[evan_au]

the concept of (a vacuum) might not be a useful thing.

I would put it a different way...

The concept of a vacuum is a useful concept.
- We can't create a perfect vacuum
- But we can certainly create something that is usefully close enough to a vacuum for many practical purposes
- The LHC needs a pretty good approximation of a vacuum so they can inject their protons and expect them to make their way around all the ring
- While for some purposes (eg analysing radio wave patterns), normal atmospheric pressure is close enough to a vacuum to get excellent results

[alancalverd]

Creating a perfect  vacuum is as difficult as drawing a straight line, but who cares? Maths deals with straight lines, physics deals with weightless strings*, and in real life we just accept whatever is good enough.


*My father swore that he read an Indian exam paper with the phrase "you may ignore the weight of the elephant".

[jeffreyH]

Let's think about the vacuum. Before the big bang everything was densely packed. Not even particles, as we understand them, existed. You still had energy. What about the vacuum in this scenario?

When energy is that dense then it must flood the vacuum. There is no zero point energy. Then we have the big bang and a continuous drop in energy density. Whereas we hold that the big bang emerged out of the vacuum shouldn't it be the other way round?

[Colin2B]

the concept of (a vacuum) might not be a useful thing.

I would put it a different way...

Thanks Evan, you’ve picked up the spirit of what I was trying to say. I view absolute vacuum in the same way as absolute zero, maybe not achievable but still useful to know it’s value and what happens as you approach it. Trying to get there gives us many benefits including discovery of superconductivity.

Let's think about the vacuum. Before the big bang everything was densely packed. Not even particles, as we understand them, existed. You still had energy. What about the vacuum in this scenario?

What form was the energy in?
You might be interested in this (popsci) article on quark soup - why does my mind think duck soup (quark, quark!). Haven’t read the original papers so not sure how accurate the reporting.

Whereas we hold that the big bang emerged out of the vacuum shouldn't it be the other way round?

no room for a vacuum? Interesting thought.

The concept of virtual particles arises in perturbation *theory* of quantum field *theory*. Virtual particles are therefore *theoretical*, as is quantum field *theory*. I rest my case.

Much better, ‘concept’ is ok. Remember I was responding to your original statement:

But the 'virtual particle' is a theory

We use a lot of concepts in a theory, but they are not theories in their own right, nor do the have a reality associated with them. In general physics these concepts are usually obvious as such, but in quantum theories they might not be so obvious. For example, we use the concept of an N dimensional phase space to help calculate the motion and characteristics of a particle, but no one would reasonably believe that this describes particles being in a real universe of N spacial dimensions.
When we come to the vacuum there are 2 concepts that help us with the calculations.
The first is harmonic oscillators - basically interconnected balls and springs at each point in space. The second is via virtual particles - not particles at all, but a calculation aid in Feynman diagrams - which were introduced to the public with the caveat “don’t take these literally. The problem is if I say to you imagine the vacuum to be full of little balls on springs, you would immediately recognise this as an analogy, but with virtual particles the public and popsci press immediately forget the caveat and do take it literally.
Nor should we refer to virtual particles as theoretical, any more than we would refer to balls and springs as such. It would be correct to refer to the Higgs Boson as theoretical, before it was discovered, because it had been predicted by a scientific theory.  Virtual particles are not predicted to exist by any standard particle theory or to be tested/discovered in any lab.
I also think you may be mistaking a quantum energy/time relationship as being a Heisenberg uncertainty relationship, whereas it is in fact the uncertainty of an operator compared to the variation of its expectation value.
If you are interested there is a thread here which covers both issues in one. It’s worth noting that the misunderstandings around these topics is a constant source of irritation to those who understand quantum theories.
https://physics.stackexchange.com/questions/439946/heisenberg-energy-uncertainty

[pensador]

folks you are confusing your selves and me not that that is difficult.

The vacuum of space is filled with virtual particles as is evidenced by the Casimir effect, and further evidenced by the dynamic Casimir effect. Space is not a empty vacuum. https://en.wikipedia.org/wiki/Vacuum_state

The only way you could possibly argue that the Casimir effect is not evidence off virtual particles is to ascribe it to the Van de Walls forces. The Dynamic Casimir effect can not be ascribed to anything but virtual particles. Hawking radiation depends on Virtual particles, are you denying that Hawking radiation is probable, but unprovable.


