I read an abstract here:
Due to the bosonic nature of the photon, increasing the peak intensity through a combination of raising the pulse energy and decreasing the pulse duration will pile up more and more photons within the same finite region of space. In the absence of material, this continues until the vacuum is stressed to the point of breakdown and virtual particles become real. The critical intensity where this occurs for electrons and positrons – the so-called Schwinger limit – is predicted to be ~ 10^29 W/cm2.
According to it virtual particles can become real at certain conditions. I guess that means that the virtual particles of vacuum fluctuation can become real. I cannot judge with my limited knowledge whether this is true. Can somebody verify it and if it is true explain how and why it happens?