Are Virtual Particles Real or Just a Concept in Physics?

[michael879]

exchange forces have always confused me. I get the feeling they're made up to explain forces. The exchange of a photon between two electrons makes some sense to me. Conservation of momentum alone would make them repel each other. It makes some sense that on a larger scale this "force" would average out to what we see as the electromagnetic force.

However, what about the attraction between a proton and an electron? How do the two particles exchanging a photon make them move towards each other?? Momentum is not conserved here...

 

[jtbell]

See http://www.physics.adelaide.edu.au/~dkoks/Faq/Quantum/virtual_particles.html, from the Usenet Physics FAQ, in particular the second section.

 

[michael879]

ok that link raises more questions than it answers. Why would photons basically "tunnel" in a full circle to hit the oppositely charged particle from the opposite direction? Its like the photon somehow "knows" the particle is of an opposite charge. Explaining attractive forces like this says to me that exchange particles is a made-up concept used to explain something more complicated in layman's terms. However I have never seen any reference to this more complicated concept.

Also, there seem to be different types of virtual particles. Those that actually exist (like matter-antimatter pairs) and those that don't exist (like exchange particles). Is there actually a difference?

One more question is why force-fields seem to have been abandoned in quantum theories. Why was it substituted for the exchange particle idea? Do exchange particles scale up to the macroscopic world? Could you describe the force between two magnets in terms of virtual photons being exchanged??

 

[meopemuk]

ok that link raises more questions than it answers. Why would photons basically "tunnel" in a full circle to hit the oppositely charged particle from the opposite direction? Its like the photon somehow "knows" the particle is of an opposite charge. Explaining attractive forces like this says to me that exchange particles is a made-up concept used to explain something more complicated in layman's terms. However I have never seen any reference to this more complicated concept.

Also, there seem to be different types of virtual particles. Those that actually exist (like matter-antimatter pairs) and those that don't exist (like exchange particles). Is there actually a difference?

These are excellent questions. In my opinion, virtual particles are "made up" concepts. The truly fundamental concept in any quantum theory, including QED, is its Hamiltonian, which contains all information about particle interactions and their time evolution. QED attempts to calculate another important object, called S-matrix, which contains information about particle scattering and some other properties (e.g., energies of bound states), which can be measured in experiments. Usually, the S-matrix is calculated from the Hamiltonian by using perturbation theory. These calculations involve a large number of rather complicated integrals. In 1949 Feynman invented an ingenious technique of representing these integrals by diagrams. Each line and vertex in the diagram corresponded to a certain factor in the integrand. This technique enormously simplified manipulations with integrals of the perturbation theory.

The diagrams looked so nice that many people (including Feynman) started to use them to "explain" in layman terms what occurs in scattering events. Lines were interpreted as virtual particles that "move" between vertices, etc. etc. These explanations became so "sticky" that many people now believe that at large magnification a collision of electrons really looks like a web of virtual particles jumping back and forth. In my opinion, these beliefs have nothing to do with reality.

One more question is why force-fields seem to have been abandoned in quantum theories. Why was it substituted for the exchange particle idea? Do exchange particles scale up to the macroscopic world? Could you describe the force between two magnets in terms of virtual photons being exchanged??

There are few people (including myself) who think that it is too early to abandon the idea of force-fields. We are developing an alternative formulation of QFT, which uses ordinary force-field particle interactions to describe all effects usually attributed to exchanges of virtual particles, vacuum polarization, etc. In particular, in this (dressed particle) approach, the Lamb shift in hydrogen atoms arises as a consequence of small corrections to the electron-proton Coulomb potential. You can find more details in http://www.arxiv.org/physics/0504062

 

[michael879]

wow thanks a lot, that answered a lot of my questions. Ill be sure to check out that link, it looks interesting. I always wondered what the quantum-level force fields of particles would look like, since they don't even have a definite position.

I still have one question that probably got lost in my long post. There ARE "virtual" particles that actually exist right? For example the matter-antimatter pair swarms around charged particles that change their magnetic moments? I've seen these called virtual particles too and it seems to me like people use the same term for two different concepts.

 

[michael879]

another example of a "virtual" particle would be the production of the W-plus particle in neutron decay. Although it typically interacts virtually as a force-carrying particle, in this interaction it seems like its a real particle. An up quark decays into a down quark (might have that backwards) and a W-plus particle. The W-plus particle immediately decays into an electron and an anti-neutrino. This would make it natural to refer to the W-plus in this case as a virtual particle even if it wasn't actually one (Im not saying it actually exists, I am just saying it would make sense if it actually existed). Then again, it has to be real in SOME interaction since it's been detected..

 

[meopemuk]

wow thanks a lot, that answered a lot of my questions. Ill be sure to check out that link, it looks interesting. I always wondered what the quantum-level force fields of particles would look like, since they don't even have a definite position.

Why do you think that particles cannot have definite positions? There is a perfectly well-defined position operator in relativistic quantum mechanics (it is called the Newton-Wigner operator). Eigenstates of this operator are well-localized.

