Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality  Manjit Kumar (2009)
TIMELINE
1858 
23 April: Max Planck is born in Kiel, Germany. 
1871 
30 August: Ernest Rutherford is born in Spring Grove, New Zealand. 
1879 
14 March: Albert Einstein is born in Ulm, Germany. 
1882 
11 December: Max Born is born in Breslau, Silesia, Germany. 
1885 
7 October: Niels Bohr is born in Copenhagen, Denmark. 
1887 
12 August: Erwin Schrödinger is born in Vienna, Austria. 
1892 
15 August: Louis de Broglie is born in Dieppe, France. 
1893 
February: Wilhelm Wien discovers the displacement law for blackbody radiation. 
1895 
November: Wilhelm Röntgen discovers Xrays. 
1896 
March: Henri Becquerel discovers that uranium compounds emit previously unknown radiation that he calls 'uranic rays'. 
June: Wien publishes a distribution law for blackbody radiation that is in agreement with the available data. 

1897 
April: J.J. Thomson announces the discovery of the electron. 
1900 
25 April: Wolfgang Pauli is born in Vienna, Austria. 
July: Einstein graduates from the Federal Polytechnikum in Zurich. 

September: The breakdown of Wien's distribution law is confirmed beyond any doubt in the far infrared part of the blackbody spectrum. 

October: Planck announces his blackbody radiation law at a meeting in Berlin of the German Physical Society. 

14 December: Planck presents the derivation of his blackbody radiation law in a lecture to the German Physical Society. The introduction of the quantum of energy is barely noticed. At best, it is deemed to be a theorist's sleight of hand to be eliminated later. 
1901 
5 December: Werner Heisenberg is born in Würzburg, Germany. 
1902 
June: Einstein begins work as an 'Expert Class III' at the Patent Office in Bern, Switzerland. 
8 August: Paul Dirac is born in Bristol, England. 

1905 
June: Einstein's paper on the existence of lightquanta and the photoelectric effect is published in Annalen der Physik. 
July: Einstein's paper explaining Brownian motion is published in Annalen der Physik. 

September: Einstein's paper 'On the Electrodynamics of Moving Bodies', outlining his special theory of relativity, is published in Annalen der Physik. 

1906 
January: Einstein receives his PhD from Zurich University at the third attempt with a thesis entitled 'A New Determination of Molecular Dimensions'. 
April: Einstein is promoted to 'Expert Class II' at the Patent Office in Bern. 

September: Ludwig Boltzmann commits suicide while on holiday near Trieste, Italy. 

December: Einstein's paper on the quantum theory of specific heat is published in Annalen der Physik. 

1907 
May: Rutherford takes up the post of professor and head of physics at Manchester University. 
1908 
February: Einstein becomes privatdozent at Bern University. 
1909 
May: Einstein is appointed extraordinary professor of theoretical physics at Zurich University, effective the following October. 
September: Einstein delivers the keynote lecture at the annual meeting of the Gesellschaft Deutscher Naturforscher und Ärzte, held that year in Salzburg, Austria. Einstein says that 'the next stage in the development of theoretical physics will bring us a theory of light that may be conceived of as a sort of fusion of the wave and of the emission theory of light'. 

December: Bohr receives his Master's degree from Copenhagen University. 
1911 
January: Einstein is appointed to full professorship at the German University in Prague. The appointment begins in April 1911. 
March: Rutherford announces the discovery of the atomic nucleus at a meeting in Manchester, England. 

May: Bohr receives his doctorate from Copenhagen University with a thesis on the electron theory of metals. 

September: Bohr arrives at Cambridge University to begin postgraduate work with J.J. Thomson. 

30 October–4 November: The first Solvay conference is held in Brussels. Einstein, Planck, Marie Curie and Rutherford are among the invited participants. 

1912 
January: Einstein is appointed professor of theoretical physics at the Eidgenossische Technische Hochschule (ETH) in Zurich, the new name for the Federal Polytechnikum where he was a student. 
March: Bohr transfers from Cambridge to Rutherford's laboratory at Manchester University. 

September: Bohr is appointed privatdozent and assistant to the professor of physics at Copenhagen University. 

