What Does Physics Say Is Real?

Physics is our most precise science.

But precision does not automatically settle ontology.

Physics tells us how systems behave with extraordinary success.

It is much harder to answer:

What does physics say is actually real?

The Ontological Question

A physical theory contains:

• mathematical objects,
• state variables,
• equations.

Which of these correspond to real features of the world?

Not every symbol in an equation must be an entity.

Classical Intuition

Classical mechanics suggests a simple ontology:

• particles,
• positions,
• velocities,
• forces.

This picture feels concrete.

Modern physics complicates every part of it.

Particles

A classical particle is:

• localized,
• persistent,
• countable.

Quantum particles do not fit this image perfectly.

They exhibit:

• superposition,
• interference,
• indistinguishability.

Quantum Identity

Two electrons are not merely hard-to-distinguish objects.

Quantum theory treats identical particles as fundamentally indistinguishable.

Swapping labels may not create a new physical state.

This weakens classical individuality.

Fields

Modern physics increasingly describes reality in terms of fields.

A field assigns quantities across spacetime.

Examples include:

• electromagnetic field,
• Higgs field.

Quantum Field Theory

In quantum field theory, what we call particles are often interpreted as:

excitations of underlying quantum fields.

This shifts ontology from:

little objects

to:

field structures.

Particle Creation

Quantum field theory allows particles to be:

• created,
• annihilated.

That would be strange for classical permanent objects.

Field ontology handles it naturally.

Are Fields Real?

If fields:

• carry energy,
• mediate interactions,
• produce measurable effects,

realists have strong reason to take them seriously.

But mathematical representation and ontology must still be distinguished.

Gauge Fields

Gauge theories contain mathematical redundancy.

Different mathematical descriptions can represent the same physical state.

This warns us:

not every element of formalism is physically real.

Gauge Symmetry

A gauge symmetry may reflect freedom in description rather than a physical transformation.

Ontology should attach to gauge-invariant structure.

This is a deep lesson.

Observables

Physics often emphasizes quantities that can be:

• measured,
• compared.

But observability is not a perfect criterion for reality.

Many realists accept unobservable entities.

Wavefunction

Quantum mechanics represents a system using a wavefunction:

[ |\psi\rangle ]

or equivalent state formalism.

What is this object?

That is one of the deepest ontological questions in physics.

Wavefunction Realism

Some interpretations treat the wavefunction as physically real.

Then it is part of fundamental ontology.

But the wavefunction can live in high-dimensional configuration space rather than ordinary three-dimensional space.

This is conceptually difficult.

Epistemic Wavefunction

Other approaches treat the wavefunction as representing:

• information,
• knowledge,
• belief.

Then it is not a physical object in the same way as a field.

ψ-Ontic vs ψ-Epistemic

Philosophers distinguish:

• ψ-ontic views: wavefunction reflects physical reality,
• ψ-epistemic views: wavefunction reflects knowledge or information.

The distinction is central to quantum foundations.

PBR Theorem

The Pusey–Barrett–Rudolph theorem constrains certain ψ-epistemic interpretations under specific assumptions.

It does not prove one universal wavefunction ontology.

Interpretation remains contested.

Measurement Problem

Quantum mechanics evolves states smoothly according to equations.

Measurements appear to yield definite outcomes.

How these fit together creates the measurement problem.

Ontology depends heavily on the interpretation.

Copenhagen-Style Interpretations

Copenhagen-like approaches often resist assigning definite properties before measurement.

The theory may be viewed as a framework for predicting observations.

Ontological commitment is cautious.

Many-Worlds

Everettian interpretations keep deterministic wavefunction evolution.

All decoherent outcomes occur in different branches.

If taken realistically, the ontology becomes enormous.

Bohmian Mechanics

Bohmian mechanics includes:

• definite particle positions,
• guiding wavefunction.

It restores trajectories but introduces nonlocal structure.

Objective Collapse

Collapse theories modify dynamics.

Wavefunction collapse becomes a real physical process.

Ontology then includes stochastic collapse events or related structures.

Interpretive Underdetermination

Different interpretations can reproduce much of the same observed quantum data.

This means empirical evidence may not uniquely determine ontology.

Structure vs Objects

One response is:

perhaps physics tells us relational structure more securely than the intrinsic nature of objects.

This motivates structural realism.

Structural Realism

On structural realism, the durable content of physics lies in:

• relations,
• symmetries,
• mathematical structure.

Entities can change across theories while structure survives.

Electromagnetic Example

Classical electromagnetism can be described using:

• fields,
• potentials.

Gauge redundancy suggests physical reality may be encoded in relational structure rather than every variable.

Symmetry

Modern physics is deeply organized around symmetries.

Conservation laws and particle classifications emerge from them.

Are symmetries themselves real?

Noether’s Theorem

Noether’s theorem connects continuous symmetries with conservation laws.

Examples:

• time translation ↔ energy conservation,
• spatial translation ↔ momentum conservation.

Structure becomes physically explanatory.

Spacetime

General relativity treats gravity not as an ordinary force but as spacetime geometry.

