We began this series with a deceptively simple question:
What does a particle do?
The question led us from properties to capacities, from capacities to interactions, from interactions to structures, and from structures to new possibilities.
The electron took us toward chemistry.
The photon took us toward detection, information and semiotics.
Along the way, something rather important became clear.
A particle does not have its consequences in isolation.
An electron's charge matters because there are other charged entities, electromagnetic fields and quantum rules with which it can participate.
A photon becomes biologically significant because there are molecules capable of interacting with it, cells capable of responding to those interactions, organisms capable of exploiting the resulting differences, and environments in which those differences matter.
The significance of a capacity therefore depends upon what else exists around it.
This suggests that we need another concept.
Not merely a hierarchy of levels.
Not merely a chain of causes.
But an ecology of capacities.
Nothing does anything alone
We ordinarily describe things as though their properties belong to them independently.
An electron has charge.
A photon has energy and momentum.
An atom has a particular electronic configuration.
A molecule has a particular shape.
An organism has sensory capacities.
All of this is perfectly legitimate.
But the moment we ask what any of these things does, the isolation begins to break down.
Charge matters because charges interact.
A molecular shape matters because other molecules can interact with it.
A receptor matters because something can bind to it.
An eye matters because there is light to detect.
A nervous system matters because signals can travel through it and alter behaviour.
A hand matters because there are objects that can be grasped.
The ability to do something is therefore not simply a possession.
It is a possibility of participation.
An electron's charge is not a little power stored inside the electron waiting for the universe to provide an opportunity to use it.
Its consequences emerge through relations.
And relations are where ecology begins.
Ecology without biology
The word ecology usually makes us think of organisms: forests, food webs, predators, prey, nutrient cycles, populations and environments.
We are going to use it more broadly.
An ecology, in the sense relevant here, is a system in which entities possess capacities whose consequences depend upon the capacities and constraints of other entities.
That definition does not require life.
An atom already participates in something resembling such an ecology.
Its electrons interact with its nucleus.
Its structure determines how it can interact with other atoms.
Those interactions depend upon the properties of the other atoms.
The resulting molecules have their own capacities, which affect what further molecules can form.
The physical world is therefore filled with mutually conditioning possibilities.
The possibility of one thing depends upon the existence and behaviour of others.
This is not ecology in the biological sense.
But it is ecological in structure.
The electron needs a world
Consider once again the electron.
Its properties are fundamental.
But what those properties make possible depends upon the physical world in which the electron finds itself.
An electron in isolation does not make a molecule.
An electron near a nucleus can participate in an atom.
Electrons in atoms can participate in chemical bonding.
Electrons in particular molecular arrangements can participate in electrical conduction.
Electrons interacting with light can contribute to processes of absorption and emission.
The same fundamental entity can therefore participate in radically different organisations.
What changes is not the electron's identity.
What changes is its relational situation.
This is why a catalogue of properties is not enough.
We need to know not only what something can do, but what it can do here, with these other things, under these constraints.
Possibility is always situated.
The environment is part of the explanation
This point becomes increasingly obvious as we move upward.
A seed does not have the same possibilities in every environment.
A bacterium does not have the same possibilities in a nutrient-rich medium as in a barren one.
A bird's wing does not have the same possibilities in air as underwater.
A human hand has different possibilities when holding a hammer, a violin or a pen.
The capacities of the object have not necessarily changed.
The affordances of the relationship have.
This is why possibility cannot be located entirely inside an object.
A door has a particular physical structure.
Whether it affords passage depends upon there being an opening, an agent capable of moving through it, and sufficient space on the other side.
The affordance belongs neither simply to the door nor simply to the organism.
It belongs to the relation.
The same principle can be found at much smaller scales.
A molecular structure has chemical possibilities because other molecules and fields can interact with it in particular ways.
A receptor has a function because particular molecules can bind to it.
A photon can become information because an organisation exists capable of responding to its differences.
The world is full of relational possibilities.
Capacities meet capacities
This gives us a more precise way of thinking about interaction.
We often describe an interaction as one thing acting upon another.
But perhaps we should also think of it as:
one capacity encountering another capacity.