1913 
February: Bohr hears about Balmer's formula for the spectral lines of hydrogen for the first time, a vital clue as he develops the quantum model of the atom. 
July: The first in a trilogy of papers by Bohr on the quantum theory of the hydrogen atom is published in the Philosophical Magazine. Planck and Walther Nernst travel to Zurich to entice Einstein to Berlin. He accepts their offer. 

September: Bohr presents his new theory of the quantum atom at the British Association for the Advancement of Science (BAAS) conference in Birmingham, England. 

1914 
April: The FranckHertz experiment confirms Bohr's concept of quantum jumps and atomic energy levels. They bombard mercury vapour with electrons and measure the frequencies of the emitted radiation, which corresponds to the transitions between different energy levels. Einstein arrives in Berlin to take up professorships at the Prussian Academy of Sciences and Berlin University. 
1914 
August: The First World War begins. 
October: Bohr returns to work at Manchester University. Planck and Röntgen are among the signatories of the Manifesto of the NinetyThree, asserting that Germany bears no responsibility for the war, has not violated Belgian neutrality, and committed no atrocities. 

1915 
November: Einstein completes his general theory of relativity. 
1916 
January: Arnold Sommerfeld proposes a theory to explain the fine structure of the spectral lines in hydrogen and introduces a second quantum number as he replaces Bohr's circular orbits with elliptical orbits. 
May: Bohr is appointed professor of theoretical physics at Copenhagen University. 

July: Einstein returns to work on quantum theory and discovers the phenomena of spontaneous and induced emission of a photon from an atom. Sommerfeld adds the magnetic quantum number to Bohr's original atomic model. 

1918 
September: Pauli leaves Vienna to study at Munich University with Arnold Sommerfeld. 
November: The First World War ends. 

1919 
November: Planck is awarded the 1918 Nobel Prize for physics. At a joint meeting of the Royal Society and the Royal Astronomical Society in London, an official announcement is made that Einstein's prediction that light is deflected by a gravitational field was confirmed by measurements made by two British expeditions during a solar eclipse in May. Einstein becomes a global celebrity overnight. 
1920 
March: Sommerfeld introduces a fourth quantum number. 
April: Bohr visits Berlin and meets Planck and Einstein for the first time. 

August: A public rally at the Berlin Philharmonic Hall against relativity theory. An angry Einstein replies to his critics in a newspaper article. He visits Bohr in Copenhagen for the first time. 

October: Heisenberg enrols to study physics at Munich University and meets fellow student Wolfgang Pauli. 
1921 
March: With Bohr as its founder and director, the Institute for Theoretical Physics in Copenhagen is officially opened. 
April: Born arrives in Göttingen from Frankfurt as professor and director of the institute of theoretical physics, determined to make it the equal of Sommerfeld's institute in Munich. 

October: After obtaining his doctorate from Munich University, Pauli becomes Born's assistant in Göttingen. 

1922 
April: Preferring city life to that in a small, provincial university town, Pauli leaves Göttingen to take up an assistant's position at Hamburg University. 
June: Bohr gives a series of celebrated lectures in Göttingen on atomic theory and the periodic table. At this 'Bohr Festspiele', Heisenberg and Pauli meet the Dane for the first time. Bohr is deeply impressed by both young men. 

October: Heisenberg begins a sixmonths' sojourn in Göttingen with Born. Pauli arrives in Copenhagen to be Bohr's assistant until September 1923. 

November: Einstein is awarded the 1921 Nobel Prize and Bohr the prize for 1922. 

1923 
May: Arthur Compton's comprehensive report concerning his discovery of the scattering of Xray photons by atomic electrons is published. The 'Compton effect', as it became known, is taken as irrefutable evidence in support of Einstein's 1905 lightquanta hypothesis. 
July: Einstein's second visit to see Bohr in Copenhagen. Heisenberg just manages to obtain his doctorate from Munich University after poorly answering questions on experimental physics during his oral examination. 

September: De Broglie links waves with electrons as he extends waveparticle duality to incorporate matter. 

October: Heisenberg becomes Born's assistant in Göttingen. Pauli returns to Hamburg after a yearlong stay in Copenhagen. 
1924 
February: Bohr, Hendrik Kramers and John Slater propose that in atomic processes energy is only conserved statistically, in an attempt to counter Einstein's lightquanta hypothesis. The BKS idea is experimentally disproved in April–May 1925. 
March: Heisenberg pays his first visit to Bohr in Copenhagen. 

