Quantum Field Theory and the Relational Nature of the Cosmos
Quantum Field Theory and the Relational Cosmos
Introduction
What is a particle? The answer, according to modern physics, is far stranger than the tiny billiard balls of classical imagination. We are accustomed to thinking of matter as composed of discrete, persistent objectsâelectrons, protons, neutronsâthat exist independently of our observation. Yet quantum field theory (QFT), the most successful framework in contemporary physics, paints a radically different picture. In this essay, I will explore the QFT framework, where particles are not fundamental objects but excited states of underlying fields that permeate all of space. I will argue that this framework leads inexorably to a relational view of existence: what counts as a 'particle' depends upon the observer's motion, horizons transform the quantum vacuum into a thermal bath, and the very notion of an objective, observer-independent particle number dissolves. The universe, I will suggest, is not a collection of things but a network of relationsâthough this does not mean reality is merely subjective; rather, it means that objectivity is found in the invariant structure of relations, not in the substances that instantiate them.
The Field Theory Picture â The Quantum Vacuum and Its Fluctuations
The old quantum theory attempted a description of the world in terms of particles and waves. In the new quantum theoryâQuantum Field Theory (QFT)âthe world is made of fields that permeate all of space, existing everywhere simultaneously. There is a separate field for each fundamental particle in the Standard Model: the electron field, the photon (electromagnetic) field, the Higgs field, and so on. In this view, a particle is not a permanent object but rather a localized excitation (a vibration) of its underlying field. A mode is a specific pattern of the field that becomes occupied when energized (e.g. different momenta, or different locations).
Quantum indeterminacy (Heisenberg uncertainty) tells us that there is a fundamental limit to how precisely we can know the energy of a system over a specific time interval. This implies that over extremely short timescales, the fluctuation in energy can be significant. In the vacuum, these fluctuations manifest as transient disturbances in the quantum field. These are often modeled in perturbation theory as 'virtual' particle-antiparticle pairsâa mathematical device representing the field's inherent jitter. Unlike real particles, these virtual excitations are not independent entities that can be detected directly; rather, they are terms in a perturbative expansion that represent the field's zero-point activity. (The standard heuristic that they "borrow" energy from the vacuum via ÎE¡Ît â ħ is a useful but approximate picture; in the full theory, they are better understood as mathematical artifacts of a particular calculational method.) These virtual particles can influence real particles (e.g., causing the Lamb shift) or create measurable forces (like the Casimir Effect, which arises from changes in vacuum energy density between plates).
So the vacuum is not 'empty' in the classical senseâit is a dynamic state of minimum energy (more accurately described as a fluctuating field than as a 'foam' of discrete particles). The term 'virtual' is crucial: it reminds us that these are not real, detectable particles, but rather the mechanism by which the vacuum exerts physical influence.
The lowest-energy real-particle state that the field supports is its one-particle state, where the field is excited in a pattern corresponding to a single quantum of energy in a specific mode. In its two-particle state, the field either has two quanta both occupying the same mode or each quantum occupying a different mode:
Two Quanta in the Same Mode â both units of energy excite the exact same pattern. The field vibrates in that specific mode with double the intensity (amplitude) compared to a single-particle state. Instead of two separate ripples, you have one stronger ripple of the same shape. In this case, Mode A has an occupation number of 2, and all other modes are empty. (Note: this is an occupation number, not simply "two units of energy," since the energy of each quantum depends on the mode's frequency.)
Two Quanta in Different Modes â the field is excited in two distinct patterns simultaneously. For example, one quantum might correspond to a wave moving to the right, and the other to a wave moving to the left. Here, the field vibrates in two separate ways at onceâyou have one quantum in Mode A and one in Mode B.
So for a million particles, the field is in a million-particle state defined not by vibrations at a million separate locations but by the occupation numbers of various field modes, reflecting the indistinguishable and probabilistic nature of quantum entities. Think of the field not as a single drumhead, but as an infinite collection of independent "strings" (the modes), each capable of vibrating at a specific frequency.
