After having completed his studies with the usual diploma (1950) and doctorate (1952), with a thesis under Prof. Richhard Becker on thermal solid-state theory,[7] he was accepted as a postdoc at the Max Planck Institute for Physics (MPI), still at Göttingen at the time.

While he was there, he was among a group of physicists including Bruno Zumino, Harry Lehmann, Wolfhart Zimmermann, Kurt Symanzik, Gerhard Lüders, Reinhard Oehme, Vladimir Glaser, and Carl Friedrich von Weizsäcker.

Hagedorn brought to the Theory Division an unusual interdisciplinary background which included particle and nuclear as well as thermal, solid state and accelerator physics.

[12] Hagedorn's work started when Bruno Ferretti (then-head of the Theory Division), asked him to try to predict particle yields in the high energy collisions of the time.

[13] Based on this, Hagedorn put forth his thermal interpretation and used it to build production models which turned out to be remarkably accurate at predicting yields for the many different types of secondary particles.

Many objections were raised at the time, particularly as to what could actually be 'thermalized' in the collisions, applying straightforward statistical mechanics to the produced pions gave the wrong results, and the temperature of the system was apparently constant when it should have risen with the incident energy or with the mass of the excited fireball (according to Boltzmann's Law).

[20] Hagedorn gave this extensive summary of the historical path across 50 years of research in particle physics at his last 2-hours public lecture in Divonne 1994, which was recorded and later made available online.

The book includes contributions by contemporaneous friends and colleagues of Hagedorn: Tamás Biró, Igor Dremin, Torleif Ericson, Marek Gaździcki, Mark Gorenstein, Hans Gutbrod, Maurice Jacob, István Montvay, Berndt Müller, Grazyna Odyniec, Emanuele Quercigh, Krzysztof Redlich, Helmut Satz, Luigi Sertorio, Ludwik Turko, and Gabriele Veneziano.