Library of human understanding
Quantum Mechanics
The strangest theory ever to be right about everything it has been tested on.
In 1900 a conservative German professor patched a formula to fit the glow of a hot oven and, almost by accident, began the biggest revision in the history of physics. Within thirty years a handful of people, most of them under thirty, had rebuilt our picture of matter. Atoms, chemistry, lasers, transistors, MRI scanners, the sunlight that powers every living thing: all of it is quantum mechanics at work.
Core concepts visualized
Heisenberg's Uncertainty Principle
You cannot know both position and momentum with perfect precision. Measure one precisely, the other becomes uncertain.
Wave-Particle Duality
Without observation: electron behaves like a wave (spreading, interfering). When measured: it acts like a particle (localized, discrete).
The Measurement Problem
Before measurement, the system exists in a superposition. Measurement forces it to “choose” a single outcome.
Entanglement: Spooky Connection
Measure particle A, and B's state is instantly determined, no matter how far apart they are. Einstein called this “spooky action at a distance.”
Double-slit experiment
Interactive Bloch sphere
Explore quantum superposition: a qubit as a point on the sphere, collapsing to |0⟩ or |1⟩ upon measurement.
Superposition is a core quantum principle: a quantum system can exist in multiple states simultaneously until measured. A qubit on the Bloch sphere occupies a point in superposition; measurement collapses it to |0⟩ or |1⟩. The double-slit experiment shows wave-particle duality: without detection, particles behave like waves and interfere with themselves; with detection determining which slit they pass through, they behave like particles with no interference. Superposition isn’t the system “trying all answers”—it’s a coherent state where all possibilities coexist as a single quantum state until observation.
Four interpretations of quantum mechanics
Copenhagen
Bohr, Heisenberg
Wavefunction collapse is real. Superposition exists until measured.
Many-Worlds
Everett
All outcomes happen in parallel universes. No collapse, just branching.
Pilot-Wave
de Broglie, Bohm
Particles guided by hidden waves. Deterministic, but hidden variables.
Objective Collapse
GRW, Penrose
Wavefunction collapse is a physical process, not observation-induced.
All agree on every experimental prediction. They differ on what's actually real. Which interpretation is correct remains an open question.
Key physicists
From Planck to Bell: the scientists who built quantum mechanics.
- Max Planck1858 Kiel – 1947 Göttingen · GermanyStarted the quantum revolution by finding that energy comes in packets.
- Albert Einstein1879 Ulm – 1955 Princeton · Germany, later Switzerland and the United StatesShowed that light is made of quanta, then spent his life doubting that quantum theory was the final word.
- Satyendra Nath Bose1894 Calcutta – 1974 Calcutta · IndiaFound the correct way to count photons, and with Einstein founded Bose–Einstein statistics.
- Niels Bohr1885 Copenhagen – 1962 Copenhagen · DenmarkQuantised the atom, then led the interpretation of quantum theory from Copenhagen.
- Louis de Broglie1892 Dieppe – 1987 Louveciennes · FranceProposed that matter has a wavelength.
- Werner Heisenberg1901 Würzburg – 1976 Munich · GermanyBuilt the first complete quantum mechanics and discovered the uncertainty principle.
- Max Born1882 Breslau (now Wrocław) – 1970 Göttingen · Germany, later BritainShowed that the wave function gives probabilities.
- Erwin Schrödinger1887 Vienna – 1961 Vienna · AustriaWrote the wave equation of quantum mechanics and coined the term 'entanglement'.
- Wolfgang Pauli1900 Vienna – 1958 Zürich · Austria, later SwitzerlandExplained why matter takes up space, and predicted the neutrino.
- Paul Dirac1902 Bristol – 1984 Tallahassee · United KingdomMerged quantum mechanics with relativity and predicted antimatter.
- John von Neumann1903 Budapest – 1957 Washington · Hungary, later the United StatesGave quantum mechanics its mathematical foundation and defined the measurement problem.
- Richard Feynman1918 New York – 1988 Los Angeles · United StatesReformulated quantum theory with sums over histories and pointed toward quantum computers.
- David Bohm1917 Wilkes-Barre – 1992 London · United States, later Brazil, Israel and BritainShowed that quantum mechanics can be completed with particles that have definite positions.
- Chien-Shiung Wu1912 Liuhe – 1997 New York · China, later the United StatesExperimentalist who tested the weak force and observed correlated entangled photons in 1950.
- John Stewart Bell1928 Belfast – 1990 Geneva · Northern Ireland, United KingdomProved that no local hidden-variable theory can reproduce quantum mechanics, and turned that into an experiment.
- Hugh Everett III1930 Washington, D.C. – 1982 McLean, Virginia · United StatesProposed that the wave function never collapses: the many-worlds interpretation.
- H. Dieter Zeh1932 Braunschweig – 2018 Heidelberg · GermanyDiscovered decoherence: how the environment makes quantum systems look classical.
- Wojciech Zurekborn 1951, Poland · Poland, working in the United StatesDeveloped the decoherence programme and co-proved the no-cloning theorem.
- John Clauser, Alain Aspect and Anton ZeilingerClauser b. 1942, United States · Aspect b. 1947, France · Zeilinger b. 1945, Austria · United States, France and AustriaTested Bell's inequality and showed the quantum world is not locally real; Nobel Prize in Physics 2022.
- John Clarke, Michel Devoret and John MartinisClarke b. 1942, Cambridge · Devoret b. 1953, Paris · Martinis b. 1958, United States · United Kingdom, France and the United StatesShowed quantum behaviour in a circuit big enough to hold in the hand; Nobel Prize in Physics 2025.
Timeline
- 1900
Planck proposes energy quanta to explain black-body radiation. Max Planck
- 1905
Einstein explains the photoelectric effect with light quanta. Albert Einstein
- 1913
Bohr's quantised atom explains the hydrogen spectrum. Niels Bohr
- 1924
De Broglie proposes matter waves; Bose and Einstein found a new statistics. Louis de Broglie
- 1925
Heisenberg's matrix mechanics; Pauli's exclusion principle. Werner Heisenberg
- 1926
Schrödinger's wave equation; Born's probability rule. Erwin Schrödinger
- 1927
Uncertainty principle; the Solvay conference and Bohr–Einstein debates begin. Niels Bohr
- 1928
Dirac's relativistic equation; antimatter is predicted. Paul Dirac
- 1935
The EPR paper and Schrödinger's cat and entanglement. Albert Einstein
- 1948
Feynman's path integral and diagrams make quantum field theory practical. Richard Feynman
- 1952
Bohm revives pilot-wave theory. David Bohm
- 1957
Everett's relative-state formulation. Hugh Everett III
- 1964
Bell's theorem. John Stewart Bell
- 1970
Zeh discovers decoherence. H. Dieter Zeh
- 1982
Aspect's experiments; no-cloning theorem. John Clauser, Alain Aspect
- 2015
Loophole-free Bell tests in Delft, Vienna and Boulder. John Clauser, Alain Aspect
- 2022
Clauser, Aspect and Zeilinger receive the Nobel Prize in Physics. John Clauser, Alain Aspect
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