There is a peculiar moment in quantum physics when a familiar picture of reality begins to dissolve.
We are accustomed to imagining a world made of actual things.
An object is here.
It has a position.
It has a momentum.
It moves through space.
At some later time, it is there.
The world, on this picture, consists fundamentally of things that are, and physics describes what those things do.
Quantum mechanics complicates this almost immediately.
The wavefunction does not seem to describe a collection of already-actual properties in quite this way. Instead, it gives us a structured description of what might happen.
And this raises a deceptively simple question:
What kind of thing is a possibility before it becomes an event?
The question is not merely philosophical.
It is already implicit in the mathematics of quantum theory.
And if we take it seriously, it leads us somewhere rather strange.
Perhaps actuality is not the only fundamental category available to physics.
Perhaps, before the event, there is something that is neither an actual thing nor mere ignorance.
There is a structured space of what could happen.
The classical picture
The classical imagination is wonderfully powerful.
Imagine a billiard ball moving across a table.
At any particular moment it has a position and a velocity.
If we knew the relevant conditions with sufficient precision, classical mechanics would tell us how its trajectory develops.
The ball is not a cloud of possibilities.
It is somewhere.
The fact that we might not know exactly where does not mean that the ball itself is indeterminate.
Our ignorance is epistemic.
The ball's state is physical.
This distinction has shaped our intuition about physical reality for centuries.
We therefore tend to approach quantum mechanics with an expectation:
Surely the particle must really be somewhere, and the wavefunction merely tells us where we don't yet know it is.
That interpretation is tempting.
But quantum mechanics refuses to behave as though the wavefunction were simply a sophisticated expression of ignorance.
The wavefunction is not a probability distribution
This is the first conceptual surprise.
The wavefunction is not itself an ordinary probability distribution.
It is a mathematical object from which probabilities are obtained.
More precisely, the squared magnitude of the wavefunction gives the probability density for certain measurement outcomes, such as position.
That distinction might sound technical.
It is anything but.
Suppose an electron is described by a state whose wavefunction extends over a region of space.
The naive classical picture says:
The electron is somewhere in that region, but we don't know where.
Quantum mechanics says something more peculiar.
The quantum state assigns amplitudes to the possible outcomes of a position measurement.
Those amplitudes can interfere.
And that interference affects the probabilities we actually observe.
The structure of the possibilities therefore has physical consequences.
This is already more than ignorance.
Possibility can interfere
Imagine two possible routes by which a quantum system might reach a detector.
In classical reasoning, we might say:
The particle went through route A or route B. We simply don't know which.
If that were the whole story, we would add the probabilities associated with the two alternatives.
But quantum mechanics requires us to add amplitudes first and calculate the probability afterwards.
The alternatives can reinforce one another.
Or they can cancel.
That is the phenomenon of interference.
And this gives us a remarkable clue.
The possibilities are not merely labels for our uncertainty.
They possess a mathematical structure that affects what ultimately becomes actual.
The world behaves differently depending upon how the possible alternatives are related.
This is one reason quantum mechanics feels so strange.
The theory does not merely tell us which actual event occurred.
It tells us about a structured space of alternatives whose relations help determine the distribution of actual events.
Possibility is not yet actuality
We should nevertheless be careful.
A quantum possibility is not an actual event.
If a photon has a nonzero probability of being detected at several different locations, it does not follow that several detection events have occurred.
When the detector actually registers the photon, we have an event.
There is now a physical record.
An atom may have absorbed energy.
An electron may have changed state.
A detector may have produced a pulse.
Something has happened.
The wavefunction, by contrast, provided the quantum state from which the probabilities of possible outcomes were calculated.
So we have a distinction:
possibility is not actuality.
That sounds obvious.
But quantum mechanics forces us to take the distinction unusually seriously.
The photon and its wavelength
This helps clarify the question:
What does it mean to say that a photon has a wavelength?
