
without reference to external, extrinsic or relational factors. Thus
particles simply aren't individuals at all.
Reference
Ladyman,
James, Ross, Don. (2007) Every
Thing Must Go: Metaphysics Naturalised.
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