We are accustomed to asking what a particle is.
An electron has a charge, a mass, a spin. A photon has energy, momentum and spin. Quarks carry colour charge. Neutrinos have an extraordinarily small mass and interact only weakly with matter. Physics gives us an increasingly precise account of these properties, together with the mathematical laws governing the interactions between the entities that possess them.
But there is another question we might ask.
What does a particle make possible?
At first this sounds like a strange question to ask of physics. It seems to smuggle purpose into a domain where there is none. Electrons do not exist in order to make molecules. Photons were not invented so that organisms could see. Quarks do not have a job description.
And yet the question refuses to disappear.
Remove the electron from the universe and something much more significant disappears than a particle species. Atomic structure as we know it disappears; so does chemical bonding, and with it the enormous domain of molecular organisation that chemistry makes possible. Remove the photon and we lose not merely a convenient means of transporting electromagnetic energy, but the physical processes through which matter can interact with electromagnetic radiation—and, eventually, the possibility of eyes, cameras, photosynthesis, visual information and an astonishing range of biological and technological systems.
None of this means that electrons or photons have purposes.
It means something subtler.
Their properties enable possibilities.
That distinction may turn out to matter.
The electron does not have a job
Consider the electron.
If we describe it in the usual language of particle physics, we can say that it is a fundamental fermion with negative electric charge, a particular mass and spin, and a characteristic pattern of interactions. None of these properties says anything about chemistry. There is no little label attached to the electron reading:
Primary function: molecular bonding.
Yet molecular bonding depends upon the behaviour of electrons.
The electron's quantum-mechanical properties permit atoms to possess structures in which electrons occupy particular states around nuclei. The electromagnetic interaction permits those electrons and nuclei to participate in stable configurations. Atoms can therefore combine into molecules, and molecules can possess structures and behaviours that no isolated atom possesses.
The important point is not simply that chemistry happens to involve electrons.
It is that the possibilities of chemistry are enabled by what electrons can do.
The distinction is easy to overlook because, once a higher-level phenomenon has become familiar, we tend to treat its enabling conditions as part of the background.
We see a molecule.
We do not ordinarily see the enormous physical history of possibility that makes the molecule possible.
We see a piece of wood.
We do not see the electronic structure of its atoms, the chemical bonds that hold its molecules together, the molecular reactions through which its biological constituents were produced, or the electromagnetic interactions that make its physical structure stable.
And yet all of these remain present as conditions of possibility.
The molecule is not secretly contained inside the electron.
That would be a mistake.
The electron does not contain chemistry any more than a seed contains a tree in miniature. Rather, the properties of electrons participate in a physical organisation from which genuinely new possibilities arise.
This is the first distinction we will need.
A possibility can be enabled without being already present.
From property to capacity
Perhaps, then, we need a word between property and function.
A property is something an entity has.
A capacity is what that property makes possible when the entity enters into appropriate relations.
An electron has electric charge.
But its charge becomes consequential through electromagnetic interaction. Its quantum properties become consequential through the structures available to electrons within atoms. Those structures, in turn, participate in chemical bonding.
We might therefore trace a sequence:
property → interaction → organisation → possibility
The sequence is important because nothing teleological has been added.
The electron does not "aim" at the atom.
The atom does not "aim" at the molecule.
The molecule does not "aim" at life.
And yet each stage creates conditions under which something that was previously unavailable becomes possible.
This suggests that the language of possibility may sometimes tell us something that the language of objects alone conceals.
Physics can tell us what entities are and how they behave.
But perhaps we should also ask:
What new possibilities become available because they behave in this way?
That question does not replace the ordinary physics.
It follows it.
A particle has consequences far beyond its scale
There is something almost vertiginous about following this question.
Take the electron again.
Electron behaviour makes possible stable atoms.
Stable atoms make possible chemical bonds.
Chemical bonds make possible molecules.
Molecules make possible complex materials.
Complex molecular organisation makes possible biochemistry.
Biochemistry makes possible living systems.
Living systems make possible metabolism, reproduction and evolution.
Evolution makes possible organisms with increasingly elaborate ways of exploiting their environments.
And eventually some of those organisms develop nervous systems, eyes, brains, languages and technologies.
We should be careful here. It would be absurd to say that the electron causes language. There are too many intervening levels, contingencies and historical pathways for such a statement to mean anything useful.
But we can say something weaker and more interesting:
The possibility of language depends upon a world in which electrons can participate in the organisations they do.
That is a different kind of statement.
It does not identify a single cause.
It identifies an enabling condition.
And perhaps there is a whole class of explanations that physics usually leaves implicit because its primary task is elsewhere: explanations of how one physical possibility becomes the condition for another.
The photon is stranger still
The electron gives us a first glimpse of this way of thinking.
The photon may take us somewhere much stranger.
A photon can be understood, in the appropriate physical description, as a quantum of the electromagnetic field. It participates in electromagnetic interactions and carries energy and momentum.
But follow the consequences of its existence.