September: Heisenberg leaves Göttingen to work at Bohr's institute until May 1925. 

November: De Broglie successfully defends his doctoral thesis extending waveparticle duality to matter. Sent a copy of the thesis by de Broglie's supervisor, Einstein had earlier given it his nod of approval. 

1925 
January: Pauli discovers the exclusion principle. 
June: Heisenberg goes to the small island of Helgoland in the North Sea to recover from a severe bout of hay fever. During his stay he takes the allimportant first steps towards matrix mechanics, his version of the much soughtafter theory of quantum mechanics. 

September: Heisenberg's first groundbreaking paper on matrix mechanics, 'On a QuantumTheoretical Reinterpretation of Kinematics and Mechanical Relations', is published in the Zeitschrift für Physik. 

October: Samuel Goudsmit and George Uhlenbeck propose the concept of quantum spin. 

November: Pauli applies matrix mechanics to the hydrogen atom. Averitable tour de force, it is published in March 1926. 

December: While enjoying a secret rendezvous with a former lover in the Alpine ski resort of Arosa, Schrödinger constructs what will become his celebrated wave equation. 

1926 
January: Back in Zurich, Schrödinger applies his wave equation to the hydrogen atom and finds that it reproduces the series of energy levels of the BohrSommerfeld hydrogen atom. 
February: The threeman paper written by Heisenberg, Born and Pascual Jordan offering a detailed account of the mathematical structure of matrix mechanics is published after being submitted to the Zeitschrift für Physik in November 1925. 
1926 
March: Schrödinger's first paper on wave mechanics is published in the Annalen der Physik after being submitted in January. Another five papers follow in quick succession. Schrödinger and others prove that wave mechanics and matrix mechanics are mathematically equivalent. They are two forms of the same theory – quantum mechanics. 
April: Heisenberg delivers a twohour lecture on matrix mechanics attended by Einstein and Planck. Afterwards Einstein invites the young turk back to his apartment where the two of them discuss, Heisenberg recalled later, 'the philosophical background of my recent work'. 

May: Heisenberg is appointed Bohr's assistant and lecturer at Copenhagen University. As Bohr recovers from a severe case of flu, Heisenberg begins using wave mechanics to account for the spectral lines of helium. 

June: Dirac receives his PhD from Cambridge University with a thesis entitled 'Quantum Mechanics'. 

July: Born puts forward the probability interpretation of the wave function. Schrödinger delivers a lecture in Munich and during the questionandanswer session, Heisenberg complains about the shortcomings of wave mechanics. 

September: Dirac goes to Copenhagen and during his stay develops transformation theory, which shows that Schrödinger's wave mechanics and Heisenberg's matrix mechanics are special cases of a more general formulation of quantum mechanics. 

October: Schrödinger visits Copenhagen. He, Bohr and Heisenberg fail to reach any sort of accord over the physical interpretation of either matrix or wave mechanics. 

1927 
January: Clinton Davisson and Lester Germer obtain conclusive evidence that waveparticle duality also applies to matter as they succeed in diffracting electrons. 
February: After months of trying, tempers fray as Bohr and Heisenberg are no closer to developing a coherent physical interpretation of quantum mechanics. Bohr leaves on a monthlong skiing holiday in Norway. In Bohr's absence, Heisenberg discovers the uncertainty principle. 
1927 
May: The uncertainty principle is published after arguments between Heisenberg and Bohr over its interpretation. 
September: The Volta conference at Lake Como, Italy. Bohr presents his principle of complementarity and the central elements of what later became known as the Copenhagen interpretation of quantum mechanics. Born, Heisenberg and Pauli are among those present, but not Schrödinger or Einstein. 

October: At the fifth Solvay conference in Brussels, the EinsteinBohr debate begins over the foundations of quantum mechanics and the nature of reality. Schrödinger succeeds Planck as professor of theoretical physics at Berlin University. Compton is awarded the Nobel Prize for the discovery of the 'Compton effect'. Heisenberg, aged only 25, is appointed to a professorship at Leipzig University. 

November: George Thomson, son of J.J. Thomson, the discoverer of the electron, reports the successful diffraction of electrons employing a different technique than Davisson and Germer. 

1928 
January: Pauli is appointed professor of theoretical physics at the ETH in Zurich. 
February: Heisenberg delivers his inaugural lecture as professor of theoretical physics at Leipzig University. 