I still have one question that probably got lost in my long post. There ARE "virtual" particles that actually exist right? For example the matter-antimatter pair swarms around charged particles that change their magnetic moments? I've seen these called virtual particles too and it seems to me like people use the same term for two different concepts.

Yes, in the standard interpretation there are virtual photons and virtual particle-antiparticle pairs. These pairs also go under the name "vacuum polarization". In the "dressed particle" approach, the effect of "vacuum polarization" gets replaced with some corrections to potentials acting between real physical particles.

 

[meopemuk]

another example of a "virtual" particle would be the production of the W-plus particle in neutron decay. Although it typically interacts virtually as a force-carrying particle, in this interaction it seems like its a real particle. An up quark decays into a down quark (might have that backwards) and a W-plus particle. The W-plus particle immediately decays into an electron and an anti-neutrino. This would make it natural to refer to the W-plus in this case as a virtual particle even if it wasn't actually one (Im not saying it actually exists, I am just saying it would make sense if it actually existed). Then again, it has to be real in SOME interaction since it's been detected..

I think that in this situation W-particle plays a role that is different from "virtual exchange boson". In this case, W-particle is an intermediate decay product, and, in principle, it could be observed, e.g., by a track in a bubble chamber. However, this particle is too short-lived for us to see the track. I wouldn't call W-boson a virtual particle in this situation.

 

[michael879]

well if you say that anything that can possibly be observed isn't a virtual particle, than vacuum polarization particles arent virtual particles either since they have been observed..

 

[michael879]

what I meant was that if you take an electron in a potential well, it doesn't have a definite position. In this case, what would the electric field look like? I couldn't just be a spherical field around the electron, since the electron doesn't exist.

I now realize that this question could be wrong. It could be that the electron has no "electric field" until it's wave function collapses. i.e. coming close to it with a charged particle would make it collapse and then produce a force between the two particles.

That scenario still raises a lot of questions however, and if its wrong my original question still stands. The electric field could just be the sum of all possible electric fields weighted by their probability (if I had to guess this would be my answer). However in the case of gravity, this doesn't really work. If you have a particle of mass m in a superposition between two possible points, the above explanation would make the gravity field appear to be the same as the gravity field of two particles of m/2 mass.

 

[meopemuk]

well if you say that anything that can possibly be observed isn't a virtual particle, than vacuum polarization particles arent virtual particles either since they have been observed..

I don't think they were directly observed. It is true that there are some observable effects, like the Lamb shift of hydrogen levels, which are (partially) *attributed* to the presence of virtual pairs. However, as far as I know, nobody has directly observed electron-positron pairs inside hydrogen atom.

If these pairs are not directly observable, then I have a freedom to reformulate the theory (QED) in such a way that these pairs are no longer present. If this new theory correctly predicts all directly observable effects, then nothing is lost in this reformulation, as far as physics is concerned. This is exactly what "dressed particle" approach does. The effects that previously were explained by (non-observable) vacuum polarization are now explained by a modification of particle interaction potentials.

 

[meopemuk]

what I meant was that if you take an electron in a potential well, it doesn't have a definite position. In this case, what would the electric field look like? I couldn't just be a spherical field around the electron, since the electron doesn't exist.

I now realize that this question could be wrong. It could be that the electron has no "electric field" until it's wave function collapses. i.e. coming close to it with a charged particle would make it collapse and then produce a force between the two particles.

That scenario still raises a lot of questions however, and if its wrong my original question still stands. The electric field could just be the sum of all possible electric fields weighted by their probability (if I had to guess this would be my answer). However in the case of gravity, this doesn't really work. If you have a particle of mass m in a superposition between two possible points, the above explanation would make the gravity field appear to be the same as the gravity field of two particles of m/2 mass.

My answer is that we never measure directly electric and magnetic fields. Usually, we are measuring observables (positions, momenta, spins, etc.) of particles which interact with each other. The interaction potentials (electro-magnetic, gravitational, etc.) are parts of the Hamiltonian, which is supposed to describe particle dynamics. If we know interaction potentials (e.g., the Coulomb potential), we insert them in the Hamiltonian and we can calculate the time evolution of particle wavefunctions and other properties that can be directly compared with measurements. In these calculations, there is no need to assume that particles are in localized states. Actually, it is impossible to keep them in localized states, due to the wave packet spreading. But I don't see any problem associated with it.

 

[michael879]

havnt they seen electron-positron pairs being created in a cloud chamber? I know I've seen the picture of two opposite spirals that is supposed to be that..

 

[meopemuk]

havnt they seen electron-positron pairs being created in a cloud chamber? I know I've seen the picture of two opposite spirals that is supposed to be that..

Yes, electron-positron pairs can be easily created in a cloud chamber. These are "real" particles, not "virtual" ones. They can be seen, measured, etc.

The name "virtual particles" is reserved for internal lines in Feynman diagrams used to calculate scattering amplitudes with real particles (whose states are represented by external lines in the diagram). In some Feynman diagrams you can see internal electron-positron "loops". It is tempting to say that a pair of particles is really created and then annihilated after a short time. My point was that this literal interpretation of Feynman graphs is misleading.