### Men Who Made a New Physics

A timeline of how quantum physics came to be

__Ernest Rutherford (1871 – 1937)__

He became known as the father of nuclear physics.

Rutherford was a loud, ambitious, easily angered (but always apologetic when necessary) man who had a gift for designing experiments and apparatus and for discerning a significant fact from a mass of confusing detail.

He detested pomposity.

He disliked chemistry, and pretty much all other disciplines except physics.

He built a device for detecting radio waves not long after the discovery of electromagnetic waves. Like Marconi, but even before him. However, he had little interest in this field which he viewed as engineering.

Three people were awarded Nobel Prizes under him; James Chadwick for discovering the neutron (1932), John Cockcroft and Ernest Walton for splitting the atom (the nucleus) (1932) and Appleton for demonstrating the existence of the ionosphere ().

1895 – Travelled to England for postgrad study at the Cavendish Lab in Cambridge University.

1898 – Was appointed to the chair of physics at McGill University in Montreal, Canada.

1899 – Coined the terms alpha and beta radiation.

1900 – Married Mary Newton.

1902 – Rutherford with his collaborator, Frederick Soddy, developed the theory of radiation, which today stands largely unchanged. Much has been added but nothing has been removed. The theory basically states that all radioactive energy came from the element itself and as it emitted radiation (alpha, beta or gamma rays) it was transformed into a different element. As part of this theory they established that there were only three main families of radioactive elements, one beginning with thorium, one with actinium and one with uranium.

1907 – Went back to England as the chair of physics at the University of Manchester.

1908 – Won the Nobel Prize in Chemistry for his discoveries regarding radioactive decay.

1911 – Discovered the nucleus of the atom (on the basis of scattering experiments in which he (and Hans Geiger and Ernest Marsden) fired a beam of alpha particles at atoms) and pioneered the Rutherford model of the nucleus.

1914 – Was knighted.

1919 – Became Director of the Cavendish Laboratory.

1919 – Became the first person to transmute one element into another through a nuclear reaction (turned nitrogen into oxygen).

1921 – He theorised about the existence of neutrons.

1931 – Was created Baron Rutherford of Nelson.

1937 – Died of an umbilical hernia and was interred alongside J.J. Thomson, near Sir Isaac Newton.

__Max Planck (1858-1947)__

Planck was a reserved, quiet and softly spoken German.

He was very meticulous and organised.

1874 – Entered the University of Munich.

1877 – Went to the University of Berlin (ruled by Hermann von Helmholtz) and studied for a year. He noted that von Helmholtz’s lectures were poorly prepared and full of errors, while Gustav Kirchhoff’s lectures were dry and boring. Planck discovered thermodynamics (largely through his own reading) including Rudolf Clausius’ ‘entropy’ interpretation of the second law.

Circa 1879 – Wrote his first paper on the second law of thermodynamics. It was largely ignored.

1880 – Became a *Privatdozent* (an apprentice professor who lectured without salary) at the University of Munich.

1885 – Appointed associate professor of theoretical physics at the University of Kiel.

1889 – Named Kirchhoff’s successor at the University of Berlin.

1892 – Was appointed a professor at the University of Berlin.

1900 – Formulated his famous Planck black body radiation law, which included Boltzmann’s statistical approach to the second law of thermodynamics (which states that entropy is not absolute) and the Planck postulate (electromagnetic energy can only be emitted in quantised form according to E = hf, where h = 6.6 x 10^{-27}). This solved the black-body radiation problem called the *ultraviolet catastrophe*.

1918 – Was awarded the Nobel Prize for his discovery of energy quanta.

__Niels Bohr (1885 – 1962)__

He was Danish and became enormously respected in his country and around the world.

Like Rutherford, he was good at performing experiments.

Bohr would circle round and round a problem, examining it from many different angles until he thoroughly understood it.

He was a kind teacher who worked closely with his students and never criticised them.

1912 – He travelled to England to study under Rutherford for only six months. On his return to Copenhagen, he started teaching at the University of Copenhagen.

1913 – Discovered the formula (related to atomic structure) which described the line spectrum of hydrogen by using a technique he called correspondence (using classical physics to derive the broad strokes that the new theory must encompass and refining the new) and Planck’s quantum ideas. There is a myth (like the apple myth for Newton) surrounding this even which says that he stumbled upon the crucial formula in a book for children. In reality, he found a formula worked out by a Swiss schoolteacher, Johann Balmer in 1885, which used a mathematical ‘trick’ to account for the hydrogen spectrum. JR Rydberg also used a similar trick and a constant both Balmer and Rydberg used was termed the ‘Rydberg constant’. Bohr refined the ‘trick’ using quantum ideas and formulated a sturdier theory of the atom which derived the Rydberg constant. This solved the problem of why the electron does not continuously emit radiation and fall into the nucleus. Unfortunately, it didn’t explain itself fully. His paper had to be sent to Rutherford before publication and the two argued several points before it finally went out.

