We have spent the preceding five essays following a peculiar disturbance through modern physics.
First, things became a problem.
Then properties lost their independence.
Then the observer could no longer stand outside the physical situation.
Then the future ceased to look quite like a collection of facts that already exist but have not yet been observed.
Finally, we found that a successful physical theory need not provide a literal picture of what reality is like underneath the mathematics.
None of these developments, taken individually, requires us to abandon the ordinary philosophical picture of reality. Physics has always revised its concepts without immediately revising its metaphysics.
Taken together, however, they begin to exert a different kind of pressure.
The problem may no longer be that physics lacks a sufficiently complete description of reality.
Perhaps the problem is that we are trying to interpret what physics has discovered using a philosophical vocabulary inherited from a different conceptual world.
The philosophy hidden in common sense
It is easy to imagine that physics is free of philosophy because physicists work with mathematics, experiments and measurements.
But philosophy does not disappear when we stop talking about it.
It becomes common sense.
We assume that reality consists of things.
We assume that things possess properties.
We assume that those properties belong to things independently of their relations.
We assume that the observer can, at least in principle, be distinguished from what is observed.
We assume that the future is already determined in principle, even when it is unknown to us.
And we assume that a successful theory ultimately gives us a picture of what is really there.
These assumptions did not arrive together. They have different histories.
The substance-centred ontology of Aristotle is one source. The distinction between primary qualities and subjective experience belongs to another. The Cartesian division between thinking subject and extended world belongs to another. Newtonian mechanics supplied an extraordinarily powerful mathematical framework within which objects could possess definite states and evolve through absolute time. Laplacian determinism then provided perhaps the clearest expression of the idea that the complete present contains the complete future.
Modern science transformed all of these inheritances.
But transformation is not the same as abandonment.
The old philosophical picture can survive as the apparently obvious interpretation of the new mathematics.
That may be precisely what has happened.
What quantum mechanics has made difficult
Quantum mechanics does not simply replace one set of physical objects with another.
It unsettles the question of what counts as a physical object in the first place.
An electron is not straightforwardly a tiny classical particle carrying a complete collection of independently determinate properties. Position and momentum cannot simply be treated as classical attributes simultaneously possessed with arbitrary precision. Spin is not simply a miniature mechanical rotation. Entangled systems cannot always be understood as collections of independently specified parts.
The difficulty is not merely that quantum objects are strange.
The deeper difficulty is that the grammar in which we describe things as possessing properties may no longer be adequate to what the theory requires.
That does not mean that there are no things.
It means that thinghood may not be fundamental.
The same applies to properties.
A property does not necessarily have to be understood as something a thing possesses first and reveals later. In quantum phenomena, the physical context in which a property becomes determinate can be inseparable from the property itself.
Again, this does not mean that measurement simply creates reality.
It means that reality may be constituted through relations that cannot be treated as philosophically secondary.
The question changes from:
What properties does the thing have?
to something more like:
What becomes determinate in this physical relation?
That is a surprisingly large philosophical shift.
The observer is not outside the world
The classical picture also permits a comforting fiction.
There is a world.
There are things in the world.
And somewhere outside this arrangement there is an observer who looks at it.
Science can then be imagined as the gradual removal of subjective distortion until the observer's description approaches the world as it really is.
But there is no physical place outside the physical world from which physics can be done.
The observer is physical.
The laboratory is physical.
The apparatus is physical.
The interaction between system and apparatus is physical.
Even the production of the record on which an observation depends is a physical event.
This does not make science subjective.
Quite the opposite.
It means that objectivity cannot depend upon escaping relation.
It must be achieved through relations capable of constraining us.
The apparatus does not obediently produce whatever the observer wishes to see. The world answers back.
Objectivity is therefore not the absence of participation.
It is a form of disciplined participation.
This is not unique to quantum mechanics. But quantum mechanics makes the philosophical problem particularly difficult to ignore.
The observer is not a spectator standing outside reality.
The observer is one of the things reality is capable of doing.
And the future?
Classical physics makes it extraordinarily natural to imagine that the future already exists as a definite physical state.
If we knew everything about the present and all the relevant laws, then—at least in principle—the future would already be fixed.
Quantum mechanics complicates this picture.
Its probabilities need not be understood merely as our ignorance of a set of classical facts waiting to be uncovered. Different interpretations of quantum mechanics disagree profoundly about what this means. Some restore determinism. Some multiply outcomes. Some modify the dynamics. Some treat the quantum state in ways that resist a straightforward ontological reading altogether.
Physics itself does not simply hand us one philosophical conclusion.
But it does make one thing increasingly difficult to ignore:
possibility may have a more fundamental role in physical description than classical common sense allowed.
The future need not be arbitrary.
