When Physics Outgrows Its Philosophy: I. When Things Become a Problem

Physics begins, naturally enough, with things.

A stone has a position. A planet has a mass. An electron has a charge. A particle moves through space and time. We measure what things do, discover regularities in their behaviour, and express those regularities mathematically.

There is nothing obviously philosophical about this.

That is precisely the problem.

The assumption that reality consists fundamentally of things with properties is so deeply embedded in ordinary thought that it hardly feels like an assumption at all.

Reality, we might say, is simply what is still there when nobody is looking.

The observer discovers what already exists.

Relations between things may be complicated, but the things themselves come first.

This picture has ancient roots. Aristotle gave philosophical prominence to substance: that which exists in itself rather than merely as a property or relation of something else. Early modern science transformed the picture without entirely abandoning it. The fundamental things became bodies, particles, fields or systems, possessing measurable properties and interacting according to laws.

The vocabulary changed.

The underlying intuition survived.

And classical physics made that intuition extraordinarily successful.

The world as an inventory

Classical physics encourages us to imagine the world as an inventory.

Here are the objects.

Here are their properties.

Here are their positions and momenta.

Here are the forces or interactions between them.

If we know the relevant quantities and the laws governing their behaviour, we can calculate what happens next.

The world may be enormously complicated, but complexity is not the same as philosophical difficulty.

The things are there.

The properties are there.

The relations tell us how they behave.

This picture is so successful that it can become invisible.

We cease to notice that we have adopted a metaphysics.

Then quantum mechanics arrives.

And suddenly the things become a problem.

What is an electron?

Consider an electron.

We might naturally imagine a small physical object possessing properties such as charge, mass, momentum and position.

But quantum mechanics does not behave as though an electron simply carries a complete collection of such properties around with it, waiting for us to reveal them.

Position and momentum cannot be treated as simultaneously determinate quantities in the classical way.

Spin does not behave like the literal rotation of a tiny sphere.

Quantum states can be superposed.

Entangled systems can display correlations that resist interpretation as independent states possessed by independently existing parts.

None of this merely means that electrons are unusually strange little objects.

It raises a more fundamental question:

Is an electron the kind of thing we originally assumed it was?

Perhaps the difficulty is not that we have failed to discover the hidden properties of a familiar kind of object.

Perhaps the concept of the object itself has become inadequate.

The temptation to restore the thing

There is a powerful response to this difficulty.

If quantum mechanics does not tell us which properties the electron really possesses, perhaps the properties are nevertheless there and the theory is incomplete.

If measurement affects what can be said about the system, perhaps the measurement merely reveals something that was already the case.

If entangled particles cannot be assigned independent states, perhaps there is some deeper description in which each particle nevertheless has its own complete reality.

The temptation is understandable.

We have spent centuries learning to think of reality as composed of things.

Why abandon the picture because the mathematics has become strange?

But notice what has happened.

The physics has created a philosophical problem.

The response is often to introduce more physics in the hope of restoring the old philosophical picture.

Hidden variables.

Additional structures.

Alternative interpretations.

Modified dynamics.

Multiple worlds.

These may be serious and productive physical proposals.

But none of them removes the underlying philosophical question:

Must reality ultimately consist of independently existing things possessing determinate properties?

That is not a question that more calculation can settle by itself.

The relation may come first

There is another possibility.

Perhaps we have started from the wrong end.

Instead of asking:

What is the thing, and how does it relate to other things?

we might ask:

What relations make the thing what it is?

This is a much more radical question.

An entangled quantum system, for example, does not seem naturally describable as two independently complete objects which subsequently acquire a relation.

The relation is part of the physical state.

The whole cannot simply be reconstructed from two independently specified parts.

That does not mean that the parts are unreal.

It means that their reality may be relationally conditioned.

And this possibility has a surprisingly long philosophical history.

Spinoza challenged the assumption that individuality requires ontological independence.

Hegel made relations and differences constitutive of determinate identity.

Process philosophy later questioned whether enduring things might themselves be achievements of processes rather than primitive substances.

None of these thinkers anticipated quantum mechanics in its modern form.

Nor should we pretend that they somehow predicted it.

But they opened conceptual possibilities that quantum theory makes newly interesting.

The philosophical assumption becomes visible

This may be the real lesson.

Quantum mechanics does not simply present us with a collection of bizarre experimental results.

It exposes the assumptions with which we instinctively try to understand those results.

We expect things to possess properties.

We expect properties to exist independently of how they are measured.

We expect separate things to have separate states.

We expect observation to reveal rather than participate in the determination of what is observed.

And we expect reality to be composed of actual things, with possibility describing merely what we do not yet know.

These expectations are not laws of nature.

They are philosophical inheritances.

They became so successful in classical physics—and so deeply embedded in common sense—that they ceased to look philosophical at all.

Quantum mechanics makes them visible again.

That does not tell us which replacement is correct.

But it does suggest that the first question should not be:

How can we make quantum mechanics fit our picture of things?

It may be:

What if quantum mechanics is telling us that our picture of things was never fundamental?

That is where the philosophical problem begins.