The traditional view of particles can be said to have been articulated by Isaac Newton, who wrote the following:
“God in the beginning formed matter in solid, massy, hard, impenetrable, movable particles, of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them.”
As will be seen, just about everything in that quote will be discussed in this piece: the impenetrability of particles; the fact that particles are seen as things (with “sizes and figures”); the view that particles have “properties”; and Newton even hints (if only loosely) at particles being what philosophers call “individuals”.
Erwin Schrodinger, on the other hand, put a position (if 270 or or so years later) which is very much at odds with Newton's. He wrote:
“A careful analysis of the process of observation in atomic physics has shown that the subatomic particles have no meaning as isolated entities, but can only be understood as interconnections between the preparation of an experiment and the subsequent measurement.”
This is just one of many positions on subatomic particles which question their status as what philosophers call "individuals" - or even as things. Schrodinger emphasises the “interconnections between the preparation of an experiment and the subsequent measurement”. Other physicists and philosophers have stressed the various fields of physics, quantum entanglement, particles and their anti-particles, particles “swallowing” other particles, etc.
However, it's not strange that particles have been seen as particles when one looks at the scientific literature.
Take the case of the Irish physicist and Nobel laureate, Ernest Walton.
Here is a perfect case of mistaking effects for causes when it comes to particles. However, it is indeed probably the case that Walton wrote the following words for purely explanatory purposes. In any case, Walton wrote:
“Particles were coming out of the lithium, hitting the screen, and producing scintillations. They looked like stars suddenly appearing and disappearing.”
The fact is that Walton didn't see or even observe particles “coming out of the lithium, hitting the screen, and producing scintillations”. In addition, the particles wouldn't have “looked like stars”. What Walton would have seed or observed, and what would have looked like stars, were the experimental observed effects of the actions (or behaviour) of particles.
This problem is made clear in something the physicist Eric Allin Cornell once wrote:
“The postdoc explained to me how to distinguish different sorts of particles on the basis of the amounts of energy they deposited in various sorts of detectors, spark chambers, calorimeters, what have you.”
In the quote above it's made clear that it's the effects of particles that Eric Allin Cornell is talking about - not (really?) particles themselves. In other words, particles are inferred or posited from the “amounts of energy they deposited in various sorts of detectors, spark chambers, calorimeters”. Thus it can be said that (at least in this case) the particles were neither seen nor observed. In other words, they were (only?) “theoretical entities”.
What is a Particle?
Of course no one should get too fixated on the word “particle”. It's true that many physicists (as well as a fair few philosophers) get annoyed with what used to be called “conceptual analysis”. However, “particle” is the word which is used in physics - so surely that's the best place to start. After all, the place we start from is not necessarily the place we will end.
When it comes to basic definitions of the word “particle”, it is defined as “an extremely small piece of something”. In that sense, then, the particles of physics are extremely small pieces of something else. It can be said that electrons are “parts” of atoms; protons and neutrons are parts of atomic nuclei; and quarks are parts of neutrons and protons. Of course in philosophy what has been called “parthood” has been a very import subject of philosophical discussion. (We can ask what it is for X to be a part of Y. We can ask if X is an "essential" part of Y. We can also discuss the exact relation of X to Y and do so in either physical or metaphysical terms.)
The notions of an individual and of being separate are also found in definitions of the word “particle”.
The word “individual” also throws up philosophical problems.
If an entity (or thing) is an individual, then (on some definitions at least) an individual is defined as being “single” or “separate”. That clearly doesn't work for particles - for a whole host of reasons. It's true that on some “holistic” (or “relationist”) readings, this also applies to almost all things - not only subatomic entities. After all, it can be argued that persons are intrinsically related (or connected) to not only other persons, but also to other things. On another “essentialist” level, if it weren't for my parents, I wouldn't even exist. (Thus that may be an essential relation - something Saul Kripke noted back in the early 1980.) Nonetheless, even if we accept vital (or even essential) relations, that doesn't automatically mean that an individual can't still be separate. Or, in technical speak, “relata” may still be separate from their relations and therefore still be individuals. However, in the case of particles, a clear lack of separation is fundamental to their nature.
Perhaps, in the end, this is simply a question of what the word “separate” or “separation” is taken to mean. And, if that's the case, we can simply stipulate what we take it to mean.
It's also worth noting that the situation with subatomic particles is very different to the situation with almost all other objects or things. That's because particles of a particular type are all identical in terms of their properties. Take electrons, which have the same charge, rest mass, spin, etc. This, on the surface at least, appears to violate Leibniz’s Principle of the Identity of Indiscernibles.
There are, however, ways of distinguishing electrons even though they have identical spin, mass, charge, decay rate, etc. That is, they'll still have different spatiotemporal trajectories which can't overlap. This also entails the view that each electron is impenetrable. That is, if an electron were penetrable, then it could (or would) share a spatiotemporal trajectory with another particle.
It's also the case that some philosophers (e.g., Bas van Fraassan) have individuated particles in terms of their history.
