Question about Virtual Particles

[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?

 

[PeterDonis]

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

The fact that, as I noted in my last post, referring to internal lines in Feynman diagrams is what the term "virtual particle" was invented for; that was its original meaning. So it's perfectly reasonable, if one is trying to be precise, to use it only with that original meaning, and use other terms for other things.

One could also note that the original intent of the adjective "virtual" was to emphasize the fact that internal lines in Feynman diagrams are unobservable. From that point of view, expanding the term "virtual particle" to include things that are observable amounts to undermining the original meaning of the term.

 

[TrickyDicky]

in the theory virtual particles lead to effects - that doesn't make them real.

So what make real particles real if not their effects in the form of clicks, dots or tracks? The only distinction I can think of is that the effects of real particles are localized, while those of virtual particles are more field-like, but in QFT there are fields and their excitations so I don't understand the distinction made a about existence vs mathematical artifice. The fact that nonperturbative methods exist that don't use Feynman diagrams is not very convincing because the fact remains that of certain observed effects there is only a perturbative accurate calculation(Lamb shift, gyromagnetic moment of electron, Casimir force...). On the other hand as counterexamples there are real particles like gluons and quarks that are internal lines in FD)

the original intent of the adjective "virtual" was to emphasize the fact that internal lines in Feynman diagrams are unobservable..

The external lines are only observable by their effects, that are more localized and therefore nearer to the classic concept of particle than the effects attributed to internal lines, but as I commented above QFT also includes fields and interactions as fundamental objects, not just particles.

 

[bhobba]

So what make real particles real if not their effects in the form of clicks, dots or tracks?

That's the point - they don't leave clicks dots or tracks.

Thanks
Bill

 

[TrickyDicky]

That's the point - they don't leave clicks dots or tracks.

Thanks
Bill

But they have a different kind of effects, let's call them virtual effects from virtual fields,(like say the line separation in the Lamb shift or the pressure in the Casimir effect), what makes these less real than clicks in a counter, or in the latter case the object that "causes" the click less of a math artifact?

 

[bhobba]

But they have a different kind of effects

Yes - exactly.

Thanks
Bill

 

[Jilang]

Isn't this a bit like saying that a particle can travel multiple paths to its final destination. Are the paths real at all given that only one could ever be observed? Would we class the other paths as virtual?

 

[TrickyDicky]

Yes - exactly.

Nice, thanks. and your answer to the question is...

 

[craigi]

Isn't this a bit like saying that a particle can travel multiple paths to its final destination. Are the paths real at all given that only one could ever be observed? Would we class the other paths as virtual?

If you even believe that the particle takes any path between observation events. That depends upon interpretation.

 

[ddd123]

On the other hand as counterexamples there are real particles like gluons and quarks that are internal lines in FD

Can you expand on this? Thanks.

 

[bhobba]

Nice, thanks. and your answer to the question is...

What I said - if you don't get it - re-read it.

Thanks
Bill