There is a problem at the foundations of modern physics.
Quantum mechanics works extraordinarily well.
General relativity works extraordinarily well.
Yet when we try to bring them together, something seems to go badly wrong.
The standard response is almost automatic.
Quantum mechanics must be extended to include gravity.
Gravity must be quantised.
We need a theory of quantum gravity.
The phrase has become so familiar that it can sound almost inevitable.
But perhaps we should pause before attempting to construct the theory.
Perhaps there is a prior question.
What, exactly, are we trying to quantise?
The question sounds elementary.
It may turn out to be fundamental.
The obvious answer
The obvious answer is:
Gravity.
But what is gravity?
In Newtonian physics, gravity is a force.
That makes the question comparatively straightforward.
A force is something a physical system exerts.
We can represent it mathematically.
We can ask what happens when that interaction is subjected to the principles of quantum mechanics.
But general relativity changed the conceptual situation.
Gravity ceased to be merely a force acting within spacetime.
Instead, the gravitational field became inseparable from the geometry through which physical events are related.
The usual language says that matter curves spacetime.
Objects then move along geodesics in the resulting geometry.
This is a remarkably successful description.
But it immediately raises an ontological question.
Is gravity a thing that exists, or is it a way in which physical relations are structured?
That distinction matters.
If gravity is a thing, perhaps we should quantise it.
If gravity is a relation, the question becomes considerably less obvious.
The familiar picture
Imagine the universe as a vast arena.
There are physical objects in the arena.
There are quantum fields and particles.
And there is spacetime—the stage on which everything happens.
Gravity then modifies the stage.
Matter tells spacetime how to curve.
Spacetime tells matter how to move.
This picture is so deeply embedded in our thinking that even physicists who know perfectly well that general relativity is not literally a theory of a physical fabric can find themselves speaking as though it were.
The metaphor is useful.
But usefulness is not ontology.
The danger comes when we move silently from:
the geometry behaves mathematically as though it were curved
to:
there exists a physical thing called curved spacetime.
Once that move has been made, the next step seems obvious.
If spacetime is physical, and quantum theory describes physical things quantum mechanically, then:
we should quantise spacetime.
And there we are.
Quantum gravity.
But perhaps the first step was never justified.
What does “quantise” mean?
There is another question hiding inside the first.
What does it mean to quantise something?
In familiar cases, we begin with something that has identifiable physical degrees of freedom.
An electromagnetic field, for example, can be represented by dynamical variables.
Quantum theory then tells us how those variables behave when subjected to quantum principles.
The classical description is replaced or extended by a quantum description.
But notice what this presupposes.
There is something there to be quantised.
A field.
A system.
A degree of freedom.
A physical quantity.
Something with a determinate role in the ontology.
So when we say:
“We need to quantise gravity,”
we should be able to answer:
What is the physical entity whose classical description is to be quantised?
If the answer is “the gravitational field”, we need to know what that field is.
If the answer is “spacetime”, we need to know what kind of thing spacetime is.
And if the answer is “the metric”, we need to know whether the metric is itself a physical entity or a mathematical representation of relations among physical events.
The distinction is not pedantic.
It may determine what the problem actually is.
The relational alternative
Our investigation of the black-hole information problem suggested a different way of thinking.
We considered the possibility that spacetime is not fundamentally a substance.
Its geometry may instead express relations among spatial intervals, temporal intervals and causal possibilities.
In this view, gravity is not something that bends a pre-existing physical fabric.
It is a systematic transformation of physical relations.
Spatial intervals shorten and temporal intervals lengthen in the direction of the centre of mass.
The geometry describes these changing relations.
The geodesic then describes a structure of possible motion within those relations.
Nothing has been removed from general relativity.
But the ontology has changed.
And once that happens, the phrase quantum gravity becomes less straightforward.
If gravity is a relational structure, what would it mean to quantise the relation?
A relation is not a thing
Consider a simple example.
Suppose Alice is standing two metres from Bob.