The notion of an individual can also be tied to the parallel notions of “intrinsic” and “relational” properties. Non-intrinsic properties can be “state-dependent” and therefore cashed in terms of monadic and relational properties. In other words, the state determines the properties and therefore the particle is not (or may not) be an individual. This has the result that two particles can have the same state-dependent properties. Or, in philosophical technical jargon, according to the Principle of the Identity of Indiscernibles, such particles will be identical. In any case, particles all have the same properties even when seen in a non-state-dependent context.
Of course if a physicist takes an instrumentalist line of particles, he may not care if they're deemed to be individuals or not. Indeed not many physicists use the word “individuals”, which is a philosophical term. What's more, we also have a situation of “underdetermination” here. That is, a physicist can happily accept a theory (or position) in which particles are seen as individuals or accept one that doesn't take that position. In the end, then, it may not matter to the physicist because he may see it as a difference which doesn't (really) make a difference.
Another notion (or definition) that's stressed when it comes to individuals is that of indivisibility. It's of course the case that if an individual is deemed to be that which is indivisible, then that doesn't work for all particles. It works for quarks (though has been questioned). However, it doesn't work for neutrons, protons and indeed atoms. There's also the fact that some particles “turn into” other particles (though, technically, this may not be the best way of putting it). In addition, Higgs bosons are now said to provide mass to other particles (though, technically, this isn't the best way of putting it).
The very notion of indivisibility may also be problematic in a wider sense. Even if x or y were indivisible, it may still have “separate” parts or properties. After all, particles have mass, charge, spin, decay rate, etc. Yet these properties are also strongly interrelated. Nonetheless, it's the case that if a particle ceases to have that spin, mass, charge and decay rate, then, quite simply, it is no longer that particle. In that sense, then, if a particle is divided, then it's no longer the particle it was. It must therefore be indivisible.
What Physicists & Philosophers Have Said About Particles
Perhaps we should heed the words of Werner Heisenberg when he wrote following:
“Actually we need not speak of particles at all. For many experiments it is more convenient to speak of matter waves; for instance, of stationary matter waves around the atomic nucleus.... The use of 'matter waves' is convenient, for example, when dealing with the radiation emitted by the atom.”
Yet, since the wave-particle duality is essential to quantum mechanics, one can ask if Heisenberg was being literal about this. Indeed this situation is complicated even more when Heisenberg says that speaking of “matter waves” is “more convenient”. So, here at least, he appears to be taking an instrumentalist position on the nomenclature. In others words, perhaps the word “wave” is just as metaphorical, loose or ontologically suspect as the word “particle”.
The philosopher Ernan McMullin (in his 'A Case for Scientific Realism') also says that electrons “are not particles strictly speaking”. That's because
“electrons do not obey classical (Boltzman) statistics, as the familiar enduring individuals of our middle-sized world do”.
McMullin elaborates on this. He writes:
“The use of namelike terms, such as 'electron', and the apparent causal simplicity of oil-drop or cloud-track experiments, could easily mislead one into supposing that electrons are very small localized individual entities with the standard mechanical properties of mass and momentum. Yet a bound electron might more accurately be thought of as a state of the system in which it is is bound than a separate discriminable entity... What is meant by 'particle' in this instance reduces to the expression of a force characteristic of a particular field...”
Then again, some physicists have seen particles as particles. David Bohm, for example, was keen to argue that states didn't “collapse” into particles when observed. There are particles from beginning to end.
The American philosopher Ernest Nagel (in his 'The Cognitive Status of Theories') had a different (though related) take on particles. Firstly he discussed their “puzzling characteristics”. These puzzling characteristics seem to be “incompatible”. (Though the word “incompatible” isn't a synonym of “contradictory”.) More precisely, electrons are “construed to have features which make it appropriate to think of them as a system of waves”. Yet, “on the other hand”, electrons “also have traits which lead us to think of them as particles”. They are deemed to be particles because each one has “spatial location and a velocity”. However, “no determinate position and velocity can in principle be assigned simultaneously to any of them”. It is here that Nagel appears to deflate quantum mechanics. He does so by saying that
“many physicists have therefore concluded that quantum theory cannot be viewed as a statement about an 'objectively existing' domain of things and processes... On the contrary, the theory must be regarded simply as a conceptual schema or a policy for guiding and coordinating experiments”.
However, as with contemporary ontic structural realism (which will be discussed later), this deflation of quantum mechanics is far from being complete. Rather,
“the fact that a visualizable model embodying the laws of classical physics cannot be given for quantum theory... is not an adequate ground for denying that the quantum theory does formulate the structural properties of subatomic processes”.
In other words, “every thing must go”. And when every thing has gone, all we really have left is what Nagel calls “structural properties”.
Dirac & Feynman on How Particles Behave
Instead of questioning whether or not there are particles (or, in ontic structural realist terms, whether there are “individuals”), we can emphasise “how [electrons and protons] behave, how they move”, as Paul Dirac did. This, of course, immediately raises the following point:
Surely only things (or particles) can “behave” or “move”.