It is tempting to imagine a little particle moving through space with a tiny sinusoidal wave attached to it.
But that picture is misleading.
For a quantum state with a well-defined momentum, wavelength is related to momentum by
The wavelength therefore characterises the spatial structure associated with the quantum state.
It is not the distance between successive photons.
If we have a beam of monochromatic light, we might have many photons.
Their average spacing can vary enormously depending upon the intensity of the beam.
But the wavelength can remain exactly the same.
A weak red laser and a strong red laser can have the same wavelength while containing vastly different numbers of photons.
So wavelength is not a little ruler measuring the distance between particles.
It belongs to the spatial structure of the quantum state.
A subtlety about the individual photon
This becomes even more interesting when we ask about a single photon.
A perfectly monochromatic state has a sharply defined frequency and therefore wavelength.
But it is correspondingly extended in space.
If instead we construct a photon into a tightly localised wavepacket, we necessarily introduce a range of frequencies and wavelengths.
The trade-off is familiar from Fourier analysis and appears throughout quantum mechanics.
So even the question:
"What is the wavelength of this individual photon?"
can be more subtle than our ordinary particle language suggests.
The wavelength is not necessarily an intrinsic little length carried around by a tiny object.
It characterises the structure of the state in relation to spatial and temporal variation.
This is one reason the relational language is so useful.
A relational interpretation
We can therefore say something like this:
A quantum state does not simply describe what an isolated object possesses. It describes structured possibilities for what may occur in relation to possible interactions.
That is deliberately interpretive.
Different interpretations of quantum mechanics disagree about what, exactly, the wavefunction represents.
We should not pretend that physics has settled the metaphysical question.
But whatever interpretation one adopts, the operational structure is clear:
The quantum state determines probabilities for possible measurement outcomes.
And those probabilities depend upon the relations encoded in the state.
This gives us a powerful conceptual shift.
Instead of imagining:
particle → possesses properties → travels → interacts,
we can consider:
quantum state → structures possible relations → interaction → actual event.
The latter is not a replacement for the equations.
It is a way of thinking about what the equations are telling us.
But is the wavefunction really "potential"?
Here we need another qualification.
It is tempting to say:
"The wavefunction is potential, not actual."
That is suggestive, but too simple.
The wavefunction itself is an actual mathematical state of the physical system.
It evolves according to the Schrödinger equation.
It can be prepared experimentally.
Different quantum states produce different experimentally testable predictions.
So the wavefunction is not imaginary.
Nor is it merely a statement of subjective ignorance.
The more careful formulation is:
The wavefunction describes a physically significant structure of possibilities for actual outcomes.
Whether those possibilities should themselves be regarded as physically real is a much more difficult philosophical question.
And that question is precisely where interpretations of quantum mechanics begin to diverge.
The measurement problem waits patiently
At this point, the notorious measurement problem appears.
The wavefunction evolves continuously according to quantum dynamics.
Yet measurements appear to produce definite outcomes.
Before the measurement, the formalism can describe a superposition of possible outcomes.
Afterwards, we observe one particular result.
What exactly happens between those descriptions?
There is no universally accepted answer.
Different interpretations tell different stories.
Some regard collapse as a genuine physical process.
Some regard the apparent collapse as emergent from decoherence and branching.
Some treat the wavefunction as a description of information.
Others assign it a more directly physical status.
We need not resolve that enormous debate here.
Indeed, doing so would take us far beyond the purpose of this essay.
The point is more modest.
Quantum theory undeniably distinguishes between:
the structure of possible outcomes
and
the actual outcome registered in an event.
That distinction is enough for our purposes.
The event is relational too
There is another important point.
A quantum measurement is not simply the revelation of a property that was sitting inside the particle waiting to be uncovered.
A measurement involves an interaction.
A system interacts with a detector.
The detector interacts with its environment.
A record is produced.
What becomes actual is therefore not simply an isolated object's private possession.
It is an event in a relation.
This is especially congenial to relational approaches to ontology.
