Julian Schwinger showed in a fundamental essay from 1951 that at such field strengths the QED vacuum is unstable and decays due to the generation of electron- positron pairs. Schwinger calculated the effective QED Lagrange density for a constant external field and in a single-loop approximation. This has the imaginary part
It determines the transition to another vacuum.
Until 2014, lasers were n't powerful enough to achieve these field strengths, but future lasers may be able to do so.
literature
J. Schwinger: On Gauge Invariance and Vacuum Polarization , Physical Review 82, 664
Individual evidence
↑ Bulanov et al .: On the Schwinger limit attainability with extreme power lasers . November 5, 2010, arxiv : 1007.4306 (English).