This timeline is is a greatly expanded version of that presented in Wikipedia’s article on the history of quantum mechanics, with milestones in the development of Relativity, cosmology, and particle physics also included due to their overlap in attempts to construct a Theory of Everything (TOE). Where possible, I’ve included references to the original founding documents related to each development, along with links to online copies of the original articles (or English translations thereof) where available. Please note that some of these documents are accessible by subscription only. In such cases, the articles should be viewable from the networks of institutions with site license access, such as universities or larger libraries.
The Era of Classical Physics, and Cracks in its Foundations (Antiquity-1899)
c. 624 BCE – c. 546 BCE – Thales of Miletus (Θαλῆς ὁ Μιλήσιος), considered by many to be the “Father of Science,” predicted a solar eclipse which halted the Battle of Halys between the Lydians and Medes. Thales’ rejection of mythological explanations for natural phenomena marked the birth of the Greek tradition of Western philosophy.
c. 570 BCE – c. 495 BCE – Pythagoras of Samos (Ὁ Πυθαγόρας ὁ Σάμιος), Ionian philosopher and mathematician, was credited with many advancements in mathematics and philosophy, including the Pythagorean theorem and the Pythagorean tuning system of music (which produces purer harmonies than modern twelve tone equal temperament tuning, but without the ability to span octaves without introducing discordance).
c. 510 BCE – 428 BCE – Anaxagoras (Ἀναξαγόρας) brought philosophy to Athens, and considered matter as consisting of indestructible primary elements. He held that matter could neither be created nor destroyed, but merely modified by rearrangement of these primary elements. This view constituted an early conservation law.
c. 310 BCE – c. 230 BCE – Aristarchus (Ἀρίσταρχος) of Samos formulated a heliocentric model of the solar system. No copies of his work on the subject survive, but there are references to it in Archimedes’ The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli)
c. 300 BCE – Euclid (Εὐκλείδης) of Alexandria developed the axiomatic principles of what is now known as Euclidean geometry, described in his book, Elements, which is widely regarded as being the most influential textbook ever written.
c. 297 BCE – c. 212 BCE – Archimedes (Ἀρχιμήδης) of Syracuse, Greek mathematician, physicist, engineer, inventor, and astronomer, made many advances in the fields of hydrostatics, statics, and mechanics, as well as inventing the “method of exhaustion,” a mechanism for calculating the area under a parabolic arc, presaging the later development of integral calculus.
c. 276 BCE – c. 195 BCE – Eratosthenes (Ἐρατοσθένης) of Cyrene was the father of geography, inventing the first system of latitude and longitude. He is also credited with having performed a remarkably accurate measurement of the circumference of the Earth (yielding a value within 2% of modern measured values). When Columbus set sail in 1492 AD, he was relying upon a later and far less accurate measurement for his navigation.
c. 10 CE – 70 CE – Heron of Alexandria (Ἥρων ὁ Ἀλεξανδρεύς), Hellenistic Egyption engineer and mathematician, came to be widely known for numerous hydraulic and steam-powered inventions, including a crude precursor to the steam engine.
c. 90 CE – c. 168 CE – Claudius Ptolemy (Κλαύδιος Πτολεμαῖος) of Alexandria, Greco-Roman Egyptian mathematician, astronomer, astrologer, and geographer, was noted for three main written works: The Almagest, the Geographia, and the Tetrabiblos. His formulation of the ancient geocentric cosmological model was widely accepted until the rise of the geocentric Copernican model.
c. 201 CE – 298 CE – Diophantus of Alexandria (Διόφαντος ὁ Ἀλεξανδρεύς) developed an early form of algebra known as Diophantine analysis, published in a series of books known as Arithmetica. (An English translation of Arithmetica is included in the book Diophantus of Alexandria; a study in the history of Greek algebra by Sir Thomas Little Heath, 1910.)
c. 370 CE – 415 CE – Hypatia (Ὑπατία) of Alexandria, a Neoplatonist philosopher, mathematician, and astronomer, was tortured and murdered by a mob of Christian monks as part of a political power struggle over the control of Alexandria. Her death is widely regarded as marking the end of the Classical era of science and philosophy.
c. 780 CE – 850 CE, Abū ʿAbdallāh Muḥammad ibn Mūsā al-Khwārizmī (محمد بن موسى الخوارزميّ المجوسـيّ القطربّـليّ), a Persian astronomer, mathematician, and geographer, elaborated on earlier development of algebra with his tome, Al-Kitāb al-mukhtaṣar fī hīsāb al-ğabr wa’l-muqābala or الكتاب المختصر في حساب الجبر والمقابلة (The Compendious Book on Calculation by Completion and Balancing). It is from the word “al-ğabr” that we obtain the word algebra.
965 CE – c. 1040 CE, Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham ( أبو علي، الحسن بن الحسن بن الهيثم), the Muslim scientist also known as Alhazen, developed concepts regarding motion, gravity, and optics which would later be rediscovered by Galileo and Newton.
c. 1214 – 1294 – Roger Bacon, an English philosopher and Fransiscan friar, emphasized what is now called the scientific method, the development of deductive theories based upon observational evidence from the natural world.
1573 – Tycho Brahe (Tyge Ottesen Brahe), a Danish nobleman, published De nova stella et nullius ævi memoria prius visa stella, which chronicled his observations of supernova in 1573 (in the process, coining the term “nova” for such “new stars”). He was able to establish that the nova lacked parallax, and thus could not be as near the Earth. Over the course of his career, he conducted extensive and detailed observations of the stars and planets without the aid of a telescope.
1588 – Tycho Braha published the first volume of Astronomiæ Instauratæ Progymnasmata (“Introduction to the New Astronomy”), focucing mainly on his observations of the 1572 supernova. The remaining two volumes were not published until after his death.
1600 – Giordano Bruno, Italian Dominican friar, philosopher, astronomer, and mathematician, was burned at the stake after being found guilty of heresy by the Inquisition for advocating the notion that the Sun is merely a star, and that there might be other inhabited worlds orbiting other stars.
1609 – Johannes Kepler, a German astronomer and mathematician, published Astronomia nova, in which he laid out two of his three laws of planetary motion, derived from Tycho Brahe’s observations of the orbit of Mars.
