Question about Virtual Particles

[Gerinski]

Wikipedia says that virtual particles can not be observed, they are a handy concept for understanding what happens in quantum interactions, annihilating each other before they can be detected as real particles. However it also says that under certain circumstances, if they are moved apart from each other quickly enough, they can turn into real particles instead of annihilating each other. As I understand this is the principle behind black holes Hawking Radiation, where one of the pair particles falls into the hole while the other one can escape it.

My question is: Has the event of a virtual particle turning into a real detectable particle ever been experimentally observed?

 

[bhobba]

No - because virtual particles don't actually exist. They are simply an artefact of the mathematical methods used called perturbation theory and something called a Dyson Series:
http://en.wikipedia.org/wiki/Dyson_series

Thanks
Bill

 

[ddd123]

No - because virtual particles don't actually exist. They are simply an artefact of the mathematical methods used called perturbation theory and something called a Dyson Series:

Admittedly I've never looked into Hawking Radiation. What would it mean then if it was actually observed? A quantum gravity behavior of the field which is just pictorially explained as virtual particles going astray, but not quite?

 

[bhobba]

Admittedly I've never looked into Hawking Radiation. What would it mean then if it was actually observed? A quantum gravity behavior of the field which is just pictorially explained as virtual particles going astray, but not quite?

I am not sure of the point you are trying to make. But in the theory virtual particles lead to effects - that doesn't make them real.

In fact there is another formulation called lattice gauge theory were they are absent and allows theoretical predictions. Trouble is it can only be done on a computer and hasn't as yet achieved the accuracy of the usual method.

Thanks
Bill

 

[ddd123]

I'm not trying to make a point, if I had one I wouldn't ask a question. Even Wikipedia manages the topic in this way and this virtual pair production becoming real is popular: https://en.wikipedia.org/wiki/Hawking_radiation

Physical insight into the process may be gained by imagining that particle-antiparticle radiation is emitted from just beyond the event horizon. 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.

If the effect of the - unreal - virtual particle is for it to become real, one wonders about the reality status of the former a bit more, if the explanation is left at that. So I was asking what the field-theoretic approach actually means in this case.

 

[phinds]

Hawking has stated that the whole "virtual particle" thing is not what actually happens in Hawking Radiation but rather is just a way to describe in English what can really only be described accurately in math. It's sort of an analogy, not a description of reality.

 

[bhobba]

If you do a search you will find many threads on this forum explaining virtual particles are simply a mathematical artefact. They do not appear in other methods. They are not the cause of Hawking radiation.

Thanks
Bill

 

[Gerinski]

Wiki also says:

"The longer a virtual particle exists, the more closely it adheres to the mass-shell relation. A "virtual" particle that exists for an arbitrarily long time is simply an ordinary particle.
However, all particles have a finite lifetime, as they are created and eventually destroyed by some processes. As such, there is no absolute distinction between "real" and "virtual" particles. In practice, the lifetime of "ordinary" particles is far longer than the lifetime of the virtual particles that contribute to processes in particle physics, and as such the distinction is useful to make."

 

[bhobba]

Forget Wiki

Our FAQ gives the facts:
https://www.physicsforums.com/threads/what-are-virtual-particles.763185/

Please, please ignore pretty much anything about Quantum Field Theory outside a Quantum Filed Theory textbook. It is almost certainly WRONG.

Thanks
Bill

 

[ddd123]

If you do a search you will find many threads on this forum explaining virtual particles are simply a mathematical artefact. They do not appear in other methods. They are not the cause of Hawking radiation.

Just to report that I already read those threads long before this one. I know about it. But since this conception is so pervasive, even on wiki etc... it does confuse me. For example that above Wiki quote is pretty bewildering, it seems to positively contradict everything this forum usually says.

 

[phinds]

For example that above Wiki quote is pretty bewildering, it seems to positively contradict everything this forum usually says.

Yes, because Wiki is wrong. I don't know why more knowledgeable people don't get on there and fix it but they don't, or if they do, the ignorant come back and "fix" it back.

 

[bhobba]

Just to report that I already read those threads long before this one. I know about it. But since this conception is so pervasive, even on wiki etc... it does confuse me..

Anything written about QFT outside a QFT textbook is almost certainly WRONG. It is not an easy area and explaining without the proper technicaluties invariably leads to inaccuracies.

Thanks
Bill

 

[Jilang]

Even the CERN website describes them as real, but just very short lived.
http://pdg.web.cern.ch/pdg/cpep/unc_vir.html

 

[ddd123]

That wiki passage doesn't cite a specific source. It would be interesting to know where they got it from.

