Source of Virtual Particles in Space?

[Ty1erC]

I'm trying to understand the nature of the virtual particles that exists in empty space. I understand that they 'bubble' in and out of existence, but why do they exist in the vacuum of space? If all particles spend some of their time as virtual particles, does quantum mechanics suggest that some of the subatomic particles in my body may be disappearing and reappearing in intergalactic space, contributing the expansion and acceleration of the universe (negligibly, of course)? Or do I have no idea what I'm talking about?

 

[Naty1]

Trying to understand quantum mechanics using our classical, everyday, terminology takes some time...

I'd say, briefly, virtual particles are artifacts of our mathematical models and have not been detected.

Here is an introductory description I found in Wikipedia...

There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. .They are "temporary" in the sense that they appear in calculations, but are not detected as single particles... In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields

and another:

In the Standard Model [of particle physics] the non-zero vacuum expectation value of the Higgs field, arising from spontaneous symmetry breaking, is the mechanism by which the other fields in the theory acquire mass.

No one knows 'why' they exist in 'empty' space and they have never been detected. It is believed there is a vacuum energy that permeates all of space and 'virtual particles' may be part of that...see here: http://en.wikipedia.org/wiki/Vacuum_energy

but beware, there is no evidence that virtual particles 'blink' in and out of existence nor is there a complete consensus that the Casimir effect is proof of vacuum energy...as stated here.

...Vacuum energy is the zero-point energy of all the fields in space...the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields...

.

For more check out ZERO POINT ENERGY in wikipedia

...does quantum mechanics suggest that some of the subatomic particles in my body may be disappearing and reappearing in intergalactic space, contributing the expansion and acceleration of the universe (negligibly, of course)?

'suggest'..well, maybe you could extend it to your scenario...but it's not likely to happen during the limited age of this universe...It's about as likely as your glass of milk disappearing and reappearing on Venus...or on your neighbors table...You can read about a related effect," quantum tunneling", which plays a role in radioactive decay and semiconductors.

PS: If you search in these forums [from the SEARCH] menu at the top of this page, you'll find this subject has been explained many times, many different ways...

 

[Bill_K]

I'd say, briefly, virtual particles are artifacts of our mathematical models and have not been detected.

Must we go through this again?

 

[Ty1erC]

Virtual particles are indeed real particles. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested.

http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea

This is why I am so perplexed. There's a lot of ambiguity in the various things I read.

 

[tom.stoer]

http://www.scientificamerican.com/article.cfm?id=are-virtual-particles-rea

Gordon Kane, director of the Michigan Center for Theoretical Physics at the University of Michigan at Ann Arbor, provides this answer.

...

Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy, ...

Sorry Prof. Kane - or whoever has written this text - you should know that this is WRONG! There is no violation of conservation of energy, all there is is a violation of the on-shell relation between energy energy and momentum. We have numerous threads discussing these questions.

But to be honest I am sick and tired to comment and discuss this nonsense; sure all these experts know better, so why the hell do they always repeat the same b...sh..?

 

[JK423]

But to be honest I am sick and tired to comment and discuss this nonsense; sure all these experts know better, so why the hell do they always repeat the same b...sh..?

This should be a next thread, "Why experts insist on repeating nonsense" :tongue:
Having already a difficulty in the interpretation of QM seems not to be enough for them, they want to confuse people even more.

 

[tom.stoer]

Good point!

 

[Naty1]

Ty1er: if you haven't read this discussion,and want more insights give it a try...

https://www.physicsforums.com/showthread.php?t=671363&highlight=virtual+particles

You'll note there are different views expressed. I happen to like Tom.Stoer's, but cannot refute all of mfb's comments either...so I try to keep an open mind.

Also, I believe a related issue is particle production in expanding geometry...that is, how real particles emerge from the vacuum perturbations during, say, the inflationary era of our universe, and also how particles emerge in the presence of cosmological horizons, like Hawking radiation and the Unruh effect. These do NOT prove the existence of virtual particles, but there are a lot of effects for which we have that appear to be solid theories and for which any experimental evidence [observations] can be debated.

I don't think it's mentioned much in this discussion, but we have had extended discussions about "What is a particle."..that is a 'real' particle, like a photon or an electron, and if you read any of those you'll find it's an interesting question...and the subject of active research papers.

