How it began
By the late 19th century physics seemed nearly complete, yet the spectrum of light emitted by hot objects (black-body radiation) defied explanation. In 1900 Max Planck solved the problem by assuming that energy is exchanged not continuously but in discrete packets, or quanta. In 1905 Einstein explained the photoelectric effect by treating light itself as particles (photons), and in 1913 Niels Bohr proposed an atom in which electrons occupy only certain orbits. Heisenberg's matrix mechanics and Schrödinger's wave equation in 1925–26 completed the theory we use today.
| Year | Who | Contribution |
|---|---|---|
| 1900 | Planck | Energy quanta (black-body radiation) |
| 1905 | Einstein | Light quanta (photoelectric effect) |
| 1913 | Bohr | Model of the atom |
| 1925–26 | Heisenberg, Schrödinger | Matrix mechanics, wave equation |
| 1927 | Heisenberg | Uncertainty principle |
Core ideas
In the quantum world quantities such as energy can take only discrete values (quantisation). Electrons and light behave like waves in some experiments and like particles in others (wave–particle duality). Before measurement a particle is described as a superposition of possible states; measurement yields one outcome with a definite probability — the strangeness that Schrödinger's cat was devised to mock. The uncertainty principle says position and momentum cannot both be known exactly, and entanglement links measurement results of particles far apart. Einstein called entanglement "spooky action at a distance" and thought the theory incomplete, but Bell-test experiments by Alain Aspect and others from 1982 backed quantum predictions, earning the 2022 Nobel Prize in Physics.
Interpretations and applications
The Copenhagen interpretation of Bohr and Heisenberg holds that properties are not definite before measurement, but rivals such as the many-worlds interpretation and Bohmian mechanics remain in play. Whatever the interpretation, the predictions are the same, and quantum mechanics is among the most precisely tested theories ever devised. Transistors and semiconductors, lasers, LEDs, MRI and atomic clocks all depend on it, and quantum computers and quantum cryptography aim to put superposition and entanglement to work. The Standard Model of particle physics is built on quantum field theory, which merges quantum mechanics with special relativity.
FAQ
- What is quantum mechanics in one sentence?
- It is the theory that at very small scales matter and light behave according to probabilities rather than fixed values, with several possibilities overlapping until a measurement is made.
- Why can't quantum mechanics and relativity be combined?
- Quantum mechanics has been merged with special relativity in quantum field theory, but not yet with general relativity, our theory of gravity. String theory and loop quantum gravity are among the attempts at a theory of quantum gravity.
Sources reviewed: 2026-10-26
References
- Planck, M. (1901). Ueber das Gesetz der Energieverteilung im Normalspectrum. Annalen der Physik, 309(3), 553–563.
- Heisenberg, W. (1927). Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik. Zeitschrift für Physik, 43, 172–198.
- Aspect, A., Dalibard, J., & Roger, G. (1982). Experimental test of Bell's inequalities using time-varying analyzers. Physical Review Letters, 49(25), 1804–1807.
- Griffiths, D. J., & Schroeter, D. F. (2018). Introduction to Quantum Mechanics (3rd ed.). Cambridge University Press.
- Einstein, A. (1905). Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt. Annalen der Physik, 322(6), 132–148.
- Bohr, N. (1913). On the constitution of atoms and molecules. Philosophical Magazine, 26(151), 1–25.
- Schrödinger, E. (1926). Quantisierung als Eigenwertproblem. Annalen der Physik, 384(4), 361–376.
- Einstein, A., Podolsky, B., & Rosen, N. (1935). Can quantum-mechanical description of physical reality be considered complete? Physical Review, 47(10), 777–780.
- Bell, J. S. (1964). On the Einstein Podolsky Rosen paradox. Physics Physique Физика, 1(3), 195–200.
- Everett, H. (1957). "Relative state" formulation of quantum mechanics. Reviews of Modern Physics, 29(3), 454–462.
- Dirac, P. A. M. (1930). The Principles of Quantum Mechanics. Oxford: Clarendon Press.