ITEP-LAT/2002-26 MPI-PhT/2002-76

We report measurements of the action associated with center vortices in SU(2) pure lattice gauge theory. In the lattice units the excess of the action on the plaquettes belonging to the vortex is approximately a constant, independent on the lattice spacing a. Therefore the action of the center vortex is of order A/a, where A is its area. Since the area A is known to scale in the physical units, the measurements imply that the suppression due to the surface action is balanced, or fine tuned to the entropy factor which is to be an exponential of A/a. PACS numbers: 11.15.-q, 11.15.Ha, 12.38.Gc 1 Monopoles and vortices Lattice measurements allow for a direct study of field fluctuations in the vacuum state of Yang-Mills theories. Generically, the probability to find a field configuration is a product of the entropy and action factors: P = exp(S) · exp(−S) , (1) where S is the action and the entropy exp(S) is the number of ways in which the field configuration can be realized. Let us mention two simple examples to illustrate (1). In case of instantons, the (classical) action is Sinst = 8π /g while the entropy is provided by counting small (quantum) fluctuations in the instanton background. Another example is the zero-point vacuum fluctuations. They are dominated by the phase space, or by the entropy.