We have now reached an interesting point.
Quantum theory and general relativity are both extraordinarily successful.
Quantum theory describes the behaviour of physical systems at microscopic scales with astonishing precision.
General relativity describes gravitation, spacetime geometry and large-scale cosmological structure with equal authority.
Yet we are told that the two theories are fundamentally incompatible.
This has produced one of the great projects of modern physics:
Find a theory of quantum gravity.
But perhaps we should pause before accepting the problem in precisely that form.
Perhaps the difficulty is not that the two theories describe the same fundamental reality in incompatible ways.
Perhaps they describe different levels of organisation of the same reality.
If so, the problem is not primarily one of combining two theories.
It is a problem of understanding the relationship between two descriptions.
That distinction may be decisive.
Two descriptions, one world
Let us begin with what seems obvious.
There is one physical world.
Quantum theory describes some of its regularities.
General relativity describes others.
We should therefore expect some relationship between the theories.
But there is no logical requirement that two successful descriptions of one world must be descriptions at the same ontological level.
Consider temperature and molecular motion.
Temperature is real.
Molecular motion is real.
But we do not ordinarily expect a fundamental theory to contain “temperature particles”.
Temperature is a macroscopic description of enormous numbers of microscopic relations.
The two descriptions refer to the same physical system.
They do not compete for fundamental status.
Perhaps quantum theory and general relativity are more like this than we have assumed.
The usual picture of the problem
The conventional quantum-gravity problem is often formulated roughly like this:
Quantum theory tells us that physical systems have quantum states.
General relativity tells us that spacetime is dynamical.
Therefore:
spacetime itself must be quantum.
This seems natural.
If everything physical is ultimately quantum, then surely spacetime must be quantised.
But notice the hidden premise.
It assumes that spacetime is one of the fundamental things that exist.
Our investigation has challenged precisely that premise.
If spacetime is emergent, then the conclusion does not follow.
We would instead expect:
the underlying physical processes are quantum; the spacetime description emerges.
The question then changes completely.
Quantising the description
Suppose temperature is an emergent property of a gas.
One can certainly construct mathematical theories involving fluctuations of temperature.
Those theories can be extremely useful.
But we would not conclude that temperature itself must be a fundamental quantum object.
Likewise, if geometry is emergent, we may be able to construct quantum theories of metric fluctuations.
That does not establish that the metric is fundamental.
It may simply mean that an emergent variable can participate in quantum-effective behaviour.
This distinction is often blurred.
There is a difference between:
a quantum theory of an emergent quantity
and
a fundamental quantum ontology in which that quantity is elementary.
The first may be perfectly sensible.
The second may be unnecessary.
The level problem
Perhaps the real difficulty is therefore a level problem.
Quantum theory is being asked to describe the fundamental dynamics.
General relativity is being asked to describe the emergent organisation of those dynamics.
But if we insist that both descriptions must be expressed as fundamental theories of the same kind, we create a conceptual conflict.
We ask:
Which one is the fundamental description?
And then we become trapped.
Perhaps neither description has to be discarded.
Perhaps they occupy different levels.
The challenge is to understand the mapping between levels.
A familiar example
Consider a fluid.
At one level we can describe it using individual molecules.
At another, we describe pressure, density, velocity and temperature.
At yet another, we describe turbulence, vortices and waves.
These descriptions are not interchangeable.
A vortex is not an individual molecule.
Pressure is not an additional substance.
Temperature is not a particle.
Yet all of these descriptions are physically meaningful.
The higher-level patterns arise from lower-level relations.
A successful theory of fluids therefore does not try to make the molecular and hydrodynamic descriptions identical.
It explains how one gives rise to the other.
Perhaps quantum gravity needs the same conceptual move.
The quantum description
Within our ontology, quantum theory describes a world of:
- potential;
- actualisation;
- probability;
- correlation;
- entanglement;
- and relational structure.
The wavefunction describes potential.
Actual events instantiate possibilities.
Those events establish physical distinctions.
Those distinctions become correlated with other systems.
The resulting relational structure evolves.
This is the level at which we should look for the fundamental description.
Notice what is absent.
We have not needed fundamental spacetime.
We have not needed fundamental geometry.
We have not needed a gravitational substance.
The relativistic description
General relativity, by contrast, gives us a remarkably economical description of the large-scale relational organisation of physical processes.
The metric encodes intervals.
The causal structure determines which events can influence which others.
Geodesics describe free trajectories.
Curvature describes the systematic behaviour of neighbouring trajectories.
Einstein's equations relate this geometry to the distribution of energy and momentum.
This is an extraordinarily rich description.
But nothing in it requires us to insist that the geometric variables are the ultimate constituents of reality.
They may instead be the effective variables of a particular regime.