We need not imagine an absolutely self-sufficient object carrying a complete inventory of properties through the universe.
Instead, physical reality can be understood in terms of systems entering into relations that generate actual events.
The measurement does not merely tell us what happened.
In an important sense, the interaction is part of what happens.
From possibility to event
This suggests a simple schema:
state → possibility → interaction → event → record
The first term gives us the physical condition of the system.
The second gives us the possible outcomes structured by that condition.
The third is the interaction through which one of those possibilities becomes relevant to an actual physical encounter.
The fourth is the event itself.
The fifth is the trace that remains in the world.
A detector click is not merely information about an event.
It is itself a physical event produced by the interaction.
This distinction matters.
The world is not divided into a "real physical realm" and a separate "informational realm".
Information is carried by physical differences.
A detector records a difference because something physically happened.
Possibility is structured
Perhaps the most important lesson is that possibility is not an amorphous cloud.
The quantum state does not say:
"Anything might happen."
Quite the opposite.
It places extraordinarily precise constraints upon what might happen.
Some outcomes have high probability.
Some have low probability.
Some are impossible.
Interference changes the distribution.
Symmetries impose restrictions.
Conservation laws constrain transitions.
The quantum world is therefore not a world of arbitrary possibility.
It is a world of structured possibility.
This is exactly what makes the word "possibility" interesting.
A possibility becomes physically meaningful when the world itself constrains its relation to other possibilities.
Possibility and capacity
Now we can reconnect with the argument of The Physics of What Becomes Possible.
Throughout that series, we distinguished between a capacity and a function.
A capacity is what a physical system can do.
A function emerges when that capacity becomes consequential within an organisation.
Quantum mechanics offers us a different but related distinction.
A quantum state specifies a repertoire of possible outcomes.
An interaction actualises one outcome.
So we might place the two ideas side by side:
capacity: what a system can do
quantum possibility: what outcomes a given state makes available, with particular amplitudes
function: what a capacity does within an organised system
These are not identical concepts.
But they belong to the same conceptual family.
They all move us away from thinking only about isolated actual objects and toward thinking about structured spaces of possibility.
The wavelength is therefore not a distance between things
We can now return to our original little puzzle with a sharper vocabulary.
If wavelength were simply the distance between photons, then it would describe an arrangement of actual particles.
But it does not.
Two photons in a beam need not be separated by one wavelength.
Indeed, their detection events can be separated by enormously varying distances in a weak beam.
What remains fixed is the wavelength associated with the quantum state.
It is a relation encoded in the spatial structure of that state.
So our original intuition can be rescued by changing one word.
Not:
wavelength is the distance between particles.
But:
wavelength characterises a spatial relation within the structure of the possible outcomes associated with the quantum state.
That is a much stranger proposition.
And, I think, a much more interesting one.
Intensity reveals the difference
The distinction becomes particularly clear when we consider intensity.
For light of a given frequency, each photon carries energy
Increase the intensity while keeping the frequency fixed, and the energy per photon does not change.
Instead, you increase the amount of energy delivered per unit time and area—something that, in a photon description, can involve a greater photon flux.
So we have:
frequency → energy per photon
photon flux → total energy delivery
wavelength → momentum/spatial structure
These are different quantities.
A brighter red light is not a light whose photons have become "more red".
It is a light delivering more red photons, broadly speaking.
This simple fact is one of the places where the particle and wave descriptions fit together rather beautifully.
The wave is not a procession
There is therefore a conceptual trap in imagining a light wave as a procession of photons:
photon — photon — photon — photon
with the wavelength being the spacing between them.
That picture confuses two different structures.
The wave's wavelength belongs to the field or quantum state's spatial and temporal variation.
The number of photons belongs to the excitation of that mode.
You can change the photon number without changing the wavelength.
You can change the wavelength without changing the basic fact that photons are discrete quanta of the electromagnetic field.
The two notions coexist.