The Big Bang and Hot or Cold Baryogenesis relies on virtual particles become real due to inflation of space, this according to the standard model lead to Big bang nucleo synthesis. Are you all stating baryogenisis after the inflation epoch ever took place?

Fields are transmitted via virtual particles in QFT . ie Electrical fields are transmitted by polarisation of virtual particle pairs. virtual particles are often viewed as virtual photons, which "definately" do not have the same characteristics as real photons, ie they can transmit electric fields etc.

To state space is empty when looking at QFT and QED is WRONG. Maxwells equations depend on the ability to polarize space in QED this is done via virtual particles.   

[alancalverd]

You can't use "is" and "virtual" in the same sentence! Stuff is either actual (you can touch it) or virtual (you can model it mathematically). A good synonym for virtual is "as if".

Space is indeed an empty vacuum until you introduce real particles between which the  Casimir and Van der Walls forces become apparent.

[jeffreyH]

@Colin2B I think you forgot the link to the article on duck a l'orange.

[jeffreyH]

Vacuum energy may well be the confusing factor here. It relates to the fields that permeate all of space. A bit of light reading may help. https://en.m.wikipedia.org/wiki/Vacuum_energy

[Colin2B]

folks you are confusing your selves

No, we are not confused. We are drawing a distinction between calculations in perturbation theory and the popsci press tendency to portray these calculations as real particles ignoring the warning to ‘not take these literally” (as the words used in description of Hawking radiation). They have their place in discussion but only if used with suitable caveats.
I don’t have time to go through all of this, but would recommend you read and understand the link on vacuum fluctuation myth.
The concept of virtual particles arises in an approximation scheme in which interactions (forces) between actual particles are calculated in terms of exchanges of virtual particles which are shorthand terms for a series of processes and calculations within Feynman diagrams. The interpretation of an interaction as a sum of intermediate states involving the exchange of various virtual particles only makes sense in the framework of perturbation theory. In contrast, non-perturbative methods in QFT treat the interacting Lagrangian as a whole without any series expansion and do not introduce virtual particles.

The only way you could possibly argue that the Casimir effect is not evidence off virtual particles is to ascribe it to the Van de Walls forces. ..............The Dynamic Casimir effect can not be ascribed to anything but virtual particles.

Not so. Any non-perturbative analysis will not use Feynman diagrams and power series approximations.

Hawking radiation depends on Virtual particles, are you denying that Hawking radiation is probable, but unprovable.

Again, see the vacuum fluctuation article and associated virtual particle articles to understand how virtual particles should be viewed.. The same is true of the other examples you quote.

To state space is empty when looking at QFT and QED is WRONG

.
No one was stating that space is empty. The vacuum energy can be modelled in QFT by quantum harmonic oscillators (ball-spring model) but no one suggests space is filled with little balls and springs. There is a difference between a model and what we consider as a reality.
 

Maxwells equations depend on the ability to polarize space in QED   

Space is not polarised, and Maxwell’s equations do not depend on it being polarised.
QED starts with the free theory of the electromagnetic field and quantum theory gives rise to a photon with two polarisation states. This is done by using the Lagrangian for Maxwell’s equations in the absence of any sources, which gives an extended phase space (not a real physical space) with gauge orbits from which we can use Coulomb gauge (as in Maxwell’s equations) to identify 2 degrees of freedom which are the photon polarisation states.

[jeffreyH]

Don't mention degrees of freedom you will only confuse the chap.

[pensador]

Don't mention degrees of freedom you will only confuse the chap.

I don't think I am the one who is confused, or misunderstanding what I wrote. 

[Colin2B]

I don't think I am the one who is confused, or misunderstanding what I wrote. 