The crucial conceptual shift from the old quantum theory is that we do not say "Particle 1 is here and Particle 2 is there." Instead, we simply count: "Mode A has 2 excitations" or "Mode A has 1 and Mode B has 1." The particles are indistinguishable counts of energy in specific field patterns, not individual labelled objects.
Entanglement
To say that two particles, A and B, 'interact' is to say that both are changed in a mutually consistent manner, preserving conservation laws. Our classical intuition, shaped by examples like billiard balls, suggests that after a collision we still have two distinct objects, each bearing its own independent properties. Quantum mechanics behaves differently. After A and B interact, the information describing them is not distributed so that each particle retains its own independent state. Instead, the two-particle system is described by a single, unified quantum state, AB. The properties of the two particles are inextricably linkedâneither particle has a definite state on its own (at least, not in the case of maximal entanglement). This is what we call entanglement.
When one particle in an entangled pair, say particle B, interacts with a third, previously unentangled particle C, we have an entangled three-particle system ABC which is again described by a single, unified quantum state. Particle B's original entanglement with particle A must decrease as it becomes redistributed among the larger system. As the entanglement cascades out into the environment in this manner, the delicate quantum correlations between the original pair become 'diluted' into the vast number of degrees of freedom in the environment. The effect is that the original particle pair AB progressively lose their visible entanglement; they begin to behave like classical (i.e. approximately independent) objects when observed locally. This process is known as decoherence.
Crucially, the virtual fluctuations that characterize the vacuum state of a field are subject to quantum entanglement. Don't think of entangled particles as "sharing" a list of propertiesârather, the entangled state is the list. The properties are not distributed between particles; they are defined by the correlations between them.
The Relativity of Particle Motion
Imagine a graph with distance on the vertical axis and time on the horizontal axis. An object moving at constant velocity in flat space (i.e. space devoid of gravity) is represented by a straight line. An accelerating object in flat space is represented by a rising curved line.
When we introduce gravity, Einstein's Principle of Equivalence tells us that the experience of acceleration in flat space is locally indistinguishable from the experience of remaining stationary in a gravitational fieldâan object in free fall (moving only under gravity) feels no force. An object in free fall follows a geodesicâthe straightest possible path through curved spacetime. Even though this path might look curved on a 2D graph (e.g. an object falling from an aircraft follows a parabolic trajectory), its trajectory is unforced motion through curved spacetime.
An object resisting gravity (e.g. standing on the Earth's surface) does feel a forceâspecifically the force of the ground pushing up and preventing it from falling freely. This force redirects the object away from its natural geodesic, creating a curved world-line in spacetime. So, the rule is: If you feel no force, you are following a straight world-line (a geodesic) in curved spacetime. If you feel a force, your world-line is curved relative to that spacetime geometry.
When a clock moves relative to your reference frame, it appears to tick slower (time dilation). If it could move at the speed of light (a limit it can never reach in practice), it would appear completely frozen. Mathematically, this is modeled as a 'skewing' of the time and space axes on a spacetime diagram. For an observer at rest, the time and space axes are perpendicular (90°). As their relative velocity increases, the axes skew toward each other (the angle between them becomes more acute). But no observer can actually reach the speed of light, so the axes never fully coincide (i.e. reach 45°).
If an object moves at a constant velocity, the angle between the axes remains fixed. Constant velocity means no acceleration, so the observer feels no force. If the object accelerates, its velocity changes, and the angle between the axes no longer remains fixed (i.e., they progressively rotate or skew toward each other). It is this changeâthis progressive deviation from a straight line in spacetime (i.e. from its geodesic)âthat manifests as a force felt by the observer. In a gravitational field, an observer in free fall feels no forceâthey are moving through spacetime in a straight line (i.e. along a geodesic). Einstein's principle of equivalence tells us that the force required to resist gravity (like standing on the Earth's surface) is locally equivalent to acceleration in flat space. Thus, free fall is the true 'inertial motion,' while resisting gravity is a form of acceleration.
The Rindler Horizon and the Unruh Effect
In QFT we can count particles by detecting the amount of energy in a specific mode. But, quite unintuitively, what counts as a mode depends upon how you move along your world-line.