They are not competing descriptions of the same little row of beads.
Possibilities have structure before events occur
And this is where the physics becomes philosophically suggestive.
Before the detector clicks, there is not simply nothing.
There is a quantum state.
The state evolves.
It establishes probabilities.
Its amplitudes interfere.
It constrains what can happen.
Then an interaction occurs.
A definite event takes place.
We should not leap from this to the claim that the universe literally consists of "pure potential" before measurement.
Quantum theory does not warrant such a simple metaphysics.
But it does force us to acknowledge something that classical intuition tends to conceal:
the description of a physical system can contain highly structured information about possible events before those events occur.
Possibility is therefore not merely an afterthought.
It is built into the predictive and mathematical architecture of the theory.
A possibility is not a ghostly object
This distinction is worth emphasising.
If we say that a quantum state contains possibilities, we must not imagine those possibilities as tiny ghost-events waiting in another realm.
A possibility is not an object.
The possibility that a die will land six is not a sixth ghostly die outcome floating beside the actual one.
But neither is the probability distribution arbitrary.
It is constrained by the physical state of the die.
Quantum mechanics is stranger because the mathematical relations among possibilities can interfere.
Nevertheless, the same broad conceptual distinction remains useful:
possibility is a mode of organisation, not a second inventory of objects.
That may be the safest philosophical language.
The world does not have to be made entirely of actualities
This is perhaps where the argument becomes genuinely provocative.
We have inherited a metaphysical picture in which reality consists fundamentally of actual things.
Possibilities are then secondary.
They are possibilities for actual things.
But quantum mechanics invites us to consider whether that ordering is too simple.
Perhaps the physical state of a system is partly characterised by its modal structure: the structured range of things that can happen when it enters into different interactions.
The actual event is then one realised relation within that structure.
This does not mean that possibility is more real than actuality.
It means that actuality may be unintelligible without reference to the possibilities against which it is defined.
A detector click is an event partly because other outcomes were possible.
A particular measurement result is meaningful because it belongs to a structured space of alternatives.
Actuality is therefore relational to possibility.
Possibility is constrained by the world
And here we reconnect with a theme from our previous series.
We discovered there that constraints are not merely prohibitions.
They are productive.
A constraint can make a new form of organisation possible.
Quantum mechanics gives us perhaps the most fundamental example.
The structure of quantum states and the laws governing their evolution severely constrain possible outcomes.
Those constraints are precisely what give rise to the characteristic patterns of atoms, molecules and matter.
If everything were possible, nothing would be stable.
The world would have no architecture.
It is because possibilities are structured and constrained that stable possibilities can exist.
The universe is not interesting because everything can happen.
It is interesting because so much can happen within remarkably precise limits.
From quantum possibility to chemistry
This is where our earlier electron can return.
The electron's quantum state is not merely a list of actual positions.
Quantum mechanics describes the allowed states of electrons in atoms.
Those states determine the structure of atoms.
Atomic structure constrains chemical bonding.
Chemical bonding constrains molecular form.
Molecular form constrains chemical interaction.
The chemistry of life emerges from this enormous hierarchy of constrained possibilities.
So the architecture of possibility begins astonishingly early.
The higher-level possibilities we encountered in our series—molecules, cells, organisms, perception—are not detached from quantum possibility.
They are built upon it.
The electron's possibilities become the conditions for chemical possibilities.
Chemical possibilities become the conditions for biological possibilities.
And so on.
The possibility of information
The photon then gives us another route.
A photon's quantum state determines possible interactions with matter.
Matter can respond selectively.
Selective response can preserve differences.
Preserved differences can carry information.
Information can become significant within an organism.
And eventually those differences can enter semiotic systems.
We therefore have another chain:
quantum possibility → physical interaction → detectable difference → information → significance
Again, no magic is required.
But each stage introduces a new organisation of what was already physically possible.
The measurement is not the end of the story
There is a temptation to think that once a possibility becomes actual, the interesting part is over.
