Can virtual particles interact with eachother? I refer here to
pair-production virtual particles of the vacuum, rather than force
mediating virtual particles.

I was considering whether an electron from a virtual electron-positron
pair could bond with a proton from a virtual proton-antiproton pair to
make an atom. This would involve emission of a photon, so it seems
forbidden, not to mention the short timescale it would have to happen
on.

But is it the case that absolutely no interactions are allowed at all?

Indeed, if one virtual particle exchanges energy with anything,
including real particles, doesn't that make it real? What happens to
it's partner? Do you get lone virtual particles wandering the vacuum
looking for annihilation?

Also, if you get a virtual particle pair popping out of the vacuum, and
one of the particles is an electron, what really is it's partner? Would
it be better to call it a negative energy electron, rather than a
positron? How can it be a positron when they don't give off gamma
radiation when they recombine? If it really is an electron/positron
pair, then what is it that remembers that they must return? What is
doing the book-keeping?

br

Dr Photon wrote:
Can virtual particles interact with eachother? I refer here to
pair-production virtual particles of the vacuum, rather than force
mediating virtual particles.


Some Frequently Asked Questions About Virtual Particles
http://hermes.physics.adelaide.edu.a...particles.html

Dr Photon wrote:
Can virtual particles interact with eachother? I refer here to
pair-production virtual particles of the vacuum, rather than force
mediating virtual particles.

I was considering whether an electron from a virtual electron-positron
pair could bond with a proton from a virtual proton-antiproton pair to
make an atom. This would involve emission of a photon, so it seems
forbidden, not to mention the short timescale it would have to happen
on.

But is it the case that absolutely no interactions are allowed at all?

Indeed, if one virtual particle exchanges energy with anything,
including real particles, doesn't that make it real? What happens to
it's partner? Do you get lone virtual particles wandering the vacuum
looking for annihilation?

Also, if you get a virtual particle pair popping out of the vacuum, and
one of the particles is an electron, what really is it's partner? Would
it be better to call it a negative energy electron, rather than a
positron? How can it be a positron when they don't give off gamma
radiation when they recombine? If it really is an electron/positron
pair, then what is it that remembers that they must return? What is
doing the book-keeping?

br

There is no interaction that happens with "real" particles that does
not happen with "virtual" particles. The Feynman diagrams are the same,
and at each vertex, the usual set of conservation rules most hold.

The only difference is that initial state particles and final state
particles must be at least somewhat close to being "on-shell", meaning
that their energy and momenta have to conspire to give appropriate
masses for those particles.

For the virtual-atom case that you mentioned, the initial invariant
mass of the vacuum would not come close enough to the invariant mass of
an atom+photon final state to be allowed.

PD

Sam Wormley wrote:
Dr Photon wrote:
Can virtual particles interact with eachother? I refer here to
pair-production virtual particles of the vacuum, rather than force
mediating virtual particles.


Some Frequently Asked Questions About Virtual Particles
http://hermes.physics.adelaide.edu.a...m/virtual_part...

note all considerations in this FAQ are for force-mediating virtual
particles (also I've read this faq before...)

The difference I had in mind is that for a two-real-particle
interaction, such as electron-electron repulsion, the virtual particles
are a component of the system, so are (somehow) a by-product of the
electrons (I even did the full QM of it at one stage, though my memory
is embarrassingly rusty), whereas in an empty vacuum they are (somehow)
a product of "the field". That just seems a bit different to me. As I
recall, when the electrons are there, the distortion of their
interacting wavefunctions can be equivalent to saying there is an extra
particle in the system. But in vacuum, there is no wavefunction to
distort, unless the vacuum has its own wavefunction. I suppose it is
all "the field", as the electrons are just as much properties of the
field as are the virtual photons, and of course the field does the
bookkeeping. The only problem left is my visualization! (drat!)

br

PD wrote:
There is no interaction that happens with "real" particles that does
not happen with "virtual" particles. The Feynman diagrams are the same,
and at each vertex, the usual set of conservation rules most hold.


well, I had also heard that, but I was doubting it

The only difference is that initial state particles and final state
particles must be at least somewhat close to being "on-shell", meaning
that their energy and momenta have to conspire to give appropriate
masses for those particles.


this is where I start having problems. This requirement seems to say
that there is *almost* no interaction. My doubt was that if they are
not exactly "on-shell", then there is some residue which will not be
destroyed once the particles re-coalesce, and I was not comfortable
with that. I suppose it just keeps ringing around the field like a tiny
probability distortion (somehow).


For the virtual-atom case that you mentioned, the initial invariant
mass of the vacuum would not come close enough to the invariant mass of
an atom+photon final state to be allowed.


PD

thanks! just as I suspected... which also goes to say that only *very*
minor interactions are permissible.

Even something like electron-electron repulsion would hardly be
allowed? If two virtual electrons popped up next to eachother and
repelled strongly, how could they recombine with their respective
positrons again? Wouldn't the momenta be all wrong?

Also, if the particles undergo "normal" interactions, why doesn't the
electron and positron emit a gamma ray when they recombine? (I know
conservation of energy rules it out, but *how* is that kept track of?)

br

Hi Dr photon ........

you are right the whole situation is a big drat
i would say much more bluntly it is a big fraud ed presentation
of the existing understanding of any attraction force
without some breakthrough idea we will all of us dash in the muddy for
the next
thousand years!!]

and of course all those nonsense ideas about attributing properties to
vacuum are just confusion - not leading to anywhere.

