There is a powerful intuition behind scientific realism.
A successful scientific theory, we naturally think, tells us what the world is really like.
The equations describe reality.
The entities in the theory are things that exist.
The mathematical structure represents the underlying physical world.
If a theory is successful enough, surely we should eventually be able to say what it is telling us about what is actually there.
This intuition is reasonable.
It is also philosophical.
And quantum mechanics makes it surprisingly difficult to sustain in its simplest form.
The demand for a picture
When a physical theory becomes difficult to interpret, we often ask for a picture.
What is the electron really?
What is the wave function really?
What is happening during measurement?
What is the particle actually doing when it is not being observed?
These questions seem unavoidable.
But notice what they assume.
They assume that the mathematical formalism must correspond to some underlying physical picture that could, at least in principle, be described in familiar terms.
The equations may be difficult.
The entities may be strange.
But somewhere beneath the mathematics there should be a comprehensible world of things doing things.
That expectation has deep roots in physics.
It also has deep roots in philosophy.
From mathematics to mechanism
Classical physics encouraged the idea that mathematical laws describe mechanisms involving physical entities.
Newton's equations tell us how bodies move.
The equations can be connected to trajectories.
A trajectory is a picture.
We can imagine a planet following a path through space.
We can imagine a projectile moving through the air.
Mathematics and physical imagery reinforce one another.
The theory tells us what happens, and the picture tells us what is happening.
Quantum mechanics breaks this comfortable relationship.
The mathematical formalism is extraordinarily precise.
But translating it into an intuitive physical picture is notoriously difficult.
The wave function evolves.
A measurement produces an outcome.
The mathematical rules work.
Yet the question “What is physically happening?” can generate radically different answers.
Perhaps the problem lies not with the theory but with the demand for the picture.
The wave function problem
Consider the wave function.
Is it a real physical entity?
Is it a description of our information?
Is it a catalogue of possibilities?
Is it something that exists in a high-dimensional configuration space?
Does it collapse?
Does it never collapse?
Does it describe a single world or many?
These are not merely competing mathematical formulations.
They are competing ontological interpretations.
And notice how quickly we reach for familiar categories.
We want the wave function to be a thing.
Or we want it to be information about a thing.
Either way, the old structure survives:
there is a physical reality, and the theory either describes it or represents our knowledge of it.
But perhaps the wave function does not fit neatly into either category.
Perhaps the demand that it do so is part of the problem.
The map is not the territory
There is an older philosophical lesson here.
A representation is not identical with what it represents.
A map is not the landscape.
A mathematical model is not necessarily a miniature physical mechanism.
This does not make the representation arbitrary.
A good map is constrained by the terrain.
A successful physical theory is constrained by experiment.
The distinction is therefore not between “mere mathematics” and “real reality.”
It is between a formal structure that successfully tracks physical phenomena and a literal picture of what reality must be like behind that structure.
The first can be extraordinarily secure without the second being uniquely determined.
This matters enormously in quantum mechanics.
The same empirical predictions can sometimes be associated with very different ontological stories.
The success of the mathematics therefore does not automatically select one metaphysical picture.
The underdetermination problem
This is sometimes described as underdetermination.
Evidence can constrain theories without uniquely determining their interpretation.
But there is a deeper point.
Perhaps the world does not owe us a representation in the categories we find intuitive.
A theory may successfully describe the relations among observable phenomena without providing a classical-style inventory of underlying objects.
That possibility is uncomfortable because we tend to think that understanding means being able to picture what is happening.
But perhaps there are forms of understanding that are not pictorial.
We understand a mathematical structure without turning every element of it into a physical object.
We understand a musical score without imagining that the notes are tiny objects sitting inside the performance.
We understand an ecological system without requiring every relation to be reducible to a single visible mechanism.
Why should physical theory be different?
Instrumentalism is not the only alternative
There is a danger here.
Once we say that theories need not provide literal pictures, it is tempting to retreat into instrumentalism.
The theory predicts observations.
It works.
Nothing more needs to be said.
That would be too easy.
The opposite of naïve realism is not “anything goes.”
