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 objects -- electrons, protons, neutrons—that 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 'reality' dissolves. The universe, I will suggest, is not a collection of things but a network of relations.
The Field Theory Picture -- 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 theory -- Quantum Field Theory (QFT) -- the 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 of particle physics: 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. The so-called 'vacuum state' is the lowest energy state of a field. In the vacuum state, these fluctuations manifest as transient disturbances in the quantum field, often modelled as 'virtual' particle-antiparticle pairs. Unlike real particles, these virtual excitations are not independent entities that can be detected directly -- they are mathematical representations of the field's jitter. They do not 'borrow' energy in order to exist in the traditional sense, but rather represent the inherent 'zero-point' activity of the field. The vacuum is best thought of as the ground state of the field with non-zero fluctuations in the field operators.
These fluctuations (interpreted as virtual particle pairs) 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 sense -- it is a dynamic state of minimum energy (more accurately described as a fluctuating field rather than 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.
Although the energy of the vacuum state is not zero, the field contains no "real particles." The lowest-energy real-particle state that the field supports is its one-particle state -- 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 has two quanta occupying one or more modes; this is described mathematically by an increased "occupation number" rather than simply vibrating "more intensely."
In a two-particle state, the field is excited by two quanta of energy that are distributed among its available modes. 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" (modes), each capable of vibrating at a specific frequency. The statement "two quanta occupying one or more modes" describes how these two units of energy are arranged:
Two Quanta in Different Modes -- 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 once -- you have one unit of energy in Mode A and one unit in Mode B.
Two Quanta in the Same Mode -- 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 contains two units of energy, and all other modes are empty.
The crucial conceptual shift from 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.
It is important to note that QFT does not assign a separate field to each individual electron. Instead, there is one universal electron field. All electrons are excitations of this single field. The same is true for each other type of particle we observe. These fields are not like classical force fields that spread out from a source and weaken with distance. They are fundamental entities defined at every point in spacetime. The electron field, for example, is a single universal field whose ripples are interpreted as electrons and positrons. Different fields can occupy the same region of space because they represent independent types of disturbance. Why nature uses precisely these fields (one for electrons, one for up quarks, the photon field, the Higgs field, and so on) rather than some other set is not explained by the theory — QFT has been built on this basis because it is consistent with what has already been discovered.
Entanglement
To say that two particles '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 carrying its own independent properties. Quantum mechanics behaves differently. After two particles interact, the information describing them is not 'split' or distributed so that each particle retains its own independent state. Instead, the system is described by a single, unified quantum state. The properties of the two particles are inextricably linked -- neither particle has a definite state on its own. This is what we call entanglement.
When one of these entangled particles interacts with a third particle (in the environment) in a way that constitutes a "measurement," the outcome for that property becomes definite for the entire system. Consequently, the corresponding property of the other particle is also immediately determined, as the pair effectively acts as a single entity. As the system undergoes further interactions with the environment, the delicate quantum correlations between the original pair become 'diluted' or dissipated into the vast number of degrees of freedom in the environment. The particles progressively lose their entanglement and begin to behave as independent objects. This process is known as decoherence.
Crucially, the virtual particle pairs that characterise the vacuum state of a field are subject to quantum entanglement. Don't think of entangled particles as "sharing" a list of properties -- rather they ARE the list.
The Relativity of Particles: Observers and Modes
Imagine a graph with distance on the vertical axis and time on the horizontal axis. An object moving at constant velocity in flat space is represented by a straight line. An accelerating object is represented by a rising curved line. In relativity, these paths through spacetime are called worldlines. We can count particles by detecting the amount of energy in a specific mode, but crucially what counts as a mode depends upon how you move along your worldline
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 standing still in a gravitational field (note that in General Relativity, gravity is not a force). An object in free fall (moving only under gravity) feels no force. It follows a "geodesic" -- the straightest possible path through curved spacetime. Even though this path might look curved on a 2D graph (e.g. with the object falling downward), it is the natural, unforced motion.
