Can virtual particles become real? 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?
 A: We often come across the need to explain how fundamental physics works to the general public unfamiliar with the physical and mathematical prerequisites. We usually end up with analogies – a powerful tool which can, without explanation, give the audience a rough idea of how the thing works. But analogies can only be taken seriously so far – attempts to use them to explain complicated phenomena usually lead to apparent paradoxes, misunderstanding and confusion.
The same thing has been happening over and over on this forum with virtual elementary particles. See, when physicists speak of virtual particles, they refer to a specific type of fluctuation in the quantum field – the same field that gives rise to ordinary (real) elementary particles. That fluctuation has a precise mathematical meaning as a part of the asymptotic series, describing a fundamental object in the theory – the scattering matrix describing interactions between real elementary particles. That's why an analogy is usually employed: those fluctuations are said to be "virtual particles" which "mediate" interactions.
This analogy addresses the correct issues, and tells an unprepared audience a lot about the underlying phenomena. But it is only an analogy, and it has its limitations. Most of the newbie questions about virtual particles can and should be addressed in the full mathematical framework which is interacting Quantum Field Theory. Any kind of explanation involving virtual particles is just hand-waving.
A: Virtual particles have no dynamics. The latter is always tied to a state, which -
 unlike virtual particles - necessarily respects causality. Hence they cannot ''become'' anything. See https://www.physicsforums.com/insights/misconceptions-virtual-particles/
Talk about virtual particles doing something is therefore always just an illustration of some underlying formula, without any intent of physical accuracy. 
A: A new experiment was done with ions , the inside of an atom with no electrons , 2 beams of these were accelerated  and a particle accelerator , the islands themselves carried virtual photons. When the beans got closer the photons themselves reacted making electron and positron pairs , like beams of real protons would do. The proton antiproton pairs acted as real pairs , annihilating each other creating real photons
A: Yes, virtual particles can be real - see details in my last question
What kind of particles can be virtual? Only those in the table of the Standard Model?
One of the experiments described by the Savasta team succeeded to transform virtual photons into real by means of a 3-level artificial atom. Another experiment, in course of implementation, generates virtual photons as intermediate steps in exciting two artificial atoms with one single photon.
In particular, I'd like to stress that it's not true that virtual particles can't exist in reality. What is true is that they can't be detected because of their problematic features (mass and others). But can appear in intermediate stages of experiments, and their effects can be tested on the final data. 
A: An experiment was done in which virtual photons are transformed to real photons
R. Stassi, A. Ridolfo, O. Di Stefano, M. J. Hartmann, and S. Savasta, "Spontaneous Conversion from Virtual to Real Photons in the Ultrastrong Coupling Regime",arXiv:
1210.2367v2
Here is the essence of the experiment:
"we consider a three level emitter where the transition between the two upper levels couples ultrastrongly to a cavity mode and show that the spontaneous relaxation of the emitter from its intermediate to its ground state is accompanied by the creation of photons in the cavity mode (see Fig. 1).  . . . .
The Hamiltonian of a realistic atom-cavity system contains so-called counter-rotating terms allowing the simultaneous creation or annihilation of an excitation in both atom and cavity mode. These terms can be safely neglected for small coupling rates $Ω_R$ in the so called rotating-wave approximation (RWA). However, when $Ω_R$ becomes comparable to the cavity resonance frequency of the emitter or the resonance frequency of the cavity
mode, the counter-rotating terms are expected to manifest"
A: The term "virtual particle" was used in the past in Feynman's diagrams, just for making easier the calculi. Such "particles" probably do not exist in the nature.
In the article that I recommended
R. Stassi, A. Ridolfo, O. Di Stefano, M. J. Hartmann, and S. Savasta, "Spontaneous Conversion from Virtual to Real Photons in the Ultrastrong Coupling Regime",arXiv: 1210.2367v2,
is described an experiment in which appear, out of the vacuum, photons called by the authors "virtual" because they cannot be detected individually: they appear in the intermediary stages of the described processes, and violate the energy conservation. Therefore, these stages can be only guessed, but not observed. 
The term "virtual" used for these photons causes confusion with the virtual particles from Feynman's diagram. But these are two different types of virtual particles. To the difference from Feynman's virtual particles, Savasta's virtual photons do not differ, by their properties, from real photons. As said above, what is "virtual" with Savasta's photons is that their intervention in the process violates the conservation of energy. This is why they don't appear in the initial and final stage of the process, which are testable stages, but in intermediary stages, which can be only guessed, not detected. 