 

[craigi]

Wikipedia says that virtual particles can not be observed, they are a handy concept for understanding what happens in quantum interactions, annihilating each other before they can be detected as real particles. However it also says that under certain circumstances, if they are moved apart from each other quickly enough, they can turn into real particles instead of annihilating each other. As I understand this is the principle behind black holes Hawking Radiation, where one of the pair particles falls into the hole while the other one can escape it.

My question is: Has the event of a virtual particle turning into a real detectable particle ever been experimentally observed?

Yes.

http://arxiv.org/abs/1105.4714

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed if it might instead be possible to more directly observe the virtual particles that compose the quantum vacuum. 40 years ago, Moore suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. This effect was later named the dynamical Casimir effect (DCE). Using a superconducting circuit, we have observed the DCE for the first time. The circuit consists of a coplanar transmission line with an electrical length that can be changed at a few percent of the speed of light. The length is changed by modulating the inductance of a superconducting quantum interference device (SQUID) at high frequencies (~11 GHz). In addition to observing the creation of real photons, we observe two-mode squeezing of the emitted radiation, which is a signature of the quantum character of the generation process.

 

[bhobba]

One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence.

Incorrect - as many threads on this forum explain - including our FAQ.

Its not just this forum:
http://physics.stackexchange.com/questions/4349/are-w-z-bosons-virtual-or-not/22064#22064
For anyone still tempted to associate a physical meaning to virtual particles as a specific quantum phenomenon, let me note that Feynman-type diagrams arise in any perturbative treatment of statistical multiparticle properties, even classically, as any textbook of statistical mechanics witnesses.

More specifically, the paper http://homepages.physik.uni-muenchen.de/~helling/classical_fields.pdf shows that the perturbation theory for any classical field theory leads to an expansion into Feynman diagrams very similar to those for quantum field theories, except that only tree diagrams occur. If the picture of virtual particles derived from Feynman diagrams had any intrinsic validity, one should conclude that associated to every classical field there are classical virtual particles behaving just like their quantum analogues, except that (due to the lack of loop diagrams) there are no virtual creation/annihilation patterns. But in the literature, one can find not the slightest trace of a suggestion that classical field theory is sensibly interpreted in terms of virtual particles.

The reaon for this similarity in the classical and the quantum case is that Feynman diagrams are nothing else than a graphical notation for writing down products of tensors with many indices summed via the Einstein summation convention. The indices of the results are the external lines aka ''real particles'', while the indices summed over are the internal lines aka ''virtual particles''. As such sums of products occur in any multiparticle expansion of expectations, they arise irrespective of the classical or quantum nature of the system.

Now can we move on please.

Thanks
Bill

 

[bhobba]

That wiki passage doesn't cite a specific source. It would be interesting to know where they got it from.

Its a common misconception even amongst those that actually know QFT. But misconception it is.

Thanks
Bill

 

[craigi]

My question is: Has the event of a virtual particle turning into a real detectable particle ever been experimentally observed?

So you have two answers to your question:

No based upon semantic objections to your question.
Yes from a well respected group of physicists, who made the observation themselves.

You choose.

 

[bhobba]

You choose.

I choose logic from understanding what virtual particles are - as per the link I gave.

But if you don't agree feel free to contact its author Professor Neumaier about it:
http://arnold-neumaier.at/

Thanks
Bill

 

[ddd123]

http://physics.stackexchange.com/questions/4349/are-w-z-bosons-virtual-or-not/22064#22064

This is the most complete answer so far. But the comments on it... There doesn't seem to be a definitive nail on the issue, at least for someone who is still slightly outside of the field. Here is a chat log where it's dissected:

http://chat.stackexchange.com/rooms/5906/discussion-between-arnold-neumaier-and-user1247

It seems very similar to the interpretational issues of QM. I guess that for you it's not. But it's hard to see. For example, the argument that "a single Feynman diagram is meaningless so virtual particles are unphysical" isn't definitively convincing: the diagrams could be seen as a weak form of superposition (okay they're not "states"), but we don't observe the pure state wavefunction either, only the measurement. So while classical fields are completely deterministic (and their Feynman diagrams with only tree levels remain formal) quantum theory is peculiarly probabilistic and involves superpositions, which we usually talk about as being "real".

The other argument about virtual particles being absent in Lattice theories is more convincing. But this could be seen as just a different viewpoint that a theory offers, while virtual particles retaining value in the continuum picture.

