How Close Can Lasers Come to the Nonlinear Vacuum? An Intensity Ledger for Present and Planned Facilities

The vacuum becomes a nonlinear optical medium at the critical field of 1.323e+18 volts per metre, an intensity of 2.324e+29 watts per square centimetre. No laser approaches that intensity; the strongest operate near 10^23 watts per square centimetre, some six orders of magnitude below. This paper computes what that gap means in the vacuum's own terms: the field ratio at each facility class, the resulting nonlinear index change, and the phase shift accumulated over a focal interaction length. The conclusion is that laser facilities test the Euler-Heisenberg theory in its perturbative foothills, where the response is quadratic in the field ratio and phase shifts are at the edge of interferometric reach, rather than anywhere near the critical regime where pair production would dominate. The ledger is computed from stated facility intensities, not measured, and it corrects a common exaggeration in which record intensities are described as approaching the vacuum's breakdown when the vacuum's own scale says otherwise.