An electron's electromagnetic capacity encounters another charge.
A molecule's chemical capacity encounters a complementary molecular structure.
A photoreceptor's sensitivity encounters electromagnetic radiation.
A predator's sensory system encounters the signals produced by prey.
A hand's grasping capacity encounters an object with a suitable shape.
In each case, something becomes possible because two systems are compatible in their capacities.
The world is therefore not merely a collection of things that happen to collide.
It is a web of possible encounters.
Some capacities complement one another.
Some interfere.
Some block one another.
Some amplify one another.
Some create conditions under which still other capacities can operate.
This begins to look remarkably like an ecology.
Compatibility is productive
Consider chemical bonding.
Atoms do not form arbitrary molecules in arbitrary ways.
Their electronic structures constrain the combinations that are available.
But those constraints do more than prevent.
They select patterns of compatibility.
Carbon can bond with carbon.
Hydrogen can bond with oxygen.
Certain molecular structures fit together in particular ways.
Other combinations are unstable or impossible.
The resulting chemical world is therefore not a random space of combinations.
It is a structured ecology of compatibilities.
Some molecules enable reactions.
Some catalyse them.
Some inhibit them.
Some store energy.
Some provide scaffolds.
Some carry information.
And once such molecules exist, they change the possibilities available to other molecules.
A molecule can therefore become part of the environment in which another molecule's capacities become consequential.
This is how possibility can become self-amplifying.
Catalysts change the neighbourhood
A catalyst provides a particularly beautiful example.
A catalyst does not simply "cause" a reaction in the ordinary sense.
It changes the pathway by which a reaction can occur, often by lowering the relevant activation barrier.
This means that a reaction that was previously inaccessible or extremely slow can become readily available.
The catalyst has therefore changed the possibility landscape.
And once catalysts themselves participate in networks, the possibilities become even richer.
One reaction can produce a molecule that catalyses another reaction.
That reaction can produce another molecule that affects the first.
Chemical networks can therefore become dynamically coupled.
At this point, the important unit is no longer the individual molecule.
It is the network of capacities.
The behaviour of each component depends partly upon the presence and behaviour of the others.
This is ecological organisation in miniature.
From networks to systems
A network becomes especially interesting when it can maintain some form of organisation through flows of matter and energy.
A living cell is an extraordinary example.
It contains thousands of interacting molecular processes.
Membranes regulate exchange with the environment.
Metabolic pathways transform energy and matter.
Enzymes accelerate particular reactions.
Genetic systems preserve and reproduce information.
Regulatory systems alter activity in response to changing conditions.
None of these components is "the cell."
The cell is the organisation of their capacities.
And the cell's own capacities then become the environment in which its components operate.
The membrane creates a chemical difference between inside and outside.
That difference makes certain reactions possible.
Those reactions help maintain the membrane.
The membrane therefore helps maintain the conditions for the reactions that maintain the membrane.
We have entered a loop.
The organisation becomes partly responsible for maintaining the conditions under which the organisation can continue.
This is a profound change.
The ecology becomes self-maintaining
A rock does not ordinarily maintain the conditions of its own existence through an elaborate network of regulated processes.
A living system does.
It actively maintains itself within a range of conditions compatible with its continued organisation.
Temperature changes.
Chemical concentrations change.
Resources become available or scarce.
The organism responds.
Its responses alter the environment around it.
The altered environment changes the conditions under which further responses occur.
We therefore have:
capacity → interaction → environmental change → new conditions → new capacity.
This is a feedback loop.
And feedback transforms an ecology.
The environment is no longer simply a background in which an organism happens to exist.
The organism becomes an active participant in constructing the environment relevant to its own continued existence.
This will become increasingly important when we return to function.
For now, the important point is that physical capacities can become organised into systems that actively maintain the conditions under which those capacities remain available.
A capacity can create the conditions for another capacity
This may be the central idea of the essay.
A capacity does not merely produce an outcome.
It can produce conditions under which another capacity becomes possible.
The electron's behaviour contributes to molecular structure.
Molecular structure makes chemical reactions possible.
Chemical reactions can create gradients.
Gradients can power molecular machines.
