  The Clifford algebra, generated by the real (Majorana) gamma-matrices and by
a hermitian gamma_5, gives room to the reductive Lie algebra u(2,2) of the
conformal group extended by the u(1) helicity operator. Its unitary positive
energy ladder representations, constructed by Gerhard Mack and the author 50
years ago, opened the way to a better understanding of zero-mass particles and
fields and their relation to the space of bound states of the hydrogen atom.
They became a prototypical example of a minimal representation of a non-compact
reductive group introduced during the subsequent decade by Joseph.
  By the mid 1980's I have developed an analytic approach to compactified
Minkowski space, suited for extending the notion of vertex operator algebras to
higher dimensions. Another 20 years later Nikolay Nikolov and I realized that
thermal correlation functions of massless fields are doubly periodic elliptic
functions while the logarithmic derivative of the corresponding partition
function is a modular form reproducing Planck's black body radiation law.
