## Definition **Quantum Mechanics** (also called quantum theory, or the theory of quanta) is the fundamental framework describing the behaviour of matter and energy at atomic and subatomic scales. Its defining features are the quantisation of energy, intrinsic probability, and the relational character of physical properties: a system's attributes are not absolute but defined only through interactions with other systems. ## Origins and Key Contributors Quantum mechanics was born in 1900 when Max Planck calculated the electromagnetic field in thermal equilibrium inside a hot cavity. To match experimental measurements, he had to treat energy as distributed in discrete packets ("quanta") — a mathematical trick he did not fully understand. Five years later, Einstein showed that the quanta are real: light itself consists of discrete particles (photons). Bohr (1910s) extended quantisation to atomic electrons, showing that they occupy only specific energy levels and emit or absorb a photon whenever they jump between levels — the *quantum jump*. Heisenberg (1925) wrote the complete equations of the theory. ## Core Principles **Quantisation** — Energy (and other physical quantities) comes in indivisible chunks. The energy of a photon of frequency $\nu$ is: $ E = h\nu $ where $h$ is Planck's constant. **Superposition and the wavefunction** — Between interactions, a quantum system is described by a wavefunction $\psi$ that does not reside in ordinary space but in an abstract mathematical space. The wavefunction encodes probabilities, not definite values. **Intrinsic probability** — When a quantum system interacts with another, the outcome is not determined by hidden variables but is irreducibly probabilistic. Born's rule gives the probability of observing a particular outcome: $ P(\text{outcome}) = |\psi|^2 $ **No definite position without interaction** — A particle has no well-defined position (or other property) between interactions. Heisenberg's insight: electrons exist only when they collide with something. In Rovelli's words, "it is as if God drew reality not with a firm line but with a faint dotted one." ## Successes The Schrödinger equation (1926) — the wave-mechanics form of the quantum equations — explains the entire periodic table of elements: each chemical element is a solution of the equation for a nucleus with the corresponding charge. Without quantum mechanics there would be no transistors, no lasers, and no modern electronics. ## The Interpretational Problem A century after its birth, quantum mechanics retains an aura of mystery. The equations describe how a system appears to another system — not what a system "is" in itself. Three live interpretations: - **Copenhagen** (Bohr): the wavefunction is a calculation tool; asking what happens between measurements is meaningless. - **Many-worlds** (Everett): all outcomes occur, in branching universes. - **Relational QM** (Rovelli): physical properties are real but always relative — there is no absolute observer-independent state. "Reality is only interaction." ## Incompatibility with General Relativity General Relativity describes a smooth, continuous, curved spacetime. Quantum Mechanics operates on a fixed, flat background and generates discrete, probabilistic events. The two frameworks cannot both be correct in their current forms — resolving this tension is the goal of quantum gravity research. See [[The Problem of Quantum Gravity]] and [[Loop Quantum Gravity]]. ## Related - [[General Relativity]] - [[Photon]] - [[Standard Model of Particle Physics]] - [[Loop Quantum Gravity]] - [[The Problem of Quantum Gravity]] - [[Black Holes and Hawking Radiation]] ## Sources - [[Seven Brief Lessons on Physics (Rovelli 2014)]]