The distance between Alice and Bob is real.
We can measure it.
We can change it.
We can calculate with it.
But is the distance a third object sitting between Alice and Bob?
No.
The relation is real without being a thing.
Now imagine that Alice and Bob move.
The distance changes.
We do not need to imagine that the “distance object” has changed shape.
The relation has changed.
This seems almost trivial.
But physics becomes much more complicated when the relation is geometrically fundamental.
A spatial interval is not merely an object with a property.
It is a relation among events.
A temporal interval is a relation among events.
A causal relation is a relation among events.
And the geometry of spacetime expresses a structured system of such relations.
If this is what spacetime fundamentally is, then asking for a quantum theory of spacetime may be rather like asking for a quantum theory of the distance between Alice and Bob.
The question is not meaningless.
But it needs reinterpretation.
The crucial distinction
We should therefore distinguish two possibilities.
Ontology 1: spacetime as thing
Spacetime exists as a physical entity.
It possesses a geometry.
Matter and fields exist within it.
Gravity corresponds to changes in its physical state.
Then it makes considerable sense to ask:
How do we quantise spacetime?
Ontology 2: spacetime as relation
Physical events stand in spatial, temporal and causal relations.
The geometry represents the structure of those relations.
Gravity is a transformation of that relational structure.
Then the question becomes:
How do quantum possibilities participate in relational geometry?
These sound similar.
They are not.
The first asks for the quantum state of a thing.
The second asks how two aspects of physical description—quantum potential and relational geometry—fit together.
The second may be the more fundamental question.
Quantum theory gives us another warning
Our relational interpretation of quantum theory provides a useful parallel.
We have been treating the wavefunction not as a physical object but as a structured field of potential instantiations.
The distinction is important.
The wavefunction tells us what actual quantum events are possible, and with what structure.
A particular particle event is an actual instantiation.
The wavefunction is not itself one of those actual events.
It is potential structure.
Now suppose someone asked:
“How do we quantise the wavefunction?”
The question would immediately sound strange.
The wavefunction is already part of the mathematical framework of quantum theory.
But the deeper reason for the strangeness is ontological.
We are asking a description of potential possibilities to become itself a physical object requiring quantisation.
Something similar may happen with spacetime.
Perhaps we are trying to quantise something whose fundamental role is not that of an independent physical object.
Perhaps we are trying to quantise a description of relational structure.
Again, the question is not necessarily meaningless.
But it may be asking for the wrong kind of thing.
The climate analogy returns
Our earlier analogy between climate and weather becomes useful here.
Weather is actual.
A particular storm occurs.
Climate is a structured description of the conditions under which possible weather patterns occur.
We would not normally think of climate as another kind of weather-object.
It is a higher-level relational description of potential actual weather.
Now imagine someone saying:
“We have a theory of weather and a theory of climate. They don't quite fit together. Therefore we need quantum climate.”
That might be a sensible project.
But before beginning, we would want to know what the problem was.
Are we trying to quantise climate itself?
Or are we trying to understand how the microscopic physical processes generating weather produce the statistical structures we call climate?
Those are very different projects.
Perhaps something analogous is happening with quantum gravity.
Perhaps the question is not:
How do we quantise spacetime?
but:
How does relational geometry arise from the more fundamental quantum dynamics?
That is a different research programme.
The direction of explanation may be wrong
This possibility is worth taking seriously.
The conventional picture tends to encourage a synthesis:
quantum theory + general relativity → quantum gravity
But perhaps the relationship is instead hierarchical.
Perhaps quantum theory describes something more fundamental:
potential physical instantiations and their transformations.
And perhaps general relativity describes an emergent relational structure among actualised events.
If so, then we should not expect to obtain quantum gravity by simply quantising general relativity.
We might instead expect to obtain general relativity as some large-scale relational consequence of quantum dynamics.
This would invert the problem.
Instead of asking:
How do we quantise spacetime?
we ask:
How does spacetime emerge?
