Physics

1.1INTRODUCTION: WHY PHYSICS COMES FIRST

Any credible account of reality must be disciplined by our best scientific theories. We do not first intuit the nature of existence and then check whether physics agrees. We observe the world, formulate our best theories about its structure, and then ask what picture of reality is most consistent with what we have learned. Physics provides the data, and philosophy provides the interpretation, but the data must come first. The questions that will occupy the subsequent chapters — what exists, what time is, whether we are free, how we ought to live — cannot be properly formulated, let alone answered, without knowing what kind of universe we inhabit. What exists at the fundamental level depends on what quantum field theory tells us about the nature of particles and energy. What time is depends on what relativity tells us about the structure of spacetime. And so on: in each case, the physical findings are not peripheral to the philosophical question but constitutive of it.

This chapter is not a physics textbook. It will not derive equations or report experimental results. Its purpose is to lay out the physical picture of reality that emerges from the last four centuries of scientific inquiry and to identify those features of that picture that are indispensable for the arguments that follow. There are four features: the nature of matter as energy, the geometry of spacetime, the thermodynamic arrow of time, and the unresolved puzzle of quantum mechanics. Each will be treated with the depth necessary to ground the philosophical system of this work, and no more.

1.2ENERGY, FIELDS, AND THE FUNDAMENTAL LEVEL

At the most fundamental level accessible to our current understanding of physics, the fabric of reality is not made of matter in the ordinary sense. What we call particles — electrons, quarks, photons, gluons — are not tiny solid objects. They are excitations of underlying quantum fields: patterns of energy vibrating in specific ways within the fabric of spacetime. A photon is a ripple in the electromagnetic field. The apparent solidity and permanence of macroscopic objects — tables, rocks, human bodies — is an emergent property, a consequence of how vast numbers of these excitations interact and aggregate at scales accessible to our senses. This insight is one of the most profound results of twentieth-century physics, and it is encapsulated in Albert Einstein's equation E = mc². The equation establishes that mass and energy are interconvertible: a quantity of mass is equivalent to an enormous quantity of energy, and vice versa. In nuclear reactions, mass is converted into energy; in the early universe, energy condensed into mass. The distinction between matter and energy is therefore not a distinction between two kinds of thing; it is a distinction between two configurations of the same underlying reality. At the deepest level, there is only energy in its various configurations. This is a point of considerable philosophical significance, and it will be developed in the Metaphysics chapter that follows.

Our best current description of this fundamental level is the Standard Model of particle physics. It describes twelve fundamental particles — six quarks and six leptons — together with the force-carrying bosons that mediate their interactions, and the Higgs boson, which endows certain particles with mass. The Standard Model accounts for three of the four known fundamental forces of nature: the electromagnetic force, which governs the interactions between charged particles and is responsible for light, chemistry, and the structure of atoms; the strong nuclear force, which binds quarks together into protons and neutrons and holds atomic nuclei together; and the weak nuclear force, which governs certain forms of radioactive decay. The fourth force, gravity, is described not by the Standard Model but by Einstein's General Theory of Relativity, and the unification of gravity with the other three forces remains one of the great unsolved problems of physics. The history of physics is, in one of its most important dimensions, a history of unification. Isaac Newton demonstrated that the force which makes an apple fall to the ground is the same force that keeps the Moon in orbit around the Earth — he unified terrestrial and celestial mechanics under a single law. James Clerk Maxwell showed that electricity and magnetism, which had been studied as separate phenomena, are manifestations of a single electromagnetic field. Einstein unified space and time into spacetime, and then showed that gravity is not a force acting within spacetime but a feature of spacetime's geometry. The electroweak theory of Sheldon Glashow, Steven Weinberg, and Abdus Salam unified the electromagnetic and weak nuclear forces. At each stage, what had appeared to be separate laws governing separate domains of nature were revealed to be aspects of a single, deeper law operating at a more fundamental level.

This trajectory is not merely a historical curiosity. It suggests something about the structure of reality itself: that the multiplicity of laws and forces we observe is not fundamental but emergent, and that at the deepest level, the physical world may be governed by a single principle. Whether this expectation will be vindicated — whether a theory of everything that unifies all four forces within a single framework will eventually be found — cannot be known in advance. But the trajectory of unification provides strong evidence that the direction is the right one, and it is reasonable to expect that the process will continue. What we currently describe as many separate laws may well be glimpses, from different angles and at different scales, of one law.