Kepler’s Laws of Planetary Motion
1. The Law of Orbits: Planets orbit the sun in elliptical orbits, with the Sun at one of the two foci of the ellipse.
2. The Law of Areas: An imaginary line from the sun to the planet sweeps out equal areas in equal times.
3. The Law of Periods: The square of the period of a planet’s orbit is proportional to the cube of the semi-major axis of the the orbit.
1632 – Galileo Galilei published Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems). It was this work, a discourse comparing Ptolemaic and Copernican views of the solar system, which resulted in Galileo being convicted of “grave suspicion of heresy.”
1704 – Sir Isaac Newton published Opticks: or, a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light.
1736 – Sir Isaac Newton published The Method of Fluxions and Infinite Series; with its Application to the Geometry of Curve-Lines, his book on differential calculus. (He referred to integral calculus as “fluents.”)
Coulomb (1785) “Premier mémoire sur l’électricité et le magnétisme,” Histoire de l’Académie Royale des Sciences, pages 569-577.
Coulomb (1785) “Second mémoire sur l’électricité et le magnétisme,” Histoire de l’Académie Royale des Sciences, pages 578-611.
Coulomb (1785) “Troisième mémoire sur l’électricité et le magnétisme,” Histoire de l’Académie Royale des Sciences, pages 612-638.
Coulomb (1786) “Quatrième mémoire sur l’électricité,” Histoire de l’Académie Royale des Sciences, pages 67-77.
Coulomb (1787) “Cinquième mémoire sur l’électricité,” Histoire de l’Académie Royale des Sciences, pages 421-467.
Coulomb (1788) “Sixième mémoire sur l’électricité,” Histoire de l’Académie Royale des Sciences, pages 617-705.
Coulomb (1789) “Septième mémoire sur l’électricité et le magnétisme,” Histoire de l’Académie Royale des Sciences, pages 455-505.
1812 – Michael Faraday constructed a voltaic pile (a simple battery) from seven halfpence coins stacked with interleaved sheets of zinc and pieces of paper soaked in salt water.
1820 – Hans Christian Ørsted discovered that an electric current produces a magnetic field.
1821 – Michael Faraday, inspired by Ørsted’s discovery, constructed the first crude electric motor.
1826 – André-Marie Ampère published Mémoire sur la théorie mathématique des phénomènes électrodynamiques uniquement déduite de l’experience (“Memoir on the Mathematical Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience”) in which he introduced the mathematical formulation of Ørsted’s discovery, known as Ampère’s Law.
1831 – Faraday began a series of experiments in which he discovered electromagnetic induction.
1831-1835 – Joseph Henry conducted a series of experiments in which he discovered electromagnetic induction (concurrently with and independently of Faraday) and self-induction, as well as perfecting the electromagnet and inventing the electric relay. He also worked on early electric motor designs.
1831, 1844 – Faraday published two volumes on his experiments with electricity, Experimental Researches in Electricity, vols. i. and ii.
1835 – Johann Carl Friedrich Gauss, one of the most prolific mathematicians and physicists of his day, formulated what we now know as Gauss’s Law, which relates electric charge distribution to the resulting electric field.
1855-56 – James Clerk Maxwell delivered a two-part paper providing a mathematical analysis of Faraday’s work.
James C. Maxwell, “On Faraday’s Lines of Force“, Transactions of the Cambridge Philosophical Society, Vol. X Part I (Dec. 10, 1855)
James C. Maxwell, “On Faraday’s Lines of Force, Part II: On Faraday’s ‘Electro-tonic State’“, Transactions of the Cambridge Philosophical Society, Vol. X, Part II (Feb. 11, 1856)
1861-1862 – James Clerk Maxwell consolidated the work of Gauss, Faraday, and Ampère in a four part paper, “On Physical Lines of Force,” in which he presents what are now known as Maxwell’s Equations. In retrospect, Maxwellian electrodynamics represents the first (and simplest) gauge field theory, as well as the first step towards grand unification of the fundamental forces of nature.
Gauss’s Law for Magnetism:
Faraday’s Law of Induction:
1865 – James Clerk Maxwell published “A Dynamical Theory of the Electromagnetic Field,” in which he demonstrated that light is an electromagnetic phenomenon.
1873 – Maxwell published A Treatise on Electricity and Magnetism, a two-volume textbook encompassing his earlier work on the subject.
James C. Maxwell, A Treatise on Electricity and Magnetism. Oxford: Clarendon Press (1873).
1874 – George Johnstone Stoney postulated the existence of the electron has a fundamental quantity of electricity. He introduced the word “electron” itself in 1891. See Stoney, G.J. (1894). “Of the “Electron,” or Atom of Electricity”. Philosophical Magazine 38 (5): 418-420. doi:10.1080/14786449408620653
1877 – Ludwig Boltzmann suggested that the energy states of a physical system could be discrete. He used this primarily as a mathematical tool, subdividing the energy continuum into cells or elements in order to construct a probabilistic derivation in his calculations of entropy and velocity distribution in a gas.
L. Boltzmann, “Über die Beziehung eines allgemeine mechanischen Satzes zum zweiten Hauptsatze der Warmetheorie” (“On the Relation of a General Mechanical Theorem to the Second Law of Thermodynamics”), Sitzungsberichte Akad. Wiss., Vienna, part II, 75, 67–73 (1877); reprinted in Boltzmann’s Wissenschaftliche Abhandlungen, Vol. 2, Leipzig, J. A. Barth, 1909, pp. 116–22].
(English translation in Kinetic Theory, Vol. 2, by S.G. Brush, New York: Pergamon Press (1966), pp. 362-367.)
1885 – Johann Jakob Balmer discovered the Balmer series, visible light (with some ultraviolet) spectral lines of hydrogen corresponding to transitions to the n’=2 state. Balmer used this spectroscopic data to construct the Balmer formula:
1887 – By bending cathode rays in a vacuum tube with magnetic fields, J. J. Thomson measured the charge/mass ratio of the electron, establishing the corpuscular nature of electricity.
Rydberg, J. “Recherches sur la constitution des spectres d’émission des éléments chimiques,” in Kungliga Svenska vetenskapsakademiens handlingar, n.s. 23. no. 11 (1890)
1895 – Wilhelm Conrad Röntgen discovered X-rays.
