  It is taken for granted that bound systems are made of massive constituents
that interact through particle exchanges (charged particles interacting via
photon exchanges, quarks in elementary particles interacting via gluon
exchanges, and nucleons in nuclei interacting via meson exchanges). However, as
was recently theoretically found, there exist systems dominated by exchange
particles (at least for the zero exchange masses). In these systems, the
contribution of massive constituents is negligible. These systems have a
relativistic nature (since they are mainly made of massless particles moving at
the speed of light), and therefore, they cannot be described by the
Schroedinger equation. Though these results were found so far in the simple
Wick--Cutkosky model (spinless constituents interacting via the ladder of
spinless massless exchanges), the physical ground for their existence seems to
be rather general.