Molecular machines can maintain cellular organisation.
Cellular organisation can maintain the gradients.
The gradient then powers the machines again.
We have a circular economy of possibility.
This is why the ecological metaphor is more than decoration.
In an ecology, the existence of one participant alters the conditions under which other participants can operate.
The same is true here.
Physical capacities can create, preserve or transform the environments in which other capacities become available.
Reality becomes generative.
Possibility is contagious
Perhaps we can now formulate a slightly provocative proposition:
Possibility can be contagious.
Not literally, of course.
But when one organisation creates the conditions for another, the space of possible behaviour expands.
A molecule makes another reaction possible.
That reaction produces another molecule.
The new molecule opens another reaction.
A cell creates a gradient.
The gradient enables a molecular machine.
The machine enables movement.
Movement changes the organism's environment.
The altered environment produces new sensory information.
The sensory information changes behaviour.
Behaviour changes the environment again.
At each stage, possibility propagates through the system.
The important thing is that the new possibility need not have existed as a distinct entity beforehand.
What existed were conditions from which it could emerge.
This is one of the deepest senses in which organisation can be generative.
The ecology of the photon
Let us return to the photon.
A photon enters a photoreceptor.
But the photoreceptor exists because of a cell.
The cell exists because of a metabolic system.
The metabolic system exists because of a network of chemical processes.
The organism exists within an ecological environment.
And the organism's ability to detect photons has consequences because those photons correlate with features of that environment.
The photon therefore participates in an astonishing ecology.
It is not simply:
photon → eye.
It is:
photon → molecular interaction → cellular response → neural organisation → perception → behaviour → environmental change.
And then the consequences return.
The organism's behaviour changes its environment.
The changed environment alters the light reaching the organism.
The new light alters perception.
The organism responds again.
The photon is now participating in a closed ecology of causal and informational relations.
The physical interaction has become part of a living loop.
The environment becomes a participant
This is where the word ecology becomes especially valuable.
An organism is not a self-contained machine sitting inside an external environment.
Its capacities are continuously coupled to what surrounds it.
A fish's swimming capacity depends upon water.
A bird's flight depends upon air.
A plant's photosynthetic capacity depends upon light, carbon dioxide, water and temperature.
An animal's visual capacities depend upon electromagnetic radiation and objects that reflect or emit it.
A nervous system's capacity to guide behaviour depends upon a world sufficiently regular for its signals to be useful.
The environment is therefore not merely where the organism operates.
It is part of the space of possibilities in which the organism exists.
And the organism, in turn, modifies that environment.
The relationship is reciprocal.
The possibility of a niche
This reciprocal relation gives us the concept of a niche.
A niche is not simply a place.
It is a set of relationships between an organism and its environment.
It includes the resources available to the organism, the conditions it can tolerate, the interactions it has with other organisms and the ways in which its own activities alter its surroundings.
In our broader sense, a niche is therefore a structured field of affordances and constraints.
The organism's capacities determine what it can exploit.
The environment determines what is available to exploit.
And the interaction between the two helps determine which possibilities become real.
This makes the ecological metaphor surprisingly close to the conceptual trajectory of our series.
The physical world provides capacities.
Organisations combine them.
Environments constrain them.
Interactions exploit them.
And the resulting organisations alter the environments in which further possibilities become available.
The hierarchy becomes a web
At the beginning of the series, we could draw a simple sequence:
particle → atom → molecule → organism.
We can no longer be satisfied with that.
The electron participates in the atom.
The atom participates in molecules.
Molecules participate in networks.
Networks participate in cells.
Cells participate in organisms.
Organisms participate in ecosystems.
But the arrows also run back.
The organism changes its molecular environment.
The cell changes the conditions under which its molecules operate.
The molecular environment affects the behaviour of the cell.
The ecosystem alters the conditions under which organisms can survive.
The organisms alter the ecosystem.
So reality does not look like a ladder.
It looks like a web.
And the important relations are not simply relations of composition.
They are relations of enablement and constraint.
Constraints become ecological
We encountered constraints in the second essay.
Now we can see how they become relational.
A cell membrane restricts molecular movement.
But that restriction enables chemical gradients.