The descriptions meet in the middle
This suggests that the relationship between the theories should not be imagined as:
quantum theory + general relativity = quantum gravity.
Perhaps it should instead be:
fundamental quantum-relational dynamics
↓
emergence
↓
effective geometric organisation
↓
general relativity
This is not merely a different diagram.
It changes what counts as the central problem.
We are no longer trying to force geometry into the quantum ontology.
We are trying to derive the geometric description from it.
What would emergence mean here?
The word emergence can become dangerously vague.
It is sometimes used to mean little more than:
“At large scales, something different happens.”
That is not enough.
If geometry genuinely emerges, we need to know what that means physically.
At minimum, we would expect many different microscopic relational configurations to produce approximately the same macroscopic geometry.
We would expect geometric variables to become robust under changes in microscopic detail.
We would expect causal and metric structure to become stable.
And we would expect the effective dynamics of that structure to reproduce general relativity.
Emergence must therefore be a controlled relationship between descriptions, not a philosophical escape hatch.
The role of coarse-graining
Coarse-graining may provide part of the conceptual machinery.
A microscopic description contains enormous detail.
A macroscopic description retains only structures that remain relevant at larger scales.
Imagine that the fundamental relational state is extraordinarily complicated.
We do not need to preserve every microscopic distinction in order to describe the large-scale world.
Instead, we identify equivalence classes of microscopic states that behave similarly at macroscopic scales.
If those equivalence classes exhibit stable relational properties, we can assign them effective geometric variables.
The metric would then represent something like:
the large-scale invariant relational structure shared by many microscopic configurations.
That is a much more plausible candidate for emergent geometry than the idea that a tiny piece of spacetime somehow becomes smooth when viewed from far away.
Why the metric is so powerful
This also explains something that otherwise seems mysterious.
Why does a relatively simple mathematical object—the metric—capture so much physical behaviour?
Because an emergent variable can be enormously informative.
Temperature summarises vast amounts of microscopic molecular behaviour.
Pressure summarises countless molecular collisions.
A fluid velocity field summarises the collective behaviour of enormous numbers of particles.
The metric could play an analogous role.
It would be a compressed representation of the relational structure that matters at macroscopic scales.
Its power would then be evidence of effective universality, not necessarily evidence of fundamental existence.
Universality
Universality is particularly important.
Different microscopic systems can produce the same macroscopic behaviour.
The details of the molecules in two fluids can differ substantially while both obeying the same hydrodynamic equations.
This is why macroscopic theories can be so robust.
They do not depend upon every microscopic detail.
If spacetime geometry is similarly universal, then many different microscopic relational structures could produce the same effective spacetime.
This would be an enormous conceptual advantage.
It would explain why general relativity could be so successful without being fundamental.
The theory would describe a universal large-scale regime.
The classical world as a fixed point
We can push this idea further.
In many physical theories, coarse-graining drives systems toward effective descriptions that are insensitive to microscopic details.
One can think of these as fixed points or stable regimes.
Perhaps classical spacetime is one such regime.
At microscopic scales, the ontology is quantum and relational.
Under coarse-graining, enormous microscopic complexity is suppressed.
A stable causal and metric structure emerges.
At the fixed point of this large-scale organisation, the effective dynamics become those of general relativity.
If so, general relativity would not be a rival to quantum theory.
It would be the large-scale expression of the quantum relational theory.
The classical limit would then be ontological
We often speak of the “classical limit” as though it were merely a mathematical approximation.
But perhaps it is more than that.
The classical world may represent a genuinely different regime of organisation.
The underlying reality remains quantum.
But the relational structures produced through enormous numbers of actualisations become sufficiently stable that classical concepts acquire determinate meaning.
Objects become persistent.
Trajectories become approximately definite.
Geometry becomes smooth.
Time becomes measurable.
The world acquires the appearance of an arena populated by things.
That arena would be real.
But emergent.
This changes the measurement problem
Our earlier discussion of actualisation now becomes relevant again.
If actual events generate stable correlations, then the classical world need not be introduced by a mysterious collapse postulate.
Repeated physical interactions can produce increasingly robust patterns.
The environment becomes correlated with particular outcomes.
Macroscopic structures record those outcomes.
The resulting patterns become effectively classical.
This does not automatically solve every interpretive problem in quantum theory.
But it gives us a natural route by which classical actuality can emerge without requiring classical spacetime to be fundamental.
The arrow of description
There is also an important asymmetry in the explanatory direction.
We can derive temperature from molecular dynamics.
But we do not normally derive molecular dynamics from temperature.
We can derive hydrodynamic equations from underlying microscopic models.
But the hydrodynamic description does not contain enough information to reconstruct every molecular detail.
