From entropy's side all processes should be irreversible, in that they all cost some 'energy', described as heat loss mostly, as I've seen. So you can't really turn back the process to its original state. As I too often state, I don't know what 'energy' is, except something describing transformations. But somehow the idea of a universe in where nothing gets lost must contain whatever disappears in a transformation. Which then points to 'heat' thermodynamically.

'Virtual particles' is a very faceted idea that hurts my head. Over the years it seem to have gone from an idea to a 'fact', without no experimental evidence I know of. The idea of the Casimir effect proving it is, as far as I know, still disputed. I've seen it explained as a effect of the matter involved, as well as of 'virtual particles'.

"Virtual particles must not be considered real since they arise only in

a particular approach to high energy physics - perturbation theory

before renormalization - that does not even survive the modifications

needed to remove the infinities. Moreover, the virtual particle content

of a real state depends so much on the details of the computational

scheme (canonical or light front quantization, standard or

renormalization group enhances perturbation theory, etc.) that

calling virtual particles real would produce a very weird picture of

reality.

Whenever we observe a system we make a number of idealizations

that serve to identify the objects in reality with the

mathematical concepts we are using to describe them. Then we calculate

something, and at the end we retranslate it into reality. If our initial

initialization was good enough and our theory is good enough, the final

result will match reality well. Because of this idealization,

'real' real particles (moving in the universe) are slightly different

from 'mathematical' real particles (figuring in equations).

Modern quantum electrodynamics and other field theories are based on

the theory developed for modeling scattering events.

Scattering events take a very short time compared to the

lifetime of the objects involved before and after the event. Therefore,

we represent a prepared beam of particles hitting a target as a single

particle hitting another single particle, and whenever this in fact

happens, we observe end products, e.g. in a wire chamber.

Strictly speaking (i.e., in a fuller model of reality), we'd have to

use a multiparticle (statistical mechanics) setting, but this is never

done since it does not give better information and the added

complications are formidable.

As long as we prepare the particles long (compared to the scattering

time) before they scatter and observe them long enough afterwards,

they behave essentially as in and out states, respectively.

(They are not quite free, because of the electromagnetic self-field

they generate, this gives rise to the infrared problem in quantum

electrodynamics and can be corrected by using coherent states.)

The preparation and detection of the particles is outside this model,

since it would produce only minute corrections to the scattering event.

But to treat it would require to increase the system to include source

and detector, which makes the problem completely different.

Therefore at the level appropriate to a scattering event, the 'real'

real particles are modeled by 'mathematical' in/out states, which

therefore are also called 'real'. On the other hand, 'mathematical'

virtual particles have nothing to do with observations, hence have no

counterpart in reality; therefore they are called 'virtual'."

And

"Virtual particles are an artifact of perturbation theory that

give an intuitive (but if taken too far, misleading) interpretation

for Feynman diagrams. More precisely, a virtual photon, say,

is an internal photon line in one of the Feynman diagrams. But there

is nothing real associated with it. Detectable photons are never

virtual, but always real, 'dressed' photons.

Virtual particles, and the Feynman diagrams they appear in,

are just a visual tool of keeping track of the different terms

in a formal expansion of scattering amplitudes into multi-dimensional

integrals involving multiple propaators - the momenta of the virtual

particles represent the integration variables.

They have no meaning at all outside these integrals.

They get out of mathematical existence once one changes the

formula for computing a scattering amplitude.

Therefore virtual particles are essentially analogous to virtual

integers k obtained by computing

log(1-x) = sum_k x^k/k

by expansion into a Taylor series. Since we can compute the

logarithm in many other ways, it is ridiculous to attach to

k any intrinsic meaning. But ...

... in QFT, we have no good ways to compute scattering amplitudes

without at least some form of expansion (unless we only use the

lowest order of some approximation method), which makes

virtual particles look a little more real. But the analogy

to the Taylor series shows that it's best not to look at them

that way."

By Arnold. Neumaier

How real are 'virtual particles'?