1.3SPACETIME: FROM NEWTON TO EINSTEIN

Before the twentieth century, our understanding of space and time was Newtonian. In Newton's mechanics, space and time are absolute: space is an unchanging, three-dimensional stage upon which events take place, and time is a universal parameter that flows at the same rate for all observers regardless of their state of motion. Distances between objects are fixed, time intervals are invariant, and all observers can agree on which events are simultaneous with which. This picture is intuitive — it corresponds closely to how we experience the world — and for over two centuries it served as the unquestioned foundation of physics.

Einstein's Special Theory of Relativity, published in 1905, overturned this picture entirely. The theory rests on the combination of two postulates. The first, the relativity postulate, asserts that the laws of physics are the same for all observers moving at constant velocity relative to one another — that there is no experiment one can perform to determine whether one is at rest or in uniform motion. The second, the light postulate, asserts that the speed of light in a vacuum is the same for all observers, regardless of the velocity of its source or its observer. This second postulate is deeply counterintuitive: if two beams of light are emitted simultaneously, one from a person standing still and another from a person moving at great speed, one would expect the second beam to travel faster. It does not. The speed of light is absolute and cannot be added to other velocities. If velocity does not change, then space and time must. And this is exactly what the theory predicts. As an object approaches the speed of light, distances in the direction of motion contract and time slows down — a moving clock ticks more slowly than a stationary one, a phenomenon known as time dilation. These are not perceptual illusions; they are real, physical consequences of the geometry of spacetime, confirmed empirically innumerable times in particle accelerators and in precision measurements involving satellites and atomic clocks. The implications are far-reaching. If you were to travel to a star one hundred light-years away at ninety-nine per cent of the speed of light, the journey would take you roughly fourteen years. At ninety-nine point ninety-nine per cent, it would take approximately one year. But upon returning to Earth, you would find that over two hundred years had passed for those who remained behind. Both durations are equally real; neither observer's clock is wrong. Time itself passes at different rates depending on one's state of motion.

The most consequential implication of Special Relativity for the arguments of this work is the relativity of simultaneity. In Newton's universe, all observers agree on which events are happening at the same time. In Einstein's universe, they do not. Two events that are simultaneous for one observer may not be simultaneous for another observer moving at a different velocity. There is no objective, observer-independent fact about which events are happening "right now" across the universe. The concept of an absolute present — a single set of simultaneous events that constitutes the totality of what exists at a given moment — has no counterpart in the physics. Special Relativity describes a spacetime that is flat as it applies in the absence of gravity. Einstein's General Theory of Relativity, published in 1915, extends the framework to include gravity. The central insight is that gravity is not a force in the Newtonian sense of a pull exerted between masses across empty space, but a consequence of the curvature of spacetime itself. Mass and energy curve the fabric of spacetime, and what we perceive as gravitational attraction is the tendency of objects to follow the straightest possible paths through that curved geometry. Spacetime is therefore not a passive backdrop against which physical events unfold; it is itself a dynamic, physical entity, shaped by the matter and energy it contains and in turn shaping their behaviour.

1.4COSMOLOGY AND THE ARROW OF TIME

The universe has a history. This is among the most remarkable discoveries of modern science, and one whose philosophical implications have yet to be fully understood. For most of human civilisation, the cosmos was assumed to be eternal and essentially unchanging — a view held by Aristotle, and shared in various forms by most thinkers until the early twentieth century. It was the combination of General Relativity with astronomical observation that overturned this assumption. Einstein's equations predict that spacetime is not static; it is either expanding or contracting. Observations by Edwin Hubble and others in the 1920s confirmed that the universe is expanding by proving that distant galaxies are receding from one another at speeds proportional to their distance. Running this expansion backward in time leads to an extraordinary conclusion: at some point in the distant past, all the matter and energy in the observable universe was concentrated in a state of extreme density and temperature. This is what we call the Big Bang. The Big Bang is the earliest state that our current understanding of physics can describe. Whether it constitutes the actual beginning of the Universe as we know it or whether it is a phase transition within a larger structure that extends beyond it through cyclic models, a multiverse, or something we cannot yet conceive, is a question that remains unanswered. We simply do not know. What we do know is the history that follows from it, and that history is one of extraordinary evolution from simplicity to complexity.