Likewise, if general relativity is emergent, we should expect the mapping to be asymmetric.
Quantum-relational dynamics could produce effective geometry.
But the geometry alone might not uniquely determine the underlying quantum structure.
This matters enormously.
It means that searching for a fundamental theory by simply “quantising general relativity” may be asking the emergent description to contain information it was never designed to retain.
Information may be lost from the description without being lost from reality
This distinction also returns us to our earlier investigation of information.
Coarse-graining discards microscopic distinctions from the description.
That does not necessarily mean that physical information has been destroyed.
It means that the higher-level description does not retain it.
This is exactly what happens with temperature.
Knowing the temperature of a gas does not tell us the exact state of every molecule.
But the missing microscopic information has not thereby ceased to exist.
Likewise, a metric description may omit enormous amounts of microscopic relational information.
The geometry can therefore be completely accurate while still being radically incomplete ontologically.
That is what an effective theory means.
The black-hole problem looks different
This gives us another perspective on the black-hole information paradox.
If the geometric description is emergent, then a black hole described by general relativity is itself an emergent structure.
Its horizon is an emergent causal boundary.
Its entropy may count underlying microscopic degrees of freedom.
Its temperature may describe a collective phenomenon.
Then the apparent tension between information preservation and geometric black-hole evaporation may arise partly because we are asking an effective description to answer questions about microscopic information.
The paradox may therefore be another instance of the same ontological confusion we encountered earlier.
We mistake the effective description for the underlying reality.
The horizon as a warning
The event horizon is particularly instructive.
From the geometric description, it appears to be an extraordinarily sharp boundary.
But if geometry is emergent, the horizon may not correspond to a fundamental boundary in the underlying ontology.
It could instead be a large-scale causal regularity.
This does not make it unreal.
A horizon has measurable consequences.
But its ontological status would be analogous to a phase boundary or a thermodynamic surface.
Real.
Stable.
Observable.
Yet emergent.
That possibility should make us cautious about interpreting the horizon as a fundamental object.
The same problem appears everywhere
We can now see a recurring pattern.
In quantum theory, we reified the wavefunction.
In black-hole physics, we reified information.
In relativity, we reified spacetime.
In gravity, we reified the gravitational field.
In quantum gravity, we have tried to quantise these reifications.
Perhaps the common problem is not any particular physical theory.
Perhaps it is our tendency to turn descriptions into things.
We observe a stable pattern.
We give it a name.
The name becomes a noun.
The noun becomes an entity.
The entity is then assigned causal powers.
And eventually we are trying to explain how entities that were originally products of our descriptions interact with one another.
The relational ontology interrupts this process.
It asks:
What relations does the concept actually describe?
Physics is full of nouns
This may sound like a linguistic observation.
It is not entirely accidental that our ontology is sensitive to language.
Physics necessarily uses concepts.
Concepts compress patterns.
Nouns are useful because they allow us to talk about stable regularities as though they were objects.
“Electron.”
“Field.”
“Particle.”
“Spacetime.”
“Gravity.”
“Information.”
These terms are indispensable.
But grammatical form does not determine ontological status.
A noun does not guarantee a thing.
A field can describe a pattern.
An event can describe a process.
A law can describe a regularity.
Information can describe a relation.
Geometry can describe an organisation.
Gravity can describe a behaviour.
The world need not share the grammar of our descriptions.
This is where the relational ontology earns its keep
A relational ontology does not demand that we eliminate nouns.
It asks us to distinguish things from the patterns through which things become identifiable.
An electron can be real as a stable pattern of possible and actual interactions.
A field can be real as a structured relational organisation.
A particle can be real as an actual instance.
A spacetime geometry can be real as an emergent pattern.
Gravity can be real as a systematic constraint on physical becoming.
Information can be real as a structure of distinctions and correlations.
Reality remains fully real.
What disappears is the assumption that reality must ultimately consist of independently existing things.
What would a fundamental theory look like?
If this picture is right, the fundamental theory may look rather different from what we have been expecting.
We may not begin with:
- spacetime;
- particles;
- fields;
- or even geometry.
We may begin with a structure of potential relations and rules governing actualisation.
The fundamental entities, if “entities” is even the right word, might be closer to:
- possible relations;
- actualisation events;
- correlations;
- constraints;
- and transformations among relational structures.
From these, persistent patterns could emerge.
Some would behave like particles.
Some like fields.
Some like objects.
Some like information-bearing structures.
Some like geometry.
And one particularly stable family of relations would appear to us as gravity.
This would be a profoundly different picture of physical reality.
But what would make it physics?
Here we must once again resist the temptation to celebrate too early.
An ontology is not a physical theory.
We have not supplied equations for the underlying relational dynamics.