The gradient enables energy-transducing processes.
Those processes maintain the membrane.
The constraint therefore becomes part of a self-maintaining loop.
Likewise, an organism's sensory limitations constrain what it can detect.
But those limitations also shape the niche it can exploit.
A bird does not need to detect every wavelength.
It needs to detect the differences relevant to its way of life.
A bee's visual capacities differ from ours.
Neither is simply "better."
Each opens a different field of possibilities.
This suggests that constraints do not merely reduce an organism's options.
They help define the world of possibilities in which that organism can operate.
An ecology is a selective world
There is therefore no single undifferentiated space of possibility available equally to everything.
Different systems inhabit different possibility landscapes.
The electron can participate in electromagnetic interactions.
The molecule can participate in chemical reactions.
The cell can exploit gradients.
The plant can exploit light.
The animal can exploit movement and sensory information.
The human can exploit language.
Each level inherits possibilities from the levels beneath it.
But each also acquires new constraints and capacities.
This produces something like a nested ecology of possibility.
The possibilities available to a system depend upon the organisation that system has achieved.
And that organisation depends upon the possibilities available at earlier stages.
There is a circularity here.
Organisation depends upon possibility.
Possibility depends upon organisation.
But it is a productive circularity, because each new organisation can open possibilities that were previously inaccessible.
From possibility to function
We can now return to the word that started this entire adventure.
Function.
If we say that the electron "functions" in molecular bonding, what do we mean?
We mean that its behaviour plays a role in making a particular organisation possible.
If we say that a catalyst functions to facilitate a reaction, we mean that its presence changes the possibilities available within a chemical system.
If we say that a photoreceptor functions to detect light, we mean that its organisation enables an organism to respond selectively to electromagnetic differences.
The word function therefore begins to look less mysterious.
It can mean simply:
the contribution a capacity makes to the possibilities of an organisation.
That formulation avoids both extremes.
We do not need to deny function because we fear teleology.
Nor do we need to attribute intention to matter.
Function can be entirely natural.
It can describe how one part participates in what a larger system can do.
And function can evolve
Biology adds another extraordinary possibility.
A structure can become functionally significant because organisms that possess it leave more descendants.
Over generations, natural selection can preserve and elaborate capacities that contribute to biological persistence and reproduction.
Now a structure's function becomes historically established.
The eye functions to detect light because organisms with visual systems can, under appropriate circumstances, exploit information about their surroundings.
The wing functions in flight because its contribution to flight has been incorporated into an evolutionary lineage.
The enzyme functions in a particular reaction because biological organisation has stabilised that role.
Function now has a history.
But that history remains grounded in the ecological organisation of physical capacities.
The biological function did not descend from the sky.
It emerged from a world in which physical structures could affect the possibilities of living systems.
The ecology becomes informational
We can now see how the previous essay fits into the picture.
An organism does not merely interact physically with its environment.
It can detect differences within it.
Those differences can become information.
Information can become significant.
Significant information can alter behaviour.
Behaviour alters the environment.
The altered environment produces new information.
So the ecological loop becomes:
physical interaction → detection → information → action → environmental change → new interaction.
The ecology is therefore simultaneously physical and informational.
The two are not separate worlds.
Information is instantiated in physical differences.
But the significance of those differences depends upon the organisation capable of exploiting them.
This gives us a bridge between physics and semiotics without pretending that the two are identical.
The remarkable thing about life
Perhaps this is what makes living systems so extraordinary.
Life does not merely persist in a physical environment.
It actively organises physical possibilities.
It constructs gradients.
It builds membranes.
It stores information.
It detects differences.
It transforms energy.
It modifies its surroundings.
It reproduces organisation.
And evolution explores new combinations of capacities.
Life therefore becomes a kind of engine for discovering what the physical world can do.
Not consciously.
Not intentionally.
But historically.
Each successful organism embodies a particular solution to the question:
What can this physical environment be made to afford?
Evolution explores that question without asking it.
A forest is an ecology of capacities
Consider a forest.
The forest is not merely a collection of trees, animals, fungi and microorganisms.
Trees alter light levels.
Roots alter soil structure.
