  We show that the unexpected metallic behavior (the so-called two-dimensional
metal-insulator transition) observed in low-density Silicon
metal-oxide-semiconductor field-effect transistors (Si-MOSFETs) is controlled
by a unique characteristic energy scale, the polarization energy. On one hand,
we perform Quantum Monte Carlo calculations of the energy needed to polarize
the two dimensional electron gas at zero temperature, taking into account
Coulomb interactions, valley degeneracy and electronic mobility (disorder). On
the other hand, we identify the characteristic energy scale controlling the
physics in eight different sets of experiments. We find that our {\it
ab-initio} polarization energies (obtained without any adjustable parameters)
are in perfect agreement with the observed characteristic energies for all
available data, both for the magnetic field and temperature dependence of the
resistivities. Our results put strong constraints on possible mechanisms
responsible for the metallic behavior. In particular, there are strong
indications that the system would eventually become insulating at low enough
temperature.
