# Can virtual particles become real?

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?

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.

• I understand that virtual particles are just a mathematical concept to solve problems. The article I linked goes against that definition by claiming that virtual particles can become real if we apply high energy laser bursts on the vacuum. The question is whether this is true and how this fits into quantum field theory. – inf3rno Dec 2 '17 at 13:38
• @inf3rno my point is that this assertion is neither true or false, but imprecise (meaning that it captures the correct intuition, but doesn't go into full detail on what really is happening). In reality, Schwinger limit is a typical value of the electric field after which electromagnetism becomes nonlinear, due to the next-to-leading order diagrams involving propagation of loops of virtual fermions between bosonic propagators. I guess it is possible to say that this means that virtual particles have a real effect, and thus "become real", but I don't like this explanation much. – Prof. Legolasov Dec 3 '17 at 1:29
• I think then you should elaborate your answer with details. – inf3rno Dec 3 '17 at 17:50

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.

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"

• "I do not participate to this site any longer. I do not accept a situation in which, a scientific answer not understood by some users, is downvoted repeatedly instead of asking clarifications from the poster of the answer..." - Yepp, same here for stackoverflow, superuser and biology.stackexchange. My questions are usually closed in 5 mins instead of downvoted. They don't even try to read and understand it. I am lucky though, because most of my problems are with questions. I saw out there good answers with minus points and totally wrong answers with 100+ points, which can be frustrating... – inf3rno Sep 21 '18 at 22:13
• @inf3rno You see, I am a long-trained researcher in "fundamenta of quantum mechanics (QM)". My answers to questions in QM represent the last-date proofs, and they were in general not understood. The bad thing is that some people here don't understand what is RESEARCH. When the person X does not understand the person Y, the person X is supposed to ask questions, to try to understand what meant Y, not to downvote. RESEARCH is QUESTIONING. Unfortunately, it is not the custom here. So, I got bored and left this site - it doesn't suit me. – Sofia Sep 23 '18 at 9:21
• I have the same problems. I am a bioengineer and I'd like to become a healthcare scientist on the long run, but these sites aren't the best for scientist. These are something like wikipedia in a Q&A format. I participated in the biology site. It was great at the beginning, because the professional : layman ratio was relative high. After that there was an Ebola epidemic and everybody started to register and ask about Ebola. The professional : layman ratio started to drop and ppl started to downvote. Quora is somewhat better, because you can give profile credentials and you have upvotes only. – inf3rno Sep 23 '18 at 10:10
• @inf3rno If you are a researcher, stack sites are not exactly what you need. For instance, as far as I remember, in StackExchange one could not ask opinions on an article he/she wrote. It seems reasonable to me, most of the users there are just students. Researchers do RESEARCH, they find new phenomena and write articles. A student can tell you what he learnt. But you need an opinion on something new that you found. – Sofia Sep 24 '18 at 14:30
• As one can see from the master equation considered, the system produces real photons. Nothing in the master equation refers to virtual photons - the latter is just an informal interpretation in terms of nonexistent (virtual=nonreal) objects. – Arnold Neumaier Nov 2 '18 at 11:58

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.

• Could you please comment on Arnold Neumair's comment below your other answer? – thermomagnetic condensed boson Nov 3 '18 at 19:23
• Virtual in Feynman diagrams are off mass shell , the price of having energy and momentum conservation at the vertices, and they are under an integral, They also are "guesses" and cannot be measured.hyperphysics.phy-astr.gsu.edu/hbase/Particles/expar.html – anna v Nov 3 '18 at 19:33

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.

• Sofia , a student asked for Bohmian mechanics books or articles here physics.stackexchange.com/questions/429767/… . Maybe you could help. BTW I agree with your But can appear in intermediate stages of experiments, and their effects can be tested on the final data. – anna v Sep 20 '18 at 7:44
• @annav Dear Anna, I more than recommend Savasta's works. I claim that at least part of them I understood well. So, if want to talk, please do not hesitate to contact me. I repeat, my domain is "fundaments of QM". It is a terrible domain, demanding extremely deep analysis and cross-comparison of our knowledge in QM. – Sofia Sep 23 '18 at 9:32