Physics has two theories that together describe everything observable.
General relativity governs the very large — the motion of planets, the bending of light around stars, and the expansion of the universe.
Quantum mechanics governs the very small — the behaviour of electrons, the structure of atoms, and the interactions of elementary particles.
Each has been tested to extraordinary precision.
Each has never been wrong within its domain, and they are fundamentally incompatible with each other.
At the Planck scale (1.616 × 10⁻³⁵ m), both theories are required and both break down. What lies beyond this limit remains one of the deepest open questions in physics.
This is a fourteen-article series examining that question from the ground up. It is divided into four groups.
Group 1 establishes the foundations of the problem — what the Planck scale is, why quantum mechanics itself is not fully understood, and why gravity alone among the four fundamental forces resists quantisation.
Group 2 examines the three serious theoretical attempts to resolve the incompatibility — loop quantum gravity, string theory, and asymptotic safety.
Group 3 follows the consequences of these frameworks into specific physical phenomena — singularities, black holes, and the holographic principle.
Group 4 reaches the deepest questions the series has been building toward — the nature of time, the arrow of entropy, dark energy, and whether the universe’s constants require explanation.
Together, this series reveals the boundary of modern physics and the unknown that lies beyond it.
In 1899, Max Planck derived a length from three fundamental constants of nature. It sits at 10⁻³⁵ metres – sixteen orders of magnitude below the reach of the most powerful accelerator ever built. What it marks is the scale at which the two most successful theories in physics simultaneously break down.
Quantum mechanics predicts experimental results to eleven significant figures. Yet it does not explain what happens when a quantum system is observed. This article examines the measurement problem and why the problem has direct consequences for any theory of quantum gravity.
Renormalisation is the procedure that made quantum field theory work. It successfully describes electromagnetism, the strong force, and the weak force with extraordinary precision. When applied to gravity, it fails. Why does it work for three of the four forces, and why is gravity structurally different?
Loop quantum gravity attempts to unify general relativity and quantum mechanics by quantising spacetime. The theory predicts that space is fundamentally discrete, with quantised units of area and volume, and offers possible resolutions to the early universe.
String theory replaces point particles with one-dimensional strings, providing a description for gravity. This article examines the theory’s development, its concrete results, and the landscape problem it has not resolved.
Every attempt to quantise gravity using standard methods produces infinities that cannot be controlled. String theory and loop quantum gravity solve this by introducing something new. Asymptotic safety asks a different question: what if nothing new is needed? What if gravity, at the highest energies, simply stops changing?
Is spacetime smooth and continuous, or does it have a fundamental granular structure? This article examines the theoretical and experimental evidence for spacetime discreteness.
What are singularities? How do they form from the death of stars? What are the types general relativity predicts, and what do the three leading approaches to quantum gravity each propose as a resolution?
The information describing a volume of space may be entirely contained on its boundary surface. This article examines where that idea came from, what the AdS/CFT correspondence established, and what it implies about the nature of space, gravity, and information.
When a black hole evaporates, is the information about what fell in preserved or destroyed?
The properties of time that feel most immediate and certain in experience are precisely the ones that appear nowhere in the fundamental equations of physics. From Newton’s mechanics to quantum theory, our core laws remain stubbornly time-symmetric, treating the past and future as fundamentally interchangeable. When we attempt to merge these theories into a single…
