  We demonstrate the extraordinary modernity of the 1924/25 "Einstein-Jordan
fluctuation conundrum", a Gedankenexperiment which led Jordan to his
quantization of waves published as a separate section in the famous
Born-Heisenberg-Jordan 1926 "Dreim\"annerarbeit". The thermal nature of energy
fluctuations caused by the restriction of the QFT vacuum to a subvolume
remained unnoticed mainly because it is not present in QM. In order to
understand the analogy with Einstein's fluctuation calculation in a thermal
black body system, it is important to expose the mechanism which causes a
global vacuum state to become impure on a localized subalgebra of QFT. The
present work presents the fascinating history behind this problem which
culminated in the more recent perception that "causal localization" leads to
thermal manifestations. The most appropriate concept which places this property
of QFT into the forefront is "modular localization". These new developments in
QFT led to a new access to the existence problem for interacting quantum fields
whose solution has remained outside the range of renormalized perturbation
theory. It also clarifies open problems about the relation of particles and
fields in particular about the incompletely understood crossing property. Last
not least it leads to a constructive understanding of integrable versus
non-integrable QFTs..