But the event becomes the starting point for another chain.
A photon is absorbed.
A molecule changes.
A receptor responds.
A neuron fires.
A neural network changes state.
An organism alters its behaviour.
The environment changes.
Another interaction becomes possible.
So even in quantum mechanics, the transition from possibility to actuality does not terminate the architecture.
It feeds back into it.
The actual event becomes a new physical state.
That state has its own possibilities.
And so:
possibility → event → new possibility
The world is continually renewing its space of possible futures.
Perhaps this is what becoming means
This gives us a rather different way to think about becoming.
Becoming is often imagined as the simple replacement of one actual state by another:
state A → state B.
But we might instead think of it as:
a structured field of possibilities → an actual event → a transformed field of possibilities.
The event matters because it changes what can happen next.
A molecule reacts.
Its products have different capacities.
A cell changes state.
Its future behaviour changes.
An organism learns.
Its repertoire of possible actions changes.
A scientific discovery occurs.
The space of possible questions changes.
Becoming is therefore not merely the production of new actualities.
It is the transformation of possibility itself.
This is why the future is not already there
The language of possibility also helps us avoid another conceptual trap.
If the quantum state determines probabilities for future outcomes, it does not follow that every possible outcome is already actual somewhere.
The future need not be a hidden warehouse containing all the events that will happen.
Possibility is not actuality in disguise.
The future can remain genuinely open while being highly constrained.
That may be the most useful philosophical lesson of all.
An open future need not mean an arbitrary future.
There can be:
real constraints + real possibilities + real events.
The future is open because several outcomes may be physically possible.
It is structured because not every outcome is possible.
And it becomes actual through events.
A relational ontology of possibility
We can now perhaps formulate the relational intuition more carefully.
A thing does not need to possess all its properties in isolation.
Some properties are intelligible only through relations.
A wavelength is intelligible through the spatial and temporal structure of a quantum state.
A momentum is related to spatial translation.
A measurement outcome is realised in interaction.
Information exists through differences that can make a difference to another system.
Function exists through a capacity's role within an organisation.
Meaning exists through relations within a semiotic system.
Across all these cases, reality becomes increasingly difficult to describe as a collection of isolated objects carrying self-contained inventories.
What we encounter instead is a world of relations that establish possibilities for further relations.
The ontology becomes dynamic.
And possibility becomes central.
But let us not get carried away
There is a danger here.
Once we discover that quantum mechanics contains probabilities, it is very tempting to turn it into a metaphysical solvent.
Quantum mechanics proves that reality is relational!
Quantum mechanics proves that consciousness creates reality!
Quantum mechanics proves that everything is potential!
Quantum mechanics proves that the universe is fundamentally information!
Such claims usually outrun the physics.
The fact that quantum mechanics is compatible with a relational ontology does not establish one unique philosophical interpretation.
The physics leaves room for philosophical work.
It does not do that work for us.
So our modest claim should be enough:
Quantum theory gives us a physical description in which structured possibilities are indispensable to predicting actual events.
That alone is extraordinary.
We need not make it stranger than it already is.
What becomes possible
And now we can see why this little essay belongs after The Physics of What Becomes Possible.
That series asked:
What do physical capacities make possible?
This essay asks a more foundational question:
What is the physical status of possibility itself?
The answer is not obvious.
But quantum mechanics gives us reason to think that possibility is not merely a human category imposed upon an otherwise completely actual world.
The formalism itself is organised around possible outcomes.
Those possibilities have quantitative structure.
Their relations matter.
They can interfere.
They determine distributions of actual events.
And when an interaction occurs, one outcome becomes actual.
So perhaps the world is not best pictured as a collection of actualities from which we infer possibilities.
Perhaps it is better pictured as a world in which actuality and possibility are deeply interwoven.
Before the event
There is therefore a peculiar interval we might call before the event.
Not necessarily a temporal interval in which the universe sits around waiting.