1929 
October: De Broglie receives the Nobel Prize for the discovery of the wave nature of the electron. 
1930 
October: The sixth Solvay conference in Brussels, the second round of the EinsteinBohr debate as Bohr refutes Einstein's 'clockinthebox' thought experiment challenging the consistency of the Copenhagen interpretation. 
1931 
December: The Danish Academy of Sciences and Letters selects Bohr as the next occupant of the Aeresbolig, 'The House of Honour', a mansion built by the founder of the Carlsberg breweries. 
1932 
John von Neumann's book The Mathematical Foundations of Quantum Mechanics is published in German. It contains his famous 'impossibility proof' – no hidden variables theory can reproduce the predictions of quantum mechanics. Dirac is elected Lucasian Professor of Mathematics at Cambridge University – a post once held by Isaac Newton. 
1933 
January: The Nazis seize power in Germany. Luckily, Einstein is in America as a visiting professor at the California Institute of Technology. 
March: Einstein publicly declares that he will not return to Germany. He resigns from the Prussian Academy of Sciences as soon as he arrives in Belgium and severs all links with official German institutions. 

April: The Nazis introduce the 'Law for the Restoration of the Career Civil Service', designed to target political opponents, socialists, communists, and the Jews. Paragraph 3 contains the infamous 'Aryan clause': 'Civil servants not of Aryan origin are to retire.' By 1936 more than 1,600 scholars would be ousted, a third of them scientists, including twenty who had been or would be awarded the Nobel Prize. 

May: 20,000 books are burned in Berlin, with similar bonfires of 'unGerman' works throughout the country. Although unaffected by Nazi regulations, unlike Born and many other colleagues, Schrödinger leaves Germany for Oxford. Heisenberg stays. The Academic Assistance Council, with Rutherford as its president, is set up in England to help refugee scientists, artists and writers. 

September: As fears over his safety increase, Einstein leaves Belgium for England. Paul Ehrenfest commits suicide. 

October: Einstein arrives in Princeton, New Jersey for a scheduled visit. Intending to stay for only a few months at the Institute for Advanced Study (IAS), Einstein never returns to Europe. 

November: Heisenberg receives the deferred 1932 Nobel Prize, while Dirac and Schrödinger share the prize for 1933. 

1935 
May: The Einstein, Podolsky and Rosen (EPR) paper, 'Can Quantum Mechanical Description of Physical Reality Be Considered Complete?', is published in the Physical Review. 
October: Bohr's reply to EPR is published in the Physical Review. 

1936 
March: Schrödinger and Bohr meet in London. Bohr says that it's 'appalling' and 'high treason' that Schrödinger and Einstein want to strike a blow against quantum mechanics. 
October: Born takes up a post as professor of natural philosophy at Edinburgh University after spending nearly three years at Cambridge and a few months in Bangalore, India. He stayed until his retirement in 1953. 
1937 
February: Bohr arrives in Princeton for a weeklong stay as part of a world tour. Einstein and Bohr discuss the interpretation of quantum mechanics facetoface for the first time since the publication of the EPR paper, but talk past each other as many things are left unsaid. 
July: Heisenberg is branded a 'white Jew' in an SS journal for teaching 'Jewish' physics such as Einstein's theory of relativity. 

October: Rutherford dies aged 66 in Cambridge after surgery for a strangulated hernia. 

1939 
January: Bohr arrives at the IAS as a visiting professor for the entire semester. Einstein avoids any discussions with Bohr, and during the next four months they meet only once at reception. 
August: Einstein signs a letter to President Roosevelt raising the possibility of making an atomic bomb and the danger of the Germans constructing such a weapon. 

September: The Second World War begins. 

October: Schrödinger arrives in Dublin after stints at the universities of Graz and Ghent. He remained in Dublin as senior professor at the Institute for Advanced Studies until 1956 when he returned to Vienna. 

1940 
March: Einstein sends a second letter to President Roosevelt concerning the atomic bomb. 
August: Pauli leaves wartorn Europe and joins Einstein at the Institute for Advanced Study in Princeton. He remained there until 1946 when he returned to Zurich and the ETH. 

1941 
October: Heisenberg visits Bohr in Copenhagen. Denmark had been occupied by German forces since April 1940. 
1943 
September: Bohr and his family escape to Sweden. 
December: Bohr visits Princeton to have dinner with Einstein and Pauli before heading to Los Alamos in New Mexico to work on the atomic bomb. It was the first meeting between Einstein and Bohr since the Dane's visit in January 1939. 

