Origins of the quantum theory Sabina Chiriotti IFAE Thursday meeting, March 26th 2009

To know the Revolutionary impact of quantum physics one need first to look at pre-quantum physics: The old quantum theory

Max Planck 1900 : Max Plank introduced the concept of energy radiated in discrete quanta. Found  relationship between the radiation emited by a blackbody and its temperature. E=hѵ quanta of energy is proportional to the frequency with which the blackbody radiate assuming that energies of the vibrating electrons that radiate the light are quantized  obtain an expression that agreed with experiment. he recognized that the theory was physically absurd, he described as "an act of desperation" .

Albert Einstein The photoelectric effect Not explained by Maxwell's theory since the rate of electrons not depended on the intensity of light, but in the frequency. 1905: Einstein applied the idea of Plank's constant to the problem of the photoelectric effect  light consists of individual quantum particles, which later came to be called photons (1926). Electrons are released from certain materials only when particular frequencies are reached corresponding to multiples of Plank's constant .

Niels Bohr 1913 : Bohr quantized energy  explain how electrons orbit a nucleus. Electrons orbit with momenta, and energies quantized. Electrons do not loose energy as they orbit the nucleus, only change their energy by "jumping" between the stationary states emitting light whose wavelength depends on the energy difference. Explained the Rydberg formula (1888), which correctly modeled the light emission spectra of atomic hydrogen Although Bohr's theory was full of contradictions, it provided a quantitative description of the spectrum of the hydrogen atom

Two theorist, Niels Bohr and Max Planck, at the blackboard.

By the late 1910s : 1916 Arnold Sommerfeld : - To account for the Zeeman effect (1896): atomic absorption or emission spectral lines change when the light is first shinned through a magnetic field, - he suggested “elliptical orbits” in atoms in addition to spherical orbits. In 1924, Louis de Broglie: - theory of matter waves - particles can exhibit wave characteristics and vice versa, in analogy to photons. 1924, another precursor Satyendra N. Bose: - new way to explain the Planck radiation law. - He treated light as if it were a gas of massless particles (now called photons).

Scientific revolution 1925 to January 1928 • Wolfang Pauli: the exclusion principle • Werner Heisemberg, with Max Born and Pascual Jordan, - discovered matrix mechanics first version of quantum mechanics. • Erwin Schrödinger: - invented wave mechanics, a second form of quantum mechanics in which the state of a system is described by a wave function, • Electrons were shown to obey a new type of statistical law, Fermi- Dirac statistics • Heisenberg :Uncertainty Principle. • Dirac :contributions to quantum mechanics and quantum electrodynamics

Many physicists have also contributed to the quantum theory: Max Planck : Light quanta Einstein “photon”: photoelectric Louis de Broglie: Matter waves Erwin Schrödinger: waves equations Max Born: probability waves Heisenberg: uncertainty Paul Dirac: Spin electron equation Niels Bohr: Copenhagen Feynman: Quantum-electrodynamics John Bell: EPR Inequality locality David Bohm: Pilot wave (de Broglie) ... Paul Dirac and Werner Heisemberg in Cambrige,1930.

The first Solvay Congress in 1911 assembled the pioneers of quantum theory.

Old faces and new at 1927 Solvay Congress

Scientific revolution: Heisenberg & the uncertainty principle

Werner Karl Heisenberg : Brief chronology 1901 - 5Dec: He was born in Würzburg, Germany 1914 :Outbreak of World War I. 1920 he entered at the University of Munich  Arnold Sommerfeld admitted him to his advanced seminar. 1925. 29 June Receipt of Heisenberg's paper providing breakthrough to quantum mechanics 1927. 23 Mar. Receipt of Heisenberg's paper on the uncertainty principle. 1932. 7 June Receipt of his first paper on the neutron-proton model of nuclei. 1933 .11 Dec. Heisenberg receives Nobel Prize for Physics (for 1932). 1976. 1 Feb. Dies because of cancer at his home in Munich.