Rather, a conceptual distinction:
Before an interaction produces a definite result, the quantum state specifies a structured range of possible outcomes.
After the interaction, there is an actual event.
And that event becomes part of the physical condition from which further possibilities arise.
The distinction is simple:
possibility is what an interaction may realise; actuality is what the interaction has realised.
But the relationship between them is profound.
For the actual event is not intelligible without the possibility structure from which it emerged.
And the possibility structure is not merely a catalogue of imagined alternatives.
It is physically constrained.
It evolves.
It has measurable consequences.
Perhaps actuality is not the opposite of possibility
We can now return to our opening question.
What kind of thing is a possibility before it becomes an event?
Perhaps the safest answer is:
It is not a thing at all.
It is a structured relation among what a physical state permits, what an interaction can realise, and what outcomes the theory assigns amplitudes or probabilities to.
That may sound less metaphysical than "potential reality".
But perhaps that is precisely its virtue.
Possibility need not be a ghostly substance.
It can be a real feature of the organisation of physical relations.
And this brings us to a final thought.
We tend to imagine actuality and possibility as opposites.
Either something is real or it is merely possible.
But perhaps that opposition is too crude.
An actual event is an event within a space of possibilities.
A possibility is a possibility for becoming an actual event.
The two concepts therefore define one another.
The world before the click
A photon approaches a detector.
We should resist imagining a tiny bead with a predetermined trajectory rushing toward a predetermined point.
The quantum state specifies a structured set of possibilities.
The detector is another physical system.
The two interact.
One outcome occurs.
The detector clicks.
A record now exists.
The world has changed.
And that new state of the world carries new possibilities.
There is no need to imagine a mysterious moment when possibility is magically converted into reality.
There is simply an unfolding sequence of physical relations:
state, possibility, interaction, event, transformed state, new possibility.
Perhaps that is all becoming ever was.
One last return to the photon
The photon that prompted our question now looks rather different.
It is not simply a particle travelling through space.
Nor is it simply a classical wave smeared through space.
It is a quantum excitation whose state has a mathematically precise structure.
That structure is related to wavelength, momentum, frequency and energy.
It determines possible interactions.
Those interactions can produce actual events.
Those events can create records.
Records can become information.
Information can become significant to an organism.
And significance can become meaning.
The photon therefore stands at a remarkable intersection between possibility and actuality.
It carries energy.
It participates in causation.
It can become a detectable difference.
And, in the right organisation, that difference can become something about something else.
The little quantum of light can therefore participate in a journey from:
possibility → event → information → meaning.
The physics of becoming
Perhaps this is where our earlier series finally takes us.
We began with properties.
We discovered capacities.
Capacities entered into relations.
Relations produced organisation.
Organisation generated functions.
Functions opened new possibilities.
Now quantum mechanics has taken us underneath the whole process and shown us something remarkable:
Even at the most fundamental level, physical description is not exhausted by actuality. It includes structured possibilities for what can happen.
That does not mean that possibilities are hidden objects.
It means that the physical world is not adequately described by saying only what is.
We must also say what, given what is, can happen next.
And what can happen next is constrained by the structure of the world.
The future is therefore neither completely predetermined in the simple classical sense nor an arbitrary realm of anything-goes possibility.
It is a structured field of becoming.
Perhaps, then, we can end where we began:
What is a possibility before it becomes an event?
Not a thing.
Not nothing.
Not merely ignorance.
Perhaps it is best understood as a real structure of potential relation: a constraint on what the world can do next.
And when a relation occurs, one possibility becomes an event.
The event changes the state of the world.
And the changed world opens onto further possibilities.
So perhaps the deepest lesson is not that reality consists of possibilities rather than actualities.
It is something subtler:
Reality is a continual passage between what can happen and what has happened.
And perhaps actuality is not the fundamental opposite of possibility.
Perhaps actuality is what possibility looks like when a relation has occurred.
🍷🙂
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