1896-1897 – Pieter Zeeman observed the splitting of spectral lines in the presence of a static magnetic field (“The Zeeman effect“).
P. Zeeman. “On the influence of Magnetism on the Nature of the Light emitted by a Substance”. Phil. Mag. 43: 226. (1897). (Reprinted in The Effects of a Magnetic Field on Radiation: Memoirs by Faraday, Kerr, and Zeeman, edited by E.P. Lewis, American Book Company, NY (1900), pp.65-82)
P. Zeeman. “Doubles and triplets in the spectrum produced by external magnetic forces”. Phil. Mag. 44: 55. (1897). (Reprinted in The Effects of a Magnetic Field on Radiation: Memoirs by Faraday, Kerr, and Zeeman, edited by E.P. Lewis, American Book Company, NY (1900), pp. 83-97)
1896 – Henri Becquerel discovered radioactivity. His work was subsequently expanded upon by Pierre and Marie Curie.
Henri Becquerel. “Sur les radiations invisibles émises par les corps phosphorescents” (“On the invisible rays emitted by phosphorescent bodies”). Comptes Rendus 122: 501 (1896). (English translation by Carmen Giunta)
“Old quantum theory” (1900-1924)
Built on the work of Planck, Einstein, Bohr, and Sommerfeld, and others, “old quantum theory” was never fleshed-out as a fully complete or self-consistent theory, but provided a mechanism for performing calculations involving simple systems such as the hydrogen atom. This era also witnessed the birth of the Special and General Theory of Relativity.
1900-1901 – Max Planck introduced the concept of energy “quanta” to avoid the “Ultraviolet Catastrophe” in the black body radiation problem, suggesting that electromagnetic energy could only be emitted in energies which are integer multiples of E = hν, where h is Planck’s constant, and ν is the frequency.
M. Planck, Ann. d. Phys. 1, p. 99 (1900)
Planck, Max. (1900). “Entropy and Temperature of Radiant Heat.” Annalen der Physik, 1, no 4. April, pg. 719–37.
1900 – Poincaré defined what came to be known as the Poincaré Group of symmetry transformations, of which Lorentz transformations are a subset.
Henri Poincaré, (1900) “La theorie de Lorentz et la Principe de Reaction” (“The Theory of Lorentz and the Principle of Reaction“), Archives Neerlandaises, V, 253–78.
1901 – Frederick Soddy and Ernest Rutherford discovered nuclear transmutation when they observe the decay of thorium into radium.
1905 – Albert Einstein publishes his four annus mirabilis papers:
A. Einstein, “Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt” (“On a heuristic point of view concerning the production and transformation of light”), Annalen der Physik 17 (1905) 132-148 (in which Einstein explains the photoelectric effect)
Einstein, Albert (1905). “Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen” (“Investigations on the theory of Brownian Movement”). Annalen der Physik 17: 549–560. (in which Einstein used an analysis of Brownian Motion to prove John Dalton’s Atomic Theory of Matter)
Einstein, Albert (1905). “Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?” (“Does the Inertia of a Body Depend Upon Its Energy Content?”). Annalen der Physik 18: 639–641 (Mass-energy equivalence in Special Relativity)*
* Interestingly enough, Einstein’s famous equation appears nowhere in this paper. Instead, he expressed it in a different form. But, alas, does not roll off the tongue as smoothly.
1905-1906 – Henri Poincaré pointed out that Lorentz transformations are equivalent to rotations through 4-dimensional space-time with the time component being an imaginary quantity.
1906-1914 – Theodore Lyman discovered the Lyman series, ultraviolet spectral lines in hydrogen corresponding to transitions to the n’=1 state.
Lyman, Theodore (1906), “The Spectrum of Hydrogen in the Region of Extremely Short Wave-Length”, Memoirs of the American Academy of Arts and Sciences, New Series 13 (3): 125–146, ISSN 0096-6134, JSTOR 25058084
1907 – Einstein’s initial paper on General Relativity, in which he introduced the Correspondence Principle, the bending of light paths by gravity, and the extension of the equivalence of mass and energy to include gravitational mass as well as inertial mass.
A. Einstein, “On the relativity principle and the conclusions drawn from it”, Jahrbuch der Radioaktivitaet und Elektronik 4 [English translation in The Collected Papers of Albert Einstein, vol. 2 (tr. by Anna Beck and consultant Peter Havas), Princeton University Press, 1989.]
1907-1909 – Herman Minkowski expanded upon Poincaré’s work and constructed his famous Minkowski space-time geometry. This proved to be the appropriate space-time metric for Special Relativity. Later, with the development of General Relativity, the Minkowski metric came to be seen as an approximation of the locally-flat Reimann metric for an object in free-fall.
- H. Minkowski, “Die Grundgleichungen für die elektromagnetischen Vorgänge in bewegten Körpern” (“The Fundamental Equations for Electromagnetic Processes in Moving Bodies”), Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse, pp. 53–111 (1907/1908)
- H. Minkowski, “Raum und Zeit” (“Space and Time”), Jahresberichte der Deutschen Mathematiker-Vereinigung, 1-14, B.G. Teubner (1908/1909)
1907-1911 – Ernest Rutherford (assisted by Hans Geiger and Ernest Marsden) performed his scattering experiments, resulting in the nuclear model of the atom, disproving the “plum pudding model.” For an excellent informal account of this discovery, see “A Little Nut.”
1908 – Friedrich Pacshen discovered the Pacshen series, infrared spectroscopic lines in hydrogen corresponding to transitions to the n’=3 state.
Paschen, Friedrich (1908), “Zur Kenntnis ultraroter Linienspektra. I. (Normalwellenlängen bis 27000 Å.-E.)“, Annalen der Physik 332 (13): 537–570, Bibcode 1908AnP…332..537P, doi: 10.1002/andp.19083321303
1909 – Geoffrey Ingram Taylor demonstrated that interference patterns of light are present in Young’s slit diffraction experiment even when light passes through the apparatus one photon at a time, laying the foundations for the concept of wave-particle duality.
1911 – Einstein’s second paper laying the foundations of General Relativity, containing a specific prediction of the deflection of light rays by the sun and time dilation in a gravitational field.