Influences Studied with three of the world’s leading atomic theorists: Sommerfeld, Max Born and Niels Bohr. In 3 of the world’s leading centres for theoretical atomic physics: Munich, Göttingen and Copenhagen. - Max Born “From Sommerfeld I learn optimism, from the Göttigen people mathematics and from Bohr physics” –Heisemberg Arnold Sommerfeld (left) and Niels Bohr Wolfgang Pauli - In Munich he began a life-long friendship with Wolfgang Pauli.

During 1920 Heisenberg’s travels and teachers during help him to become one of the leading physicists of his time. Goal fortune of entering in the “world atomic physics” just in the right moment for breakthrough. Found that properties of the atoms predicted from the calculations did not agree with existing experimental data. “The old quantum theory”, worked well in simple cases, but experimental and theoretical study was revealing many problems  crisis in quantum theory. The old quantum theory had failed but Heisenberg and his colleagues saw exactly where it failed.

Quantum mechanics 1925-1927 The leading theory of the atom when Heisenberg entered at University was quantum theory of Bohr. Although it had been highly successful, three areas of research indicated that this theory was inadequate: light emitted and absorbed by atoms the predicted properties of atoms and molecules The nature of light, did it act like waves or like a stream of particles? 1924 physicists were agreed old quantum theory had to be replaced by “quantum mechanics”.

Heisenberg set the task of finding the new quantum mechanics: The breakthrough to quantum mechanics: Heisenberg set the task of finding the new quantum mechanics: Since the electron orbits in atoms could not be observed, he tried to develop a quantum mechanics without them. By 1925 he had an answer, but the mathematics was so unfamiliar that he was not sure if it made any sense.  These unfamiliar mathematics contain arrays of numbers known as “matrix”. Born sent Heisenberg’s paper off for publication. “All of my meagre efforts go toward killing off and suitably replacing the concept of the orbital path which cannot observe” Heisemberg, letter to Pauli 1925

The first page of Heisenberg's break-through paper on quantum mechanics, published in the Zeitschrift für Physik, 33 (1925), “The present paper seeks to establish a basis for theoretical quantum mechanics founded exclusively upon relationships between quantities which in principle are observable”. Heisemberg, summary abstract of his first paper on quantum mechanics

The wave-function formulation 1926: Erwin Schrödinger proposed another quantum mechanics, “wave mechanics”. Appealed to many physicists because it seemed to do everything that matrix mechanics could do but much more easily and seemingly without giving up the visualization of orbits within the atom. “I knew of [Heisemberg] theory, of course, but I felt discouraged, not to say repelled, by the methods of transcendental algebra, which appeared difficult to me, and by the lack of visualizability.”- Schrödinger in 1926.

The Uncertainty Principle 1926: The rout to uncertainty relations lies in a debate between alternative versions of quantum mechanics: - Heisenberg and his closest colleagues who espoused the “matrix form” of quantum mechanics - Schrödinger and his colleagues who espoused the new “wave mechanics ”. May 1926, Matrix mechanics and wave mechanics, apparently incompatible  proof that gave equivalent results. “The more I think about the physical portion of Schrödinger’s theory, the more repulsive I find it.. What Schrödinger writes about the visualizability of his theory is not quite right, in other words it’s crap” Heisenberg, writing to Pauli, 1926

In 1927 the intensive work led to Heisenberg’s uncertainty principle and the “Copenhagen Interpretation” “The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa” Heisenberg, uncertainty paper, 1927 After that, Born presented a statistical interpretation of the wave function, Jordan in Göttingen and Dirac in Cambridge, created unified equations known as “transformation theory”. The basis of what is now regarded as quantum mechanics. The uncertainty principle was not accepted by everyone. It’s most outspoken opponent was Einstein. .

Conclusion The history of Quantum mechanics it’s not easy, many events pass simultaneously  difficult period. Quantum mechanics was created to describe an abstract atomic world far removed from daily experience, its impact on our daily lives has become very important. Spectacular advances in chemistry, biology, and medicine… Quantum information The creation of quantum physics has transformed our world, bringing with it all the benefits—and the risks—of a scientific revolution.

Bibliography http://www.aip.org/history/heisenberg/p08.htm http://www.4physics.com/phy_demo/QM_Article/article.html http://www.vcpc.univie.ac.at/~ian/hotlist/qc/qm.shtml http://www.slac.stanford.edu/pubs/beamline/30/2/30-2-carson.pdf