Fungi alter nutrient availability.
Microorganisms transform chemical compounds.
Animals transport seeds.
Leaves alter humidity and temperature.
Predators alter populations.
The capacities of each participant modify the possibilities available to the others.
A tree makes a habitat possible.
A fungus makes nutrients available.
An animal makes seed dispersal possible.
A fallen tree creates new possibilities for other organisms.
Death becomes an ecological resource.
The forest therefore exhibits the same general pattern we have been tracing from the particle upward:
one organisation changes the possibility landscape for another.
The difference is scale.
The underlying principle is remarkably similar.
The universe as an ecology of capacities
We should now be in a position to make a larger claim.
Perhaps the deepest common feature connecting particles, molecules, cells and organisms is not that they are all "things."
It is that they are bearers and participants of capacities.
A particle has capacities defined by its physical properties.
A molecule has capacities defined partly by its structure.
A cell has capacities defined by its organisation.
An organism has capacities defined by its biological organisation.
An ecosystem contains interacting populations whose capacities alter one another's possibilities.
At every level, therefore, we can ask the same question:
What can this system do, given what it is connected to?
That question is ecological before it is biological.
And it may offer us a more unified way of thinking about emergence.
The world does not merely contain possibilities
There is, however, a subtle shift in our understanding now.
At first, we thought of possibilities as things that physical laws permit.
Now that seems incomplete.
Possibilities are also generated by organisation.
An electron makes certain atomic structures possible.
Atoms make molecular structures possible.
Molecules make chemical networks possible.
Chemical networks make cellular organisation possible.
Cells make organisms possible.
Organisms make ecological and semiotic relationships possible.
Each new organisation therefore does more than occupy the world.
It changes the world of possibilities available within it.
This is perhaps the deepest meaning of emergence.
A new level does not merely add another object to reality.
It adds another way for reality to act.
The ecology of what becomes possible
We can now return to the title of our series.
The Physics of What Becomes Possible.
The phrase "becomes possible" is doing more work than we perhaps realised at the beginning.
Possibility is not simply sitting in a cosmic warehouse waiting to be selected.
Physical capacities interact.
Interactions form structures.
Structures create new capacities.
Capacities alter environments.
Environments select among possibilities.
Some organisations persist.
Some disappear.
Some create conditions for others.
And the resulting world contains possibilities that were not accessible before.
Reality is therefore not merely a sequence of events.
It is a history of changing possibility landscapes.
And perhaps this is what an ecology really is
We began by wondering whether our project might be called an "ecology of particle physics."
Perhaps that was not such a strange intuition after all.
An ecology, in the broadest sense relevant here, is not merely a collection of living things interacting with nature.
It is a world in which the capacities of one thing alter the possibilities available to another.
By that definition, the idea reaches surprisingly deep into physics.
Particles interact.
Atoms constrain and enable one another.
Molecules form networks.
Networks become systems.
Systems modify their environments.
Organisms exploit and transform those environments.
And eventually, organisms create worlds of signs, symbols and shared meanings.
The layers are not separate.
They are nested, coupled and historically connected.
The ecology grows richer as new capacities appear.
The next threshold
We have now reached an important point in our journey.
We began by asking what a particle does.
We discovered that its behaviour can contribute to structures.
Structures can open possibilities.
Possibilities can become capacities.
Capacities can enter ecological relationships.
And ecological relationships can produce systems that maintain themselves, exploit information and acquire functions.
But this leaves us with a question.
At what point should we actually call one of these capacities a function?
Is there a meaningful distinction between a capacity that merely happens to contribute to an organisation and a capacity that has become a function of that organisation?
The eye detects light.
The heart circulates blood.
The enzyme catalyses a reaction.
The electron participates in chemical bonding.
Are these instances of one general phenomenon, or are we equivocating between very different senses of "function"?
Perhaps the answer lies in the way organisations come to depend upon particular capacities.
And that may allow us, in the next essay, to return to the word that began our adventure without fear of either teleology or reductionism.
We can ask:
When does a capacity become a function?
Because perhaps function is not something that was present in matter from the beginning.
Perhaps it is what a capacity becomes when an ecology learns to depend upon it.