A theory is constrained by a world that resists it.
Its mathematical structures are not arbitrary.
Its predictions can fail.
Its conceptual distinctions can reveal previously inaccessible phenomena.
A successful theory therefore tells us something real.
The question is what kind of thing it tells us.
Perhaps what it reveals most securely is not a collection of classical objects hidden behind appearances, but a structure of relations, constraints, probabilities and transformations.
That would still be realism.
It would simply be a different kind of realism.
Structural realism
This possibility has attracted considerable philosophical attention under the heading of structural realism.
The basic intuition is that science may tell us more securely about the structure of physical reality than about the intrinsic nature of the entities that occupy that structure.
The idea is attractive in quantum physics because relations and correlations are often much more stable features of the theory than classical pictures of what carries them.
But even “structure” can be misunderstood.
A structure is sometimes imagined as a framework imposed upon independently existing things.
A more relational conception is possible.
Perhaps what persists across scientific change is not a set of things with fixed intrinsic properties, but patterns of relations and constraints through which physical systems can behave.
Then scientific progress need not be the gradual discovery of an increasingly accurate inventory.
It can be the discovery of increasingly adequate ways of articulating how physical possibilities are organised.
Theories can change what can be seen
There is another reason not to treat theories merely as pictures.
A scientific theory does not simply describe phenomena that were already available to an untheorised observer.
It can change what becomes observable.
New concepts lead to new experiments.
New instruments make new phenomena accessible.
New mathematical structures reveal patterns that could not previously be distinguished.
The theory therefore participates in the history of inquiry.
It does not stand outside the world and photograph it.
It becomes part of the relation through which the world is investigated.
This is not a return to the idea that scientists invent reality.
The world remains resistant.
But what can be asked, measured and recognised depends partly upon the conceptual and technological structures through which inquiry is conducted.
A theory is therefore not simply a picture of reality.
It is also an instrument for entering into new relations with reality.
The philosophical inheritance
The demand for a picture rests upon several assumptions we have now encountered repeatedly.
Reality consists fundamentally of things.
Things possess intrinsic properties.
Those properties exist independently of relations.
The observer stands outside what is observed.
The future is already determinate.
And a successful theory should represent the underlying things as they really are.
These assumptions belong to different philosophical traditions and should not be collapsed into one doctrine.
But they reinforce one another.
Together they create a powerful common-sense image of science.
Quantum mechanics disrupts each part of it.
And yet the image remains remarkably persistent.
When the theory becomes strange, we search for a stranger picture.
When the picture becomes difficult, we search for a deeper picture.
When no picture works, we sometimes conclude that quantum mechanics is simply incomprehensible.
Perhaps another possibility deserves more attention.
Perhaps the demand for a picture is itself the inheritance we need to reconsider.
Understanding without a final image
A theory can be intelligible without being pictorial.
We can understand what a theory says about possible outcomes, relations among measurements, transformations of states and constraints on physical processes without pretending that these must correspond to familiar objects moving around in a hidden classical world.
Indeed, demanding a final image can sometimes make understanding harder.
We become so concerned with what the mathematics is “really describing” that we stop asking what the mathematics itself has forced us to give up.
Perhaps quantum mechanics does not need to be translated back into classical ontology.
Perhaps it is telling us that the world is organised in ways for which classical ontology was never designed.
The task is then not to find the right picture behind the equations.
It is to discover what kind of reality the equations require us to think.
Beyond the picture
This brings us back to the central argument of this series.
Physics may have outgrown some of its inherited philosophy not because its theories are becoming less intelligible, but because they are becoming intelligible in a different way.
The success of a theory does not guarantee that it supplies a literal picture of what exists.
It may instead reveal structures of relation, possibility and constraint that no classical image can adequately capture.
That would not make physics less realist.
It might make realism more demanding.
Reality would no longer be whatever fits most comfortably into our inherited picture of a thing.
It would be whatever continues to constrain our theories, resist our expectations and reveal new possibilities through experiment.
A theory need not be a picture.
It may be something more interesting:
a way of entering into a more adequate relation with a world that no picture can finally contain.