[I find it helpful to keep in mind what Richard Feynman said...something to the effect that 'You must accept nature as she is, absurd.' I personally think a major problem may be that it not always clear which absurdity she is allowing to be revealed to us.]

 

[Ty1erC]

I've found some more articles online, and I think I've come to the conclusion that my questions are unanswerable at the moment. The hypotheses about dark/vacuum energy that are out there have yet to become substantial theories. I'm not educated on the math that goes into this stuff, so I can't fully grasp some of these concepts until they become more concrete in the arena of science.

 

[jtbell]

The "virtual particles are real" and "virtual particles are not real" camps agree on the mathematics of quantum field theory, and the predictions that it makes for things that we can actually observe experimentally. They disagree on the words that should be used when discussing QFT in non-mathematical language, among themselves or with laymen.

 

[tom.stoer]

The "virtual particles are real" and "virtual particles are not real" camps agree on the mathematics of quantum field theory, ...

Not so sure about that. I have the impression that some virtual particle fans do not understand the formalism, its limitations and the reason why and when it fails. My impression is that some of them have not seen anything else but perturbative treatment of QED.

 

[Naty1]

I think I've come to the conclusion that my questions are unanswerable at the moment.

we have plenty of answers! You get to pick!

You will often discover the more finely tuned your questions the less finely tuned are answers.

The hypotheses about dark/vacuum energy that are out there have yet to become substantial theories.

They ARE 'substantial', I think, but in the absence of agreed upon observable parameters, sorting out details is, well, theoretical. That's because we have more math than seems to fit our universe; some fits, some doesn't; The ability to make accurate predictions and verify those predictions via experimental observations is a big determinant about what people can agree upon is an appropriate 'theory'.

 

[Bill_K]

Not so sure about that. I have the impression that some virtual particle fans do not understand the formalism, its limitations and the reason why and when it fails. My impression is that some of them have not seen anything else but perturbative treatment of QED.

Tom, Please don't assume that if someone disagrees with you, it is due to lack of education. There may be other reasons.

 

[Ty1erC]

They ARE 'substantial', I think, but in the absence of agreed upon observable parameters, sorting out details is, well, theoretical. That's because we have more math than seems to fit our universe; some fits, some doesn't; The ability to make accurate predictions and verify those predictions via experimental observations is a big determinant about what people can agree upon is an appropriate 'theory'.

Substantial in their own right, yes, but not on the level of relativity or evolution. (not yet, anyway.) Those are concepts that are clear and understandable and answer huge scientific questions. Maybe it's just the nature of quantum mechanics, but several varying and ambiguous answers don't satisfy my curiosity. I guess I'm guilty of wanting the simple answer that just isn't there.

 

[micky_gta]

If all particles spend some of their time as virtual particles, does quantum mechanics suggest that some of the subatomic particles in my body may be disappearing and reappearing...

By the time you finish reading this post the particles that make you you have already disappeared and reappeared millions of times. For all practical purposes, you(your body) are not the same now as it was before you started reading this post.

 

[Naty1]

By the time you finish reading this post the particles that make you you have already disappeared and reappeared millions of times.

Via what process do you think electrons, atoms, and so forth appear and disappear instantaneously?

What do you think the half life of stable atoms is, say, like hydrogen?

 

[bahamagreen]

Whether "real" or not, is it OK to ask questions like these?

Do virtual particles engage in gravitational interaction while they exist?

Sometimes questions about gravity ask things like, "What if the Sun instantly disappeared?" or "What is the impetus to initial motion for a mass placed at rest in curved space?" These questions are usually dismissed as asking about a physically impossible scenario. But what about virtual particles? Do they appear and disappear in a such a way to ask these questions of them?

If they do, what about after they exist? Wouldn't their gravitational attraction continue to interact with distant matter long after the virtual particle no longer existed?

This would be like the answer to the "how does gravitational influence get out of a black hole?" question where the gravitational influence is assigned to the space around the thing rather than to the singularity. If virtual particles interact gravitationally, does that influence remain after they are gone similar to the black hole?

Is the point of pair creation allowed to have any velocity?

Virtual particle pair creation conserves momentum, but doesn't that require that we allow the point location of the pair creation to have a velocity with respect to our observation frame of reference? If the creation points are not allowed to have relative velocities wouldn't that allow a special frame of reference in which all the pair creations do conserve momentum?