A. Einstein, “Ueber den Einfluss der Schwerkraft auf die Ausbreitung des Lichtes”, Annalen der Physik 35, 1911, 898-908.
1911-1912 – Henri Poincaré published a groundbreaking (from the standpoint of the philosophy of science) analysis of the evolution of Natural Law, as well as a mathematical proof of Planck’s quanta. (Thus far, I have found little of his work translated into English, but much of his work in the original French is available here, here, and here.) An overview of Poincaré’s study of quantum phenomena is given by F.E. Irons, “Poincaré’s 1911-12 proof of quantum discontinuity interpreted as applying to atoms”, American Journal of Physics, Vol. 69, No. 8, pp. 879-884 (August 2001).
H. Poincaré, “L’evolution des lois” (“The Evolution of Laws”), Scientia, v. IX , pp. 275-292 (1911), (This essay on the evolution of our understanding of natural law was based upon a speech given at the Congresso di Filosofia di Bologna on 8 April 1911. It later was made into the first chapter of his book, Dernières Pensées (Last Essays), published in Paris in 1913. Digital scans of this book can also be seen here.)
H. Poincaré, “Sur la theorie des quanta” (“On the Theory of Quanta”), Comptes Rendus de l’Academie des Sciences, 153:pp. 1103-1108 (1912)
H. Poincaré, “Sur la theorie des quanta” (“On the Theory of Quanta”), Journal de Physique theorique et appliquee, 2: pp. 5-34 (1912)
H. Poincaré, “L’hypothese des quanta” (“The Quanta Hypothesis”), Revue Scientique, 50: pp. 225-232 (1912)
H. Poincaré, “Les rapports de la matiere et l’ether” (“Reports of Matter and Ether”), Journal de physique theorique et appliquee, ser 5, 2, pp. 347-360 (1912)
1912 – Victor Hess discovered cosmic radiation by sending aloft sensitive radiation detectors aboard high-altitude balloons, both manned and unmanned (the former at great risk to himself).
1912 – Einstein begins to realize that the relativistic theory of gravity he is developing cannot allow time to warp while keeping space flat.
A. Einstein, “Lichtgeschwindigkeit und Statik des Gravitationsfeldes”, Annalen der Physik (ser. 4), 38, 355–369, link
A. Einstein “Theorie des statischen Gravitationsfeldes”, Annalen der Physik (ser. 4), 38, 443–458, link
1913 – Einstein enlisted the help of his old ETH friend Marcel Grossmann to incorporate Riemannian geometry into his nascent theory of General Relativity.
A. Einstein & M. Grossmann, “Entwurf einer verallgemeinerten Relativitaetstheorie und einer Theorie der Gravitation”, Zeitschrift fuer Mathematik und Physik 62, 1913, 225-261.
1913 – Robert Millikan published the results of his famed oil drop experiment, in which he measured the charge of the electron. This measurement made it possible to calculate Avogadro’s number, thus enabling the determination of atomic weights.
1913 – Johannes Stark and Antonino Lo Surdo discovered the shifting and splitting of atomic spectra in the presence of an external electric field (the “Stark effect“).
J. Stark, “Beobachtungen über den Effekt des elektrischen Feldes auf Spektrallinien I. Quereffekt” (“Observations of the effect of the electric field on spectral lines I. Transverse effect”), Annalen der Physik, vol. 43, pp. 965-983 (1914). Published earlier (1913) in Sitzungsberichten der Kgl. Preuss. Akad. d. Wiss.
1913-1914 – Niels Bohr proposed his “shell” model of the atom, which explained, at a basic level at least, the spectral lines of the hydrogen atom consistent with the Rydberg formula. Long-supplanted by the quantum probability cloud model, this image of the atom persists in the minds of the general public to this day.
1914 – Einstein presented the “Hole Argument” to explain why he felt it would be impossible to construct gravitational field equations in his theory which satisfy general covariance.
“Die formale Grundlage der allgemeinen Relativitaetstheorie”, Preussische Akademie der Wissenschaften, Sitzungsberichte (1914), 1030-1085.
1915 – Einstein presented four iterations of his General Theory of Relativity to the Prussian Academy of Science (with the third containing an explanation for the precession of the perihelion of Mercury, and the last containing the final form of his field equations), which physicist to this day are still attempting to reconcile with QM.
The Einstein Field Equations:
where is the Ricci curvature tensor and is stress-energy tensor.
A. Einstein, “Zur allgemeinen Relativitaetstheorie”, Preussische Akademie der Wissenschaften, Sizungsberichte (1915), 778-786.
A. Einstein, “Zur allgemeinen Relativitaetstheorie (Nachtrag)”, Preussische Akademie der Wissenschaften, Sizungsberichte (1915), 799-801.
A. Einstein, “Erklaerung der Perihelbewegung des Merkur aus der allgemeinen Relativitaetstheorie”, Preussische Akademie der Wissenschaften, Sizungsberichte (1915), 831-839.
A. Einstein, “Die Feldgleichung der Gravitation”, Preussische Akademie der Wissenschaften, Sizungsberichte (1915), 844-847.
1916 – Einstein published the final form of his paper on the General Theory of Relativity.
1916 – The Schwarzschild metric for General Relativity was introduced.
K. Schwarzschild, “Über das Gravitationsfeld eines Massenpunktes nach der Einsteinschen Theorie” (”On the gravitational field of a mass point according to Einstein’s theory“, Sitzungsber. K. Preuss. Akad. Wiss., Phys.-Math. Kl. 189-196 (1916).
1918-1923 – N. Bohr formulated the Correspondence Principle, which states that quantum mechanics should reproduce classical physics in the limit of large quantum numbers (or, equivalently, as h → 0).
N. Bohr, “Über die Serienspektra der Elemente”, Zeitschrift für Physik 2 (5): 423–478 (1920)
N. Bohr, “Über die Anwendung der Quantentheorie auf den Atombau I. Die Grundpostulate der Quantentheorie.” Zeitschrift für Physik Volume 13, Number 1: 117-165.
1919 – Ernest Rutherford experimentally verified the existence of protons.
1921 – Chadwick and Bieler determine that the force holding nuclei together must be strong compared to the electromagnetic force.