*Summary of the differences between the Bohr Model and the Classical Model:*

Classical: the electron circles the nucleus in various orbits which are infinite in number. The orbit is equal to the energy. Energy is continuous.

Bohr: the electron only moves in orbits where its energy is proportional to Planck’s constant multiplied by a whole number. Energy is discontinuous.

Classical: the electron revolves around the core radiating energy and spirals into the core.

Bohr: the electron can shift from one orbit to another but may not go below the innermost orbit.

Classical: the electron always emits radiation and the frequency is the same as the ‘oftenness’ with which the orbit is completed.

Bohr: the electron only emits radiation after energy has been added to the atom, pushing electrons from their low orbit into an outer one. As they fall back to their inner orbits (of lower energy) they emit radiation of a frequency equal to the difference in energy between the two orbits divided by Planck’s constant.

1914 – James Franck and Gustav Hertz performed an experiment which they claimed ionised mercury. They added just enough energy to the atom so that an electron was lost and the mercury would be positively charged. According to Bohr, this experiment should have revealed the line spectrum of mercury, one line at a time, but it didn’t. Fortunately, the experiment was flawed and the results ended up backing up Bohr’s idea of discrete atomic energies.

1916 – Became a professor at the University of Copenhagen.

1920 – Started “The Institute for Theoretical Physics” or “Bohr’s Institute.”

1922 – Bohr won the Nobel Prize for his contributions to the structure of atoms and their radiation.

1927 – Finally formulated the Copenhagen Interpretation of quantum mechanics (with Heisenberg and Pauli) which basically accepted the inconsistencies and limits and codified the notion that the observer could not be considered separate from the observed.

__Wolfgang Pauli (1900 – 1958)__

Pauli (Austrian-Swiss) excelled at criticism and he was blunt in delivering it.

He was completely useless at conducting experiments and this spurred the coining of “the Pauli effect.”

1921 – Pauli wrote an article about general relativity which became extremely well-known. Even Einstein himself was said to have commented that after reading it he understood the theory better.

1921 – Spent a year at the University of Gottingen as assistant to Max Born.

1922 – Spent a year at the Institute for Theoretical Physics in Copenhagen with Niels Bohr.

1923-28 – Was a lecturer at the University of Hamburg.

1924 – Proposed a quantum spin number for electrons.

1925 – Formulated the Pauli exclusion principle, which states that no two electrons in an atom can have the same four quantum numbers.

1925 – Used Heisenberg’s theory to derive the hydrogen spectrum.

1930 – Proposed the existence of the neutrino (named by Enrico Fermi). This was confirmed in 1956.

__Werner Heisenberg (1901 – 1976)__

Heisenberg was a handsome, blond, muscular German.

He was able to see straight to the truth of problems with huge flashes of insight.

1924-27 – Was a *privatdozent* at the University of Gottingen.

1924 – Went to Copenhagen with Niels Bohr for about eight months.

1925 – After returning to Gottingen, he came up with an idea for a turning Bohr’s correspondence system into a precise science. This was further developed by Max Born and Pascual Jordan, who used matrix mechanics, and later by Heisenberg himself until they formulated the matrix mechanical view of quantum mechanics (particle based).

1927 – Formulated the Heisenberg uncertainty principle which states that momentum and position are not actually independent quantities. They are in fact, related in such a way that they cannot both be known at the same time. The uncertainty in one or the other must equal at least Planck’s constant.

__Louis Victor de Broglie (1892 – 1987)__

De Broglie was a French Prince who submitted his doctoral thesis in 1924 on the idea of electron waves and the wave-particle duality of matter. He reasoned that if light had a dual nature then maybe matter could as well. His thesis examiners were unsure of the merits of his paper and submitted it to Einstein who thoroughly endorsed it. De Broglie created an equation which said that wavelength = Planck’s constant / momentum. This was later confirmed experimentally (by mistake by an American, Clinton Davisson) for electrons and later for other elements.

__Erwin Schrodinger (1887 – 1961)__

Schrodinger was an Austrian physicist. In 1926, he took de Broglie’s wave-particle duality of matter idea and by creating the Schrodinger wave equation, created the general system of wave mechanics. He also showed that the spectrum of hydrogen could be derived from it.

__Paul Dirac (1902 – 1984)__

Dirac was a British physicist who worked alone. He was famously simple and economical with his words. In 1925, he saw Heisenberg’s matrix formulation of quantum mechanics and using Poisson brackets, was able to form a new description of quantum mechanics (particle based, like Heisenberg’s). In 1928, he built on Pauli’s non-relativistic spin systems by proposing the Dirac equation (a relativistic equation of motion for the wavefunction of an electron) which led directly to the notion of antiparticles.