 

[micky_gta]

Via what process do you think electrons, atoms, and so forth appear and disappear instantaneously?

What do you think the half life of stable atoms is, say, like hydrogen?

1st question: They don't appear and disappear instantaneously just very fast. I am not familiar through what process is used.

2nd question: I'm not sure i understand the question, Hydrogen is not radioactive? although some Isotopes of hydrogen are and can decay. I don't understand the relevance of your question regarding my statement?

 

[Naty1]

micky...my point is that stable particles do not disappear...nor reappear..as you have stated. They do not go anywhere'...

The half life of stable atoms is waaay longer than the age of the universe...

The body does replace it's constitutents over time but not because anything 'disappears and reappears'...the skin, for example, sloughs off old dead surface skin and grows new from within...

 

[Naty1]

Do virtual particles engage in gravitational interaction while they exist?

lots of good discussion here...
https://www.physicsforums.com/showthread.php?t=671363

demystifier puts it this way in one view:

" virtual particles don't cause decoherence {nor gravity} simply because virtual particles don't exist. .."

 

[Bill_K]

Do virtual particles engage in gravitational interaction while they exist?

Yes they do, absolutely. Gravity couples to energy and momentum. Everything that carries energy and momentum interacts with gravity, and this includes virtual particles too.

If they do, what about after they exist? Wouldn't their gravitational attraction continue to interact with distant matter long after the virtual particle no longer existed?

Just as the source of EM (charge and current) must be conserved, the source of gravity must be conserved. Which is why energy cannot simply appear or disappear, even momentarily. For every interaction, the total energy of the incoming particles must equal the total energy of the outgoing ones. If a virtual particle "disappears", something else must appear in its place and carry the energy along. Thus a positron-electron pair may annihilate into one or more photons.

 

[JK423]

Yes they do, absolutely. Gravity couples to energy and momentum. Everything that carries energy and momentum interacts with gravity, and this includes virtual particles too.

Just as the source of EM (charge and current) must be conserved, the source of gravity must be conserved. Which is why energy cannot simply appear or disappear, even momentarily. For every interaction, the total energy of the incoming particles must equal the total energy of the outgoing ones. If a virtual particle "disappears", something else must appear in its place and carry the energy along. Thus a positron-electron pair may annihilate into one or more photons.

The words "interaction", "energy" and "momentum" (in bold) are attributes of quantum states. Only quantum states can interact (with other quantum states), and be characterized by energy and momentum. Do your virtual particles acquire a quantum state during the time of their supposed existence?

 

[Naty1]

Just for an outside perspective, here is what Lisa Randall [Physics professor, Harvard] says regarding virtual particles in her book WARPED PASSAGES... pg 225...227
my comments enclosed {}..

Virtual particles interact with gauge bosons and alter forces so their effect depends on distance...{sounds like they exist} ...Virtual particles have the same interactions and the same charges as physical particles but they have energies that look wrong. {sounds like they are not physical} A virtual particle can have enormous speed but no energy...virtual particles can have any energy that is different from the energy carried by the corresponding true particle...If it had the same energy it would be a true particle, not a virtual particle...the uncertainty principle allows particles to have the wrong energy...for such a short time they can never be measured...Virtual particles have measurable consequences because they influence the interactions of the real physical particles that enter and leave the interaction region...{referring to an illustration}...the photon which was exchanged to generate the classical electromagnetic force was in fact a virtual photon...it only needed to last long enough to communicate the electromagnetic force and make the real charged particles interact.

What a 'fanstastic' statement...sounds like a politician describing something... something for everybody...something to support anybody's view...
did ANYBODY edit that??

The only thing that everybody may agree upon is that we can't directly observe virtual particles.

/////////

 

[JK423]

Just for an outside perspective, here is what Lisa Randall [Physics professor, Harvard] says regarding virtual particles in her book WARPED PASSAGES... pg 225...227
my comments enclosed {}..

...

What a 'fanstastic' statement...sounds like a politician describing something...
something for everybody...something to support anybody's view...

The only thing that everybody may agree upon is that we can't directly observe virtual particles.

Wow.. After reading this post as well, i would like to make a hypothesis
Is it plausible that maybe even some experts don't know what the heck they're talking about?
I don't want to make such statements, since i am too "small" too judge Harvard professors, but still it doesn't make sense.