The main problem is that it's not just profane people that allow this idea, but even scientists in that very field. The CERN website says it. So since you put the issue at the level of something uncontroversial and established, it's confusing. The other threads about virtual particles don't go into this detail. Maybe we should talk about this more, not less. I don't agree with craigi in that many scientists have often been wrong anyway: it's called a "argument from authority" fallacy.

 

[bhobba]

There doesn't seem to be a definitive nail on the issue,

You mean that its mathematically exactly the same as what can be done in classical field theory doesn't convince you its just part of the mathematical formalism?

Forget others confusion - you know the facts and can reason for yourself.

Thanks
Bill

 

[ddd123]

Yes, to conclude that I would have to first study that paper and see for myself (which I intend to do, but it'll take time). I still have to finish my QFT course. I will certainly make up my mind when I see the theory for myself.

 

[Gerinski]

What about this other quote from Wiki?

"In an accelerating frame of reference, the virtual particles may appear to be actual to the accelerating observer; this is known as the Unruh effect. In short, the vacuum of a stationary frame appears, to the accelerated observer, to be a warm gas of actual particles in thermodynamic equilibrium."

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

 

[bhobba]

What about this other quote from Wiki?

Same answer.

No one is claiming they don't lead to effects. Its simply they are mathematical by-products of the perturbation methods used.

Thanks
Bill

 

[ddd123]

Do you agree with this sentence from the Unruh Effect wiki page? It uses "fields" instead of "virtual particles" as I suggested in my first post in the thread:

In modern terms, the concept of "vacuum" is not the same as "empty space": space is filled with the quantized fields that make up the universe. Vacuum is simply the lowest possible energy state of these fields.

The energy states of any quantized field are defined by the Hamiltonian, based on local conditions, including the time coordinate. According to special relativity, two observers moving relative to each other must use different time coordinates. If those observers are accelerating, there may be no shared coordinate system. Hence, the observers will see different quantum states and thus different vacua.

 

[Nick666]

So they're something, but not particles ?

 

[bhobba]

Do you agree with this sentence from the Unruh Effect wiki page? It uses "fields" instead of "virtual particles" as I suggested in my first post in the thread:

Did you read what Professor Neumaier wrote?

Thanks
Bill

 

[bhobba]

So they're something, but not particles ?

As per the above.

Just a comment here. The facts have been laid out. Its not too hard to see how to answer questions like the above based on that.

Thanks
Bill

 

[ddd123]

Did you read what Professor Neumaier wrote?

Thanks
Bill

Yes, also the whole comments and chat with the other user I linked. But they were in the context of scattering matrix elements, I don't know how to translate this to the Unruh effect.

 

[bhobba]

Yes, also the whole comments and chat with the other user I linked. But they were in the context of scattering matrix elements, I don't know how to translate this to the Unruh effect.

Translate what? Virtual particles are simply mathematical artefacts of the perturbation methods used. They could be called Jaberwokys - it won't make any difference - so that's what I will call them. Now these Jaberwokys look different in an accelerated frame - so?

Thanks
Bill

 

[Jilang]

So... If the distinction between real and virtual lies in the eye of the beholder, is it a useful distinction?

 

[ddd123]

Translate what? Virtual particles are simply mathematical artefacts of the perturbation methods used. They could be called Jaberwokys - it won't make any difference - so that's what I will call them. Now these Jaberwokys look different in an accelerated frame - so?

Thanks
Bill

Well no in the accelerated frame they are real particles this time, not mathematical artifacts. But it could be solved if we interpreted them as a way to handle the behavior of a field. That's why I asked you if you agree with that other wiki quote instead, which doesn't mention virtual particles. It may be easy to deduce from Neumaier's explanation but not for me :D

 

[anorlunda]

Yes, because Wiki is wrong. I don't know why more knowledgeable people don't get on there and fix it but they don't, or if they do, the ignorant come back and "fix" it back.

Hmmm, does Wikipedia allow links to PF as valid sources?

 

[phinds]

Hmmm, does Wikipedia allow links to PF as valid sources?

Don't know but i doubt it. There are many threads here where there is a lot of wrong information included. It DOES get corrected (or the thread gets closed) but it can make for a messy read and I suspect wiki is looking for more academic references (or at least I hope they are, I haven't looked a lot at what kind of references they give since I don't use them for anything but basic definitions and those they generally get right).

 

[bhobba]

Well no in the accelerated frame they are real particles this time, not mathematical artifacts.

"In an accelerating frame of reference, the virtual particles may appear to be actual to the accelerating observer

Thanks
Bill

 

[ddd123]

What's the difference? Aren't we in General Relativity? There are no standard inertial frames. That's not the quote I was referring to, I meant the one that I quoted.