J. Chadwick, E.S. Bieler, “Collisions of α Particles with Hydrogen Nuclei“, Phil. Mag. 42 (1921) 923
1921 – The introduction of Kaluza-Klein theory, an early attempt to unify gravitation with electromagnetism.
Kaluza, Theodor (1921). “Zum Unitätsproblem in der Physik”. Sitzungsber. Preuss. Akad. Wiss. Berlin. (Math. Phys.) 1921: 966–972.
Klein, Oskar (1926). “Quantentheorie und fünfdimensionale Relativitätstheorie”. Zeitschrift für Physik a Hadrons and Nuclei 37 (12): 895–906. doi:10.1007/BF01397481
[English translations of both articles can be found in Modern Kaluza-Klein Theories by Applequist, Chodos, and Freund, (Addison-Wesley, 1987). Many thanks to Prof. Matt Strassler for pointing this out.]
1922 – Otto Stern and Walther Gerlach performed the Stern-Gerlack experiment, demonstrating that particle spin is quantized. (See the 2003 Physics Today article, “Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics.”)
Gerlach, W.; Stern, O. (1922). “Das magnetische Moment des Silberatoms”. Zeitschrift für Physik 9: 353–355. doi:10.1007/BF01326984
Stern, O. (1921). “Ein Weg zur experimentellen Pruefung der Richtungsquantelung im Magnetfeld”. Zeitschrift für Physik 7: 249–253. doi:10.1007/BF01332793
1922-1924 – Friedmann Cosmology – Alexander Freidman found a solution to Einstein’s general relativity field equations for an expanding universe, predating the theoretical work of Lemaître and the observations of Edwin Hubble.
A. A. Friedman, “Uber die Krümmung des Raumes”, Zeitschrift fur Physik, 10:377-387 (1922).
Friedman, A. (1924). “Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes”. Zeitschrift für Physik 21 (1): 326–332. doi:10.1007/BF01328280.
1922-1923 – Using X-ray scattering experiments, Arthur Compton discovered what is now known as the “Compton effect,” an inelastic scattering of photons in matter.
1923-24 – Louis de Broglie posited his theory of matter waves, paving the way for the birth of modern quantum mechanics. de Broglie suggested that an electron possesses a wavelike aspect with a wavelength equal to Planck’s constant divided by the electron’s momentum.
The Birth of Modern Quantum Mechanics (1924-1941)
In this period, quantum theory was mathematically formalized.
1924-1927 – The Copenhagen interpretation of QM began to form. Much of the consensus regarding the Copenhagen interpretation arose from discussions held in 1927 at the Fifth Solvay International Conference on Electrons and Photons and the accompanying Bohr-Einstein debates.
Principles of the Copenhagen Interpretation
- A wave function ψ completely describes a system and represents an observer’s knowledge of the system. (Heisenberg)
- Rather than being deterministic (as was the case in Newtonian mechanics, quantum descriptions of nature are probabilistic. The probability of an event is given by the square of the amplitude of the wave function related to it. (Born rule, due to Max Born)
- Heisenberg’s uncertainty principle states that it is not possible to know the values of all of the properties of the system simultaneously with arbitrary precision.
- Complementarity principle: matter exhibits a wave-particle duality. Matter can exhibit particle-like properties or wave-like properties (depending upon how it is observed), but not both at the same time. (Niels Bohr)
- Measuring devices should be treated as classical devices. They measure classical properties such as position and momentum, which represent probabilistic expectation values of the corresponding quantum properties.
- The correspondence principle of Bohr and Heisenberg: the quantum mechanical description of large systems should converge upon the classical description when taken to the classical limit (as h → 0).
1924 – Satyendra Nath Bose derived Planck’s quantum radiation law without making any reference to classical physics. His approach paved the way for Bose-Einstein statistics, the theoretical description of Bose-Einstein condensates. Bosons, the class of particles obeying Bose-Einstein statistics, are named in his honor. Initially, journals refused to publish his work since he was interpreting what appeared to be an error as an indication of new physics, but a letter from Einstein to Zeitschrift für Physik persuaded them to publish Bose’s paper.
A. Einstein, “Quantentheorie des einatomigen idealen Gases”, Sitzungsber. Kgl. Preuss. Akad. Wiss., 261 (1924), 3 (1925).
1924 – Max Born coined the term “quantum mechanics.”
M. Born, “Über Quantenmechanik,” Z. Phys. 26, 379–395 (1924).
1924-1925 – Walther Bothe and Hans Geiger performed Compton scattering experiments with X-rays and coincidence detection to show that the X-rays were truly discrete particles.
W. Bothe and H. Geiger, Z.Physik 32 (1925) 639-663.
1925 – Wolfgang Pauli formulated his exclusion principle, which holds that no two identical fermions may simultaneously occupy the same quantum state. This principle forms the foundation of chemistry, dictating the chemical interactions of the various elements.
W. Pauli, “Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren” (”On the Connexion between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra“), Z. Phys. 31:765 (1925).
1925-1926 – Werner Heisenberg, Max Born, and Pascual Jordan formulated the matrix formalism of quantum mechanics, and Erwin Schrödinger invented wave mechanics and constructed the non-relativistic Schrödinger equation (built upon de Broglie’s theory). Schrödinger later showed that matrix mechanics and wave mechanics are equivalent.
W. Heisenberg, “Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen” (”Quantum-Theoretical Re-Interpretation of Kinematic and Mechanical Relations”), Z. Phys. 33:879 (1925).
M. Born, W. Heisenberg, and P. Jordan, “Zur Quantenmechanik II” (“On Quantum Mechanics II“), Z. f. Phys. 35, 557-615 (1926). (This paper is frequently referred to as the Dreimännerarbeit, or “Three-Man Paper,” and marks the birth of quantum matrix mechanics.)
M. Born, “Zur Quantenmechanik der Stoßvorgänge” (”Quantum Mechanics of Collision”), Z. Phys. 37:863 (1926).
(It is in this paper that Born put forth the Born Rule.)
E. Schrödinger, “Quantizierung als Eigenwertproblem (Erste Mitteilung)” (”Quantization as an Eigenvalue Problem. Part I.”), Annalen der Physik., 79:361-76 (1926).
