# Virtual Particles

1. Sep 25, 2006

### Duhoc

Perhaps someone can explain virtual particles. My intuitive notion of them is that particles come in and out of existence to communicate to other particles how to behave and if you try to observe them they disappear. How is the Higgs mechanism responsible for particle masses? What is a Higgs field? What is the hierarchy problem and has new evidence been obtained to resolve it?

Duhoc

2. Sep 25, 2006

### arivero

I have a different question: Where did you first heard about virtual particles? I was amazed to know that it was a typical topic from newbies in the physics newsgroups, and I can only think that it is a cultural issue, perhaps from some pupular book in English-speaking countries.

3. Sep 25, 2006

### Duhoc

Yes

From a survey type book. I had difficulty understanding the concept. I first posted it on the cosmology board and it was moved. I would like to understand it as it leads into the other concepts I mentioned.

Duhoc

4. Sep 25, 2006

### arivero

What title? I'd like to know if there is some common book to blame.

Of course if *doesn't* leads in a straighforward way to the other concepts you mentioned, but every can be convolved in a survey book. For instance I can tell that virtual particles are respoinsible of renormalisation and that renormalisation is responsible of the hierachy. But I could also say that virtual particles drive to techicolor condensation and then to models without higgs and without hierarchy.

5. Sep 25, 2006

### Duhoc

patronizing

Strange how you answer a question with a question. Anyway, what does pupular mean? Semicolons are not used at the end of sentences. What do you mean a cultural issue? Also what did you mean by "if "doesn't leads" "Respoinsible" is spelled r-e-s-p-o-n-s-i-b-l-e. Just in case you are interested. When you start a sentence with "for instance" or "for example" you use a comma. And really, I thought I might get a response from someone who is an illiterate. So thanks for nothing.

Duhoc

6. Sep 25, 2006

### CarlB

7. Sep 25, 2006

### Duhoc

A functional illiterate then. Anyway, I will not post here again.

Duhoc

8. Sep 26, 2006

### Severian

One should keep in mind that all particles are in fact virtual. They are distributed randomly in a gaussian distribution around their mass-shell, so the probability of them being exactly on-shell is precisely zero. We only regard 'real' particles as on-shell because they have to be very close to mass-shell to live any appreciable time.

On a sidenote, may I ask Alejandro why these papers have not been published?

9. Sep 26, 2006

### CarlB

I've been working on rewriting QM to the density marix formalism and one of the things that comes out of this is that all particles have to be treated as if they were virtual. That is, one ends up rewriting Feyman diagram amplitudes so that the initial and final states are treated the same as the intermediate states.

A short way of stating this is that in the usual state vector formalism an amplitude looks like:
$$\langle I | M | F\rangle$$

where I and F are the initial and final state vectors and M is some complicated thing. In rewriting this in density operator form, M is easy to convert as it is already in operator form. And the conversion for I and F is to turn them into density operators and rewrite the above amplitude into operator form:
$$|I \rangle \langle I | M | F\rangle\langle F|$$

But the internal (virtual) propagators already were in density matrix $$|\psi\rangle\langle \psi|$$ form, so written in density matrix form, everything looks to be in virtual form.

What happens in the above is that a complex number gets replaced with what turns out to be a complex multiple of an operator. You can then get the squared magnitude (i.e. a real number) by the usual technique, but it ends up coming out multiplied by a density operator (which, unlike the state vectors, do not have arbitrary complex phases).

Carl

Last edited: Sep 26, 2006
10. Sep 27, 2006

### LURCH

arevero,
Firstly; allow me to apologize on behalf of English-speaking people everywhere. I find it highly ironic when someone who speaks only one language (as is the case for nearly all Anglophones) refers to a multilingual individual as "illiterate". However, you may take some complement from his remarks. I believe that Duhoc has mistaken you for one of the millions of people for whom English is a first language (indeed, their only language) yet their speaking and writing is filled with the same kind of little mistakes for which you were being criticized.

Secondly; yes, virtual particles have become a part of common culture in most English-speaking countries in recent years. I believe that most laypeople were first introduced to the idea of virtual particles when popular publications started talking about Hawking Radiation. Since Duhoc has stated that his first post was originally submitted to the Astronomy Forum, this would certainly seem likely in his case.

11. Sep 27, 2006

### arivero

Thanks. :!!) Intentional or not, it does an incredible apology (meaning, defense)