by Manjit Kumar
Although he never managed to deliver a decisive blow in his encounters with Bohr, Einstein’s challenge was sustained and thought-provoking. It encouraged men like Bohm, Bell and Everett to probe and evaluate Bohr’s Copenhagen interpretation when it was all-prevailing and few distinguished theory from interpretation. The Einstein-Bohr debate about the nature of reality was the inspiration behind Bell’s theorem. The testing of Bell’s inequality directly or indirectly helped spawn new areas of research including quantum cryptography, quantum information theory, and quantum computing. Among the most remarkable of these new fields is quantum teleportation, which exploits the phenomena of entanglement. Although it appears to belong to the realm of science fiction, in 1997 not one but two teams of physicists succeeded in teleporting a particle. The particle was not physically transported, but its quantum state was transferred to a second particle located elsewhere, thereby effectively teleporting the initial particle from one place to another.
After having been marginalised during the last 30 years of his life because of his criticism of the Copenhagen interpretation and his attempts to slay his quantum demon, Einstein has been vindicated, in part. Einstein versus Bohr had little to do with the equations and numbers generated by the mathematics of quantum mechanics. What does quantum mechanics mean? What does it say about the nature of reality? It was their answers to these types of questions that separated the two men. Einstein never put forward an interpretation of his own, because he was not trying to shape his philosophy to fit a physical theory. Instead he used his belief in an observer-independent reality to assess quantum mechanics and found the theory wanting.
In December 1900, classical physics had a place for everything and almost everything in its place. Then Max Planck stumbled across the quantum, and physicists are still struggling to come to terms with it. Fifty long years of ‘conscious brooding’, said Einstein, had not brought him any closer to understanding the quantum.39 He kept trying to the end, taking solace in the words of the German playwright and philosopher Gotthold Lessing: ‘The aspiration to truth is more precious than its assured possession.’40
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 X-rays.
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 light-quanta 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 Franck-Hertz 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.
August: The First World War begins.
October: Bohr returns to work at Manchester Univers
ity. Planck and Röntgen are among the signatories of the Manifesto of the Ninety-Three, 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 six-months’ 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 X-ray photons by atomic electrons is published. The ‘Compton effect’, as it became known, is taken as irrefutable evidence in support of Einstein’s 1905 light-quanta 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 wave-particle duality to incorporate matter.
October: Heisenberg becomes Born’s assistant in Göttingen. Pauli returns to Hamburg after a year-long 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 light-quanta 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 wave-particle 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 all-important first steps towards matrix mechanics, his version of the much sought-after theory of quantum mechanics.
September: Heisenberg’s first ground-breaking paper on matrix mechanics, ‘On a quantum-Theoretical 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. A veritable 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 Bohr-Sommerfeld hydrogen atom.
February: The three-man 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.
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 two-hour 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 question-and-answer 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 wave-particle 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 month-long skiing holiday in Norway. In Bohr’s absence, Heisenberg discovers the uncertainty principle.
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 Einstein-Bohr 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.