What do you think the E in EM fields stands for?
They are not interchangeable terms, nor do they describe the same phenomena. A static electric field has no magnetic component and therefore no photon.
A static electric field is a reference frame dependent object. If you hold in your hands what you think is a static electric field with no magnetic component, and I whiz past you at half the speed of light, I am going to see a field with both electric and magnetic components. Which one of us is correct? We both are. This is why physicists insist that E and M are two parts of the same phenomenon.
I’ve tried answering the OP’s question three times now, and came each time close to something along these lines, but couldn’t seem to find an end. So I’ll just endorse this, and furthermore link to an excellent description of how the electromagnetic force emerges; it’s got a bit of math, but I think the text can be followed without in-depth understanding of the equations. Clicky here!
Though my knowledge of QFT only really extends to a few tehnical to ‘technical-lite’ introductions, there are severe problems as viewing virtual particles as anything other than convienent mathematical fictions (though it’s clear some if not many physicists do view them as more).
It’s all to do with the fact they represent terms in peturbation series. By giving them an ontological value you’re effectively giving special billing to the non-peturbed system even though in the basic theory of QFT there’s no reason for this. The fact that we view the petrubed system as a peturbation of the non-petrubed system is purely practical. Plus it may be possible to view a system as the petrubation(s) of two different systems, which means which virtual particles appear would be present is entirely due to the arbitary chocie of which system you have.
If you’ve got enough quantum theory not to be scared away by a few equations and a bit of technical language and a few equations, but wouldn’t consider yourself an expert this paper by Hrvoje Nikolic makes interesting reading. Section 9.3 is the relevant section for this psot.
Arrgh! I need a 12 pack now! Who’s buying?
I will not stand in between a man and his 12-pack, but to reduceit to simpler terms:
Let’s say we have a physical system A which we wish to (mathematically) describe. One way of describing it is to take another simlair physical system A[sub]0[/sub] which we already know how to describe and then adding various ‘corrections’ A[sub]1[/sub], A[sub]2[/sub], etc which make it more and more like A. I.e.:
A = A[sub]0[/sub] + A[sub]1[/sub] + A[sub]2[/sub] …
With each term becoming successively smaller.
This is the basic idea behind peturbation theory and what virtual particles represent mathematically are the terms in the series representing correction i.e. A[sub]1[/sub] and so on.
Now the problem with viewing virtual particles as ‘real things’ is that it’s saying system A[sub]0[/sub] is special and that A should only be viewed in terms of ‘corrections’ of A[sub]0[/sub]. Of course A is a physical system just as much as A[sub]0[/sub] so there’s no reason for this.
Secondly, the total of any sum can be decomposed in to a different sum. e.g. the sums ‘2 + 2’ and ‘1 + 3’ are different sums as they have different terms however their totals are the same. We could (in theory at least) use system B[sub]0[/sub] as our starting point rather than A[sub]0[/sub] such that:
A = B[sub]0[/sub] + B[sub]1[/sub] + B[sub]2[/sub] …
As B[sub]0[/sub] is different to A[sub]0[/sub], the set of terms {B[sub]1[/sub], B[sub]2[/sub], …} will be different to the set of terms {A[sub]1[/sub], A[sub]2[/sub], …} and hence will represent a different set of virtual particles. This is despite the fact they are both describing the same physical system the only difference being that we choose a different starting point.
I think this is really a problem that’s more general than just whether or not virtual particles exist (on which issue I tend to agree with what you said). But it’s very often the case that you can describe the same physics, as in observations, with different mathematics, making reference to different mathematical entities. Which description should be given ontological priority? Take something like AdS/CFT, where you can describe the same physical situation both as a gravitational theory in some volume, and as a quantum field theory on the boundary of that volume – both formulations don’t even agree on the number of spatial dimensions, so how many are there really? Or simpler, take gauge invariance: different choices of potential lead to the same electromagnetic field, so which one is real? Is any?
My own personal view* is that in a sense, there is no matter of fact to the distinction. The laws of the universe are the laws of a certain universality class of systems – i.e. systems which may be ‘microscopically’ different, yet have the same macroscopic description, such that there is no effective way of finding out the ‘true’ microscopic ontology. Something like this is natural in a computational universe: there’s no difference between universes written in perl and run on a mac and universes written in lisp and run on a windows machine; from the inside, thanks to computational universality, they’ll look exactly the same, though their ‘microscopic’ implementation may differ. Moreover, I don’t think there’s really a fact of the matter as to the actual microscopic implementation: they’re all equally real, or equally unreal, just as the paths some particle takes are. Asking about the ‘true’ microscopic laws is like asking about how the knight in chess ‘really’ moves: one to the side, two straight, or one diagonally, one straight, or two straight, one to the side… It just doesn’t matter, and as far as the rules of chess are concerned, it’s just not a sensible question to ask.