1945 
May: Germany surrenders. Heisenberg is arrested by Allied forces. 
August: Atomic bombs are dropped on Hiroshima and then Nagasaki. Bohr returns to Copenhagen. 

November: Pauli is awarded the Nobel Prize for the discovery of the exclusion principle. 
1946 
July: Heisenberg is appointed director of the Kaiser Wilhelm Institute for Physics in Göttingen, later renamed the Max Planck Institute. 
1947 
October: Planck dies in Göttingen aged 89. 
1948 
February: Bohr arrives at the IAS as a visiting professor until June. Relations with Einstein are more cordial than during previous visits as both men continue to disagree over the interpretation of quantum mechanics. In Princeton, Bohr writes an account of the debate with Einstein at the Solvay conferences of 1927 and 1930 as his contribution to a volume of papers to celebrate Einstein's 70th birthday in March 1949. 
1950 
February: Bohr is at the IAS until May. 
1951 
February: David Bohm publishes his book Quantum Theory. It contains a novel and simplified version of the EPR thought experiment. 
1952 
January: Two papers by Bohm are published in which he does what von Neumann said was impossible: he offers a hidden variables interpretation of quantum mechanics. 
1954 
September: Bohr is at the IAS until December. 
October: Bitterly disappointed at being overlooked when Heisenberg was honoured in 1932, Born is finally awarded the Nobel Prize for 'his fundamental work in quantum mechanics and especially for his statistical interpretation of the wave function'. 

1955 
April: Einstein dies in Princeton aged 76. After a simple ceremony, his ashes are scattered at an undisclosed location. 
1957 
July: Hugh Everett III puts forward the 'relative state' formulation of quantum mechanics, later known as the many worlds interpretation. 
1958 
December: Pauli dies in Zurich aged 58. 
1961 
January: Schrödinger dies in Vienna aged 73. 
1962 
November: Bohr dies in Copenhagen aged 77. 
1964 
November: John Bell's discovery that any hidden variables theory whose predictions agree with those of quantum mechanics must be nonlocal is published in a littleread journal. Known as Bell's inequality, it derives limits on the degree of correlation of the quantum spins of entangled pairs of particles that have to be satisfied by any local hidden variables theory. 
1966 
July: Bell shows conclusively that von Neumann's proof ruling out hidden variables theories, published in 1932 in his book The Mathematical Foundations of Quantum Mechanics, is flawed. Bell had submitted his paper to the journal Review of Modern Physics at the end of 1964, but an unfortunate series of mishaps delayed its publication. 
1970 
January: Born dies in Göttingen aged 87. 
1972 
April: John Clauser and Stuart Freedman at the University of California, Berkeley, having conducted the first test of Bell's inequality, report that it is violated – any local hidden variables cannot reproduce the predictions of quantum mechanics. However, there are doubts about the accuracy of their results. 
1976 
February: Heisenberg dies in Munich aged 75. 
1982 
After years of preliminary work, Alain Aspect and his collaborators at the Institut d'Optique Théoretique et Appliquée, Université ParisSud, subject Bell's inequality to the most rigorous test then possible. Their results show that the inequality is violated. Although certain loopholes remain to be closed, most physicists, including Bell, accept the results. 
1984 
October: Dirac dies in Tallahassee, Florida aged 82. 
1987 
March: De Broglie dies in France aged 94. 
1997 
December: A team at the University of Innsbruck led by Anton Zeilinger reports that it has succeeded in transferring the quantum state of a particle from one place to another – in effect, teleporting it. An integral part of the process is the phenomenon of quantum entanglement. A group at Rome University, under the leadership of Francesco DeMartini, also successfully carries out quantum teleportation. 
2003 
October: Anthony Leggett publishes a Belltype inequality derived on the basis that reality is nonlocal. 
2007 
April: An AustrianPolish team led by Markus Aspelmeyer and Anton Zeilinger announce that measurements of previously untested correlations between pairs of entangled photons show that Leggett's inequality is violated. The experiment rules out only a subset of possible nonlocal hidden variables theories. 
20?? 
A quantum theory of gravity? A Theory of Everything? A theory beyond the quantum? 