# Trying to understand

1. Nov 4, 2006

### wolram

What a virtual particle *is* in theories of quantum gravity.

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

You must understand that a non scientist may want to know how things work, and it is unfair for you guys to use terms like *virtual* without
giving some explanation as to what it means.

2. Nov 4, 2006

### arivero

I already told in other thread, I would like understand the psicologocal or semiotical implications of the word "virtual" when used in "virtual particle", because it is probably the most repeated physics question in the internet (check the FAQ of physics in USENET). Not that people asks about particles under Heisenberg uncertainty or about particles in Off-shell Feynman diagrams... they ask about virtual particles.

3. Nov 4, 2006

### wolram

I can just about understand Heisenberg uncertainty, but how does this help
in understanding the real world, it is you guys that use these words, i just want to know how they help in understanding the blossoming of the universe.

4. Nov 22, 2006

### actionintegral

Interesting that I found this post on a search for the precise definition of "virtual". Apparently there is a gap between the "popular" notion of virtual and the "scientific" notion.

5. Nov 23, 2006

### Demystifier

Here is a citation from
http://arxiv.org/abs/quant-ph/0609163

Virtual particles?

The calculational tool represented by Feynman diagrams
suggests an often abused picture according to which
real particles interact by exchanging virtual particles".
Many physicists, especially nonexperts,
take this picture literally, as something that
really and objectively happens in nature. In fact, I have
{\em never} seen
a popular text on particle physics in which this picture was
{\em not} presented as something that really happens.
Therefore, this picture of quantum interactions as processes
in which virtual particles exchange is one of the
most abused myths, not only in quantum physics, but in
physics in general. Indeed, there is a consensus among experts
for foundations of QFT that such a picture should
not be taken literally. The fundamental principles
of quantum theory do not even contain a notion of a
virtual" state. The notion of a
virtual particle" originates {\em only} from a
specific mathematical method of calculation, called perturbative
expansion. In fact, perturbative expansion
represented by Feynman diagrams can be introduced even in
{\em classical} physics \cite{thorn,penco}, but nobody
attempts to verbalize these classical Feynman diagrams
in terms of classical virtual" processes.
So why such a verbalization is tolerated in quantum physics?
The main reason is the fact that the standard interpretation
of quantum theory does not offer a clear canonical" ontological picture
of the actual processes in nature, but only provides
the probabilities for the final results of measurement outcomes.
In the absence of such a canonical" picture,
physicists take the liberty to introduce
various auxiliary intuitive pictures that sometimes help them
think about otherwise abstract quantum formalism. Such auxiliary
pictures, by themselves, are not a sin. However, a potential
problem occurs when one forgets why such a picture has been introduced
in the first place and starts to think on it too literally.

6. Nov 24, 2006

### Hans de Vries

They will basically tell you that a 'virtual' particle in QFT is off the mass shell

$$E^2 - c^2p^2 \neq\ m^2c^4$$

This hasn't anything to do with Heisenberg's uncertainty principle and the
idea that a particle "can have a different Energy for a short time"

On the contrary. The laws of conservation are always observed
exactly in QFT, for each and every vertex. The fact that they are off
the mass shell is simply because the particles are not free solutions but
are interacting. Every real particle which interacts is "off the mass shell"
One could use the terms "free" and "interacting" particles instead of
the terms real and virtual

In more technical terms:

The equation of motion of a virtual electron contains the interaction term
from the photon “it has absorbed” while the equation of motion of a
virtual photon contains the transition current representing the change
of motion of the electron which has emitted the photon. This is the reason
why they are off the mass shell.

Regards, Hans

Last edited: Nov 24, 2006
7. Nov 24, 2006

### vanesch

Staff Emeritus
Great text

8. Nov 24, 2006

### Demystifier

Thanks vanesch!

9. Nov 24, 2006

### Hans de Vries

This may all be written out of some sort of preoccupation that a picture
is taken to literally for your taste or gut feeling , however, the text has
become way to dogmatic for my personal taste.

Those who consider a literal interpretation seriously are likely somewhat
dumb (associate them with non-experts). "Kindly" you grant that they are
not necessary evil though. (no sin). But still you cast doubt that their
viewpoint should be tolerated at all.....