In the first moments after the Big Bang, the universe was an almost uniform sea of energy at immense temperature. As it expanded and cooled, this primordial energy condensed into the simplest subatomic particles. Within the first few minutes, protons and neutrons combined into the nuclei of the lightest elements: hydrogen, helium, and traces of lithium. For hundreds of millions of years, the universe remained a dark expanse of gas, gradually cooling. Then gravity began its work. Slight irregularities in the distribution of matter — regions fractionally denser than their surroundings — attracted more matter, grew denser still, and eventually collapsed under their own weight to form the first stars. Within the cores of these stars, nuclear fusion — the process by which lighter atomic nuclei are pressed together under extreme heat and pressure to form heavier ones — forged heavier elements: carbon, oxygen, nitrogen, silicon, iron. When the most massive stars exhausted their fuel and exploded as supernovae — catastrophic explosions that briefly outshine entire galaxies — they scattered these elements across space, seeding the interstellar medium with the raw materials for chemistry, for planets, and eventually for life. This is not a story of design. There is no blueprint according to which hydrogen becomes stars and stars become the periodic table and the periodic table becomes DNA. The process is driven by the laws of physics operating on initial conditions, and the results, however extraordinary, are consequences rather than goals. But there is a direction to the process, and that direction requires explanation. Why does the universe evolve from simplicity to complexity rather than the reverse? Why do structures form, persist, and give rise to ever more elaborate structures, rather than dissolving immediately into disorder?

The answer lies in thermodynamics, and specifically in the second law: the total entropy of a closed system always stays the same or increases. Entropy can be understood, roughly, as a measure of disorder — more precisely, as a measure of the number of microscopic configurations that are compatible with a given macroscopic state. A state of low entropy is one in which the microscopic constituents are arranged in a highly specific way; a state of high entropy is one in which they could be arranged in a vast number of equivalent ways. The second law tells us that systems naturally evolve from less probable (low-entropy) states to more probable (high-entropy) states. This is the source of the arrow of time — the reason why ice melts in warm water but warm water does not spontaneously form ice cubes, why eggs break but do not unbreak. It might seem paradoxical that a universe tending towards disorder should produce stars, planets, and life — structures of extraordinary complexity. The explanation is that the second law governs the total entropy of a closed system, not the entropy of every part within it. Local decreases in entropy — such as the formation of a star from a diffuse cloud of gas, or the growth of an organism from simple chemical ingredients, both of which involve the organisation of dispersed matter into highly structured forms — are possible as long as they are compensated by greater increases elsewhere. It is the entropy gradient itself, the vast difference between the low-entropy initial state and the high-entropy future, that powers the formation of complex structures. Complexity does not arise despite the second law; it arises because of it, powered by the gradient between the ordered past and the disordered future.

The crucial point, for the arguments of this work, is that the arrow of time is thermodynamic, not temporal. The fundamental laws of physics — Erwin Schrödinger's equation, Maxwell's equations, the equations of General Relativity — are time-symmetric: they work just as well in reverse. There is nothing in these equations that distinguishes past from future or that mandates a preferred direction of time. The asymmetry we experience — the irreversibility of macroscopic processes, the sense that time flows from past to future — is not built into the laws of nature. It is a consequence of a fact about the universe: that entropy was extremely low in the past, near the Big Bang, and has been increasing ever since. The arrow of time is the entropy gradient. This has a further consequence that connects to the work of Ludwig Boltzmann in statistical mechanics. Boltzmann showed that entropy can be calculated according to a precise formula relating the macroscopic state of a system to the number of its possible microscopic configurations. When this formula is applied, the results almost invariably indicate that a system evolves from lower to higher entropy. The sun, for example, is a source of low entropy: it sends high-energy photons to the Earth, which re-radiates them as a larger number of lower-energy photons. This process increases the total entropy of the system. The sun itself derives its low entropy from the still lower entropy of the primordial gas cloud from which it formed. Tracing the chain backward leads ultimately to the extraordinarily low-entropy initial state of the universe near the Big Bang.

Why the initial entropy of the universe was so low remains unexplained. It is, in a sense, the question of why there is a history at all, why the universe is not simply a featureless equilibrium. No fully satisfactory answer has yet been given, and it may be that the answer lies in physics we have not yet discovered. What can be said is this: since the initial entropy was low, the subsequent evolution — from the primordial gas to atoms to stars to heavy elements to planets to life to consciousness — follows from the laws of physics and the entropy gradient, without the need to invoke purpose or design.