Anyway, as to your last comment; we can all agree that virtual particles (defined as internal lines in Feynman diagrams of perturbation theory) are not described by quantum states, hence they are not quantum systems,
.
.
.
thus they don't exist.

 

[Bill_K]

The words "interaction", "energy" and "momentum" (in bold) are attributes of quantum states. Only quantum states can interact (with other quantum states), and be characterized by energy and momentum.

Some quantum states are multiparticle states. That is, their properties including energy and momentum are shared by more than one particle. A virtual particle is generally one particle in a multiparticle state.

here is what Lisa Randall [Physics professor, Harvard] says regarding virtual particles in her book WARPED PASSAGES.

Considering that it is adapted to a popular account, this is a very accurate statement and a very clear one.

The only thing that everybody may agree upon is that we can't directly observe virtual particles.

No, that's exactly backwards, it's the "real" particles that are the fictitious idealizations and cannot be observed. A real particle is a solution to the free wave equation, e.g. a plane wave. To be on the mass shell a particle must exist undisturbed forever, from the infinite past to the infinite future. If you observe it, i.e. interact with it, necessarily it will be slightly off the mass shell and hence slightly virtual.

In fact, interactions can take place only with the participation of virtual particles. An electron, for example, cannot emit a photon if it and the photon are both on the mass shell, for there is no way for it to conserve both energy and momentum.

 

[JK423]

Some quantum states are multiparticle states. That is, their properties including energy and momentum are shared by more than one particle. A virtual particle is generally one particle in a multiparticle state.

Okay, so you say that your virtual particles acquire a quantum state during the time of their existence. I want to ask you one more thing, in order to understand if we define virtual particles in the same way:
Do you regard virtual particles as the internal lines in Feynman diagrams of perturbation theory? Or you have something else in mind?

 

[1977ub]

Does Hawking radiation belong in this conversation?

"This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles." - http://en.wikipedia.org/wiki/Hawking_radiation

 

[tom.stoer]

This is another example where Hawking's math and Hawking's popular explanations are not consistent. The math does not talk about virtual particles at all. Virtual particles are artifacts of perturbation theory; they are used to treat interactions. But the Hawking radiation does not require an interacting field, it arises in free field theories, it does not depend on any interaction vertex or propagator.

 

[Naty1]

B ill_K posts:

No, that's exactly backwards, it's the "real" particles that are the fictitious idealizations and cannot be observed.

I always read your posts and most I understand, but that oft repeated one leaves me scratching my head!{see below} For the time being I mostly like this view as expressed by Carlo Rovelli:
[maybe because I think I 'understand' it??

Rovelli: “…we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”

BUT!

Rovelli does acknowledge some issues which seem to support Bill_K's view:
[From the What is a Particle? discussion]

https://www.physicsforums.com/showthread.php?t=386051

http://arxiv.org/abs/gr-qc/0409054

...On curved spacetime, in general there is no symmetry group, no preferred set of modes and no preferred decomposition into positive and negative frequency.
As a consequence, there is no preferred vacuum state, and the interpretation of the field states in term of particles appears to be difficult...The defining properties of the particles, mass and spin (or helicity), are indeed the invariants of the Poincar´e group. Now, strictly speaking we do not live in a Poincar´e invariant region of spacetime: does this mean that, strictly speaking, the world around us has no particles?...

{no}

...Such arbitrariness and ambiguity of the particle concept have led some theoreticians like Davies to affirm that “particles do not exist” , a view shared by several relativists. ... other theoreticians ... hold that QFT is fundamentally a formalism for describing processes involving particles, such as scattering or decays... A typical example of this position is Weinberg who cannot certainly be suspected of ignoring general relativity. These difficulties become serious in a background-independent quantum context. For instance, in loop quantum gravity quantum states of the gravitational field are described in terms of a spin network basis. Can we talk about gravitons, or other particle states, in loop quantum gravity. A common view among relativists is that we cannot, unless we consider the asymptotically flat context...

In 'Unfinished Revolution' Carlo Rovelli makes this observation:

Relational Quantum Mechanics
http://arxiv.org/PS_cache/quant-ph/pdf/9609/9609002v2.pdf

...I have concluded that two observers give different accounts of the same physical set of events (main observation). If different observers give different descriptions of the state of the same system, this means that the notion of state is observer dependent...There is neither an absolute state of the system, nor absolute properties that the system has at a certain time. Physics is fully relational, not just as far as the notions of rest and motion are considered, but with respect to all physical quantities…..Does this mean that there is no relation whatsoever between views of different observers? Certainly not; it means that the relation itself must be understood quantum mechanically rather than classically….(The relation between two views is not absolute either.) …..
the values of physical quantities are relational and their consistency is only probabilistically required …

.