There are many physicist who do take the literal interpretation very
seriously. Nobel price winners like Feynman and Veltkamp (edit: Veltman)
For the latter the only thing which really makes sense in QFT is the
diagrammatica.

Interestingly. These people are/were down-to-earth, no nonsense people
in a quest for a non mystical, logical picture of Quantum Mechanics. Such
a picture however does not necessary need to coincide with the most
common primary reactions like QM should be Brownian motion or Bohmian...

Regards, Hans.

Last edited: Nov 24, 2006
10. Nov 24, 2006

### Careful

Actually, your previous (and this) answer is the only one with content to it in this thread

Careful

Last edited: Nov 24, 2006
11. Nov 24, 2006

Staff Emeritus
Oh yeah? Just because you liked that part and not the latest? Strikes me that this thread is getting pretty heavy on the personal theory side. Let us all remember that the great physicists who built field theory up to the level of the standard model were not fools, and may even have understood more deeply than we do. ALL of us!

12. Nov 24, 2006

### Careful

I clearly meant post 6 and 9 written by Hans. There is nothing personal about it, he gives a fairly objective point of view on the issue. I think we misunderstood each other here. Of course I agree with the last post, it is well known that Veltman is quite sceptical towards QFT.

Last edited: Nov 25, 2006
13. Nov 24, 2006

Staff Emeritus
if you disagreed you could have said you disagreed instead of the insulting expression you did use.

I don't know why so many savants from Europe think that this kind of trash talk is appropriate in discussing controversial issues.

14. Nov 24, 2006

### Careful

The thing is that there is no controversy in what Hans said (and neither is there about this topic), some other comments on this thread were much more insulting (as correctly pointed out by him). By the way, I believe Freidel (?) developped a theory of 2+1 quantum gravity with point particles entirely based upon a fairly literal'' interpretation of the Feynman diagrams.

15. Nov 24, 2006

### Hans de Vries

Martinus Veltman (sorry, my typo) strongly beliefs in Feynman
diagrams (diagrammatica) but is not so impressed by how they
are derived, quote:

And further, in relation to the discussion on this thread:

Regards, Hans

Last edited: Nov 24, 2006
16. Nov 24, 2006

### arivero

IHans, about this explanation, could you be more concrete for instance in the case of electroweak interaction by exchange of a W or Z0 particle? My understanding is that two electrons, or two neutrinos, can interact by exchange of a Z0 even if their kinetic energy is a lot less than the mass of the Z0.

17. Nov 24, 2006

### Hans de Vries

Good remark. For neutral weak processes there are vertices where a
neutrino emits a Z. (Like in Griffiths 2.4). Now consider conservation
of momentum and energy at the vertex if the Z has its usual mass
of 91 GeV...

Regards, Hans.

18. Nov 24, 2006

### CarlB

Thanks for the note. I feel great sympathy for that point of view and will look around for Veltman's writings.

Okay, I've ordered a copy of Veltman's introduction to QFT through Feynman diagrams:
http://www.abebooks.com/servlet/SearchResults?&isbn=0521456924&nsa=1

A great Veltman article:

Perturbation Theory and Relative Space
Martinus Veltman, 2006
The validity of non-perturbative methods is questioned. The concept of relative space is introduced.
http://arxiv.org/abs/hep-ph/9404358

By the way, I've just added a section to my book on the elementary particles that has to do with the concept that Feynman diagrams are more fundamental than the Lagrangian. It's section 7.6:
http://www.brannenworks.com/dmaa.pdf

Carl

Last edited: Nov 24, 2006
19. Nov 25, 2006

### Careful

Nah, I should have noticed it, the only excuse I have is that it was late in the evening (and I must have thought about ice skating or so)

Last edited: Nov 25, 2006
20. Nov 25, 2006

### Ratzinger

Unfortunately I'm such a non-expert who takes it too literally.

To describe the interaction of two unaccelerated charges on each other ( a conservative static force field) we need the concept of virtual particles. Unless we believe in forces-at-distance.

What am I missing?