1.5QUANTUM MECHANICS AND THE MEASUREMENT PROBLEM

Quantum mechanics is one of the most successful physical theories ever devised. Its predictions have been confirmed to extraordinary precision across an immense range of phenomena. And yet, nearly a century after its formulation, there is no consensus on what the theory tells us about the nature of reality. The mathematics works; the interpretation remains contested. This is not a peripheral difficulty. It concerns the foundations of the theory, and it has a name: the measurement problem. The problem arises from the tension between two components of quantum mechanics. The first is Schrödinger's dynamics: the state of a quantum system is described by a mathematical object called the wave function — a complete catalogue, so to speak, of everything that can be known about the system. The wave function evolves over time according to Schrödinger's equation, a law that is both deterministic (the future state is fully determined by the present state) and linear (if two states are possible, their combination is also possible). This linearity gives rise to superposition: a particle need not be in one definite state but can be in a combination of multiple states simultaneously. An electron can be in a superposition of two different locations at once, and this is not a matter of ignorance — according to the formalism, the electron genuinely has no definite position until the superposition is resolved. The second component is what physicists call the collapse postulate — the claim that when a measurement is performed on a system in superposition, the superposition is abruptly destroyed and the system is found in one definite state, with probabilities given by Max Born's rule, a mathematical formula that extracts the likelihood of each possible outcome from the wave function. This process is abrupt and stochastic — governed by probabilities, not by the deterministic evolution of Schrödinger's equation.

The measurement problem is the problem of reconciling these two descriptions. If Schrödinger's equation is the complete law of nature, superpositions should persist at all scales, including the macroscopic. We should, in principle, observe measuring devices displaying contradictory results simultaneously. We never do. But if the collapse postulate is also fundamental, then we have two incompatible laws — one for unmeasured systems and one for measured systems — and the theory is incomplete until we can specify when and why one gives way to the other. Saying that the collapse occurs "upon measurement" only pushes the problem back: what counts as a measurement? The theory leaves the term undefined. A significant advance came with the discovery of decoherence. When a quantum system interacts with its environment — any sufficiently large collection of particles — the interference effects — the observable signatures of superposition, such as the ability of a particle to behave as though it takes multiple paths simultaneously — are rapidly destroyed. Decoherence explains why macroscopic superpositions are never observed: the entanglement of a macroscopic system with its environment is so rapid and so thorough that quantum coherence vanishes almost instantaneously. However, decoherence does not solve the measurement problem. It explains why superpositions become unobservable, but after decoherence the formalism still describes the system as being in a superposition — the different branches have merely ceased to interfere with one another. Whether a definite outcome genuinely occurs, or whether all branches remain equally real, is precisely what separates the main interpretations of quantum mechanics.

What decoherence does accomplish is a reformulation of the problem. The question is no longer about the mysterious role of the word "measurement" in the formalism. It is about the relationship between the microscopic world, where superpositions are ubiquitous, and the macroscopic world, where they are never observed. This is why recent literature has renamed it the macro-objectification problem: the problem of explaining how definite, classical outcomes emerge from the quantum substrate.

1.6INTERPRETATIONS: HOW THE WHOLE PRESERVES ITS NECESSITY

The measurement problem has generated a wide range of proposed solutions, and there is no consensus among physicists or philosophers of physics as to which is correct. The landscape of interpretations includes, among others, various forms of the Copenhagen interpretation, dynamical collapse theories, hidden-variables theories, the many-worlds interpretation, relational quantum mechanics, QBism, consistent histories, and superdeterminism. Each proposes a different strategy for reconciling Schrödinger's dynamics with the definite outcomes we observe. Rather than surveying this entire landscape, this section examines four interpretations that represent the main types of response to the problem: the Copenhagen interpretation in its von Neumann-Wigner version (consciousness causes collapse), the Ghirardi-Rimini-Weber theory (spontaneous dynamical collapse), Bohmian mechanics (hidden variables), and Hugh Everett's many-worlds interpretation (no collapse).

The Copenhagen interpretation, in its von Neumann-Wigner version, holds that the collapse of the wave function is caused by the consciousness of the observer. The view was influential for much of the twentieth century, but it has been decisively refuted. Experiments — notably those in which the information about a possible collapse is split between observers so that neither can know alone whether it has occurred — have demonstrated that the physical interaction between a quantum system and a macroscopic apparatus is sufficient to produce a definite outcome, without any conscious observation being involved.

The Ghirardi-Rimini-Weber theory modifies Schrödinger's equation by introducing a spontaneous collapse mechanism: particles undergo random localisations at a rate calibrated so that microscopic systems collapse extremely rarely, while macroscopic systems collapse almost instantaneously. The theory has the virtue of providing a precise, universal dynamics. But it faces serious difficulties. The collapse mechanism does not actually eliminate macroscopic superpositions; it renders one branch overwhelmingly dominant while the other retains a non-zero amplitude. The superposition is suppressed, not removed. Furthermore, the two constants upon which the theory depends are introduced without theoretical justification. A theory that solves a fundamental problem by introducing unexplained parameters has not fully solved it.