And instead of asking:
What is the quantum state of gravity?
we ask:
What quantum relational structure gives rise to gravitational geometry?
That is a much more radical possibility.
It is also much closer to the ontology we have been developing.
But emergence is not magic
We should be careful here.
To say that spacetime might be emergent is not to say that it is unreal.
A thing can be emergent and perfectly real.
Temperature is real.
Pressure is real.
Climate is real.
None of these is fundamental in the sense of being independent of the underlying physical processes.
The same could be true of geometry.
If spacetime geometry emerges from more fundamental relational processes, then spacetime is not an illusion.
It is a real structural level of physical organisation.
The important question is therefore not:
“Is spacetime real?”
but:
What kind of reality does spacetime have?
That is an ontological question.
And it is exactly the question that can be obscured when we immediately ask how to quantise it.
What if the metric is a relation?
The metric tensor is often treated as the central object of general relativity.
It tells us how to calculate intervals.
It determines the geometry.
It participates in the field equations.
It changes dynamically.
It is therefore natural to speak of the metric as a physical field.
But there is another possibility.
Perhaps the metric is fundamentally a representation of the relational structure among events.
It tells us:
how spatial intervals compare;
how temporal intervals compare;
which paths are geodesic;
which events can be causally connected;
and how physical clocks and rulers relate.
On this interpretation, the metric is not a physical substance occupying spacetime.
It is the mathematical expression of how physical events stand in relation.
Then a “quantum metric” is not obviously a fluctuating piece of cosmic fabric.
It might instead represent a quantum structure of possible relations among events.
That is a very different picture.
Quantum possibilities and relational geometry
We can now see the deeper problem.
Quantum theory gives us a structured field of possibilities.
General relativity gives us a structured field of relations among events.
Perhaps the fundamental problem is to understand how these two structures are connected.
A quantum possibility is not yet an actual event.
A geometric relation, however, is typically described between events.
So there is an intriguing sequence:
potential → actualisation → relational structure
Perhaps geometry belongs downstream of actualisation.
Not necessarily in a simple temporal sense.
Rather, ontologically.
The actualisation of physical events establishes a network of relations.
At sufficiently large scales, that network may exhibit the smooth geometric structure described by general relativity.
If so, quantum gravity is not the quantisation of geometry.
It is the explanation of how geometry arises from quantum actualisation.
That would be a profound shift.
The temptation to quantise everything
There is a broader philosophical habit at work here.
When a successful classical theory encounters a domain where quantum theory matters, we naturally try to quantise its fundamental variables.
It worked for fields.
It worked for many mechanical systems.
So why not spacetime?
The reasoning is understandable.
But it contains an assumption:
Whatever appears as a fundamental variable in the classical theory must correspond to a fundamental quantum entity.
That assumption may be false.
Classical variables can sometimes be emergent descriptions.
A successful theory can be ontologically effective without being fundamental.
The temperature of a gas is a perfectly legitimate physical quantity.
But there is no single molecule whose quantum state is the temperature.
Temperature emerges from the collective relational state of many degrees of freedom.
Likewise, perhaps spacetime geometry is a legitimate physical description without being a fundamental quantum object.
If so, quantising the geometry directly could be analogous to quantising temperature.
It might be mathematically possible.
But it would not necessarily answer the fundamental question.
What would we be quantising?
We can now return to our title.
Suppose we say:
We need quantum gravity.
The next question should be:
What are we trying to quantise?
Gravity?
If so, what is gravity?
The gravitational field?
What kind of physical thing is that field?
The metric?
Is the metric a thing, or a mathematical representation of relations?
Spacetime?
Is spacetime a physical substance, or a relational structure?
The geometry?
Is geometry fundamental, or emergent?
If the answers keep taking us back to relations, then perhaps the original formulation has become unstable.
We may be asking a quantum theory to be applied to something whose ontological status is not that of an independent quantum object.
The stronger question
Perhaps, then, the foundational question is not:
How do we quantise gravity?
