Measurement-free definition of Virtual particles (MWI)

[Dmitry67]

Can anyone provide it?

I just examined wiki article just for fun, it is a mess of different concepts (because, as I understand, there was an attempt to make it interpretation-neutral)

So, can anyone provide a definition which is MWI-compatible? For example:
in MWI any 'particle' behavior is a result of a measurement and decoherence, on a fundamental level thee is a Universe Wavefunction (UW) (which is a wave). So the very beginning of wiki article 'In physics, a virtual particle is a particle that' does not make any sense.

In MWI measurement is a mapping of UW into some basic, chosen arbitrary (to some extent). So, as I understand it, in Feynman diagrams the distinction of the ‘incoming and outgoing’ particles and virtual ones is based on a measurement. Looks like wiki admits this weakness in one place (bold is mine), using the word detected/observed in all explanations:

As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles.
…
they never appear as the observable inputs and outputs of the physical process being modeled
…
Furthermore, in a vacuum, a photon experiences no passage of (proper) time between emission and absorption. This statement illustrates the difficulty of trying to distinguish between "real" and "virtual" particles as mathematically they are the same objects and it is only our definition of "reality" which is weak here. In practice, a clear distinction can be made: real photons are detected as individual particles in particle detectors, whereas virtual photons are not directly detected; only their average or side-effects may be noticed, in the form of forces or (in modern language) interactions between particles.

Even the mathematical attempt fails:

A virtual particle is one that does not precisely obey the m2c4 = E2 − p2c2 [2] relationship for a short time

Because it implicitly assumed the possibility of measuring E and p when a ‘particle’ is detected

So my question is,
What are virtual particles in MWI?
What is an observation-free definition?
Are they a part of UW or not? (UW and its evolution is the only thing which exists)

 

[Demystifier]

I think one cannot understand virtual particles in any particular interpretation (including MWI) before one understands virtual particles in the shut-up-and-calculate "interpretation" (SUACI). It's not that I am a fan of SUACI of QM in general, but I am definitely a fan of SUACI of virtual particles.

 

[Dmitry67]

I understand your sarcasm,
but I am sure that you have understood virtual particles in SUACI,
so you probably have answer to my queston?

 

[ConradDJ]

Though I have nothing positive to say about MWI, I think this may help clarify the basic issue --

QM says that in order to predict what will actually happen, you need equations that take into account everything that could actually happen. These equations describe the underlying structure of the world in terms of possibilities, not in terms of actual facts.

These equations are useful to the extent we can provide actual inputs (from prior measurements) and get actual measurements as output, to compare with the results of the equations.

To the extent it's meaningful to describe what we observe in terms of particles, it's also meaningful to describe all the possibilities we don't observe in exactly the same way. Everything it makes sense to say about "real" (observed) physics also makes sense with respect to the "virtual" background of physics that could have happened but in fact didn't.

Now so far as I understand it, MWI says everything actually happens -- i.e. instead of measurement you have branching. To me that seems like a statement with no content. I at least know something about how to measure things, but "branching" is just a word.

But to answer your question, in MWI the equivalent of "virtual particles" must be all the other branches. Just as in standard QM, so in MWI you can't calculate the probabilities of what will happen in your future branch without taking into account all the other possible branchings.

Hope that helps.

 

[Demystifier]

1. What are virtual particles in MWI?

1. An auxiliary mathematical tool that sometimes may be useful to calculate the wave function. (For example, in low-energy QCD it's not very useful at all.)

In fact, virtual particles are even less than a mathematical tool. They are a picturesque name for certain mathematical terms appearing as a part of the mathematical tool called perturbative expansion.

2. What is an observation-free definition?

2. See 1.

3. Are they a part of UW or not? (UW and its evolution is the only thing which exists)

3. They are not.