The confusion lies with the use of shorthand jargon by popsci press without expanding the caveats. Having worked with them on articles I can understand why; they have limited column inches and want to get as many wow factors into their pages as possible. They also know that their main audience has never studies quantum theories, and those who have will understand the reality of the jargon, and so they aim at the layman, but sometimes at a playschool level. The problem comes when speculation is made on the back of the top level statements.
We are not a popsci site, although we try to explain things for a non-science audience.
We are not saying that the term ‘virtual particle’ should be banned, what we are trying to do is make it clear that some things should not be taken literally as there is important detail behind the jargon.
This topic arose because of the comment about virtual particles popping in and out of existence. As one of the links you provided in a previous topic said “We routinely say things like "virtual photons ... are constantly being emitted and re-absorbed by the electron" but that isn't really what we mean.  ..... Nothing at all is going on. The words about things fluctuating around are a rough way to convey one of the peculiar properties of quantum fields.”
Anyone who is interested in understanding the issue is welcome to read the links provided on vacuum fluctuations and the associated links on virtual particles.
It is also worth reading Prof Strassler’s article where he repeatedly says:
“Physicists often say, and laypersons’ books repeat, that the two electrons exchange virtual photons. But those are just words, and they lead to many confusions if you start imagining this word “exchange” as meaning that the electrons are tossing photons back and forth as two children might toss a ball.”
“The Feynman diagram is actually a calculational tool, not a picture of the physical phenomenon”
“One says "the electron emits and reabsorbs a virtual photon", but this is just shorthand for the physics “
“these quantum fluctuations are sometimes described as being due to two or more ‘virtual particles’, but this name really reflects a technical issue (i.e., how you can calculate the fluctuations’ properties using Feynman’s famous diagrams) more than it guides you as to how you should really think about them”
“The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle.”

If anyone can get beyond the popsci descriptions there awaits a far richer description of what is happening which in the case of the dynamical Casmir effect lies on the border between quantum theory and relativity, but it does not depend on simplistic, jargon descriptions of virtual particles ‘popping in and out of existence’ because DCE does not prove the existence of virtual particles.
eg https://arxiv.org/pdf/quant-ph/9802017.pdf
It’s also worth noting that in Hawking’s paper on Hawking radiation he mentions virtual particles only once p. 202 but says at the bottom of the page: ''It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally.''
 What really happens is that the high energy of the gravitational field of the black hole creates real particle-antiparticle pairs - although before that event it could be viewed in terms of Feynman diagrams as virtual is completely irrelevant to his argument.

So, do you want to stay at the layman/playschool-of-physics stage or go deeper for a fuller understanding?
Let’s try and see what lies beneath.

[pensador]

I don't think I am the one who is confused, or misunderstanding what I wrote. 

The confusion lies with the use of shorthand jargon by popsci press without expanding the caveats.

So, do you want to stay at the layman/playschool-of-physics stage or go deeper for a fuller understanding?
Let’s try and see what lies beneath.

I think everyone wants to learn more. Incorrect pop science is often misleading. Over simplified answers are equally misleading. That is why I stated above I am not the one confused. Space is not empty nothingness, it has properties, and substance. 

I went out and bought a good text book on QFT/QED, which starts out with the Quantum Vacuum based on Lagrangians and Hamiltonians and eigenvectors etc.

Does anyone have a recommendation for QCD, or is that limited to those with super computers who produce pop science links. like this one http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/

[alancalverd]

Most humans define "space" as that which has neither properties nor substance. Everything else is "stuff".

[jeffreyH]

I don't think I am the one who is confused, or misunderstanding what I wrote. 

The confusion lies with the use of shorthand jargon by popsci press without expanding the caveats.

So, do you want to stay at the layman/playschool-of-physics stage or go deeper for a fuller understanding?
Let’s try and see what lies beneath.

I think everyone wants to learn more. Incorrect pop science is often misleading. Over simplified answers are equally misleading. That is why I stated above I am not the one confused. Space is not empty nothingness, it has properties, and substance. 

I went out and bought a good text book on QFT/QED, which starts out with the Quantum Vacuum based on Lagrangians and Hamiltonians and eigenvectors etc.

Does anyone have a recommendation for QCD, or is that limited to those with super computers who produce pop science links. like this one http://www.physics.adelaide.edu.au/theory/staff/leinweber/VisualQCD/Nobel/

The problem you have is you try to come across as someone who knows what they are talking about. I learn something new here quite often. I have a lot to learn, an awful lot. Reading textbooks can be hard going. Ask some questions about what you have read. This will help to clarify some of the harder concepts.

[pensador]

I dont try and come across as anything other than what I am. Sorry people think that.
I will stop answering questions, and leave that to others. I had hoped that by answering a few simple questions people might reciprocate and answer some of my musings. I may have waisted my time.

I might ask questions in the future ref the finer points of QFT, but at the moment, my maths is holding up, and I have been able to use google for clarification on points so far, it is good bed time reading zzzzzzzzzzzzz.  My curiosity ref QCD is limited to pop science at the moment and a book recommendation to get me started.

Cheers

[pensador]

Most humans define "space" as that which has neither properties nor substance. Everything else is "stuff".

Perhaps I am not human, but at least I am not an idiot.