Reality Is Not What It Seems

Carlo Rovelli traces the history of humanity's evolving understanding of reality, from the ancient Greek atomists to the frontiers of modern physics. His central argument is that the two great theories of the twentieth century, general relativity and quantum mechanics, can be reconciled into a single framework called loop quantum gravity. This theory, which combines general relativity with quantum mechanics, reveals a world radically unlike everyday experience: a world without continuous space or flowing time, woven instead from a network of discrete quantum events and probabilistic interactions.

Rovelli begins with the intellectual revolution in the ancient Greek city of Miletus during the sixth and fifth centuries BCE. There, thinkers such as Thales and Anaximander pioneered a new way of understanding the world through observation, reason, and critical thought rather than myth. Their decisive innovation was a culture in which students could improve upon and even reject their teachers' ideas. Around 450 BCE, Leucippus traveled from Miletus to Abdera, where he and his student Democritus developed atomism: the proposal that the universe consists of boundless empty space filled with innumerable indivisible atoms. Democritus argued that matter cannot be divided infinitely, since extensionless points could never reassemble into extended objects. Whether space itself is continuous or granular, Rovelli stresses, is the very question quantum gravity must answer.

Most of Democritus's works were lost during the systematic destruction of pagan texts following the Christianization of the Roman Empire, but the Latin poet Lucretius preserved atomist ideas in his poem De rerum natura (The Nature of Things). Rediscovered in 1417, the text profoundly influenced the scientific revolution. Rovelli notes that Albert Einstein's 1905 proof of atoms, using the random jittering of particles suspended in fluid (Brownian motion), echoes a passage in Lucretius describing dust dancing in sunbeams as evidence of invisible atomic motion.

Rovelli then traces classical physics through Ptolemy's mathematical astronomy, Copernicus's heliocentric revision, Kepler's planetary laws, and Galileo's discovery that mathematical laws govern earthly motion. Isaac Newton synthesized these achievements by showing that the gravitational acceleration governing falling objects also holds celestial bodies in their orbits. The next major shift came with Michael Faraday, a self-educated Londoner who intuited that forces between charged bodies are carried by an entity diffused throughout space, the field, and James Clerk Maxwell, who translated Faraday's intuitions into equations. Maxwell discovered that the equations predicted waves traveling at the speed of light, revealing light as a vibration of the electromagnetic field.

Rovelli devotes substantial attention to Einstein's two theories of relativity. Special relativity resolved a contradiction between Maxwell's equations, which fix the speed of light, and Newton's mechanics, in which velocity is always relative. Einstein discovered that absolute simultaneity does not exist: Between the past and future of any event there is an intermediate zone that grows with distance, meaning what we perceive as a universal "now" is an illusion. Special relativity also fused mass and energy, yielding the formula E = mc². General relativity, which Rovelli calls the first pillar of quantum gravity, identified the gravitational field with space itself. Space is not an inert container but a dynamic entity that bends under the influence of matter. Using the mathematics of curved spaces developed by Carl Friedrich Gauss and Bernhard Riemann, Einstein wrote an equation stating that spacetime curvature is proportional to the energy of matter, yielding confirmed predictions including the bending of light by the sun, black holes, gravitational waves, and the expansion of the universe.

The second pillar is quantum mechanics. Rovelli traces it from Max Planck's 1900 discovery that electromagnetic energy comes in discrete packets, through Einstein's proof that photons are real, to Niels Bohr's atomic model with discrete electron orbits. The decisive breakthrough came from Werner Heisenberg, who conceived that an electron need not always have a definite position; it exists only when it interacts with something else. Paul Dirac then built the complete mathematical framework, showing that particles are quanta of fields. The resulting quantum field theory forms the basis of the Standard Model of particle physics. Rovelli distills three core lessons: granularity (information about any system is finite), indeterminacy (the future is genuinely unpredictable), and relationality (properties exist only in interactions between systems).

With both pillars established, Rovelli turns to their incompatibility. General relativity describes spacetime as continuous and curved, while quantum mechanics describes fields as granular and probabilistic. Where both theories matter simultaneously, inside black holes or at the Big Bang, neither works alone. Rovelli credits Matvei Bronštejn, a young Russian physicist executed by Stalin's police in 1938, with first recognizing that combining the two theories implies a minimum length, the Planck length (approximately 10⁻³³ centimeters), below which space ceases to exist. Subsequent work by John Wheeler, Bryce DeWitt, Abhay Ashtekar, Lee Smolin, and Ted Jacobson led to loop quantum gravity.

The theory's core results redefine space and time. Its fundamental structures are lines of force of the gravitational field, but because the gravitational field is space itself, these lines are the threads from which space is woven. Physical content resides in "nodes" carrying discrete volumes and "links" carrying discrete areas, forming graphs called spin networks. The theory shows that volume and area can take only certain discrete values: Space is composed of "atoms of space" vastly smaller than atomic nuclei. Time also disappears at the fundamental level. To describe processes, Rovelli introduces spinfoams, histories of evolving spin networks whose probabilities are computed by summing over all possible internal spacetimes. The resulting picture is that the world consists of quantum fields defined without a fixed background spacetime; these fields do not live in spacetime but generate it through their interactions. Time re-emerges at large scales through "thermal time": Just as heat arises from averaging over many microscopic variables, time emerges as an artifact of our coarse-grained interaction with countless microscopic degrees of freedom.

Rovelli applies the theory to cosmology and black holes. Loop quantum gravity predicts that the universe was not crushed to a point at the Big Bang; quantum repulsion caused a contracting universe to bounce, a "Big Bounce" through a phase where space and time dissolved into quantum probabilities. The theory also addresses Stephen Hawking's discovery that black holes emit thermal radiation, explaining this heat as arising from quantum fluctuations of the quanta of space on the black hole's surface. Matter falling into a black hole does not collapse to infinite density but bounces, and the black hole may eventually explode.

Regarding experimental evidence, Rovelli notes that measurements have not yet been precise enough to confirm or refute loop quantum gravity but identifies promising avenues. Recent discoveries, including the Higgs boson at CERN, cosmic background radiation measurements, and the detection of gravitational waves, all confirmed existing theories without revealing exotic new particles, leaving general relativity, quantum mechanics, and the Standard Model as the sole key ingredients, consistent with loop quantum gravity's approach. The cosmic background radiation may carry footprints of the quantum bounce.

In his concluding chapters, Rovelli argues that quantum gravity eliminates pathological infinities in both general relativity and quantum field theory by removing the infinite divisibility of space. He proposes that the concept of information, as formalized by Claude Shannon, the information theorist who defined information as the measure of possible alternatives in bits, is central to understanding reality. The world is a network of correlations, and the structure of quantum mechanics can be expressed in two postulates: that information in any system is finite and that new information can always be obtained. Rovelli closes by emphasizing that science is reliable not because it offers certainty but because it provides the best available answers while remaining open to correction. The world revealed by quantum gravity is strange and still largely mysterious, but coherent and open to future confirmation.