Cristina Marchetti signs the Ehrenfest wall after delivering the Colloquium Ehrenfestii in Leiden on April 19, 2019.

See here for the long and illustrious history of the Colloquium that was started by Paul Ehrenfest in 1912.

The colloquium culminates with the speaker signing the “Ehrenfest’s wall”, a tradition that dates
back to the first colloquia by Einstein, Bohr, Planck and many other great contributors to modern physics.