（1） 有些人认为，虚粒子只是个数学记号，或者是图语言方便，顶多是费曼或费曼图（或微扰论）给我们洗脑后的错觉。这似乎是主流观点。这篇写得蛮早的科普式文章不长：International Journal of Theoretical Physics, Vol. 3, No. 6 (1970), pp. 507-508。
（2） 有些人认为虚粒子与实粒子一样真（”否则你如何解释’电子靠虚光子传递电磁交互作用’这句话？”,”virtual is not imaginary!” “virtual particles do keep popping in and out of existence .”）。 Michigan的Gordon Kane的公开答复具代表性： http://www.scientificamerican.co ... rtual-particles-rea 把夸克，W粒子Z粒子，Casimir力等当作虚粒子是”真实”的例证。
I think, like almost everything in quantum theory, you have to define the question you ask first, then the answer follows. The question of what a virtual particle is really depends on what we mean by particle, or real particle.
The point of view 3 is wrong in detail, electrons are stable. However, it does underline the fact that we do have to be careful about what we mean by particle, and width will inevitably play a role for unstable particles in that definition.
Usually, we tend to link particle to a lump of energy which has a well defined momentum and propagates freely. This does not have to be the definition, and it is not necessarily more correct than other definitions. However, it is a useful one especially in weakly coupled theories, since such a picture for particle is the same as the asymptotic states we like to use to define S-matrix. In particular, in this case, you will be able to say particle 1 collide with particle 2, and particles a and b come out in the end. For this picture to be true, we encounter the most common way people define a real particle, a degree of freedom can be produced for which the mass is much bigger than the width. We can then sensibly talk about such a degree of freedom being ``on-shell", which can propagate to a distance much longer than its de Broglie wave-length (we can measure it as a wave packet), or its energy can be measured more precisely than its mass, i.e., ``free" propagation. Although not necessarily true in every process, such a degree of freedom can be produced on-shell, since such a concept makes sense in this case.
Of course, you would ask what I mean by ``much bigger". I will answer that is indeed imprecise language used to make a simplified point. The real point is that, as long as the mass is larger than the width, you can think of this is a degree of freedom which can be produced as a particle. However, the size of the width exactly tells you the limitation of such a picture, such this particle can not be completely a free propagating one. In the limiting case in which the width is the same as the mass, this "particle" will decay on the same time scale we need to see it is a particle.
For virtual particles, of course you are not the mass-shell, typically off by an amount bigger than the width. It cannot propagate as free state. However, whether they exist really depend on what you mean by "exist". If you only recognize things which propagate freely, you won't call virtual particles particles. But, something exists. In more technical language, the existence of virtual particle change the structure of the Green's function. In particular, it introduces poles to the Green's function although you are not necessarily going through the pole, depending of the kinematics of the external legs.
Gordy's explanation is probably for popular audience. The precise way to say it is through the pole argument above. And more precisely, virtual particle is just off-shell, where energy momentum are always conserved, by the Poincare symmetry of the underlying theory.
I have not read the text of that paper in 1970. However, the abstract looks absurd. And, I would not believe anything written about quantum field theory 39 years ago, unless it was from someone whom you can really trust.
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发表于 2009-5-9 23:53:44 
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发表于 2009-5-10 01:14:33 
Blackwood
发表于 2009-7-15 08:24:43 
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