For an observer in inertial motion (free fall), the field modes appear as simple sine waves. For such an observer, the vacuum state has a particle occupation number of zero. By contrast, an accelerating observer (or equivalently an observer resisting gravity) moves through spacetime in a curved path. For such an observer, there will be a distance behind them beyond which the velocity of separation effectively reaches the speed of light, creating what is known as a "Rindler Horizon." This is a boundary that hides part of the universe from the accelerating observer, since information in the space beyond this horizon would have to exceed the speed of light to reach the observer.
To such an observer, the fixed frequency sine waves of an inertial observer appear as a chirp or spectrum of different frequencies when measured against his own clock, because the time axis of the accelerating observer is continuously changing. Remarkably, then, the vacuum state (zero particles) of the inertial observer looks like a thermal bath of real particles to the accelerating observer. This is known as the Unruh effect. The upshot is that although the field itself is the same for all observers, the definition of a 'particle' depends on how the observer divides spacetime between its time and space axesâi.e., on their choice of what physicists call a foliation of spacetime. The accelerating observer sees particles where the inertial observer sees nothing.
A technical note: the distinction between "positive frequency" and "negative frequency" modes is crucial here. In QFT, positive frequency modes (which evolve forward in time) correspond to particle states, while negative frequency modes (which evolve backward in time) correspond to antiparticle states. In the vacuum state, positive and negative frequency modes are paired through quantum entanglement. An inertial observer's time coordinate naturally separates these modes in a way that yields no particles. An accelerating observer's time coordinate, however, mixes positive and negative frequency modes, transforming the vacuum into a thermal bath.
In short, in a flat universe (no gravity) an inertial observer sees the vacuum as empty space with simple sine-wave modes, and an accelerating observer sees a thermal bath of real particles. Unruh radiation is the direct consequence of acceleration: the horizon created by the accelerating observer's motion breaks the quantum entanglement between positive and negative frequency modes, hiding the partners required to cancel the positive ones. By concealing them, cancellation is prevented, leaving the positive modes 'naked' and excited. The positive frequency modes, which were previously in the vacuum state (no particles), appear excited (many particles) to the accelerating observer. So particle number is not an absolute property of the universeâit is a relational property between the observer's motion and the quantum field.
Hawking Radiation
A stationary observer hovering above a black hole (curved space) experiences the same thermal bath as an observer accelerating in flat spacetime, with the local temperature proportional to the required acceleration. This is referred to as Hawking radiation. Locally, it is equivalent to Unruh radiationâthe difference lies in the global boundary conditions and the fact that Hawking radiation can escape to infinity and carries negative energy into the black hole, causing it to evaporate.
A horizon (such as a Rindler horizon, or the event horizon of a black hole) is a boundary in spacetime that separates an observer from a portion of the field. Portions of a mode that extend beyond the horizon become inaccessible, and their quantum entanglement with the observable region is effectively brokenânot in the sense of being destroyed, but because the partner on the other side can no longer be observed.
As previously discussed, in the global vacuum state a mode is composed of entangled positive and negative frequency pairs. At a horizon, the negative frequency modes are hidden behind the horizon. The precise cancellation between these partners that makes the vacuum appear empty to an inertial observer fails for the accelerating (or hovering) observer because the partner is missing. Consequently, the remaining accessible modes appear excited. What the observer sees is a thermal bath of real particles filling their space.
In a volume of space lying beyond an event horizon, all field modes become inaccessible to an external observer. The observer's frame of reference is effectively 'cut off' from this region, as no causal signal can pass from inside to outside.
Quantum vacuum fluctuations near the horizonâthe entangled positive-negative frequency pairsâmay be separated by the horizon: one partner falls into the black hole, while the other escapes. Since the external observer cannot access the interior partner, they must ignore or trace out the interior degrees of freedom. This averaging process converts the pure entangled state into a thermal mixed state. To the external observer, the radiation appears as random thermal noise (no entanglement visible), but this is an illusion caused by the inaccessibility of the interior. Although the entanglement still exists in the global state, linking the radiation to the black hole's interior, the horizon creates a radical separation of informationânot a lack of quantum connection. The "entanglement across the horizon" is what makes the black hole a thermal object.
