  It is a long-standing non-trivial prediction of quantum electrodynamics that
its vacuum is unstable in the background of a static, spatially uniform
electric field and, in principle, sparks with spontaneous emission of
electron-positron pairs. However, an experimental verification of this
prediction seems out of reach because a sizeable rate for spontaneous pair
production requires an extraordinarily strong electric field strength $|\mathbf
E|$ of order the Schwinger critical field, ${\rm E}_c=m_e^2/e\simeq 1.3\times
10^{18}\ {\rm V/m}$, where $m_e$ is the electron mass and $e$ is its charge.
Here, we show that the measurement of the rate of pair production due to the
decays of high-energy bremsstrahlung photons in a high-intensity laser field
allows for the experimental determination of the Schwinger critical field and
thus the boiling point of the vacuum of quantum electrodynamics.