An object resisting gravity (like standing on the Earth's surface) feels a force (the ground pushing up against its tendency to fall 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 worldline (a geodesic) in curved spacetime. If you feel a force, your worldline is curved relative to that 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 quite reach -- the time dimension would appear completely frozen relative to space. Mathematically, this is modelled as a skewing of the time and space axes on a spacetime diagram. For an observer at rest, the axes are perpendicular (90°). As velocity increases, the axes skew toward each other, but no observer can actually reach the speed of light so the axes never fully coincide.
If an object moves at a constant velocity, the angle between the axes remains fixed; the observer feels no force. If the object accelerates, its velocity changes, and the angle between the axes no longer remains fixed (i.e., it progressively rotates or skews in the plane of the diagram). It is this progressive deviation from a straight line in spacetime (i.e. from its geodesic) that manifests as a force felt by the observer. In a gravitational field, an observer in free fall feels no force -- they are moving through spacetime in a straight line (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 Unruh Effect and the Rindler Horizon
An observer in inertial motion (free fall) defines field modes as simple sine waves. For this observer, the vacuum state has a particle occupation count of zero. By contrast, an accelerating observer (or one resisting gravity) moves through spacetime in a curved path (their 'time' axis is progressively rotating). For such an observer, there will be a distance behind them at which the velocity of separation reaches 'c,' creating what is known as a "Rindler Horizon." This is a boundary that hides part of the universe from the accelerating observer. To such an observer the simple sine waves of the inertial observer appear as a mix of different sine waves. Remarkably, the vacuum state (zero particles) of the inertial observer appears as 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 axes. The accelerating observer sees particles where the inertial observer sees nothing. Particle number, then, is not an absolute property of the universe -- it is a relational property between the observer's motion and the field.
In a flat universe, an inertial observer sees the vacuum as empty space with simple sine-wave modes. However, an accelerating observer has a different definition of time (and thus energy/frequency). Because their time axis is constantly shifting, the "simple sine waves" of the vacuum appear to the accelerating observer as a mixture of many different frequencies, including those corresponding to a thermal bath of particles. This is the Unruh radiation -- the perception of heat generated purely by acceleration in a vacuum.
Unruh radiation is the direct consequence of this acceleration due to the principle of equivalence: the horizon created by the accelerating observer's motion breaks the 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.
Hawking Radiation
A stationary observer hovering above a black hole 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, and is locally indistinguishable from Unruh radiation -- the difference lies in the global boundary conditions and the fact that Hawking radiation can escape to infinity.
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 broken -- not in the sense of being destroyed, but because the partner on the other side can no longer be observed.
In the global vacuum state, a mode is composed of entangled pairs -- a 'positive frequency' part accessible to the observer and a 'negative frequency' partner hidden behind the horizon. The precise cancellation between these partners that makes the vacuum appear empty to an inertial observer fails for the accelerating 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.
However, this does not mean that quantum entanglement across the horizon is impossible. In fact, entanglement across the horizon is the fundamental mechanism behind Hawking radiation. Quantum vacuum fluctuations near the horizon generate entangled particle pairs. One particle falls into the black hole, while the other escapes. Because the external observer cannot access the interior particle, they must '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. The entanglement still exists in the global state, linking the radiation to the black hole's interior. Thus, the horizon creates a radical separation of information, not 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 fluctuations -- normally unobservable -- are 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.
Close to the event horizon of a black hole, a stationary observer sees intense Hawking radiation, which is physically the same as 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, it manifests as a thermal bath filling the entire volume of the observer's accessible universe, because the horizon breaks the entanglement between virtual pairs throughout that region. The vacuum state is a single, global quantum object -- one giant, entangled wavefunction spanning the entire universe -- so 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 virtual fluctuations into real particles throughout the entire volume.
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 feature -- it is a global constraint on all the information they can ever access—but it leaves no local trace in their immediate surroundings. The upshot is that an observer passing a black hole sees a completely different universe to that of an observer falling into the black hole.
