Natural Order and Science

[Alex K]

Basic definition: two electrons interact, they exchange a photon. That photon is "virtual". It's not going to be a photon that any of my detectors  - eyes, voltmeters, or whatever apparatus I have at hand - will register. The photon goes from one electron to the other. Period. It's virtual.

Now, should a photon be emitted by an electron and not absorbed - for whatever reason, then that photon can be detected (by my eyes or any other detector). That photon is observable, so it's not a virtual photon. The reason you can see is that there are gazillions of those photons floating around, bouncing everywhere and then.

Now if you understand that basic definition, then we can talk about interactions, what are they, how are they calculated, what role does the Heisenberg Principle play, what are Feynman diagrams, what's their use, what's their limitations, what's renormalization, what's gauge theory, what role does symmetry play in QFT, what's the advantages of going through the Hamiltonian as opposed to the Lagrangian formulation, what about the Feynman path integral, what's the core idea behind QFT, and why do we need QFT instead of QM, and a dozen other topics. But if you have trouble of understanding basic definition, we're not going to get anywhere.

Look, thanks for the table of contents of a field theory textbook you just gave, I think I know the basic definition well enough, but I also think that these categories are not very useful once you get to the dirty details
( but I'm still unhappy by one point in your explanation. You say that virtual photons are those that are exchanged between electrons. And real photons are emitted but not absorbed. But then if they are supposed to be registered by a detector, they need to be absorbed by an electron or proton in the detector, and that, at the end of the day, is an exchange, right?)

But - I think you misunderstood me, my point was not that there cannot be any technical distinction between virtual particles and others (though I don't find these categories the way you use them too useful). I was merely pointing to the technical distinction and arguing that it is philosophically dubious to say based on that, that one kind "really exist" whereas the other is just an artefact of the calculation. My point always was that such an ontological distinction is not as obviously justified as was stated upthread.