at that site Arnold Neumaier, a theoretical physicist, explains principles to the layman (like me) about all the aspects of quantum mechanics. An excerpt of one of his writings on virtual particles reads thusly:
"the literature readily shows that the details of Feynman
diagrams strongly depend on the perturbation scheme used:
In light front calculations, one gets a completely different set of
diagrams than in the more traditional covariant form. And in
nonperturbative approaches such as lattice gauge theory or conformal
field theory, the predictions do not involve virtual particles at all.
How can anything be real if its existence depends on a particular way
of viewing the world? How can an experiment (verifying the Casimir
effect, say) can be said to prove the existence of virtual particles
if the same experiment can be explained by a method of calculation not
involving virtual particles at all?" http://www.mat.univie.ac.at/~neum/physfaq/topics/virtual
Now I have not read (or even understood) all of his writings, but 1 thing is nagging at me, what is magnetic attraction/repulsion if not the exchange of virtual particles as I had been led to believe?
Virtual particles are merely a useful way of describing the terms in a perturbation expansion – they shouldn’t be thought of as “real.”
Magnetic attraction/repulsion effects can be calculated using perturbation theory. This means that you can describe the force in terms of the exchange of virtual particles. It doesn’t mean anything more than that. There is nothing controversial about this viewpoint.
On the other hand, philosophically you are more than welcome to think of them as “real”. Physicists casually refer to them as though they were real, because they are such a useful description in certain circumstances. For example, consider the decay of the muon. It decays to a neutrino and a virtual W-boson. You can think of the virtual W-boson as “real” if you want – it behaves very much as though it were a real W-boson, except that it must decay more quickly because it is virtual. On the other hand, you can also think of it as a useful description for something that happens during the process of the muon decaying. In either case you never “see” the W-boson directly – at the end of the day the muon decays to a couple neutrinos and an electron.
Of course, one can also make the argument that the only particles we ever detect are virtual particles. When I look up into the night sky and see a star, what’s happening is that there’s a photon with one vertex at some electron or proton in the star’s photosphere, and another vertex at an electron in my retina. Since the photon just connects two charged particles together and does not propogate off to (or from) infinity, it’s not, strictly speaking, a real particle (just really, really close to being real).
One way of thinking about what Chronos just said is that the only way of ever “seeing” a photon is through its effect – it exerts a force inside your eye. Ultimately all we know is that there is a force inside our eyes, and it is up to you how you want to interpret it. What we call a “photon” is just an incredibly convenient description of how a particular kind of force is conveyed between objects – it comes in discrete lumps with certain properties, such as energy and velocity.
Basically you’d have a really hard time justifying the ‘relaity’ of virtual particles. To do so would attach special significance to the system that is being peturbed over the systems you wish to study as peturbations of that system.
To take analogy from another area of physics: linearized gravity is a first order approximation of general relatvity theory. In it the metric tensor filed g is approximated as g = η + h where η is a known exact solution to Einstein’s field equations. The metric tensor field h has no physical signifcance other than representing the approximate difference between the spacetime we use as our starting point and the one we wish to study. This is essentially what virtual particles represent i.e. approximate differences between the peturbed system and the one which we wish to study.
This reminds me of the situation late in the 19th century regarding atoms - they were convenient for describing the ratios at which elements combine, and other phenomena, but there was strong resistance among some scientists against considering them “real” because they could not be observed. One of Einstein’s contributions to physics was his analysis of Brownian motion, which provided an estimate of atomic size which was consistent with estimates based on entirely different physical phenomena, which convinced almost all of the holdouts of the reality of atoms.
Note: I am not suggesting that we’ll someday have an equivalent view of the reality of virtual particles, just that the current status of virtual particles as not real but darn useful to consider as real is not new in physics
But there’s a difference in that there exists an atomic theory of matter, whereas there doesn’t really exist a ‘virtual particle theory of interactions’.
I’m slightly out of my depth here (i.e. my knowledge of the practicalities is very limited) so I hope the following is correct: lets say we have a physical system we wish to study which we’ll call system A and we choose to study it as petrubation of another system, system B. Okay that’s fine and dandy we get our petrubation series with the terms representing virtual particles. What though if we choose to study system A as a petrubation of system C (i.e. a distinct system from system B)? The petrubation series must be different so we get a different set of virtual particles. The key point being there’s no reason to give primacy to system B.
I agree - like I said, it was just an interesting fact that physicists often find it convenient to talk about things that are not necessarily real, but that sometimes they turn out to be real.
My own view is that both viewpoints can be equally “real”. Ultimately, one can suppose that there is no such thing as any particle at all. Everything – electrons, protons, photons – none of them we see “directly”. The best we can do is to describe things using a certain model, and often there are multiple models that are isomorphic to one another. In some cases one model is more parsimonious than another, but ultimately the question of which is more “real” is philosophical.
I think this is a debate about what “real” means or should mean, in disguise.
It’s pretty easy to demonstrate that light comes in particles and not waves. It is also pretty easy to demonstrate the opposite. Proving two mutually exclusive things may just be a sign that our thinking congregates into ideas that lack something.
Nobody wants anybody to do anything with real or virtual particles. The only thing people try to do with this stuff is to understand what we observe and be able to predict what else we can make happen. And, understanding what we observe, which sounds like a real activity, is really just part of learning to predict what else we can make happen. The only proof is in nifty and surprising inventions and demonstrations.
It’s not so much about modelling as such, it’s a way of finding approximate solutions in the models that we already have.
I think even a philosopher would baulk at assigning ontology to the peturbative solutions which are approximations of the non-petrubative solution (which will not contain artifacts like virtual particles).
You could argue their usefulness may be suggestive of their existance, BUT in current theory they don’t exist in any model insetad, they’re a conceit used to approximate models.
I agree, and yet everything is a conceit. The “real” (ie non-virtual) unclothed electrons described by a non-renormalized lagrangian are a conceit: a low-order approximation, valid only at tree-level. The only thing that exists in our current models independent of perturbation theory are quantum fields. All particles are just useful conceits. But even the quantum fields are likely a low-energy approximation. But even if they aren’t, they may be dual to another theory (AdS/CFT, for example), rendering any guess at what is “real” a matter of taste.
