Reality is not what it seems : the journey to quantum gravity

The author shares his journey in quantum gravity research, framing the book as a historical exploration from ancient philosophy. While his previous work offered a glimpse, this book delves deeper into the quantum nature of space and time, acknowledging that cutting-edge research remains a subject of scientific debate. Science continually redefines reality, moving beyond self-obsession to reveal surprising truths, such as a universe where space and time are emergent properties of granular events.

Scientific thinking began in Miletus, 6th century BCE, with thinkers like Anaximander seeking natural explanations through reason and observation. Leucippus and Democritus introduced ancient atomism, proposing the universe is made of indivisible grains in a vacuum, a vision that challenged divine purpose. This idea faced opposition but endured, eventually supported by Einstein's proof of granular matter in 1905, resolving Zeno's paradoxes.

Aristotle's physics described motion, but subsequent science embraced Pythagorean and Platonic mathematics. Copernicus advanced a sun-centered model, while Kepler found mathematical laws for orbits. Galileo pioneered experimental science, discovering the law of falling bodies. Isaac Newton unified these insights with universal gravitation, explaining that a single force governs earthly and celestial mechanics. Later, Michael Faraday envisioned forces carried by fields, mathematically formalized by James Clerk Maxwell, revealing light as electromagnetic vibrations.

Albert Einstein revolutionized physics in 1905 with special relativity, challenging the understanding of space and time. He showed that absolute simultaneity doesn't exist, leading to the concept of spacetime where space and time are intertwined. He also unified mass and energy with E=mc², dispelling the illusion of a universal present. Later, Einstein's general relativity revealed the gravitational field as spacetime itself, curving in the presence of matter to explain motion. This theory accurately predicted phenomena like Mercury's orbit, light bending, and black holes.

Quantum mechanics, though conceptually challenging, is highly successful. Max Planck introduced quanta in 1900, suggesting energy is distributed in finite packets. Einstein later confirmed light is composed of photons. Niels Bohr applied granularity to atoms, proposing quantized electron orbits. Werner Heisenberg suggested electrons manifest only during interactions, and Paul Dirac developed the mathematical framework, introducing indeterminacy and fundamental chance into reality. Dirac also unified particles and fields, leading to the Standard Model. Three cornerstones are granularity, indeterminacy, and the relational nature of reality.

Twentieth-century physics drastically reshaped the Newtonian worldview with general relativity and quantum mechanics. These theories demanded a complete rethinking of space, time, matter, and energy, forming the foundation for current research into the quantum nature of gravity. The aim is to unify these two successful but incompatible frameworks into a single, coherent vision.

The search for quantum gravity aims to unite general relativity and quantum mechanics, reconceptualizing space and time as quantum entities. Matvei Bronštejn first theorized a limit to space divisibility, defining the Planck length. John Wheeler visualized quantum space as a "frothing foam." Loop quantum gravity proposes space is woven from discrete gravitational field lines, leading to quantized volume and area. This granular structure resolves Zeno's paradoxes. Spin networks, consisting of nodes and links, represent these quantum states. Fundamentally, time disappears from equations, emerging as a macroscopic approximation.

Black holes, extreme regions where time nearly stops, require quantum mechanics for full understanding, explaining Hawking radiation and evaporation. Quantum gravity challenges the classical singularity, proposing matter bounces out due to quantum pressure. This could manifest as slow explosions (fast radio bursts) to outside observers. Furthermore, loop quantum gravity suggests the Big Bang was a "Big Bounce," where a contracting universe rebounded, implying a previous cosmic continuum. The theory also eliminates the concept of infinity, establishing Planck scale as the minimum length, resolving mathematical divergences.

Information, defined as a measure of possible alternatives, is a crucial tool for understanding the physical world. It represents physical correlations between systems, updating ancient atomism with a view of reality as a network of reciprocal interactions. Quantum mechanics can be summarized by two postulates: finite relevant information and the constant possibility of new information. Black hole entropy is interpreted as missing information about internal area quanta. Furthermore, everyday time is an emergent phenomenon, arising from averaging countless microscopic variables and human ignorance of microstates, rather than a fundamental continuum.

Scientific thinking thrives on an acute awareness of ignorance and the humble recognition of knowledge limits. Science offers the best available answers, continually open to improvement, embracing good ideas while being ready to discard them for more accurate descriptions. Quantum gravity presents a world without infinity or absolute time, composed of interacting quantum fields that generate space, time, and matter. This finite, coherent structure dissolves the need for calming fables, revealing a reality that is profound and still full of mysteries to be explored, defining the human spirit's drive to understand the unknown.