Matter tells spacetime how to curve.

Spacetime tells matter how to move.

Is Spacetime a Thing?

Two broad positions reappear:

• substantivalism,
• relationalism.

Is spacetime an entity?

Or only relations among events?

General Relativity Complicates the Choice

Spacetime geometry is dynamical.

It can:

• propagate waves,
• interact.

Gravitational waves make spacetime seem physically active.

This supports realist interpretations.

Hole Argument

Einstein’s hole argument and later philosophical discussion show that treating spacetime points as independently individuated creates problems.

Relational structure may be more fundamental.

Events

Physics often represents reality as events in spacetime.

An event is:

something happening at a spacetime location.

Perhaps events are more basic than enduring objects.

Worldlines

A particle’s history can be represented as a worldline.

In relativity, the four-dimensional trajectory may be more fundamental than a sequence of three-dimensional positions.

Block Universe

Relativity motivates a four-dimensional view where spacetime as a whole is real.

This connects directly to the next essay on time.

Quantum Gravity

At very small scales, spacetime itself may not be fundamental.

Candidate theories suggest it could emerge from deeper structures.

No empirically confirmed quantum-gravity theory yet settles the matter.

Emergent Spacetime

Some approaches explore spacetime emerging from:

• entanglement,
• information,
• networks.

If successful, spacetime would be real but nonfundamental.

Causal Sets

Causal-set theory models spacetime using discrete causal relations.

The continuum could emerge from ordered events.

This is one example of deeper ontology.

Loop Quantum Gravity

Loop quantum gravity quantizes geometric structure.

Area and volume may have discrete spectra.

Its ontology differs from classical spacetime.

String Theory

String theory replaces point particles with extended objects such as strings.

It also introduces additional dimensions in many formulations.

Whether these entities are physically real remains unconfirmed.

Effective Field Theory

Modern physics often treats theories as effective.

A theory works within an energy range.

It need not be final.

This encourages ontological humility.

Effective Ontology

Quasiparticles are real in condensed matter despite being emergent.

Examples:

• phonons.

They behave like particles but arise from collective dynamics.

Phonons

A phonon is a quantized vibration mode in a material.

It is not a fundamental particle.

Yet it can:

• carry energy,
• scatter.

Emergent entities can be physically real.

Quasiparticles

Condensed matter contains many quasiparticles with effective properties.

This demonstrates that “particle” does not automatically mean fundamental object.

Reality by Scale

Physics may require different ontologies at different scales.

At one scale:

molecules.

At another:

fields.

At another:

effective excitations.

No single vocabulary dominates everywhere.

Renormalization

Renormalization explains how descriptions change with scale.

Different variables become relevant at different levels.

This supports layered realism.

Universality

Very different microscopic systems can exhibit the same macroscopic behavior.

Universal behavior depends more on large-scale structure than microscopic detail.

Reality is organized by scale.

Fundamental vs Effective

A thing can be:

effective

and:

real.

The idea that only fundamental entities are real is too restrictive.

Causal Efficacy

Quasiparticles and fields produce measurable effects.

Causal and explanatory roles support realism.

Instrumental Success

Instrumentalists remind us:

predictive success does not force literal ontology.

A model can work without being a photograph of reality.

Scientific Realist Reply

Realists answer:

systematic success across independent contexts is best explained by approximate truth.

The debate remains philosophical.

Theory Change

Physics has replaced:

• ether,
• classical particles

with new structures.

This teaches caution about declaring current ontology final.

Yet Continuity Exists

Later theories often preserve earlier structures as approximations.

Newtonian mechanics reappears in suitable limits.

This supports structural continuity.

Correspondence Principle

New theories must recover old successful predictions in domains where old theories worked.

Reality constrains theory change.

Measurement Devices

All physical evidence arrives through interactions.

A detector itself is a physical system.

Observation is part of physics.

No View from Outside

Physics does not stand outside the universe.

Its instruments and observers are inside the system being described.

This creates reflexive philosophical depth.

Does Physics Describe Everything Real?

Even a complete physics might not eliminate:

• biology,
• psychology,
• social reality.

Higher-level patterns can remain real.

Physics and Consciousness

Physics describes neural processes.

Whether it exhausts subjective experience is the hard problem.

A physical ontology may be complete causally yet incomplete descriptively.

Physics and Mathematics

Physics uses mathematical structures.

Whether the structures themselves are physically real remains separate.

Mathematics is not automatically ontology.

Theory Is Not Reality

The equations are representations.

Nature is whatever makes the successful equations work.

This distinction should never be forgotten.

The Philosophical Lesson

Physics does not currently give us one uncontested ontology.

Depending on theory and interpretation, reality may involve:

• fields,
• particles,
• wavefunctions,
• spacetime,
• deeper structures.

What physics tells us most securely may be not the intrinsic essence of things but the lawful structure of their relations.

The Next Question

If physics is fundamental, does that make only atoms and fields truly real?

What about:

• persons,
• minds,
• experiences?

Are they merely convenient descriptions?

The next essay asks:

Are People and Consciousness as Real as Atoms?