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Can virtual particles become real? A case study
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As a case study for the way how loose, purely figurative talk by 
physicists about virtual particles is related to the true underlying 
science, I discuss here one particular case:

The Science Online artice at
   Volume 277, Number 5330 Issue of 29 August 1997, p 1202
   http://www.slac.stanford.edu/exp/e144/science1202.html
quotes the experimental physicist Adrian Melissinos explaining new 
results of their group, as follows:
   ''Melissinos views the result as the first direct demonstration of
     "sparking the vacuum," a long-predicted phenomenon. In it, the 
     energy of a very strong electromagnetic field promotes some of 
     the fleeting, "virtual" particles that inhabit the vacuum, 
     according to QED, to become pairs of real particles.''

What is the scientific evidence behind this claim? Science Online says 
in the introduction:
   ''In the 1 September Physical Review Letters, the researchers 
     describe how they collided large crowds of photons together so 
     violently that the interactions spawned particles of matter and 
     antimatter: electrons and positrons (antielectrons).''
This is already written on a more factual level - all talk about 
virtual particles is gone. Looking up the September issue mentioned,
we find a refereed research paper with the title
    Positron Production in Multiphoton Light-by-Light Scattering 
    Phys. Rev. Lett. 79, 1626 (1997).
The relevant part of the abstract says:
    ''The positrons are interpreted as arising from a two-step process 
      in which laser photons are backscattered to GeV energies by the 
      electron beam followed by a collision between the high-energy 
      photon and several laser photons to produce an electron-positron 
      pair. These results are the first laboratory evidence for 
      inelastic light-by-light scattering involving only real photons.''
Not the slightest mention is made of virtual particles becoming real, 
neither in the abstract nor in the body of the research paper. The real 
particles produced are electrons and positrons. and they were created 
from real photons.

The research paper mentions the word ''virtual'' precisely three times 
- in each case it is about a virtual photon, and describes a 
particular Feynman diagram. This is the scientifically correct way to 
communicate a diagram without having to draw it: Virtual particles 
refer to the internal lines of the diagram; they have _no_other_ 
meaning. 

Why then did Melissinos in the interview with Science Online talk of a 
virtual electron-positron pair becoming real? (And indeed, _many_ 
particle physicists like this sort of loose talk involving virtual 
particles.) The reason is that, on the figurative level (i..e, with 
figures on paper or in the mind), this talk invokes some loose sort of 
understanding why the experimental findings make sense; and _this_ fast 
way of communicating is the sole purpose for talking like this to 
collegues: 

The Feynman diagram representing the experimentally observed process 
(two gamma in, e^- and e^+ out) and the Feynman diagram representing 
the process responsible for detecting the positrons (e^- and e^+ in, 
two gamma out) put together (assuming that the two electrons are the 
same - which they aren't in practice) form a typical 1-loop diagram 
for photon-photon scattering via a virtual e^-/e^+ pair. 

But it should be clear, that this is a purely symbolic metaphor that 
hasn't anything to do with what happened in the laboratory - where 
(as the abstract says), colliding photons create an electron-positron 
pair.

 
Of course, in some sense, all physical language apart from the 
equations in a model is metaphorical (and in some sense, even the 
equations themselves). But there are degrees of metaphors. One can say 
in equations what it means to collide and to have an electron-positron 
pair. The latter are described by a 2-particle state, and the 
collision process is given by the scattering matrix. To say that 
''colliding photons create an electron-positron pair'' is concise 
language that tells something about in-states and out-states, which 
are supposed to be real, preparable things. 

But one cannot prepare or measure Feynman diagrams and/or their 
internal lines. What they mean depends on the perturbative formalism 
used, and except at tree level, usually only an infinite sum of them 
makes any computational sense. Thus virtual particles have a far less 
tangible relation to reality as colliding photons creating an 
electron-positron pair. The do not deserve to be called real - it only 
creates confusion.