Natural Order and Science

[little_monkey]

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?)

Yes, of course the photons are absorbed by the electrons in your eyes, but if you understand the definition, those electrons in your eyes are part of the detector. So by definition, those electrons are not VIRTUAL. You think you understand the basic definition, but you don't. BTW, that table of contents, I can teach you every single topic in that table of content. I'm not bragging, I'm just stating a fact.

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.

 Before discussing any topic or the validity of any theory, it would help if you can understand basic definitions, otherwise we will pass each other and never get anywhere. Secondly, I've never made the claim that one photon is "more real" than the other. Go back to my earlier post where I stated, "For the record: virtual particles are particles you can't see. Not that they are not real or some kind of illusion." 

Just to elaborate on this topic. From classical physics, we would look at these interactions in the following way: two electrons are closed to each other, and we would see that they get off their initial position. We would conclude that there was a force between them, in this case an electric repulsive force. In QFT, the language changes to: two electrons interact by exchanging virtual photons, and so they go off their initial position. We don't get to "see" the virtual photons, how could we,  but by making that assumption, QFT gives results that agree with lab confirmation to the tune of one part in 10-8. So far, there isn't any other theory that can best that result. 

But you could argue, how do I know this is taking place - this exchange of virtual particles? It's no more different than Einstein postulating that molecules are in motion, and then explaining Brownian motion on the basis of that hypothesis. Do we really see those molecules in motion? Not really. And so this is science in action: you make an hypothesis that gives a theory in the mathematical form of an equation, you make calculations, then you check your (theoretical) results against what's observed in the lab. If you get confirmation, then it's a green light for your hypothesis/theory/equations. No confirmation means go back back to square one, and start all over.