E. Schrödinger, “Quantizierung als Eigenwertproblem (Zweite Mitteilung)” (”Quantization as an Eigenvalue Problem. Part II.”), Ann. Phys. 79: 489-527 (1926).
E. Schrödinger, “Der stetige Übergang von der Mikro-zur Makromechanik, Die Naturwissenschaften” (“The Science of the Continuous Transition from Micro- to Macro-mechanics), 14. Jahrg. Heft 28:664-666 (1926).
E. Schrödinger, “Über das Verhältnis der Heisenberg-Born-Jordanschen Quantenmechanik zu der meinen” (”On the Relation Between the Quantum Mechanics of Heisenberg, Born, and Jordan, and that of Schrödinger”), Ann. Phys. 79:734-56 (1926).
E. Schrödinger, “Quantisierung als Eigenwertproblem (Dritte Mitteilung)” (“Quantization as an Eigenvalue Problem. Part III.”), Ann. Phys. 80:437-90 (1926).
E. Schrödinger, Quantisierung als Eigenwertproblem (Vierte Mitteilung) (“Quantization as an Eigenvalue Problem. Part IV.”), Ann. Phys. 81:109-39 (1926).
1926 – Enrico Fermi and P.A.M. Dirac developed Fermi-Dirac statistics, which govern the behavior of fermions (particles with fractional spin).
E. Fermi, “Zur Quantelung des Idealen Einatomigen Gases” (”On Quantizating an Ideal Monatomic Gas“), Z. Phys. 36, 902 (1926).
1926 – Gilbert N. Lewis coined the term “photon.”
1927 – Clinton Davisson and Lester Germer performed the Electron diffraction experiment, an analog to Young’s double slit experiment, demonstrating the wavelike nature of electrons.
1927 – Monsignor George Lemaître, a Belgian astronomer and Roman Catholic priest, proposed that the universe is in a state of expansion.
Lemaître, G. (1927). “Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques”. Annals of the Scientific Society of Brussels 47A: 41.(French)
(Translated in: G. Lemaître, “A Homogeneous Universe of Constant Mass and Growing Radius Accounting for the Radial Velocity of Extragalactic Nebulae”. Monthly Notices of the Royal Astronomical Society 91: 483–490. 1931.)
1927 – Heisenberg formulated the Uncertainty Principle:
1927 – Publication of Ehrenfest’s Theorem (represented below in the later Dirac bra-ket notation), which demonstrates computation of the time derivative of the expectation value for a quantum operator by taking the commutator of that operator with the Hamiltonian of the system.
P. Ehrenfest, “Bemerkung über die angenaherte Gultigkeit der klassischen Mechanik innerhalb der Quantenmechanik” (“Remarks on the approximate validity of classical mechanics in quantum mechanics”). Z. Phys. 45:455 (1927).
1927 – Wolfgang Pauli modified Schrödinger’s equation to include spin, resulting in the Pauli Equation.
Wolfgang Pauli (1927) Zur Quantenmechanik des magnetischen Elektrons Zeitschrift für Physik (43) 601-623
1927-1930 – Dirac constructed a relativistic version of Schrödinger’s equation now known as the Dirac Equation, thus unifying QM with Special Relativity. This provided a description of spin-1/2 particles and predicted the existence of the positron. Dirac’s 1930 textbook also made extensive use of operator theory.
The Dirac equation:
P.A.M. Dirac (1927). “The Quantum Theory of the Emission and Absorption of Radiation”.Proceedings of the Royal Society of London A 114 (767): 243–265. Bibcode1927RSPSA.114..243D. doi:10.1098/rspa.1927.0039 (which marks the origin of the theory of quantum electrodynamics, or QED, which at this point is plagued with divergences, a problem later solved by Feynman, Schwinger, and Tomonaga.)
P.A.M. Dirac, Principles of Quantum Mechanics, Oxford: Clarendon Press (1930)
1929 – Edwin Hubble put forth his analysis of the redshift of distant celestial objects, laying the foundations for the Big Bang Theory.
1930 – Wolfgang Pauli postulated the existence of neutrinos in order to preserve conservation of energy, momentum, and angular momentum in beta decay reactions. He originally called this particle a neutron, but that name was appropriated in 1932 by James Chadwick when he discovered the more massive particle now known by that name. Enrico Fermi coined the name neutrino in 1934 in the course of developing his theory of beta decay.
1931 – In light of Hubble’s work, Lemaître expanded upon his earlier work and posited the Big Bang Theory. (That appellation was later coined by Fred Hoyle as a pejorative. Hoyle advocated a steady state cosmology.)
1931 – Dirac proposed the existence of magnetic monopoles as a way of explaining charge quantization. To date, no magnetic monopoles have been detected experimentally. In the same paper, he predicted (based upon interpretation of his eponymous equation) the existence of positrons.
1932 – James Chadwick discovered the neutron.
Chadwick, J., “Possible Existence of a Neutron“. Nature 129 (1932) 312.
Chadwick, J., “The Existence of a Neutron”. Proc. Roy. Soc A136 (1932) 692.
1933 – Carl Anderson publishes confirmation of the experimental detection of the positron predicted from the Dirac Equation.
Carl D. Anderson, “The positive electron”, Physical Review 13 March 1933, vol. 43, p491.
1932-1949 – John von Neumann formulated the mathematical basis for QM in terms of operator algebras.
P. Jordan, J. von Neumann & E. Wigner: On an algebraic generalization of the quantum mechanical
formalism. Ann. Math. 35, 29-64 (1934)
Neumann, J. von, 1937, “Quantum Mechanics of Infinite Systems”, first published in (Rédei and Stöltzner 2001, 249-268). [A mimeographed version of a lecture given at Pauli’s seminar held at the Institute for Advanced Study in 1937, John von Neumann Archive, Library of Congress, Washington, D.C.]
Ibid., 1955, Mathematical Foundations of Quantum Mechanics, Princeton, NJ: Princeton University Press. [First published in German in 1932: Mathematische Grundlagen der Quantenmechank, Berlin: Springer.]