 

[bhobba]

What's the difference? Aren't we in General Relativity? There are no standard inertial frames. That's not the quote I was referring to, I meant the one that I quoted.

Well if you meant your quote then there is even less to worry about. They see different 'vacua' ie they see different Jaberwokys. So?

Thanks
Bill

 

[ddd123]

So you're against it as well? It's not mentioning virtual particles there. It did seem pretty reasonable to me, all the objections against virtual particles don't hold anymore for the field in general (the classical field exists instead of the virtual particles exhibited in classical Feynman diagrams; the lattice gauge theory handles the field; etc).

 

[bhobba]

So you're against it as well? It's not mentioning virtual particles there.

It mentions 'vacua' and how the vacua looks different to different observers.

In pertubative QFT the vacua is teeming with Jaberwockeys in constant creation and annihilation. It is responsible in the theory for all sorts of things like spontaneous emissions, Lamb Shift and charge screening. Its also responsible for the infinite energy of the vacuum. That should be a clue something is rotten if you consider them real. So you go and look why it happens. Lo an behold you soon find - its that space-time is treated as a continuum. So we do a cut-off. But guess what - that's lattice gauge theory and in lattice gauge theory you get no virtual particles.

So if you think them real how do you explain the vacuum energy isn't infinite? The usual work around is to renormalise it by subtracting infinity from it to give zero. Again rather fishy. In fact its one of the first instances of renormalisation.

Thanks
Bill

 

[ddd123]

Okay I've heard about all this, but if an observer sees real particles which were absent in another reference frame (there being no preferred inertial one in GR) how do we interpret this in the Jabberwock picture? At least some form of vacuum energy must really exist for this to be possible.

 

[bhobba]

Okay I've heard about all this, but if an observer sees real particles which were absent in another reference frame (there being no preferred inertial one in GR) how do we interpret this in the Jabberwock picture?

Didn't you see the highlighted bit - APPEAR.

I am sure Lattice Gauge Theory would predict the same thing without Jaberwocky's - it better or we are in deep do do.

Thanks
Bill

 

[ddd123]

Yes but there are no apparent forces in GR, at least that's the only way I can interpret the "appear". A thermal bath can't just appear, either it's there or it isn't. Otherwise you may be appearing to me this moment and fall into a Jabberwock if I accelerated in a certain way wrt you.

 

[ddd123]

Anyway I give up, I'll come back when I have more technical knowledge.

 

[bhobba]

Yes but there are no apparent forces in GR, at least that's the only way I can interpret the "appear". A thermal bath can't just appear, either it's there or it isn't. Otherwise you may be appearing to me this moment and fall into a Jabberwock if I accelerated in a certain way wrt you.

That's true - but my view is even more basic. If Jaberwockys were the actual reason then other methods to calculate it that don't have them wouldn't work.

Thanks
Bill

 

[Vanadium 50]

, but if an observer sees real particles which were absent in another reference frame (there being no preferred inertial one in GR)

That would be a problem, if it happened. But it doesn't.

 

[ddd123]

That would be a problem, if it happened. But it doesn't.

How about this paper: http://arxiv.org/pdf/gr-qc/9707012.pdf in the Hawking Radiation chapter, there's "particle production in non-stationary spacetimes". This is where I've been led to on the topic by my GR professor.

 

[haushofer]

I am not sure of the point you are trying to make. But in the theory virtual particles lead to effects - that doesn't make them real.

In fact there is another formulation called lattice gauge theory were they are absent and allows theoretical predictions. Trouble is it can only be done on a computer and hasn't as yet achieved the accuracy of the usual method.

Thanks
Bill

I believe one has also the Kadyshevski formulation (or something like that) of QFT, in which virtual particles are also absent.

 

[Vanadium 50]

How about this paper: http://arxiv.org/pdf/gr-qc/9707012.pdf in the Hawking Radiation chapter, there's "particle production in non-stationary spacetimes".

Imagine a box that counts particles and displays how many it detects in a bright LED. Observers, both accelerated and non-accelerated will agree on the number displayed on the LED. They may well disagree on the source or histories of the particles, but there is no dispute as to the number.

 

[PeterDonis]

So you have two answers to your question:

No based upon semantic objections to your question.
Yes from a well respected group of physicists, who made the observation themselves.

You choose.

First we need to be clear about what we are "choosing". The issue here is that the term "virtual particle" is being used in this thread to mean two different things:

Bhobba is using "virtual particle" to mean, roughly speaking, "an internal line in a Feynman diagram". It's impossible to observe one of those, so the answer with this meaning is obviously "no". While it's true that many sources don't use the term "virtual particle" with this meaning, it does happen to be the original meaning of the term, since describing internal lines in Feynman diagrams was what the term was invented for. The fact that so many sources have not respected this original usage illustrates the problems you get into when you try to use ordinary language instead of mathematics to describe scientific theories.