We have not shown that three-dimensional space emerges.
We have not derived the Lorentz group.
We have not derived the Einstein field equations.
We have not explained the numerical values of the physical constants.
We have not demonstrated that quantum probabilities emerge from the proposed ontology.
These are enormous tasks.
But perhaps we have done something that has to come first.
We have identified a possible direction of explanation.
And that direction may be more important than another attempt to manipulate the existing equations without questioning their ontological assumptions.
The problem was perhaps backwards
We can now formulate the central suspicion of this series.
Perhaps the question:
“How do we combine quantum mechanics and general relativity?”
is backwards.
Perhaps we should ask:
“Why do quantum theory and general relativity describe such different aspects of reality?”
And then:
“What underlying relational process could give rise to both descriptions?”
The first question assumes that both theories are fundamental.
The second allows that they may be descriptions of different levels.
That small shift could change the entire research programme.
What if there is no quantum gravity?
This is now the provocative possibility that our investigation has been approaching.
Perhaps there is no fundamental entity called quantum gravity.
Not because gravity is unreal.
Not because quantum theory fails.
Not because general relativity is wrong.
But because gravity may be an emergent relational phenomenon, while quantum theory describes the deeper level from which the gravitational regime arises.
In that case, “quantum gravity” would be like asking for a quantum theory of temperature.
One can certainly study quantum fluctuations in systems whose temperature is meaningful.
But temperature itself need not be fundamental.
Perhaps the same distinction applies to gravity.
Then what are we looking for?
We would no longer be looking for:
the quantum of spacetime.
We would be looking for:
the dynamics of potential and actual relational structure from which spacetime emerges.
We would not ask:
What is the fundamental gravitational field?
We would ask:
What relational organisation produces effective gravitational behaviour?
We would not ask:
What is the quantum state of the metric?
We would ask:
Under what conditions does a relational quantum system acquire an effective metric description?
These are radically different questions.
And they seem to follow naturally from the ontology we have developed.
Perhaps the descriptions should not be combined
There is therefore a final irony.
The search for a theory of quantum gravity has been driven by the desire to combine two descriptions.
But perhaps the correct theory will not combine them at the same level.
Instead, it will explain why they cannot be combined at the fundamental level.
Quantum theory and general relativity may remain distinct descriptions because they describe different regimes.
The fundamental theory would sit beneath both.
Quantum theory would emerge as the appropriate description of potential and actualisation.
General relativity would emerge as the appropriate description of large-scale relational geometry.
The two theories would then be related not by direct unification, but by emergence.
That may be a more profound kind of unity.
A hierarchy rather than a synthesis
The picture we are approaching might therefore look like this:
fundamental relational potential
↓
actualisation
↓
physical events
↓
correlations and persistent patterns
↓
effective quantum systems
↓
classical objects and fields
↓
stable causal and metric structure
↓
general relativity
The exact hierarchy will certainly need revision.
Some of these levels may be simultaneous rather than sequential.
Some may collapse into one another.
But the essential idea is that reality may be stratified by organisation rather than by substance.
Different theories describe different organisational regimes.
There is no contradiction in this.
The deepest question
This leaves us with perhaps the deepest question yet.
If quantum theory describes potential and actualisation, and general relativity describes emergent geometry, then what determines the stability of the transition between them?
Why does the universe settle into a world in which:
- objects persist;
- causal relations stabilise;
- geometry becomes smooth;
- gravity obeys Einstein's equations;
- and classical reality emerges from quantum possibility?
We have come a long way.
But we have not yet answered that question.
And perhaps this is where the real investigation begins.
Because we are no longer trying to repair two incompatible theories.
We are trying to understand how a world acquires the form in which those theories become applicable.
The next question
We began this series by asking what we are trying to quantise.
The answer has gradually become uncomfortable.
Perhaps we were trying to quantise things that were never fundamental things.
First the wavefunction became potential rather than substance.
Then spacetime became an emergent relational structure.
Then gravity became a pattern rather than a gravitational entity.
And now the very project of combining quantum theory with general relativity has begun to look different.
Perhaps the problem was not:
How do we combine two fundamental descriptions?
Perhaps it was:
How do two different effective descriptions emerge from one deeper relational ontology?
If so, the search for quantum gravity may indeed be a kind of wild goose chase—not because the physicists pursuing it are mistaken about the physics they are describing, but because the ontological question has been posed at the wrong level.
The real problem is not to quantise gravity.
It is to understand how gravity becomes possible.
And not merely how gravity becomes possible.
We must understand how a classical world becomes possible at all.
That is where the investigation must finally lead.
VIII. Beyond Quantum Gravity: What Are We Actually Looking For?
🍷🙂
No comments:
Post a Comment