F.J. Murray & J. von Neumann: On rings of operators i, ii, iv. Ann. Math. 37, 116-229 (1936),
Trans. Amer. Math. Soc. 41, 208-248 (1937), Ann. Math. 44, 716-808 (1943)
J. von Neumann: On rings of operators iii, v. Ann. Math. 41, 94-161 (1940), ibid. 50, 401-485
1934 – Enrico Fermi published his theory of beta decay.
Fermi, E. (1934). “Versuch einer Theorie der β-Strahlen. I”. Zeitschrift für Physik 88 (3–4): 161-177. Bibcode1934ZPhy…88..161F. doi:10.1007/BF01351864 (English translation: “Attempt at a Theory of β Rays”)
1935 – Chandrasekhar presented his analysis of the dynamics of white dwarfs and black holes.
1935 – A quantum field theory for nuclear interactions associated with the strong nuclear force was proposed by Hideki Yukawa, specifically including a mechanism for beta decay. The Yukawa interaction was later also used to describe coupling with the Higgs field. The Yukawa interaction forms the foundation for later work in Gauge Theory, such as Yang-Mills Theory and Electroweak Theory, ultimately culminating in the Standard Model.
1935 – Erwin Schrödinger, in a response to the EPR paradox, giving us the Schrödinger’s cat paradox.
Schrödinger, Erwin (November 1935). “Die gegenwärtige Situation in der Quantenmechanik (The present situation in quantum mechanics)”. Naturwissenschaften. 23: pp.807-812; 823-828; 844-849 (English translation)
1935 – Euler and Heisenberg published a famous analysis of Dirac’s theories.
1937-1947 – The muon was discovered in cosmic rays, but initially mistaken for Yukawa’s predicted “meson.” It was not until the 1947 that this mistake was recognized, prompting I.I. Rabi’s famous comment, “Who ordered that?”
Neddermeyer, S.H.; Anderson, C.D.;
“Note on the Nature of Cosmic-Ray Particles”
Phys. Rev. 51 (1937) 884
Street, J.C.; Stevenson, E.G.;
“Penetrating Corpuscular Component of the Cosmic Radiation”
Phys. Rev. 51 (1937) 1005
Street, J.C.; Stevenson, E.G.;
“New Evidence for the Existence of a Particle of Mass Intermediate Between the Proton and Electron”
Phys. Rev. 52 (1937) 1003;
Nishina, Y.; Takeuchi, M.; Ichimiya, T.;
“On the Nature of Cosmic-Ray Particles”
Phys. Rev. 52 (1937) 1198;
Rossi, B.; Van Norman Hilbery, H.; Hoag, J.B.;
“The Disintegration of Mesotrons”
Phys. Rev. 56 (1939) 837;
Williams, E.J.; Roberts, G.E.;
“Evidence for Transformation of Mesotrons into Electrons“
Nature 145 (1940) 102;
“Mean Life of Slow Mesotrons“
Phys. Rev. 59 (1941) 613;
“Disintegration of Slow Mesotrons“
Phys. Rev. 60 (1941) 198;
Rossi, B.; Nereson, N.;
“Experimental Determination of the Disintegration Curve of Mesotrons“
Phys. Rev. 62 (1942) 417;
Nereson, N.; Rossi, B.;
“Further Measurements on Disintegration Curve of Mesotrons“
Phys. Rev. 64 (1942) 199;
Conversi, M.; Pancini, E.; Piccioni, O.;
“On the Disintegration of Negative Mesons“
Phys. Rev. 71 (1947) 209;
1939 – Weisskopf demonstrated ultraviolet divergences in the higher-order terms of Dirac’s QED theory.
1939 – Dirac introduced “bra-ket” notation, which was subsequently incorporated into the 3rd edition of his 1930 textbook.
Towards a Theory of Everything (1942-present)
This period witnessed the growth and development of modern quantum field theory, and its offshoots quantum electrodynamics (QED) and quantum chromodynamics (QCD), electroweak theory, the Standard Model, and efforts at unification with the goal of creating a “theory of everything.” Much of this development was driven by the embrace of the path integral picture of quantum mechanics, gauge theory, and group theory.
1942 – Feynman’s PhD dissertation, The Principle of Least Action in Quantum Mechanics laid the groundwork for overcoming earlier issues with divergence in Dirac’s QED theory.
1943 – Gelfand and Naimark reformulated quantum mechanics in terms of C*-algebras.
I.M. Gelfand & M.A. Naimark: On the imbedding of normed rings into the ring of operators in
Hilbert space. Mat. Sbornik 12, 197-213 (1943)
1946-1948 – Japanese physicist Sin-Itiro Tomonaga published a series of papers on a formulation of QED which was relativistically invariant and eliminated ultraviolet divergences via renormalization. A few weeks after Feynman and Schwinger reported their own efforts in this area at the Pocono conference in March 1948, Robert Oppenheimer received a letter from Tomonaga describing his work and immediately recognized the overlap in their efforts.
Z. Koba, S. Tati & S. Tomonaga (1947). “On a relativistically invariant formulation of the quantum theory of wave fields II”. Progress of Theoretical Physics 2 (3): pp. 101-116. doi: 10.1143/PTP.2.101
Z. Koba, S. Tati & S. Tomonaga (1947). “On a relativistically invariant formulation of the quantum theory of wave fields III”. Progress of Theoretical Physics 2 (4): pp. 198-208. doi: 10.1143/PTP.2.198
D. Ito, Z. Koba & S. Tomonaga (1947). “Correction due to the reaction of ‘cohesive force field’ for the elastic scattering of an electron”. Progress of Theoretical Physics 2 (4): pp. 216-217. doi: 10.1143/PTP.2.216
1947 – Lamb and Retherford experimentally demonstrated a shifting in the spectra of hydrogen atoms which was inconsistent with the Dirac-Heisenberg-Pauli theory. On the train ride home from the Shelter Island conference where these results were announced, Hans Bethe performed a quick calculation of appropriate radiative corrections to account for the shift, an early form of renormalization. More info here and here.
1948 – Foley and Kusch experimentally measured anomalous values for the magnetic moment of the electron. Schwinger then used a renormalization technique similar to Bethe’s to perform new calculations for the value of the magnetic moment, and found the results to be in good agreement with the experimental results.
1948-1949 – Julian Schwinger published a series of articles describing his formulation of QED.
