Oskar Kleins Gastbok in Östermalm, Sweden

Oskar Klein was one of the leading physicists of Sweden, known for his pioneering work in quantum and theoretical physics, many of which are now textbook material. Klein also focused on the popularization of physics and gave many public lectures and wrote popular texts.

He also invited many of his famous colleagues over for lectures. After lectures, guests would add their names to the wall of the room, a list of signatures that quickly grew into an impressive collection. Among these signatures were names like Bohr, Pauli, and Dyson. 

The institute has since moved to a new building and the signatures did not survive, but a replica of them was reproduced in steel and hung next to the lecture room named after Klein.