It can be structured, constrained and physically intelligible without being a fully determinate actuality that merely awaits our discovery.
And once an outcome becomes actual, the situation from which subsequent possibilities arise has changed.
The future is not simply waiting for us.
The world becomes.
That possibility is philosophically significant whether or not any particular interpretation of quantum mechanics ultimately adopts it.
A theory need not be a picture
There is one final assumption to reconsider.
We often imagine that the purpose of a physical theory is to tell us what the world is really like by providing a mathematical description of its underlying constituents.
But perhaps the mathematics does something more subtle.
A theory can constrain what we can legitimately say about the world without supplying a miniature picture of what exists behind the phenomena.
It can tell us which distinctions are physically meaningful, which relations are invariant, which transformations are possible, which outcomes are constrained and which alternatives cannot consistently be combined.
The map need not resemble the territory in order to be a map.
And perhaps a physical theory is sometimes less like a picture of reality than a disciplined way of entering into relation with it.
This does not require abandoning realism.
Indeed, it may require a more demanding realism.
The question becomes not:
What picture does the theory give us of the things that are really there?
but:
What does the theory allow reality to constrain us to say?
The distinction matters.
A theory can be genuinely about the world without telling us that the world consists of entities corresponding one-to-one with the terms in its formalism.
What has physics outgrown?
We can now see the pattern.
Physics has not necessarily outgrown things.
It has outgrown the assumption that things must be philosophically prior to relations.
It has not outgrown properties.
It has outgrown the assumption that properties must belong to things independently of the relations in which they become determinate.
It has not outgrown observers.
It has outgrown the assumption that an observer can occupy a standpoint outside the physical world.
It has not outgrown prediction.
It has outgrown the assumption that prediction and determinism are the same thing.
It has not outgrown theory.
It has outgrown the assumption that a successful theory must provide a literal picture of an underlying classical reality.
These are not conclusions that every physicist accepts.
Nor are they uniquely dictated by quantum mechanics.
They are philosophical pressures generated by the encounter between our inherited conceptual picture and the behaviour of the physical world as our theories disclose it.
Different interpretations of quantum mechanics can therefore be understood, at least in part, as different responses to the question:
Which part of the inherited picture are we prepared to give up?
Perhaps we should give up determinism.
Perhaps locality.
Perhaps classical objecthood.
Perhaps a single universal state.
Perhaps the idea that the quantum state is a picture of physical reality at all.
Or perhaps we should give up something deeper:
the assumption that reality must ultimately be composed of independently existing things with intrinsic properties, related only afterwards.
Philosophy after the physics
This is where philosophy becomes unavoidable.
Physics can tell us that certain experimental results occur.
It can tell us which mathematical structures predict them.
It can tell us which interpretations are physically compatible with observation, and which are not.
But the question of what counts as a thing, a property, an observer, an actuality, a possibility or a representation is not settled simply by adding another equation.
Those are questions about the conceptual interpretation of what the equations and experiments have made possible to say.
That is not a weakness of physics.
It is a sign of its success.
The more radically physics transforms our access to the world, the more pressure it places on the concepts with which we interpret that access.
Perhaps, then, the problem is not that physicists need to do less physics and more philosophy.
It is that physics has already done enough to make some old philosophy difficult to sustain.
The philosophical work is to notice what has changed.
When physics outgrows its philosophy
A science can continue to advance while its philosophical inheritance lags behind it.
The mathematics moves forward.
The experiments move forward.
The technology moves forward.
But the picture of reality through which the results are interpreted can remain remarkably old.
That is not surprising.
Philosophical assumptions are hardest to notice when they have become indistinguishable from common sense.
The great conceptual achievement may therefore not be to discover a final interpretation of quantum mechanics.
It may be to become capable of asking a different question.
Not:
How can we make this strange quantum world fit the reality we already understand?
But:
What kind of reality would make the quantum world intelligible without first forcing it back into the categories of classical common sense?
Perhaps the lesson is not that there are no things, no properties, no observers, no futures and no theories.
Perhaps it is that none of these is fundamental in quite the way we have assumed.
Things may be relatively stable achievements of relations.
Properties may become determinate within interactions.
Observers may be participants rather than spectators.
Futures may emerge from structured possibilities rather than exist fully formed in advance.
And theories may be instruments for entering into increasingly adequate relations with a world that no final picture can contain.
Physics may therefore be approaching a curious threshold.
It has changed what we can know about the world so profoundly that the philosophy with which we interpret that knowledge may itself have to change.
Physics does not need philosophy in order to calculate.
But perhaps it needs philosophy in order to understand what its calculations have made possible to think.
Physics may not need to become more philosophical in order to become more scientific.
It may need to become more philosophical in order to understand what it has already discovered.