The Fabric of the Cosmos: Space, Time, and the Texture of Reality

Theoretical physicist Brian Greene traces three centuries of scientific investigation into the nature of space and time, arguing that modern physics has repeatedly overturned humanity's intuitive understanding of reality and that the deepest mysteries of the cosmos remain tied to questions about the very fabric in which all existence unfolds.

Greene opens by challenging the philosopher Albert Camus's assertion that questions about the structure of the universe are secondary to the question of whether life is worth living. For Greene, understanding the physical arena of existence is essential to assessing reality. He frames the book as following the progression of scientific revolutions, from Isaac Newton's classical mechanics through Albert Einstein's relativity and quantum mechanics to superstring theory, a framework that replaces point particles with tiny vibrating strings.

The investigation begins with a centuries-old debate: Is space a real physical entity, or merely an abstract way of describing relationships between objects? Newton's spinning-bucket experiment, in which a bucket of water develops a concave surface as it spins, led him to conclude that the water must be spinning relative to something real and invisible, which he called absolute space. His contemporary Gottfried Wilhelm von Leibniz countered that space without objects is meaningless. In the 1870s, the physicist Ernst Mach argued that the sensation of spinning arises from motion relative to all the matter in the universe rather than from some imperceptible backdrop, though his proposal lacked a concrete mechanism.

Einstein's special theory of relativity, completed in 1905, dismantled Newton's absolute space and absolute time by showing that measurements of distance and duration depend on an observer's motion. Observers moving relative to each other disagree about the length of objects, the rate at which clocks tick, and even which events happen at the same time. Einstein introduced a new absolute: spacetime, the four-dimensional union of space and time first articulated by the mathematician Hermann Minkowski. Different observers slice the spacetime block at different angles, yet the block itself is a fixed entity. Greene uses this framework to argue that physics provides no support for the intuitive sense of time flowing from past to future: Because observers disagree about which events are simultaneous, no single moment can be objectively designated as "now," and all moments exist equally. Einstein's general theory of relativity, completed in 1915, showed that mass and energy warp spacetime, and that this warping is what we experience as gravity. Spacetime becomes a dynamic participant in cosmic evolution; even in a completely empty universe, the equations predict that it provides a benchmark for acceleration.

Greene turns to quantum mechanics. At the subatomic level, particles do not possess definite properties until measured but instead exist as probability waves encoding the likelihood of various outcomes. The double-slit experiment illustrates this: Individual electrons fired through two slits build up an interference pattern characteristic of waves, as though each electron passes through both slits simultaneously. Most striking is quantum entanglement, in which particles that have interacted remain correlated so that measuring one instantaneously influences the distant partner's properties. Einstein, along with his collaborators Boris Podolsky and Nathan Rosen, argued in a 1935 paper that such correlations meant the particles must possess definite properties all along, and that quantum mechanics was incomplete. In the 1960s, the physicist John Bell showed that this assumption yields a testable prediction. Experiments by the physicist Alain Aspect and others in the 1980s confirmed quantum mechanics and ruled out pre-existing local properties. The result does not permit faster-than-light communication, since individual outcomes are random, but it means that space cannot fully isolate entangled particles. Greene concludes that the universe is nonlocal.

Greene devotes major attention to the arrow of time. Everyday experience presents a stark asymmetry: Eggs break but never unbreak, coffee and cream mix but never unmix. Yet the fundamental laws of physics treat past and future identically. Greene traces the resolution to entropy, formalized by the physicist Ludwig Boltzmann as a measure of disorder. The second law of thermodynamics holds that entropy tends to increase because disordered arrangements vastly outnumber ordered ones. However, since the underlying laws are time-symmetric, the same reasoning that predicts increasing entropy toward the future also predicts higher entropy in the past, absurdly implying that our memories are unreliable. The only escape is to accept that the universe began in an extraordinarily low-entropy state. The near-uniformity of the cosmic microwave background radiation, a relic of the early universe, supports this: When gravity is taken into account, a smooth distribution of matter represents extreme order, because gravity drives matter to clump. The arrow of time thus traces back to the special conditions at the universe's origin.

Building on the astronomer Edwin Hubble's 1929 discovery that distant galaxies are receding from one another, indicating that the universe is expanding, Greene explains how inflationary cosmology addresses what the standard big bang theory leaves unexplained: the bang itself. Inflationary cosmology posits a brief early burst of exponential expansion. The physicist Alan Guth realized in the early 1980s that a special field, the inflaton field, could have generated enormous repulsive gravity, causing the universe to expand by a factor of at least 10^30 in a fraction of a second. This burst explains the observed uniformity and flatness of space, and its quantum fluctuations, stretched to cosmic scales, seeded galaxy formation. Precision measurements by the Cosmic Background Explorer (COBE) satellite and the Wilkinson Microwave Anisotropy Probe (WMAP) satellite match inflationary predictions. Inflation also illuminates the arrow of time: Repulsive gravity smoothed space to extremely low gravitational entropy, creating a vast gap between actual and maximum possible entropy that the universe has been closing through gravitational clumping.

The book's final major thread concerns unifying general relativity and quantum mechanics. On ultramicroscopic scales near the Planck length (about 10^-33 centimeters), quantum fluctuations destroy smooth spacetime geometry, and the combined equations yield nonsensical infinities. Superstring theory resolves this by positing that strings' finite size imposes a minimum distance, taming the fluctuations. Different vibrational patterns of a single string type produce particles with different properties, potentially unifying all matter and forces. The theory requires 10 spacetime dimensions, with six extra spatial dimensions curled up into tiny shapes called Calabi-Yau spaces.

Greene describes the second superstring revolution of 1995, in which the physicist Edward Witten showed that five apparently distinct string theories are different descriptions of a single framework called M-theory. M-theory operates in 11 spacetime dimensions and includes higher-dimensional objects called branes. In the braneworld scenario, our three-dimensional space may be a three-brane floating in a higher-dimensional spacetime, with all known particles except the graviton (the hypothetical particle carrying gravity) confined to its surface. Greene also presents the cyclic cosmological model proposed by the physicists Paul Steinhardt and Neil Turok, in which two parallel three-branes repeatedly collide and rebound, generating successive big-bang-like events.

The book surveys experiments that may test these ideas, including searches for Higgs particles (quanta of the field thought to give particles mass) and supersymmetric partners (proposed heavier counterparts to known particles) at the Large Hadron Collider (LHC), CERN's high-energy particle collider, and the detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Greene also explores quantum teleportation, achieved for individual photons in 1997, and time travel, permitted to the future by special relativity and not definitively prohibited to the past, though practical obstacles are immense.

Greene concludes by considering the possibility that space and time may not be fundamental but instead emerge from deeper constituents, much as temperature emerges from the collective motion of molecules. Evidence from black hole entropy, the holographic principle (the idea that a region's physics can be fully encoded on its boundary surface), and string theory's geometrical dualities (in which different spatial configurations yield identical physics) suggest that spacetime may be an approximation whose underlying reality remains to be discovered.