James Ladyman and Don Ross argue that particles can’t be individuated without reference to external and relational factors. (All particles are parts of package deals.) So, to them, this means that particles simply aren’t “things” at all.
This essay is a commentary on the ‘Ontic Structural Realism and the Philosophy of Physics’ chapter of James Ladyman and Don Ross’s book Every Thing Must Go: Metaphysics Naturalised. (There are also a handful of references to — and quotes from — other parts of that book.)
The rather pompous term “ontic structural realism” basically refers to a realism about structures, not a realism about what the authors call “things” (i.e., things such as particles… or anything else other than structures).
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The philosophers James Ladyman and Don Ross’s philosophy of physics is primarily motivated by the findings and theories of quantum mechanics. Elements of their position can be traced back to — among others — the German philosopher Ernst Cassirer (who died in 1945).
Ladyman and Ross 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.”
Some readers may wonder what the words “nothing but structure” actually mean.
The usual assumption is that there can only be a structure if there are things (of some kind) that make up (or are part of) that structure. That said, this may be one of those everyday “intuitions” which the authors warn us against. Hopefully, Ladyman and Ross’s position on the nature of structure will become clear in the following. In addition, the phrase “[w]e can’t describe [a field’s] nature without recourse to the mathematical structure” is fairly commonplace — at least within physics — and has been since the late 19th century.
What Ladyman and Ross say about Ernst Cassirer’s position on objects (or “things”) is almost exactly the same as their own position on objects. Indeed it was also quantum mechanics which provided Cassirer himself with the motivation to reject (what Ladyman and Ross call) “individual objects”. Ladyman and Ross 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.”
It may seem odd that, at least within the context of 20th-century physics, an “Aristotelian idea” is being discussed at all. Not only that: we also have the technical philosophical term individual substance too. That said, these are the words of two philosophers of physics, not the words of two physicists.
One can firstly ask whether or not a commitment to the existence of objects (or things) is also automatically a commitment to “individual substances”; as well as to intrinsic (or essential) properties. After all, the bundle theory (among other theories) rejects the notion of substance; though not that of an individual.
We can also ask whether or not these positions are equally applicable to objects in the “classical” (or macro) world. Let’s put it this way. Cassirer’s, Ladyman’s and Ross’s positions are far more acceptable when applied the the quantum world than when applied to the classical world. More precisely, all this is far easier to swallow in the (to use Ladyman and Ross’s words) “context of field theory” than it is in relation to human persons, cups or trees.
Essential and Intrinsic Properties?
It does seem strange (if only 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”. As it is, mass, size and charge are (essentially) seen as intrinsic properties. In fact it’s hard to imagine elementary particles having other (i.e., inessential) properties. After all, we’re not talking about classical objects here. So one can also easily imagine a natural kind having essential properties; though, when it comes to a particular member of that kind, having contingent properties too.
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 the set {mass, size, 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 classical objects such as lions or even a given sample of water.
That said, Ladyman and Ross do cite examples of what can be taken as a particle’s accidental properties. They cite “velocity or position at a particular time”. Indeed, in commonsense terms, it can be doubted that anyone would take the position or velocity of a particle at a particular time as being anything other than an “accidental” property. (Could it be that physicists could take a particular velocity — or at least mean velocity — of a type of particle to be essential to it?)
From what’s just be said, it’s therefore no surprise that different particles of the same type can be seen as what Ladyman and Ross call “indistinguishable”. However, particles may still have properties which aren’t intrinsic. That is, they may have 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 four-dimensional objects than classical objects (see ‘Four-dimensionalism’)?
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?
Ladyman and Ross 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, many — or all — the assumptions in the classical picture above are rejected. (At the least, on some interpretations of quantum mechanics 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”. On the other hand, if particles are penetrable (or if the notion of penetrability does 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 in the case of quantum mechanics. Or as Ladyman and Ross put it:
“Thus for everyday objects and for classical particles, the principle of the Identity of Indiscernibles is true [].”
It can now be shown that the notion of a field plays an important part in Ladyman and Ross’s philosophy.
Particles as Package Deals
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 a particle from the state it (as it were) belongs to. Thus, in an example given by Ladyman and Ross, we can interpret a given field/particle situation in two ways:
1) A two-particle state.
2) 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, Ladyman and Ross adopt the “alternative metaphysical picture” which “abandons the idea that quantum particles are individuals”. Thus all we have are states. This means that the “positing individuals plus states that are forever inaccessible to them” is deemed (by Ladyman and Ross) to be “ontologically profligate”.
Ladyman and Ross back up the idea that states are more important than individuals (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.”
All this means that 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? Indeed doesn’t that also mean that everything that was said about particles can now be said about fields?
On Bohm’s picture ( if not Ladyman and Ross’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 at different times.
To Bohm, “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 of trajectory will tell us which type of particle is involved in that trajectory.
Ladyman and Ross 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 Ladyman and Ross 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 Ladyman and Ross are basically saying that if all we’ve got are trajectories (which is part of the structure?), then let’s stick with trajectories and eliminate particles (as individuals) altogether.
Ladyman and Ross go into more detail on this by arguing 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”.
Of course most physicists would have no time for a term like “haecceities”, but if Ladyman and Ross’s physics is correct, then what they say makes sense. Positing particles seems to run free of Occam’s razor. In other words, Bohm was (to mix two metaphors) filling the universe’s already-existing ontological slums with yet more superfluous entities .
One way of interpreting this position is by citing two different positions. Thus:
1) The positing of particles as individuals which exist in and of themselves.
2) The positing of particles as part of package deals which include fields, states, trajectories, etc.
Then there’s Ladyman and Ross’s position.
3) If there are never particles in splendid isolation (apart from fields, etc.), then why see particles as individuals in the first place?
Ladyman and Ross are a little more precise as to why they endorse 3) above. They make the metaphysical point that “haecceities seem to be needed for the individuality of particles of Bohm’s theory too” (see ‘Haecceity’). In other words, in order for particles to exist as individuals (as well as to be taken as existing as individuals), such particles will require what Ladyman and Ross call “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 essence.
To sum up. A specific particle is always part of a package deal. Indeed all particles simply can’t be individuated without reference to external, extrinsic or relational factors. Thus particles simply aren’t individuals (or things) at all.
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