# Existence of Virtual Particles

1. Apr 16, 2008

### Bose

As I understand it, according to the Copenhagen interpretation of QM, nothing can be said to exist until it is observed. I have also read that it is impossible to observe virtual particles in an experiment.

How is it then that virtual particles can be said to exist?

2. Apr 16, 2008

### lbrits

...if a tree falls in the forest...

3. Apr 17, 2008

### reilly

Do a search of this forum for "virtual particles" You will find much to read and consider--including the fact that virtual particles, almost always, are artifacts of perturbation theory. The "almost always" makes allowances for resonances.
Regards,
Reilly Atkinson

4. Apr 18, 2008

### Fredrik

Staff Emeritus
I would say that a description of what happens in terms of virtual particles is just one of many equivalent ways to describe the same thing. Virtual particles appear in the mathematics when a certain function is expanded in a series, kind of like the expansion exp(x)=1+x+x2/2+x3/6+...., but I don't think it would make sense to say that it's the individual terms in the series that describe what "actually" happens.

The statement "nothing can be said to exist until it is observed" is pretty strange. I guess you can say that, but in that case I'd rather treat that statement as a partial definition of what we mean by "existence" instead of as a statement about quantum mechanics. But I'd rather not say that at all.

5. Apr 18, 2008

### malawi_glenn

We can not tell wether they exists or not I think, however we can look if QED, QCD etc makes sense with experiment, and they indeed do that.

Also one has find the real counterparts of the proposed virtual particles (Z, W +/- , gluons) so that was also a gret sucess for quantum field theory.

And I am not sure wheter 'Copenhagen interpretation of QM' applies to QFT or not.

6. Apr 18, 2008

### haushofer

My understanding of virtual particles is this:

In quantum field theory, you can describe the amplitude of a field by a path integral ( for a heuristic motivation of this path integral, maybe you've read the first chapter of Zee's QFT in a Nutshell, that's very nice ) You can't calculate this path integral exactly, but you have to expand it in a power series and integrate term by term. In each term, you will encounter so called propagators. In these propagators, you integrate over all possible momentum. And that's not what you're used to do; for example, you're used to that a particle obeys

$$E^{2} - p^{2} = m^{2}$$

So these propagators appear to describe particles with energies lying "off-shell", allowing also energies which don't obey the formula above. The particles pop up during the traveling of the field from A to B. These particles are called virtual, because they don't have to obey the energy condition. But they certainly contribute to the amplitude !