That is, you can't have behaviour or movement without things/particles which display that behaviour or which move. Then again, what if it's the case that (from an anti-realist perspective) we can't get at what it is that behaves or moves. In other words, all we have is behaviour or movement.
Thus Dirac goes on to compare particles to the pieces of chess. He writes:
“I can describe the situation by comparing it to the game of chess. In chess, we have various chessmen, kings, knights, pawns and so on. If you ask what chessman is, the answer would be that it is a piece of wood, or a piece of ivory, or perhaps just a sign written on paper, or anything whatever. It does not matter. Each chessman has a characteristic way of moving and this is all that matters about it. The whole game of chess follows from this way of moving the various chessmen.”
Yet the obvious point must be made again.
Yes, chess is defined by how the pieces (to use Dirac's own terms) “behave” or “move”. Nonetheless, the chess pieces must still exist and chess can't be played without them. (I suppose there could be a purely abstract version of chess.) It's no use Dirac saying that a chess piece can be “piece of wood, or a piece of ivory” if it must still be a something – a thing. Having said that, isn't it easier to see chess as being a literally abstract game than it is to see the fundamental nature (or parts) of reality as being an abstract... something? In a certain sense, a chess game can be explained mathematically and with little physical remainder; though, surely, that isn't the case when it comes to reality or particles... Or is it?
The analogy between chess pieces and particles may break down in another way too. It's of course the case that the way chess pieces move or behave is not dependent on their being made of wood, ivory or of anything else. When it comes to particles, on the other hand, their physical nature may – or surely must - determine how they behave or move.
To put it bluntly: Dirac's position seems to be eliminativist when it comes to particles. Yet if it is eliminativist, then why speak of “particles” at all? Unless, of course, the word “particle” is simply shorthand for specific types of behaviour or movement. However, this simply raises the same question again:
What is it that behaves or moves?
Richard Feynman also had a problem with seeing particles as particles. He also hinted at the possibility that we have a wave-particle duality simply because there are no particles in the first place. (Thus there is no wave-particle duality?) He wrote:
“Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.”
So perhaps there can be particles which “do not behave like particles”! However, isn't that claim hard to make sense of? In any case, like Dirac, Feynman was emphasising behaviour, not particles or things. Yet here again we can say that surely only things can behave or move. (This seems to parallel the relations-no-relata stance of ontic structural realism.)
Quantum Field Theory
If particles aren't fundamental, then what is fundamental? Lee Smolin gives an answer which is expressed in the clearest possible terms:
“If fields are not made from matter, perhaps fields are the fundamental stuff. Matter must then be made from fields.”
Of course it needs to be said that this quote expresses a conditional. Thus Smolin leave open the possibility that fields aren't fundamental. Nonetheless, these words express the huge importance of fields in physics: from Michael Faraday's electric and magnetic fields, to the Higgs field.
Indeed Smolin sees “the geometry of space as another field”. Not only that: we also have a symmetry here in that “the geometry of space is almost the same as the gravitational field”. Finally, if we take a look at the whole picture, then Smolin finishes off by saying that “[w]e have a bunch of fields all interacting with one another, all dynamical, all influencing one another”.
So we need to know what a field is.
In broad terms, in classical physics, fields were seen as global stuff or substance. Alternatively, a field is just a way of assigning properties to various spacetime points. Thus, “[i]n the case of quantum field theory”, Paul Teller tells us that
“the field quantities are not well-defined at such points (because of difficulties in defining exact locational states in quantum field theory) but are instead regarded as ‘smeared’ over space-time regions”.
Now if we turn to the more relevant idea of quantum field theory (QFT), we can say that although fields are stressed, particles aren't thereby dispensed with. QFT retains the notion of “point particles” as well as their locality. Nonetheless, such particles are deemed to be the excited states of such fields. In other words, they are “field quanta”.
Indeed the quantum-mechanical interactions of particles are seen as interactions in their corresponding (or underlying) quantum fields.
All this raises the question as to whether the word “quantum” is simply a synonym for the word “particle”.
So what is a quantum?
A quantum is the minimum amount of any physical entity (or of a physical property) which is involved in an interaction. This immediately raises a problem - at least from a philosophical point of view. In this definition, a quantum is (in basic terms) an “amount” of a “physical entity” (or of a “property”). In more technical terms, that entity (or property) can therefore be “quantized”.That means that the entity (or property) has a magnitude. That magnitude can take on certain values. Indeed it can only take on “discrete values” which are then measured in terms of integer multiples of one quantum.
Thus, here at least, there's a distinction being made between a quantum and a physical entity/thing/particle. We have an entity/thing/particle and then we have an amount (a quantum) of that entity/thing/particle. On this reading, then, an entity's quantum (value) can't be numerically identical to that entity.
Yet a photon (for example) is indeed a single quantum of light. In fact it's referred to as a "light quantum" or as a “light particle”. So here we are back to particles! It's also the case that single particles are fired in double-slit experiments - sometimes at relatively long intervals! However, are particles really fired? Of course something must be fired. Though is it the case that some thing is fired?