J. Schwinger (1949). “Quantum electrodynamics III: the electromagnetic properties of the electron – radiative corrections to scattering”, Physical Review 76 (6): pp. 790-817. doi: 10.1103/PhysRev.76.790
1948-1950 – Richard Feynman published a series of articles describing his formulation of Quantum Electrodynamics (QED), a subset of quantum field theory describing interactions between photons and electrically-charged particles based upon the path integral formulation of quantum mechanics. Notable features of QED include the use of Feynman diagrams to assist in constructing the calculations, and the use of renormalization to avoid divergences in the calculations.
P. Feynman, “Space-Time Approach to Non-Relativistic Quantum Mechanics“, Rev. of Mod. Phys. 20:367 (1948). (This was essentially a published version of Feynman’s dissertation.)
R.P. Feynman, “Mathematical Formulation of the Quantum Theory of Electromagnetic Interaction” Phys. Rev. 80, pp. 440-457 (1950) (This paper constituted an attempt by Feynman to provide his approach with mathematical rigor and to explain why it worked.)
1948 – The Big Bang nucleosynthesis theory was proposed to describe the formation of bulk of existing hydrogen and helium in the universe during the Big Bang.
1949 – Freeman Dyson demonstrated that Tomonaga, Schwinger, and Feynman’s approaches to QED were equivalent (despite Feynman’s diagrammatic approach being qualitatively different). Dyson then went on to show that the only quantities needing to be renormalized were mass, change, and the wave function.
1949 – Victor Weisskopf published an overview of developments in quantum theory.
1950 – John Clive Ward demonstrated that all ultraviolet divergences in QED are eliminated by renormalization.
WARD, J. C. An Identity in quantum electrodynamics, Physical Review, Vol. 78, No. 2 (1950), p. 182. DOI: 10.1103/PhysRev.78.182
1954 – Locally gauge invariant Yang-Mills Theory was introduced, based in part upon Pauli’s unpublished efforts to apply Kaluza-Klein theory to QED and strong interactions. Initial versions of Yang-Mills failed to properly describe strong interactions since all bosons predicted by the theory were massless. This, however, was later corrected by the introduction of symmetry breaking concepts to the theory.
1955 – Clyde L. Cowan and Frederick Reines provided experimental confirmation of the existence of the neutrino.
C.L Cowan Jr., F. Reines, F.B. Harrison, H.W. Kruse, A.D McGuire (July 20, 1956). “Detection of the Free Neutrino: a Confirmation”. Science 124 (3212): 103–4. doi:10.1126/science.124.3212.103.PMID 17796274
1957 – Everett introduced the “Many worlds” interpretation of QM.
1961 – Murray Gell-Mann proposed “The Eightfold Way,” a system of organizing baryons and mesons.
1960-1962 Nambu-Goldstone bosons were postulated initially in the context of BCS superconductivity theory, then later generalized to quantum field theory. These bosons arise in models involving the spontaneous breakdown of continuous symmetries.
1961 – Clauss Jönsson performed the double-slit experiment with electrons, re-affirming the results of the Davisson-Germer experiment.
Jönsson C,(1961) Zeitschrift für Physik, 161:454–474
1963 – The Kerr metric, which describes the geometry of spacetime near a massive spinning body, was introduced.
1964 – Murray Gell-Mann and George Zweig both proposed the Quark model.
G. Zweig, “An SU3 model for strong interaction symmetry and its breaking“, Developments in the Quark Theory of Hadrons, pp. 22-101.
1964 – Higgs proposed a mechanism for boson mass generation via spontaneous symmetry breaking.
1965 – Penzias and Wilson detected the Cosmic Microwave Background radiation field, the “afterglow” of the Big Bang.
Sakharov, A.D. (1967). “Violation of CP Invariance, C Asymmetry and Baryon Asymmetry of the Universe”. Zhurnal Eksperimentalnoi i Teoreticheskoi Fiziki, Pisma 5: 32. (Russian)
1967 – Steven Weinberg introduced Electroweak Theory, unifying the electromagnetic and weak nuclear forces.
1968-1970 – String Theory was proposed.
1970 – Quantum Decoherence was introduced as a potential description of the emergence of classical behavior out of quantum systems, yielding the appearance of wave function collapse.
1971 – Gerard t’ Hooft analyzed the issue of determining the renormalizability of gauge fields using massless Yang-Mills fields as an example.
1973 – Asymptotic freedom in quark interactions (and in non-Abelian gauge theories in general) was described.
1974 – String theory was applied to quantum gravity.
1974 – The Standard Model of Physics fell into place: all fundamental forces of nature, except for gravity, were shown to be describable via quantum field theory (Electro-weak theory and QCD), with the hope of eventually unifying them in a Grand Unified Theory describing them as manifestations of the same fundamental interaction.
1975 – Stephen Hawking proposed a model for the emission of radiation from black holes, now commonly referred to as Hawking radiation.
1980 – Klaus von Klitzing discovered the Quantum Hall effect.
Klitzing, K. von; Dorda, G.; Pepper, M. (1980). “New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance”. Phys. Rev. Lett. 45 (6): 494–497.doi:10.1103/PhysRevLett.45.494
1981 – Alan Guth proposed Inflation Theory to resolve several problems associated with Big Bang cosmology.
1981 – Superstring theory (supersymmetric string theory) was introduced
1982 – Linde proposed what is known as chaotic inflation theory.
1982 – Alain Aspect experimentally verified quantum entanglement.
A. Aspect, P. Grangier, and G. Roger (1982). “Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell’s Inequalities”. Physical Review Letters 49 (2): 91–94.doi:10.1103/PhysRevLett.49.91
1986 – Ashtekar created a formulation of general relativity meant to simplify its integration with quantum theory.
1990 – A competitor to string theory appeared on the scene with the introduction of Loop Quantum Gravity.
1993 – ‘t Hooft introduced the Holographic Principle, an outgrowth of black hole thermodynamics.
(Subsequent experimental confirmations:)
D. Boschi; S. Branca, F. De Martini, L. Hardy, and S. Popescu (1998). “Experimental Realization of Teleporting an Unknown Pure Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels”. Phys. Rev. Lett.. pp. 80, 1121.
1995 – Formulation of M-theory, an extension of string theory utilizing 11 dimensions.