So Hawking radiation is the phenomenon whereby vacuum fluctuationsânormally unobservableâare converted into real, detectable particles due to the influence of a horizon. At the event horizon of a black hole, the effect is particularly dramatic. From the perspective of a stationary observer outside the horizon, the horizon appears to radiate particles. This observer must accelerate (i.e. fire rockets) to resist falling in, and this acceleration is the physical mechanism that generates the Hawking radiation. (Indeed, the local temperature of the radiation corresponds precisely to the acceleration required to hover at that radius.)
Close to the event horizon of a black hole, a stationary observer sees intense Hawking radiation (which is physically the equivalent of the Unruh effect caused by the extreme acceleration required to resist the black hole's gravitational pull). Far away, the radiation is weaker. Crucially, this radiation does not emanate from the horizon itself, as though the horizon were a light source. Instead, because the horizon breaks the entanglement between positive-negative frequency pairs throughout that region, it manifests as a thermal bath filling the entire volume of the observer's accessible universe. To put this another way, the vacuum state is a single, global quantum state spanning the entire universeâso this 'breaking' affects the vacuum state everywhere in the accessible volume. Every mode in that remaining space has had its entanglement partner removed, effectively turning the newly liberated vacuum fluctuations into real particles throughout the entire volume.
Recall that observers in free fall feel no force. To them, the horizon is not a physical barrier; they cross it without detecting any local change in the vacuum. The horizon is not a local physical surface that one can touch or see. It is a relational limit that defines the boundary of a specific observer's causal past and future. For a free-falling observer, the horizon is not a local featureâit is a global constraint on all the information they can ever accessâbut it is not a phenomenon in their immediate surroundings. The upshot is that an observer crossing a black hole's event horizon experiences a completely different universe to that of an observer hovering outside it.
Cosmological Horizons: de Sitter Space
For decades, it was assumed that the universe's expansion was decelerating due to the gravitational pull of all the mass contained within it. In this scenario, while there is a distance where recession velocity equals the speed of light (a particle horizon), the expansion rate slows down over time under the gravitational influence of the mass contained in the universe. This means the recession velocity of distant galaxies eventually drops below 'c,' and light emitted from those regions wins the race. Think of this like two racers: runner A running at constant speed and runner B running at an instantaneously faster speed but progressively slowing down. Runner B will pass runner A, but because he is slowing down, runner A will eventually overtake runner B. So this kind of cosmic expansion creates only a temporary block on information; in the far future, we would be able to see the entire universe. There is no permanent horizon beyond which light can never reach us.
However, the discovery that the cosmic expansion is accelerating changed everything. (The cause of this acceleration, being unknown, is referred to as dark energy by analogy with dark matter.) In this case, runner A is running at constant speed but runner B is accelerating. Once runner B has passed runner A, runner A will never catch up. This creates a permanent horizonâa boundary of what we can ever observe of the cosmos. (Technically, what matters for horizon formation is the behavior of the comoving Hubble radiusâwhether it is increasing or decreasing with time. In an accelerating universe, this radius approaches a constant value, creating a cosmological horizon.) This is referred to as a de Sitter universe, having an accompanying de Sitter (or cosmological) horizon.
We have covered Unruh radiation and Hawking radiation, and a similar phenomenon occurs due to the accelerating expansion of our universe. This expansion creates a Cosmological Horizon around us, beyond which we can never see or communicate. Just as acceleration creates a horizon and thermal radiation in flat space, the cosmic horizon creates a theoretical thermal bath known as Gibbons-Hawking radiation. The temperature of Unruh radiation depends on the observer's acceleration, which only becomes significant at extreme accelerations. The temperature of Gibbons-Hawking radiation depends on the expansion rate of the universe, and because the expansion is extremely slow, this temperature is incredibly lowâfar colder than the 2.7 Kelvin Cosmic Microwave Background left over from the Big Bang. So, while we do live in a universe with a horizon that technically generates thermal radiation, it is not the source of the 'warmth' we observe. Our universe is 'warm' (2.7 K) because of the vestigial heat from the decoupling of matter and energy in the early universeâthe microwave background radiationâand not because of the current expansion horizon. The horizon radiation is effectively a whisper in a storm.