...Quantum mechanics is the theoretical formalization of the experimental discovery that the descriptions that different observers give of the same events are not universal...
It was recently brought to my attention that Zurek ends his paper [Zurek 1982] with
conclusions that are identical to the ones developed here: “Properties of quantum systems have no absolute meaning. Rather, they must be always characterized with respect
to other physical systems” and “correlations between the properties of quantum systems are more basic that the properties themselves”

To quote an ancient lament: "oy vey" ! [LOL]

 

[Naty1]

This is another example where Hawking's math and Hawking's popular explanations are not consistent.

yes, and Hawking acknowledged that distinction...He called it an 'intuitive' explanation ...
I actually posted a source some where in these forums but have now lost track...likely
one of his popular books...

 

[Bill_K]

Rovelli: “…we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier…”

This definition leaves out a lot of particles! Many particles in the Standard Model have lifetimes too short to leave a visible track.

Consider the Z meson. It has a mass of 91 GeV and a lifetime of 3 x 10^-25 sec. Implying, at velocity c it can travel at most a tenth of a fermi before it decays, less than the diameter of a proton. And thanks to its short lifetime the Z meson has a width of 2.5 GeV. GEV! It is never on the mass shell. It always appears as an "internal line" in some Feynman diagram.

So what do you say - is the Z meson a real particle? Or is it merely an "artifact of perturbation theory".

W mesons, top quarks and Higgs bosons have equally short lifetimes. If you consider these particles somehow not real, you're drawing an artificial distinction between particles that are otherwise closely related.

 

[JK423]

This definition leaves out a lot of particles! Many particles in the Standard Model have lifetimes too short to leave a visible track.

Consider the Z meson. It has a mass of 91 GeV and a lifetime of 3 x 10^-25 sec. Implying, at velocity c it can travel at most a tenth of a fermi before it decays, less than the diameter of a proton. And thanks to its short lifetime the Z meson has a width of 2.5 GeV. GEV! It is never on the mass shell. It always appears as an "internal line" in some Feynman diagram.

So what do you say - is the Z meson a real particle? Or is it merely an "artifact of perturbation theory".

W mesons, top quarks and Higgs bosons have equally short lifetimes. If you consider these particles somehow not real, you're drawing an artificial distinction between particles that are otherwise closely related.

Please give an answer to my post #26, thanks!

Roveli talks about leaving tracks in principle! Even when the particle has a lifetime 3 x 10^-25 sec you can in principle interact with it during that time because it acquires a quantum state! In the case of an internal line of a Feynman diagram in perturbation theory, even if it had a lifetime of 1 year you wouldn't be able to interact with it all this time because it has no quantum state and it does not exist!

So, let's put it this way:

Internal lines in Feynman diagrams of perturbation theory, even if they had a A HUGE lifetime, you cannot in principle interact with them because they have no quantum state to interact with! They do not exist.

Real particles, no matter how SMALL a lifetime they have, you can in principle interact with them because they have a quantum state!

If that's not a clear distinction then what is?

 

[tom.stoer]

yes, and Hawking acknowledged that distinction...He called it an 'intuitive' explanation...

I think he says something like that in the non-math part of his famous paper.

 

[Naty1]

Bill_k...

Consider the Z meson.

Now you are just making stuff up! ...just joking,
but I could not even find Z meson on 'the list'...

http://en.wikipedia.org/wiki/List_of_mesons

But the note at the bottom of the table is interesting:

Because this table was initially derived from published results and many of those results were preliminary, as many as 64 of the mesons in the above table may not exist or have the wrong mass or quantum numbers.

Methinks we have way too many particles!jk423...good point you beat me to posting...

Rovelli talks about leaving tracks in principle! Even when the particle has a lifetime 3 x 10-25 sec you can in principle interact with it ...

 

[Bill_K]

Now you are just making stuff up! ...just joking, but I could not even find Z meson on 'the list'...

Sorry, I'm referring to the Z⁰ boson.