The paper you linked to is using "virtual particle" to mean, roughly speaking, "a mode of the quantum field". It's certainly possible to observe one of those: just induce a state transition in the mode and then have it interact with a detector. In the paper, the "mirror" (actually a SQUID device being tuned appropriately) adds energy to EM field modes, and that energy is then detected as photons--basically the field modes just transfer the energy from the SQUID to the detector, and the intermediate carrier of the energy is called a "photon"--a "virtual" photon when the corresponding field mode is in its ground state, which then turns into a "real" photon when the mode is excited by the SQUID. So on this interpretation, the answer is obviously "yes"; there are lots of ways of exciting quantum field modes and then observing the results of the excitation.

In the case of the Unruh effect, the key is that a given state of the quantum field can be a "vacuum" state to an inertial observer--i.e., an inertial detector detects no particles--zero probability of a state transition--but not to an accelerated observer, i.e., an accelerated detector has a nonzero probability of undergoing a state transition that we interpret as "detecting a particle". Once again, if we interpret "virtual particle" to mean "a mode of the quantum field", then this is just another example of a "yes" answer to the question: the accelerated detector is just another interaction with a quantum field mode. It's worth noting that, from the viewpoint of an inertial observer, this interaction looks like the emission of a particle, rather than the detection (and consequent absorption) of one; in the inertial viewpoint, what happens is that some of the energy that is being pumped into the accelerating detector in order to accelerate it gets transferred to a quantum field mode, which transitions from the "ground" state (at least, the ground state from the viewpoint of the inertial observer) to an "excited" state.

So what we actually need to choose is a single consistent interpretation of the term "virtual particle". Even better, we could taboo that term altogether for this discussion, and ask the OP to restate his question without using it. Then we would know which answer to give.

 

[craigi]

First we need to be clear about what we are "choosing". The issue here is that the term "virtual particle" is being used in this thread to mean two different things:

Bhobba is using "virtual particle" to mean, roughly speaking, "an internal line in a Feynman diagram". It's impossible to observe one of those, so the answer with this meaning is obviously "no". While it's true that many sources don't use the term "virtual particle" with this meaning, it does happen to be the original meaning of the term, since describing internal lines in Feynman diagrams was what the term was invented for. The fact that so many sources have not respected this original usage illustrates the problems you get into when you try to use ordinary language instead of mathematics to describe scientific theories.

The paper you linked to is using "virtual particle" to mean, roughly speaking, "a mode of the quantum field". It's certainly possible to observe one of those: just induce a state transition in the mode and then have it interact with a detector. In the paper, the "mirror" (actually a SQUID device being tuned appropriately) adds energy to EM field modes, and that energy is then detected as photons--basically the field modes just transfer the energy from the SQUID to the detector, and the intermediate carrier of the energy is called a "photon"--a "virtual" photon when the corresponding field mode is in its ground state, which then turns into a "real" photon when the mode is excited by the SQUID. So on this interpretation, the answer is obviously "yes"; there are lots of ways of exciting quantum field modes and then observing the results of the excitation.

In the case of the Unruh effect, the key is that a given state of the quantum field can be a "vacuum" state to an inertial observer--i.e., an inertial detector detects no particles--zero probability of a state transition--but not to an accelerated observer, i.e., an accelerated detector has a nonzero probability of undergoing a state transition that we interpret as "detecting a particle". Once again, if we interpret "virtual particle" to mean "a mode of the quantum field", then this is just another example of a "yes" answer to the question: the accelerated detector is just another interaction with a quantum field mode. It's worth noting that, from the viewpoint of an inertial observer, this interaction looks like the emission of a particle, rather than the detection (and consequent absorption) of one; in the inertial viewpoint, what happens is that some of the energy that is being pumped into the accelerating detector in order to accelerate it gets transferred to a quantum field mode, which transitions from the "ground" state (at least, the ground state from the viewpoint of the inertial observer) to an "excited" state.

So what we actually need to choose is a single consistent interpretation of the term "virtual particle". Even better, we could taboo that term altogether for this discussion, and ask the OP to restate his question without using it. Then we would know which answer to give.

A good summary which clears up much misinformation on the subject.

The remaining question, is what is the motivation for Neumaier's restriction of the term virtual particle to an internal leg of a Feynman diagram?

Are we safe to presume that he wishes to reserve the term in the literature for this since it is easy to define rigourously?