But of course, that’s just my own take on the matter.
*Argued at some length here.
I agree with you up to a point, but I think when it comes to virtual particles it’s not really made clear that they are a matehmatical artifact of a particular method of solving physical problems which is used mainly as it’s a good way of obtaining approximate solutions to problems. Virtual particles don’t fall out of the theory naturally instead they’re artifacts of a particular problem solving method and even within the confines of that solving method which virtual particles ‘exist’ and ‘don’t exist’ is fairly arbitary.
Personally I think ontology is only really as good as it is useful in helping you to get a handle on things in physics and whislt it could be argued that giving ontological existance to virtual particles is useful in that sense. However let’s be clear that virtual particles are not a vital part of quantum field theory.
I don’t want to hijack this thread, so I’ll just ask this one here, before I think about starting a separate thread.
I can understand how virtual particles aid in modeling, understanding or predicting the nature of nature. But if it’s merely a construct of the mathematics to describe nature in some sort of “language,” isn’t it important to make the distinction between what is or can be real, and what cannot (i.e. noting that the math is a description and not the actual thing(s) itself it attempts to describe)?
Yes, there is a clear distinction. Virtual particles are not real, hence the name ‘virtual’.
Ok, cool then. 
Assuming the OP was asking what an EM field is made of, in an ontological sense, saying it’s “virtual photons” doesn’t really cut the mustard.
So, the EM force just… is? That is: it is fundamental, at the moment, to our understanding. It offers an explanation for how electrons and photons do their thing, but can we offer any further depth or insight on this force (outside of mathematical modeling)?
On the other hand, you can also make the argument that, when you really get right down to it, that all particles that are ever detected are virtual, just really, really close to their mass shell. Technically, a real particle must propagate off to (or in from) infinity, and a photon that left the surface of Sirius and collided with a rod cell in my eye didn’t actually make it to infinity.
So if virtual particles don’t really exist, but all detected particles are virtual, then we never actually detect anything that really exists.
Is it generally excepted that all elementary / subatomic particles have zero dimensions?
That is, everything we’re made of is really just a complex interaction of forces, and there’s really nothing there, – as we intuitively think of matter as having dimension and “substance” – but these fundamental forces centered over a zero-dimensional point? *
*without getting into string-theory and more than 3 spacial dimensions.
From what I understand this is stretching the concept of Feynman diagrams well past breaking point. A photon thta is detected by your eye cannot in any way thoguht to be virtual.
It is worth pointing out though that the concept of ‘real particles’ isn’t as clearly defined in QFT as one might think. The paper I posted earlier goes in to this
In quantum mechanics you assume particles are point particles, i.e. they occupy a single point in space and have no volume. Now of course there’s cavaets to this as they don’t always have a well-defined postion and their wavefunction can spread out.
Quantum field theory (under discussion here) is different as it treats everything as fields, so for example electrons are described in terms of the Dirac field. In QFT there isn’t really even a basic concept of a particle.
Very interesting. Thanks, 'pants.
I think one can adopt the view that the whole particle vs. field thing in QFT is just the wave/particle dichotomy of quantum mechanics iterated once over: the quantum ‘field’ simply has both discrete and continuous aspects, the former of which one would associate with particles, while the latter are usually thought of as characteristic of fields. But neither reading need be anymore correct then the wave- or particle-reading of quantum mechanics: we’re not dealing with objects classical intuition applies to.
This may seem somewhat unsatisfactory in terms of getting answers to the fundamental questions, but it’s easy to see that something’s gotta give: if the ordinary stuff our world is made of can be explained in terms of more fundamental, but equally ordinary stuff, then we stand before the question of what this more fundamental stuff is made of, and can continue this iteration all the way down the rabbit hole. If, on the other hand, there is to be some stuff that just is, something that artificially anchors the regression, then questions regarding the origin or composition of that stuff seem to be unanswerable, which I’d think is similarly dissatisfying.
So we must, if we expect for answers to exist, eventually come to fundamentally different kind of stuff, to which our ordinary concepts don’t quite apply. It’s similar to the problem of where life came from: if the ancestor of every living thing is a living thing, then either the chain of living things stretches back infinitely far into the past, or there must be some first living thing that popped up by magic. But this problem is just a problem of human categorization: ‘living thing’ is not an as sharply defined category as one might first think. Life can gradually develop from non-life, over a series of intermediary stages that don’t quite belong to either category. Perhaps it’s the same with the stuff that exists, or that stuff is composed of; perhaps the question of whether something exists is of a similarly ambiguous nature as the question of whether something is alive, in certain circumstances.
Aren’t we all.
Uh…I have virtual hangover today.