Among the most far-reaching developments of the recent 25 years of research into the consequences of fundamental interactions is the recognition that the true physical vacuum is a state of considerable complex and physically significant dynamical structure.
Quarks play an active role in shaping the vacuum structure.
I will describe here the current status of the understanding of the vacuum structure as related to strong interactions, and take a fresh look at the possibility of an interference between QCD and QED properties of the vacuum. Ideas Experimental exploration of the vacuum is not a simple task, since in laboratory experiments it is difficult, if not indeed impossible, to perform a comparative study with a vacuum structure free manifold. The search for vacuum melting at high temperature and density of matter as arising in high energy nuclear collisions is to-date the sole widely accepted method to pursue this objective. Its value obtained from sum-rule methods is today not much different from values first noted nearly 20 years ago. Clearly, the vacuum must be field free, so that the appearance of a field correlator has no classical analog, it expresses a Bogoliubov-type rotation away from the trivial Fock space state, induced by the interactions.