Quantum: Einstein, Bohr and the Great Debate About the Nature of Reality
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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 Einstein-Bohr debate as Bohr refutes Einstein’s ‘clock-in-the-box’ 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 ‘un-German’ 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 week-long stay as part of a world tour. Einstein and Bohr discuss the interpretation of quantum mechanics face-to-face 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 war-torn 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 non-local is published in a little-read 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 hid
den 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 Paris-Sud, 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 Bell-type inequality derived on the basis that reality is non-local.
2007
April: An Austrian-Polish 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 non-local hidden variables theories.
20??
A quantum theory of gravity? A Theory of Everything? A theory beyond the quantum?
GLOSSARY
Terms in italics have an entry in the glossary.
Alkali elements Elements such as lithium, sodium and potassium in group one of the periodic table that share the same chemical properties.
Alpha decay A process of radioactive decay in which the nucleus of an atom emits an alpha particle.
Alpha particle A subatomic particle consisting of two protons and two neutrons bound together. Emitted during alpha decay, it is identical to the nucleus of a helium atom.
Amplitude The maximum displacement of a wave or an oscillation that is equal to half the distance from the top of the wave (or oscillation) to the bottom. In quantum mechanics, the amplitude of a process is a number that is linked to the probability of that process occurring.
Angular momentum A property of a rotating object akin to the momentum of an object moving in a straight line. The angular momentum of an object depends on its mass, its size, and the speed with which it is spinning. One object orbiting another also possesses angular momentum that depends on its mass, the radius of its orbit, and its velocity. In the atomic realm, angular momentum is quantised. It can change only by amounts that are whole-number multiples of Planck’s constant divided by 2.
Atom The smallest component of an element consisting of a positively-charged nucleus surrounded by a bound system of negatively-charged electrons. Since an atom is neutral, the number of positively-charged protons in the nucleus is equal to the number of electrons.
Atomic number (Z) The number of protons in the nucleus of an atom. Every element has a unique atomic number. Hydrogen, with a single proton making up its nucleus and one electron orbiting it, has an atomic number of 1. Uranium, with 92 protons and 92 electrons, has an atomic number of 92.
Balmer series The set of emission or absorption lines in the spectrum of hydrogen caused by the transitions of its electron between the second and higher energy levels.
Bell’s inequality A mathematical condition derived by John Bell in 1964 concerning the degree of correlation of the quantum spins of entangled pairs of particles that has to be satisfied by any local hidden variables theory.
Bell’s theorem A mathematical proof discovered by John Bell in 1964 that any hidden variables theory whose predictions agree with those of quantum mechanics must be non-local. See non-locality.
Beta particle A fast moving electron ejected from the nucleus of a radioactive element due to the interconversion of protons and neutrons. Faster and more penetrating than alpha particles, it can be stopped by a thin sheet of metal.
Blackbody A hypothetical, idealised body that absorbs and emits all electromagnetic radiation that strikes it. It can be approximated in the laboratory as a heated box with a pinhole in one of its walls.
Blackbody radiation Electromagnetic radiation emitted by a blackbody.
Brownian motion The erratic motion of pollen grains suspended in a fluid first observed, in 1827, by Robert Brown. In 1905 Einstein explained that Brownian motion was due to the random buffeting of the pollen grains by the molecules of the fluid.
Causality Every cause has an effect.
Classical mechanics The name given to the physics that originates from Newton’s three laws of motion. Also called Newtonian mechanics, in which the properties of particles such as position and momentum are, in principle, simultaneously measurable with unlimited accuracy.
Classical physics The description applied to all non-quantum physics such as electromagnetism and thermodynamics. Although Einstein’s general theory of relativity is regarded by physicists as ‘modern’ twentieth-century physics, it is nevertheless a ‘classical’ theory.
Cloud chamber A device invented by C.T.R. Wilson around 1911 that enables the detection of particles by observing their tracks through a chamber containing saturated vapour.
Collapse of the wave function According to the Copenhagen interpretation, until it is observed or measured, a microphysical object like an electron does not exist anywhere. Between one measurement and the next it has no existence outside the abstract possibilities of the wave function. It is only when an observation or measurement is made that one of the ‘possible’ states of the electron becomes its ‘actual’ state and the probabilities of all the other possibilities become zero. This sudden, discontinuous change in the wave function due to an act of measurement is called the ‘collapse of the wave function’.
Commutativity Two variables A and B are said to commute if A×B=B×A. For example, if A and B are the numbers 5 and 4, then 5×4=4×5. Multiplication of numbers is commutative, since the order in which they are multiplied makes no difference. If A and B are matrices, then A×B does not necessarily equal B×A. When this happens, A and B are said to be non-commutative.
Complementarity A principle advocated by Niels Bohr that the wave and particle aspects of light and matter are complementary but exclusive. This dual nature of light and matter is like the two sides of the same coin that can display either face, but not both simultaneously. For example, an experiment can be devised to reveal either the wave properties of light or its particle nature, but not both at the same time.
Complex number A number written in the form a+ib, where a and b are ordinary real numbers familiar from arithmetic. i is the square root of –1, so that (–1)2 = –1, and b is called the ‘imaginary’ part of the complex number.
Compton effect The scattering of photons by atomic electrons discovered by the American physicist Arthur H. Compton in 1923.
Conjugate variables A pair of dynamical variables such as position and momentum, or energy and time, that are related to one another through the uncertainty principle, are called conjugate variables or conjugate pairs. Cons
ervation law A law which states that some physical quantity, such as momentum or energy, is conserved in all physical processes.
Conservation of energy The principle that energy cannot be created or destroyed, but can only be converted from one form to another. For example, when an apple falls from a tree, its potential energy is converted into kinetic energy.
Copenhagen interpretation An interpretation of quantum mechanics, whose principal architect Niels Bohr was based in Copenhagen. Over the years there were differences of opinion between Bohr and other leading advocates of the Copenhagen interpretation such as Werner Heisenberg. However, all agreed on its central tenets: Bohr’s correspondence principle, Heisenberg’s uncertainty principle, Born’s probability interpretation of the wave function, Bohr’s principle of complementarity, and the collapse of the wave function. There is no quantum reality beyond what is revealed by an act of measurement or observation. Hence it is meaningless to say, for example, that an electron exists somewhere independent of an actual observation. Bohr and his supporters maintained that quantum mechanics was a complete theory, a claim challenged by Einstein.