Dutch physicist, of Austrian origin, born on January 18, 1880 in Vienna and died on September 25, 1933 in Leiden.

Student at the Technische Hochschule in Vienna, was one of the best disciples of Boltzmann, who taught him the theory of heat from 1899 to 1990. A year later he/she went to study mathematics at the University of Göttingen, where he/she had as teachers to eminent professors of the stature of Hilbertand Klein, Nernst, Stark and Schwarzschild. He/She obtained the doctorate at the University of Vienna in 1904 with a thesis on the phenomenon of diffusion under the direction of Boltzmann. This work examined the second law of thermodynamics under a statistical interpretation that hatched in a non-linear theory of Thermodynamics of systems far from equilibrium.

After a stay in Petersburgo he/she moved to the Dutch University of Leiden in 1912, where he/she became responsible for the vacancy left by Lorentz. During a brief visit to Prague he/she became a strong friendship with Einstein, who influenced him greatly. He/She was also a great friend of Bohr, one of the fathers of quantum physics, theory that both abhorred Einstein. Occasional meetings between Bohr and Einstein, which is discussing hard philosophy of quantum from antagonistic interpretations of the Vienna - represented by Einstein - School of Copenhagen - represented by Bohr - were always promoted by their mutual friend Ehrenfest.

The contributions of Ehrenfest to quantum physics were very fruitful. He/She tried to enforce the rules of quantum calculation to the rotation of bodies and studied the way of reconciling of Ampère's Molecular currents to classical statistical mechanics. The elegance of its style manifests itself in called Ehrenfest theorem, which expresses the equations of motion of Newton in the quantum view. Designates this theorem that the expected values of the position and time variables relate to each other as

m d <**r >**/DT = <**p >**

d <**p >**/DT = - <**N**V >

Despite all these contributions, Ehrenfest was unable to understand in depth the quantum physics, perhaps because he/she was too used to own a classic mental image of physical processes, something that also suffered Schrödinger, who would point out that if someone asserted that he/she understood quantum physics was because their ignorance was palpable. Perhaps influenced by his teacher Boltzmann, their desperation to realize the contradiction of develop a theory that could not understand led him to suicide in 1933.