**The Solvay Conference 1927 – Formulation of Quantum Theory**

A few years before the outbreak of World War I, the Belgain industrialist **Ernest Solvay** (1838-1922) sponsored the first of a series of international physics meetings in Brussels. Attendance at these meeting was by special invitation, and participants – usually limited to about 30 – were asked to concentrate on a pre-arranged topic.

The first five meeting held between 1911 and 1927 chronicled in a most remarkable way the development of 20th century physics. The 1927 gathering was devoted to quantum theory and attended by no less than **nine** theoretical physicists who had made fundamental contributions to the theory. Each of the nine would eventually be awarded a Nobel Prize for this contribution.

This photograph of the 1927 Solvay Conference is a good starting point for introducing the principal players in the development of the most modern of all physical theories. Future generations will marvel at the compressed time scale and geographical proximity which brought these giants of quantum physics together in 1927.

**There is hardly and period in the history of science in which so much has been clarified by so few in so short a time.**

Look at the sad-eyed **Max Planck** (1858−1947) in the front row next to **Marie Curie** (1867−1934). With his hat and cigar, Planck appears drained of vitality, exhausted after years of trying to refute his own revolutionary ideas about matter and radiation.

A few year later in 1905, a young patent clerk in Switzerland named **Albert Einstein** (1879−1955) generalized Planck’s notion.

That’s Einstein in the front row centre, sitting stiffly in his formal attire. He had been brooding for over twenty years about the quantum problem without any real insight since his early 1905 paper. All the while, he continued to contribute to the theory’s development and endorsed original ideas of others with uncanny confidence. His greatest work – the General Theory of Relativity – which had made him an international celebrity, was already a decade behind him.

In Brussels, Einstein had debated the bizarre conclusions of the quantum theory with its most respected and determined proponent, the “great Dane” **Niels Bohr** (1885-1962). Bohr – more than anyone else – would become associated with the struggle to interpret and understand the theory. At the far right of the photo, in the middle row, he is relaxed and confident – the 42 year old professor at the peak of this powers.

In the back row behind Einstein, **Erwin Schrodinger** (1887−1961) looks conspicuously casual in his sports jacket and how tie. To his left but one are the “young Turks”, **Wolfgang Pauli** (1900−58) and **Werner Heisenberg** (1901−76) – still in their twenties – and in front of them,** Paul Dirac **(1902−84), **Louis de Broglie** (1892−1987), **Max Born** (1882−1970) and Bohr. These men are today immortalized by their associate with the fundamental properties of the microscopic world: the Schrodinger wave equation; the Pauli exclusion principle ; the Heisenberg uncertainty relation, the Bohr Atom…. and so forth.

They were all there – from Planck, the oldest at 69 years, who started it all in 1900 – to Dirac, the youngest at 25 years, who completed the theory in 1928.

The day after this photograph was taken – 30 October 1927 – with the historic exchanges between Bohr and Einstein still buzzing in their minds, the conferees boarded trains at the Brussels Central Station to return to Berlin, Paris, Cambridge, Gottingen, Copenhagen, Vienna and Zurich.

They were taking with them the most bizarre set of ideas ever concocted by scientists. Secretly, most of them probably agreed with Einstein that this madness called the quantum theory was just a step along the way to a more complete theory and would be overthrown for something better, something more consistent with common sense.

But how did the quantum theory come about? What experiments compelled these most careful of men to ignore the tenets of classical physics and propose ideas about nature that violated common sense ?

Before we study these experimental paradoxes, we need some background in **thermodynamics** and **statistics** which are fundamental to the development of quantum theory.