If we look at the analogy of electrons and holes in semiconductor physics, we see that holes appear real in many ways: we can physically measure their apparent mass and all sorts of electrical properties.
However, we know holes are not real, but are simply a mental construct for convenience, because we can perform a thought experiment eliminates their reality: imagine taking an electron out of the semiconductor and you can still detect it, but you cannot take a hole out of a semiconductor! QED.
So the question is can we make some sort of thought experiment to decide the reality of virtual particles and virtual photons in QFT?
The hole in the semiconductor would also be real, according to your definition, if you would be able to convert it into a positron (i.e. a hole in vacuum). As a gedanken experiment that should definitely be possible. The right question is probably why do we find around ourselves more matter than antimatter. But if it was different (i.e. everything made of antimatter), then we would be asking the opposite question, of course. My point is that holes are as real as electrons and there is no fundamental difference however one would look at it.
The notion of "virtual particles" is a mental image associated to specific terms that appear in the perturbative expansion of the evolution operator, or the S-matrix, in QFT. It is a useful device, but it is not necessary for the theory. One can do the calculations, and obtain all experimental predictions with no reference at all to virtual particles. Hence, any thought experiment that can concretely distinguish virtual particles must go beyond QFT. This means new physics.
Well, virtual particles can't be apprehended directly - by their very definition - but can by their indirect effects.
One instance is the tunnel effect - the math (Schrödinger) clearly indicates a non zero likelihood of presence of a particle beyond an energy barrier, but what is the physical mechanism whereby a particle bores through that energy wall? The most credible mechanism is through borrowing energy from a virtual particle and relinquising it back on the other side, within a Δt of time such that the particle remain virtual.
Another illustration would likely be radioactive decay. You can't forecast accurately when an individual radioactive element molecule decays, which seems utterly random, but what intermediates the randomness? It likely is the effect of the interplay between virtual particles in the surrounding quantum froth and the element's properties, with different decay frequencies (and therefore element half-lives) ultimately stemming from the distribution of certain attributes within the population of virtual particles.
I'd like to challenge the view that you "can take out an electron of a semiconductor". Approximately, you can induce a transition from a piece of matter 'containing N electrons' to one 'containing (N-1) electrons' plus an extra electron. But the electron as an individuum does not exist in that piece of material. To me that belongs to the essence of many body quantum physics.
@Kai. Electrons have a wave as well as a particle nature, so true to a point. However, using the photoelectric effect you can kick off an individual electrons from the surface of a semiconductor. But you cannot kick out a hole. Thus a hole has no reality of its own, but the electron does.
Is there some kind of thought experiment we can do to test the reality or lack of reality of virtual particles?
@Derek: this certainly is a side track to your question. But "the quantum wave object thing" to me is what we represent as the antisymmetrized "wave function" of all electrons. There are 2 electrons in a He atom, but they cannot neither be understood nor represented as 2 (individual) things.
For the holes in semiconductors I agree (they're a highly practical and adequately simplifying mental construct) with your viewpoint. I would not enumerate them among virtual particles or excitations.
Some of the virtual particles and/or excitation language seems to me as being related to our choice of description, i.e. the basis states in which we choose to formulate what we want to describe:
- If we choose to represent the H2+ molecule in localized atomic orbitals, then there are spatial fluctuations in the ground state. If we choose the bounding state (stationary) then there are no such fluctuations.
- If we choose spin orbital product states to represent 2 interacting electrons (subject to exchange) then there are quantum (spin) fluctuations in the ground state. If I choose singlet and triplet states right away, then they are stationary and there are no fluctuations.
-likewise, we can describe e.g. superexchange as to originate from (spin dependent) virtual hopping (virtual, since the energy required to do a "real hopping" is way larger than a thermal energy available) [in a basis set language facilitating the formulation of the problem] or we can choose to diagonalize the Hamiltonian and choose the solutions (stationary states) as the basis of our description.
I am not quite sure that this view covers all aspects of "reality of virtual particles", though.
A real experiment instead of a Gedanken one: resonant Raman scattering both with lasers or synchrotron radiation (RIXS): cross sections witness the presence of a resonant excitation (at higher energy) which cannot, nevertheless, be excited but "virtually".
The same is true for phase contrast x-ray microscopy using sub threshold x-ray wavelengths - you're working far out on the (low frequency) Lorentzian tail of some xray absorption edge.
@Derek: would these be valid examples that fit your question at all?
One of the direct manifestation of the virtual particles is the so called Casimir effect:
"The Casimir effect is a small attractive force that acts between two close parallel uncharged conducting plates. It is due to quantum vacuum fluctuations of the electromagnetic field. Some comments to this topic can be found in
http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea
Once more, sorry. I am afraid there is here some confusion between quasi particles and virtual particles which is not quite the same !
Which are all virtual particles?
(Magnetism fluctuation implies existence for me).
IMO quasiparticles are collective excitations of a medium which behave like standard particles (mass,charge etc) but have no individual existence outside this medium. Virtual particles are purely quantum mechanics objects and can be interpreted as quantum fluctuations of a vacuum Their existence has measurable consequences and are necessary, for example, to make Quantum Chromodynamics (QCD) calculations effective. Reality of quasiparticles was discussed, for example, in the blog
http://2013phys4030.blogspot.com/2013/06/are-quasi-particles-real.html
Distance, space between to points, Does it create necessary fluctuations explicable with this virtual particles?
Dear Professor Ana Maria, yes, it can be a cause of fluctuations but only two points not possible. This will be due to many points.
Yes there is a thought experiment to do that. We can, virtually, kick out a virtual hole out of the semiconductor, simply by injecting an electron to fill it. ;)
I think this thought experiment should generally describes the virtual photons.
We do not know how a charged particle produces an electric field or virtual photons in quantum mechanics.
According quantum mechanics concepts of forces, a charged particle emits exchange force particles continuously. This process has no effect on the properties of a charged particle such as its mass and charge. How is it explainable? If a charged particle as a generator has an output known as a virtual photon, what will be its input? Suppose an electric field is made up of virtual photon. Fields around the two opposite charged particle are made up of same virtual photon?
How two same sign charged particles repel one another, and particles whose charges have different signs attract. In my opinion a thought experiment should be able to answer these questions.
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