Cosmological Horizons: de Sitter Space
For decades, it was assumed that the universe's expansion was decelerating due to the gravitational pull of all its mass. 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 -- i.e. it eventually catches up with us. Thus the 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 of accelerating expansion (driven by Dark Energy) changed everything. (This acceleration is attributed to Dark Energy -- a mysterious component that acts like a repulsive pressure, driving the expansion. Like Dark Matter, we name it because we don't yet know its fundamental nature, but its effect is clear: it creates a permanent, shrinking boundary of what we can ever observe.) In a universe where the expansion is accelerating (like the de Sitter case), the recession velocity of distant galaxies increases over time and light emitted from sufficiently distant regions loses the race. The space separating the light source from us expands faster than the light can traverse it. There is now a permanent Cosmological Event Horizon. Light emitted beyond this boundary today will never reach us, no matter how long we wait. We are effectively losing access to parts of the universe forever.
A similar phenomenon to Unruh radiation and to Hawking radiation occurs due to the 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. However, there is a crucial difference in scale. The temperature of Unruh radiation depends on your acceleration; for everyday accelerations, it is undetectably small, but for extreme accelerations, it becomes significant. The temperature of Gibbons-Hawking radiation depends on the expansion rate of the universe. Because the expansion is extremely slow, this temperature is incredibly low (around 10^{-30} Kelvin) -- far 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 relic heat of the Big Bang, not because of the current expansion horizon. The horizon radiation is effectively a whisper in a storm.
de Sitter space describes our expanding universe. We observe an edge to the observable universe -- the 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 of the universe, driven by dark energy, creates the de Sitter horizon. Consequently, we do not see Hawking radiation in the black-hole sense, but we do detect Gibbons–Hawking radiation -- the cosmological analogue of Unruh radiation -- which appears as a uniform thermal bath throughout our local universe.
The universe we inhabit is defined by what we can see -- the positive frequency modes -- while the 'reality' of the vacuum depends upon what we cannot see: the hidden negative frequency modes behind the horizon. The existence of particles, then, is a relational property between the observer and the horizon. The horizon hides information, and the loss of that information manifests as heat.
Every observer in a de Sitter universe has their own distinct causal patch, meaning they can access only a finite portion of the full theatre of spacetime, bounded by their personal event horizon. This is mathematically similar to how everyone sees their own rainbow -- an optical phenomenon dependent on the observer's position -- but 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 synchronise clocks across the horizon.
Each observer effectively constructs their own static coordinate system centred 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. In de Sitter space, the event horizon is not merely a limit of visibility -- it defines the causal boundary of that observer's reality. Each distinct observer lives in a 'personal universe'.
It is important to note that Unruh radiation, Hawking radiation, and Gibbons-Hawking radiation are deep predictions of the theory, and not yet confirmed by observation.
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 locations: geometry is a geometric manifestation of quantum entanglement. If this entanglement is disrupted or reconfigured -- as happens in e.g. Hawking radiation -- the 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 hypothesis that reverses 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. This is a speculative and exciting active research program rather than established fact.
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 substrate—a '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 relations: how fields interact with other fields, how observers move relative to horizons, how energy curves geometry. Reality is the consistent network of causal interactions.
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 frame. The state is the measurement relative to the observer. The 'object' we try to reconstruct is a projection of the underlying relational network.
But the 'reconstruction by integrating partial views into a whole' is not a reconstruction at all—it is the fundamental mechanism underlying the construction of reality. 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'—the observers and their reference frames—the 'object' vanishes. The universe is not a thing we observe; it is a conversation between observers. The 'reality' we seek is the grammar of that conversation, not the furniture in the room. Dynamic relations have replaced static objects. Process has replaced substance.
Conclusion
What emerges from quantum field theory, relativity, and cosmology is a picture of 'reality' without a foundation—no 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 but a network of interactions—a conversation conducted across spacetime between observers.
The old question 'What exists?' must give way to 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;' they are different perspectives on the same underlying field, each equally valid from its own frame of reference.
We are not passive witnesses to an objective 'reality' but active participants in its construction. 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 relation—and that, perhaps, is the deepest lesson of all.