So to recap, de Sitter space describes our expanding universe. We observe an edge to the observable universeâthe cosmological horizon, or de Sitter horizon. There is no local gravitational pull from this horizon, and we are not accelerating towards it. However, the accelerated expansion creates the de Sitter horizon. Consequently, we do not see Hawking radiation from the de Sitter horizon (in the black-hole sense), but we do detect GibbonsâHawking radiationâthe cosmological analogue of Unruh radiationâwhich appears as a uniform thermal bath throughout our local universe.
The universe we inhabit is defined by what we can seeâthe positive frequency modes of the quantum fieldsâwhile the 'reality' of the vacuum depends upon what we cannot see: the hidden negative frequency modes behind the various kinds of horizon. The existence of particles, then, is a relational property between the observer and the horizons. In short, a horizon hides information, and the loss of that information manifests as heat.
It must follow that every observer in a de Sitter universe has their own distinct causal patch, meaning they can access only a finite portion of the full spacetimeâthat portion bounded by their personal de Sitter horizon. This is mathematically similar to how everyone sees their own rainbowâan optical phenomenon dependent on the observer's positionâbut the physics is deeper. Unlike in flat space, where we can conceptually define a 'global' time slice, the expansion of de Sitter space makes it impossible for distant observers to synchronize clocks across the horizonâeach observer effectively constructs their own static coordinate system centered on themselves. If two observers are sufficiently separated such that their horizons do not overlap, they can never exchange signals, never see each other, and never agree on a shared definition of 'now' for events outside their respective horizons. Effectively they do not exist for each other. In de Sitter space, then, the de Sitter horizon is not merely a limit of visibilityâit defines the causal boundary of that observer's reality. Each distinct observer lives in a 'personal universe' bounded by their horizon, though these are overlapping patches of a single global spacetime, not separate universes.
The Emergence of Space
In the emerging view of quantum gravity, the 'distance' between two points in space is not a fixed background property, but is determined by the degree of entanglement between the quantum states at those locationsâi.e., geometry is a geometric manifestation of quantum entanglement. (This idea, known as the ER=EPR conjecture, suggests that spatial connectivityâEinstein-Rosen bridges or wormholesâis equivalent to quantum entanglement.) If this entanglement is disrupted or reconfiguredâas happens in Hawking radiationâthe geometric connectivity of space itself changes. A region that was once smoothly connected is effectively 'pinched off' or develops a barrier if the entanglement structure fails.
This suggests a radical reversal of our classical intuition: rather than quantum fields existing within a pre-existing space, space itself emerges from the entanglement of the quantum field. Space is not a passive void; it is a dynamic, emergent structure woven from the web of quantum correlations. In this framework, if the vacuum state's entanglement is altered everywhere, the very fabric of the 'accessible universe' is redefined. Space is not fundamental; it is a collective phenomenon arising from the underlying quantum information.
Note in passing that this generates a paradox. In the case of a black hole horizon, the cosmos encompasses both spaces, but the spaces exhibit an asymmetric relationshipâan observer outside the black hole can know nothing about whatever passed its event horizon, but an observer falling through that horizon still sees the rest of the universe (until they hit the singularity). This tension between the perspectives is a central puzzle in quantum gravity.
Philosophical Implications: Structural Realism
In modern physics, the relations are the reality, and it is an error to assume the existence of a static, solid substrateâa 'thing' that exists independently of observation or relationship. Physics has moved towards what philosophers call structural realism. Particles are not little billiard balls; they are excitations in a field. Fields are not fluids filling a void; they are the fundamental entities that define the geometry of spacetime itself. Spacetime is not a stage; it is a dynamic participant that curves and expands based upon the energy within it. There is no 'solid ground' behind the scenes. There is only the network of relationsâhow fields interact with other fields, how observers move relative to horizons, how energy curves geometry. Reality is the consistent network of causal interactions.