It is:
How do quantum possibilities become organised into the relational structures we experience as geometry and gravity?
That question does not assume that spacetime is fundamental.
It does not assume that gravity is a force.
It does not assume that the metric is a substance.
And it does not assume that the classical variables of general relativity must correspond directly to quantum objects.
It asks instead how one level of physical description gives rise to another.
That is a very different project.
This does not mean quantum gravity is impossible
We should pause here, because it would be easy to overstate the argument.
Nothing we have said establishes that a theory of quantum gravity cannot be constructed.
Perhaps some future theory will contain genuinely quantum-geometric degrees of freedom.
Perhaps spacetime itself has a quantum structure at the Planck scale.
Perhaps approaches to quantum gravity that treat geometry as fundamental will ultimately prove correct.
The point is not to rule these possibilities out.
The point is to refuse to assume them at the beginning.
Before constructing the theory, we should ask what ontology the theory requires.
That is the philosophical investigation.
And perhaps the physics will answer differently from what we expect.
The wild goose chase
There is, however, a more provocative possibility.
Perhaps the enormous effort to construct quantum gravity is partly a response to a conceptual problem that has been misidentified.
Perhaps we have assumed:
general relativity describes fundamental spacetime;
quantum mechanics describes fundamental matter;
both are fundamentally correct;
therefore spacetime itself must be quantised.
But if the first assumption is wrong, the conclusion no longer follows.
Perhaps general relativity is instead an extraordinarily successful theory of an emergent relational structure.
Then the failure to quantise it cleanly would not necessarily indicate that we have not tried hard enough.
It might indicate that we are trying to quantise the wrong thing.
This is the possibility we must investigate.
Not proclaim.
Investigate.
Because if it is true, then the search for quantum gravity could indeed be something of a wild goose chase—not because the physicists are incompetent, but because the goose was never where we thought it was.
A different research question
Our investigation therefore begins with a simple inversion.
Instead of:
How do we quantise spacetime?
we ask:
Why should spacetime be quantised?
Instead of:
What is the quantum state of gravity?
we ask:
What is the ontological status of gravity?
Instead of:
How do quantum fields live in a quantum spacetime?
we ask:
How do quantum actualisations generate the relational structure we describe geometrically?
And instead of:
How do we reconcile two incompatible fundamental things?
we ask:
What if they are not two fundamental things at all?
That is where our investigation begins.
The furrow
There is a useful image for the possibility we are exploring.
A furrow does not determine what the plough will do.
But it changes the field of possibilities.
It makes some trajectories easier than others.
It affords certain courses.
Likewise, the relational structure of physical reality may constrain and enable the actualisation of possibilities without itself being another object in the world.
The wavefunction describes potential.
Actual events instantiate possibilities.
Relations among actual events form structured patterns.
Those patterns may become what we describe as geometry.
Geometry constrains subsequent possibilities.
And those possibilities can become actual in turn.
So the sequence is not:
things → relations.
It may be:
potential → actualisation → relations → new potential.
If that is even approximately right, then spacetime is not the stage upon which becoming occurs.
It is part of what becoming produces.
And gravity is not something added to that becoming from outside.
It is a particular form of relational structure within it.
The question we carry forward
We should therefore resist the temptation to end this first essay with a solution.
We have only changed the question.
But sometimes changing the question is the beginning of the solution.
The standard programme asks:
How do we quantise gravity?
Our investigation asks:
What are we trying to quantise?
And behind that:
What is fundamental enough to be quantised?
And behind that:
What kind of reality do quantum theory and general relativity actually describe?
If the answer is relational, then the next question follows naturally:
Is spacetime fundamental at all?
That is where we should go next.
Because before we attempt to quantise the universe's geometry, perhaps we should first establish whether the universe has geometry in the ontological sense we have assumed.
Perhaps spacetime is not something waiting to be quantised.
Perhaps it is something waiting to be understood.
🍷🙂
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