Bohmian mechanics, developed by David Bohm in 1952, accepts Schrödinger's equation but supplements it with an additional postulate: particles always have definite positions at all times, guided by what Bohm called a pilot wave — a field, derived from the wave function, that steers particles along definite trajectories without itself being directly observable. The wave function never collapses; it evolves according to Schrödinger's equation at all times. The apparent randomness of measurement outcomes is a consequence of our ignorance of the exact initial positions of particles, not a feature of reality itself. Indeterminacy is epistemic, not ontic. The appeal is considerable: one world, one history, one set of definite positions. The cost is non-locality — the pilot wave must influence distant particles instantaneously, in tension with the spirit of relativity.

Everett's many-worlds interpretation takes a different route. It accepts Schrödinger's equation as the complete dynamics and denies that collapse ever occurs. When a measurement takes place, the system and the observer enter into a joint superposition. The different branches, corresponding to different outcomes, continue to exist but cease to interact due to decoherence. Each branch constitutes, in effect, a separate world. The price is ontological: reality is vastly larger than it appears, containing not one history but an immense branching tree of histories. Whether these branches are genuinely separate worlds or different regions of a single, richly structured whole is partly a question of terminology. In both cases, the determinism of the fundamental dynamics is preserved.

Both these last two interpretations preserve what matters most for the arguments of this work: the determinism of the fundamental physics. Neither interpretation treats quantum indeterminacy as a brute, irreducible feature of reality. For Bohm, it is a consequence of our ignorance; for Everett, a consequence of our perspective from within a single branch. The two disagree profoundly on the structure of reality, but they agree on the philosophically decisive point: the apparent randomness of the quantum world does not reflect a fundamental feature of nature. I do not claim to know which interpretation is correct. The choice between them is, I believe, ultimately an empirical question that depends on future developments in physics. What I believe is that quantum indeterminacy is most likely not fundamental — that the measurement problem reflects the incompleteness of our current understanding rather than a brute feature of reality. Whether this conviction will be vindicated is itself an open question. But the trajectory of physics — the progressive deepening of our understanding, the history of apparent mysteries resolved by better theories — gives reason for confidence that the apparent indeterminacy of the quantum world will, in time, receive a satisfactory explanation.

1.7TOWARDS UNIFICATION: THE UNFINISHED PROJECT

The theories discussed in this chapter are not all equally fundamental. Thermodynamics is a macroscopic framework that emerges from the statistical behaviour of vast numbers of particles; it is not a basic law of nature. Special Relativity is subsumed within General Relativity as the special case in which gravity is absent. The Standard Model, for all its precision, is a quantum theory — it operates within the framework of quantum mechanics. When we ask what our deepest descriptions of nature are, the landscape simplifies considerably: ultimately, there are two. Quantum mechanics governs the subatomic world, and General Relativity governs gravity and the large-scale geometry of spacetime. Everything else discussed in this chapter is either derived from or contained within these two frameworks. And it is precisely these two that are fundamentally incompatible with each other. General Relativity is a theory of smooth, continuous spacetime geometry; quantum mechanics is a theory of discrete, probabilistic events occurring within that geometry. When we attempt to apply both simultaneously — as we must when considering the interior of black holes, or the state of the universe in the first instants after the Big Bang, where both gravitational and quantum effects are extreme — the mathematics breaks down. The two frameworks cannot both be correct as currently formulated. A deeper theory, capable of unifying gravity with the quantum forces, is needed. This is the project of quantum gravity, and it remains one of the great unfinished tasks of physics. Several candidate approaches exist — loop quantum gravity, string theory, and others — but none has yet achieved the empirical confirmation that would elevate it from hypothesis to established theory.

The details of these programmes lie beyond the scope of this chapter. What matters here is the philosophical significance of the problem, and the fact that it continues the trajectory of unification. As outlined in Section 1.2, the history of physics is a history of unification: at each stage, what had appeared to be many laws governing separate domains turned out to be aspects of fewer, more fundamental laws. The unification of gravity with quantum mechanics would be the next step in this trajectory — and, perhaps, the final one. Whether it will be achieved cannot be guaranteed. But the trajectory provides strong evidence that the direction is the right one, and it is reasonable to expect that what we currently describe as many separate laws are approximations, glimpses from different angles and at different scales, of a deeper structural unity.