To make a comparison, this is like never being able to view the entirety of an object but only being able conceptually to 'reconstruct' it from different partial views and from the perspectives reported by other people. The propensity to think that there is some transcendent, metaphysically 'real' object is a misconception. We assume there is a 'true state' of the universe that we just have not measured yet, but recent developments in physics suggest that there is no 'true state' independent of the reference frameâat least not for properties like particle number. The state is the measurement relative to the observer. The 'object' we try to reconstruct is a projection of the underlying relational network.
However, the "reconstruction by integrating partial views into a whole" is not a reconstruction at allâit is the fundamental mechanism by which reality is constructed. There is no 'whole' to 'reconstruct' in the classical sense; the 'whole' is the network of relations itself. If we strip away the "partial views"âthe observers and their reference framesâthe 'object' vanishes. Or, more precisely, what remains is the invariant structure of relations, which is what physics describes through its equations.
It is important to address a potential objection: if everything is relational, what grounds the relations? Structural realism's answer is that the relations are the ground. There is no need for an underlying substance; the structure is ontologically primary. This is a radical metaphysical claim, but it is consistent with the physics we have examined. Different observers in the same causal patch agree on measurements because they share access to the same relational structureâthe same invariant patterns in the network of interactions. Objectivity, in this view, is not about substances but about the invariance of structure across perspectives.
Of course, this view is not without its critics. Some philosophers argue that relations require relataâthat you cannot have a network of relations without nodes that are related. Structural realism's response is that the "nodes" are themselves defined by their place in the relational network; they have no intrinsic identity apart from their relations. This is precisely what we see in QFT: particles are defined by their mode occupations and their interactions with other fields, not by any intrinsic "thingness."
What Remains Constant
To avoid the impression that the relational view leads to complete relativism, we must ask: what remains invariant across all reference frames? The answer is the structure of physical lawâthe equations that govern the fields. The field equations themselves are covariant: they take the same form in all coordinate systems. The spacetime interval is invariant. The S-matrix (which describes the scattering of particles) is invariant. What changes is the descriptionâthe particle content, the choice of coordinates, the division into modesâbut the underlying structure remains the same. This is the sense in which objectivity is preserved: not as a collection of observer-independent "things," but as a network of invariant relations that all observers can, in principle, agree upon.
This is why the relational view is not a form of idealism or solipsism. It does not claim that reality is created by consciousness or that each observer lives in a separate universe (though each has a unique causal patch). It claims that reality is relationalâthat the fundamental entities of physics are not substances but structures, and that these structures are defined by their interrelations. The cosmos is not a collection of objects, as though we were discovering pre-existing furniture in a pre-existing room. It is a network of interactions in which dynamic relations have replaced static objects, and process has replaced substance.
Conclusion
What emerges from quantum field theory, relativity, and cosmology is a picture of 'reality' without a foundationâno absolute particles, no observer-independent spacetime, no 'thingness' that persists beneath the flux. Instead, we find a web of relations: fields define particles, motion defines modes, horizons define accessibility, and observers define what is real within their causal patch. The universe, in this view, is not a collection of objects, as though we were discovering pre-existing furniture in a pre-existing room. It is, rather, a network of interactions. Dynamic relations have replaced static objects. Process has replaced substance.
The old question 'What exists?' must be supplemented by a more nuanced one: 'What exists for whom, and from where?' The accelerating observer sees a thermal bath where the inertial observer sees a vacuum. The stationary observer above a black hole detects Hawking radiation while the free-falling observer feels nothing. The de Sitter observer lives in a personal universe bounded by a horizon that no signal can cross. These are not competing descriptions of a single objective 'reality'âthey are different perspectives on the same underlying field, each equally valid from its own frame of reference, and each connected by the invariant structure of the field equations.
We are not passive witnesses to an objective 'reality' that exists independently of us. We are participants in a cosmos that is constituted by relationshipsâbetween fields and excitations, between observers and horizons, between entangled systems across vast distances. The cosmos we inhabit is not given to us; it is constituted by our relationship to the fields, the horizons, and the other observers with whom we share this vast, entangled domain. In the end, physics teaches us that to exist is to be in relationâand that, perhaps, is the deepest lesson of all.