The entity "light quantum", for which to understand Einstein exerts himself all his life - of no avail, according to his own testimony - (and which is subject to change as the concept of "photon", reflecting the endeavour for physical understanding) leads multiple dual lifes: wave-particle, photon-quark, photon-lepton, photon-hadron, ... (?).

Leptons are light material particles, like the electron; hadrons are heavier ones, like the proton.

The quark, lepton or even hadron character of the photon becomes manifest in its transformation into a massive particle and its anti-particle (pair production). This is a top theme of particle physics which is, also with a view on such kind questions, in want for highly precise installations - aside of accelerators that rush forwards into ever new energy domains, like the ring-shaped "Large Hadron Collider" (LHC) at CERN in Geneva. In an international co-operation, a linear accelerator TESLA for leptons is being developed to this end. The supraconducting accelerating cavity (figure) of the TESLA project becomes an essential element, too, of a novel, highly energetic laser in the X-ray range, X-FEL.