Einstein started the 'curved space time' that was relay revolutionary
but not good enough

just see (may be again) my Circlon suggestion

ATB Brandan

Y.Porat
----------------------

to refine my concern a bit mo

When two real particles interact via virtual particles, and the virtual
particle undergoes an "unintended" interaction, any imbalance can be
carried away by the real particles. But in the vacuum case, if there is
an "unintended" interaction, and then the virtual pair recombine, how
can the vacuum carry away any imbalance?

The imbalance must remain in the field, which implies a residual
particle is left after the virtual pair recombine?

br

Dr Photon wrote:
PD wrote:
There is no interaction that happens with "real" particles that does
not happen with "virtual" particles. The Feynman diagrams are the same,
and at each vertex, the usual set of conservation rules most hold.


well, I had also heard that, but I was doubting it

The boundary between "real" and "virtual" particles is arbitrary and
fuzzy. There can be no dynamical difference in their behavior.


The only difference is that initial state particles and final state
particles must be at least somewhat close to being "on-shell", meaning
that their energy and momenta have to conspire to give appropriate
masses for those particles.


this is where I start having problems. This requirement seems to say
that there is *almost* no interaction.

Hmm... It does mean there is an additional constraint on the integral,
but the strength of the terms themselves is the same as always. Put
another way, the higher-order terms in the perturbation series *do*
include interaction terms between the virtual particles, and these have
the same relative relation/contribution to the higher-order terms as
they would if you were considering interactions between real particles.

My doubt was that if they are
not exactly "on-shell", then there is some residue which will not be
destroyed once the particles re-coalesce, and I was not comfortable
with that.

I'm not sure why. There are e+e- -- e+e-gamma events, q + qbar --
3jet events that are all expressions of some residue in the event. As
long as the invariant mass of the system is preserved, and energy and
momentum and charge and so on are individually conserved, everythings
fine.

I suppose it just keeps ringing around the field like a tiny
probability distortion (somehow).

No, I don't think the idea of a "spirit in limbo" particle in the final
state is a good way to get around it.



For the virtual-atom case that you mentioned, the initial invariant
mass of the vacuum would not come close enough to the invariant mass of
an atom+photon final state to be allowed.


PD

thanks! just as I suspected... which also goes to say that only *very*
minor interactions are permissible.

I wouldn't characterize it as that, but perhaps it's a matter of
degree.


Even something like electron-electron repulsion would hardly be
allowed? If two virtual electrons popped up next to eachother and
repelled strongly, how could they recombine with their respective
positrons again? Wouldn't the momenta be all wrong?

Now here is where you're treating the diagrams too literally and too
classically. You are supposing that if you have a vertex where a photon
decays into an electron-positron pair, and the electron and the
positron have opposite momenta (to conserve momentum at the vertex),
then they must be heading away from each other and separating in
distance. But recall the uncertainty principle, which says the more you
know about the momenta, the less you know about their positions. Which
means that even if the momenta are in opposite directions, you still
don't know whether the particles are heading away from each other or
towards each other! You may say, but they were next to each other at
the vertex so they *must* be moving away from each other. The response
is, but that's applying knowledge of both position and momentum at the
same time, and you can't do that in quantum mechanics, only in
classical mechanics. (This is Porat's mistake, as well.)


Also, if the particles undergo "normal" interactions, why doesn't the
electron and positron emit a gamma ray when they recombine? (I know
conservation of energy rules it out, but *how* is that kept track of?)


Well, essentially by the same rule that the e+e- pair popped out of the
vacuum without a gamma in the first place. How you "borrow" energy from
the vacuum and put it back in is sort of arbitrary. I personally would
prefer to pull energy and momentum from the vacuum in the form of a
gamma, which then decays to e+e-, which then coalesces into a gamma,
which then gets reabsorbed into the vacuum. For those little "detached"
Feynman diagrams that don't have real particles in initial and final
states, it's better to start with the math and decide later how to draw
the diagram.

PD

did you ever hared me saying that we can know momentum and position at
the same time???!!

i never said that
i only said that the HUP can be bypassed by other knowledge
while there is no need at all to know momentum and position at the same
time
in order to know much more than you imagine!!!
see for instance my model in my site
i show there the geometric structure of the nuc
*without length scale* and i say 'not proportional!!
we can know for instance the relative positions of particles
as you can know the relative positions of buildings in your street
without telling what is the length scale there
and i keep on bringing a simple example that you never answered me :

if we know the Deuteron is composed of a Proton and a neutron---
can we say that they are located one next to the other??
while we know they can be separated and combined not too difficultly
and many other facts about them
so can we say:
if they are one next to the other than can we assume with a highest
probability that the shape of the deuteron is something longish??

will you answer me that question once and for all
in order of making some advance beside the
QM mumbling that you dont stop dashing in ?

and without admitting that(once and for all) all your above discussion
is one big mumbling???
IE not admitting to say:
'we have no green idea what is making any attraction force ??!!!

btw the spell checker fount about 20 mistakes in my post ....
and probably there are still more .(:-)

TIA
Y.Porat
------------------------

Learn how to use "did", Porat!