The Physicist Who Says Time Doesn't Exist

Julian Barbour introduces his groundbreaking theory challenging the widely held belief that the arrow of time is solely due to increasing entropy. He suggests that time might be an illusion, an interpretation of changing shapes in the universe, and presents evidence from Newton's theory that complexity, or order, is increasing instead of disorder.

Julian Barbour discusses his unique path as a physicist outside of academia, funding his research by translating Russian scientific journals for 28 years. He explains his core concept of time, citing Ernst Mach: time is an abstraction deduced from change, a succession of 'complete shapes' of the universe, rather than an independent duration.

Barbour details how Paul Dirac's 1958 paper, advocating for the restoration of simultaneity in a quantum theory of gravity, profoundly influenced his work. He argues that Einstein's general relativity, which allows for various 'slicings' of spacetime, presents challenges for quantum mechanics, and that a notion of simultaneity closer to Dirac's thinking better aligns with observed cosmic microwave background.

Using a cardboard diorama of triangles, Barbour clarifies that each 'slice' represents a distinct instant of time with differing shapes. He asserts that while instants are inherent, duration is a human construct. He introduces the concept of 'complexity' – an inherently increasing quantity in Newton's theory – as the true arrow of time, signifying an increase in order, contrary to the second law of thermodynamics.

Barbour delves into Mach's principle, emphasizing its relational perspective on space and time, opposing Newton's absolute concepts. He explains Mach's idea that local inertial frames are determined by the relative positions and motions of all bodies in the universe. He also clarifies Mach's operational definition of inertial mass through mutual accelerations, distinguishing it from Einstein's sometimes confused interpretation.

Barbour proposes that correlations in geometry, similar to Bell inequalities in quantum mechanics, provide the mechanism by which local phenomena 'know' about global structures. He suggests that the mysterious aspects of quantum mechanics might be a reflection of inherent relationships in space.

Barbour discusses Carlo Rovelli's relational interpretation of quantum mechanics, claiming credit for introducing the term 'relational' into physics discourse. He then outlines his own ontological starting point: point particles in Euclidean space, from which he believes all observable structure can be explained, distinguishing it from the 'gift of consciousness'.

He draws a parallel to the Greek atomists, particularly Lucretius, who sought to explain the shapes and variety in the universe, albeit by invoking different 'primordial seeds' for every form. Barbour emphasizes that the fundamental task of science is to explain shapes, not just sizes.

Barbour introduces the concept of an intrinsic measure of size, independent of external rulers, by using angles within a system, such as a triangle. He applies this to the expansion of the universe, arguing against the idea of an external ruler and suggesting that perceived expansion is an intrinsic change in relative shapes. He reads a passage from Henri Poincaré on the unobservability of universal scaling.

Barbour formally introduces his measure of 'complexity' (also called 'shape potential' by N-body physicists) as a dimensionless, scale-invariant quantity derived from inter-particle separations. He explains that this measure fulfills Leibniz's philosophical idea that the universe strives for maximal variety, as it is highly sensitive to clustering and non-uniform distributions.

He explains how the minimum of this complexity corresponds to a maximally uniform shape, suggesting a 'Newtonian Big Bang' where the universe starts uniform but evolves to become increasingly structured and ordered. This directly contradicts the second law of thermodynamics, which states entropy (disorder) increases.

Barbour explicitly challenges the second law of thermodynamics, citing Eddington and Einstein's strong belief in its unfailing applicability. He argues that thermodynamics' validity is contingent on 'systems in a box,' a condition not met by the universe as a whole. Without this 'box,' the growth of complexity and order dominates, with subsystems only locally exhibiting thermodynamic behavior.

Using the 'shape sphere' for the three-body problem, Barbour shows how probabilities of shapes can be defined without reference to infinite measure spaces. He connects this to a novel approach to quantum gravity, suggesting that rather than a wave function, the theory could be based purely on probabilities for shapes, potentially removing the need for Planck's constant and hinting at an emergent quantum mechanics.

Barbour questions whether experiments, such as the two-slit experiment, merely discover what exists or actively contribute to creating the observed phenomena. He cites Eddington's 'strange footprint' analogy to suggest that the specialized setups of experiments might be imposing correlations, leading to observed patterns like interference fringes, rather than revealing intrinsic properties of reality.

Barbour delves into his agnostic views on God, emphasizing the 'gift' of consciousness in interpreting the mathematical structures (shapes) of the universe. He explores the etymology of 'idea' (pattern/shape), 'divine' (Sanskrit for god, sky), 'pattern' (father), 'matter' (mother), and 'chaos' (chasm/gap), connecting these linguistic roots to fundamental concepts in his theory and broader philosophical considerations.