Electromagnetic radiation can interact with matter. Matter can absorb and emit photons. Different wavelengths can interact differently with different physical systems.
At some point in the history of the universe, however, something remarkable happens.
There are organisms capable of detecting these differences.
A photoreceptor responds to the absorption of a photon. Neural machinery can distinguish patterns in those responses. An organism can thereby acquire information about its surroundings.
Now a purely physical distinction has acquired a new role.
A difference in electromagnetic radiation has become a difference for an organism.
The photon itself has not changed.
What has changed is the organisation into which its interaction has entered.
And this gives us a second important transition:
A physical difference can become a detectable difference.
That is already a profound transformation of possibility.
But it does not stop there.
From difference to information
Suppose an organism can distinguish one pattern of light from another.
The difference is now capable of altering the organism's state.
If the pattern reliably correlates with something in the environment—a predator, a mate, a source of food, the boundary between day and night—then the physical difference can acquire an informational role.
We should resist the temptation to say that the photon itself contains meaning.
It doesn't.
The meaning arises from the organisation of the system capable of responding to it.
This gives us another sequence:
physical difference → detectable difference → informational difference → meaningful difference
That sequence will eventually take us far beyond particle physics.
But its beginning is already physical.
And this is why the question "What does a particle do?" is more interesting than it first appears.
It does not ask what the particle was designed to accomplish.
It asks what its capacities can become within larger organisations.
Function without purpose
This brings us to the dangerous word with which we began: function.
Biology is full of functional language.
The heart has the function of circulating blood. The eye has the function of detecting light. The photoreceptor has the function of responding to particular physical signals.
But the electron has no biological function.
So perhaps we should not simply extend biological functional language downward until electrons acquire purposes.
Instead, we might move in the opposite direction.
Perhaps what we call function in biology is a particularly sophisticated form of a more general relation:
a capacity becomes functionally significant when an organisation comes to depend upon it.
The electron does not have the function of forming molecules.
But molecular organisation depends upon electronic behaviour.
The photon does not have the function of enabling vision.
But vision depends upon organisms being able to exploit electromagnetic interactions.
The difference matters.
A function need not be a purpose imposed from outside. It can be a role that emerges within an organisation of dependencies.
This does not make electrons biological.
It makes biological function intelligible as something that could emerge from a world already structured by capacities and constraints.
The world is not merely a collection of things
There is perhaps a deeper lesson here.
When we describe a particle by its properties, we are describing what it is capable of doing.
But the consequences of those capacities cannot be found by inspecting the particle in isolation.
They appear through relations.
The electron becomes chemically consequential because of its relations with nuclei and electromagnetic fields.
The photon becomes biologically consequential because of its relations with matter and, eventually, with organisms.
The organism becomes capable of seeing because its components are organised in ways that allow physical differences to propagate and acquire significance.
At every stage, therefore, something new becomes possible because existing capacities have entered into a new organisation.
This gives us a rather different picture of physical reality.
Instead of imagining reality as a catalogue of objects, each carrying a list of properties, we might imagine it as an architecture of possibilities.
Objects have properties.
Properties confer capacities.
Capacities enter into relations.
Relations can form organisations.
Organisations open new possibilities.
And those possibilities can become the conditions for still further organisations.
The world begins to look less like a warehouse and more like a landscape.
But does the landscape exist in the physics?
We should pause here.
It would be very easy to become intoxicated by this language.
"Possibility," "affordance," "organisation," "function," "information," "meaning": these are powerful words, and they can make a familiar story sound profound without actually adding anything to it.
So we should be suspicious of our own metaphor.
Does the electron really afford molecular bonding?
Does the photon really enable vision?
Or are we simply describing the consequences of physical laws in a more evocative vocabulary?
Perhaps.
That is one of the questions this series will have to earn the right to answer.
For now, however, there is a reason to take the question seriously.
The language of enabling possibility draws our attention to something real that ordinary descriptions can obscure: the dependence of higher-order phenomena upon lower-order capacities without reducing the higher-order phenomena to those capacities.
Chemistry is not merely "electrons."
Vision is not merely "photons."
Yet chemistry would be impossible without the physical possibilities opened by electrons, and vision would be impossible without the electromagnetic possibilities exploited by biological systems.
The interesting territory lies precisely between those two claims.
What does a particle do?
Perhaps, then, we can reformulate our original question.
We should not ask:
What is the electron for?
We should ask:
What does the electron make possible?
And then:
What becomes possible because those possibilities exist?
And then:
What new organisations can exploit those possibilities?
These questions take us somewhere the usual vocabulary of particles and properties does not quite reach.
They ask us to follow possibility through the world.
The electron does not know about chemistry.
The photon does not know about vision.
Neither knows about life, information or meaning.
And yet the universe in which they behave as they do is a universe in which chemistry, vision, life, information and meaning can arise.
Perhaps that is not a marginal fact about physics.
Perhaps it is one of the most important things physics has quietly been telling us all along.
The particle does not have a purpose.
But it has consequences.
And among those consequences are new possibilities.
That may be where our adventure begins.