Question about Virtual Particles

[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

 

[PeterDonis]

Are the paths real at all given that only one could ever be observed?

No, no path is observed for virtual particles that are internal lines in Feynman diagrams.

 

[PeterDonis]

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?

The words "virtual" and "real" in this connection (i.e., referring to internal vs. external lines in Feynman diagrams) IMO are best viewed as technical terms; they're just labels attached to particular mathematical objects. No ontological claims are intended--or at least, using the terminology does not, IMO, require you to commit yourself to any ontological claims. If you don't like the words "virtual" and "real" because they seem to you to imply ontological claims, then just use different words (like "internal lines" vs. "external lines"). The math and the physical predictions are the same.

The distinction between types of effects IMO is a question of what types of effects are reasonably called "particle" effects. A track in a cloud chamber seems like an obvious case of a "particle" effect. A shift in a spectral line or a tiny force between uncharged plates, not so much. This distinction happens to correspond pretty closely with the distinction between internal and external lines in Feynman diagrams, which is why the terms "virtual particle" vs. "real particle" were originally chosen. But both types of effects are certainly real (in the ordinary ontological sense). It's just a question of what label you consider appropriate for the different types of effects.

 

[TrickyDicky]

Can you expand on this? Thanks.

Simply, since it's being claimed that internal lines of Feynman diagrams are mathematical artifacts I just showed a couple of elementary particles of the SM that are always internal lines and that are considered real indirectly by their effects(look up jets and hadronization). Why effects like electric repulsion, spectral line separation or Casimir force don't deserve the same treatment is what I was asking, but the best answerso far is by bhobba:just because.

 

[TrickyDicky]

The words "virtual" and "real" in this connection (i.e., referring to internal vs. external lines in Feynman diagrams) IMO are best viewed as technical terms; they're just labels attached to particular mathematical objects. No ontological claims are intended--or at least, using the terminology does not, IMO, require you to commit yourself to any ontological claims. If you don't like the words "virtual" and "real" because they seem to you to imply ontological claims, then just use different words (like "internal lines" vs. "external lines"). The math and the physical predictions are the same.

Hmmm, all the answers to the OP and ddd123 rest on the idea that something "doesn't exist", period. That looks pretty ontological to me. So your answer is reasonable but doesn't seem to correspond to what is being argued here by most. All the claims have been clearly ontological.

The distinction between types of effects IMO is a question of what types of effects are reasonably called "particle" effects. A track in a cloud chamber seems like an obvious case of a "particle" effect. A shift in a spectral line or a tiny force between uncharged plates, not so much.

That's why I used the word "virtual field", more appropriate to a theory of quantum fields. But honestly, I don't think this is about semantics, it seems to be about ontology.

 

[PeterDonis]

all the answers to the OP and ddd123 rest on the idea that something "doesn't exist", period

That's why I took the trouble to point out the difference in terminology; before you can even ask the ontological question, you have to first be sure what you are asking it about. It's perfectly reasonable to have different answers to the question "do virtual particles exist?" for different interpretations of the term "virtual particle"; for example, one could say that internal lines in Feynman diagrams don't exist (because they are artifacts of a particular model, perturbation theory) but quantum field modes that get excited and register in detectors do exist (because the detectors interact with them). But two people who have those two different viewpoints don't necessarily disagree with each other; they could both be right.

 

[ddd123]

But does this mean that the former camp also believes that quarks and gluons don't really exist?

 

[TrickyDicky]

But does this mean that the former camp also believes that quarks and gluons don't really exist?

Certainly not. But most, at least judging by the "official" position in this site will insist that any internal line in a Feynman diagram(other than those in the SM of elementary particles) doesn't really exist.

 

[PeterDonis]

That's why I used the word "virtual field", more appropriate to a theory of quantum fields.

But the fields are either never "virtual" or always "virtual", depending on how you look at it. They are never directly observable (so always "virtual" in that sense). The expansion in perturbation theory is an expansion of the amplitude in powers of the coupling constant, not powers of the field--the field doesn't even appear (so fields are never "virtual" in that sense).

 

[PeterDonis]

I just showed a couple of elementary particles of the SM that are always internal lines

This is not correct. The correct statement is that quarks and gluons are confined. But confinement does not prohibit you from looking at diagrams that have external quark or gluon lines. For example, the standard diagram for a weak interaction decay has external quark lines (the only internal line at lowest order is the W particle).

One could also argue that perturbation theory doesn't really work for the strong interaction anyway, because the coupling constant is large enough that the higher order terms are as large as the lower order terms. At higher energies, the coupling constant gets weaker, so this argument doesn't apply--but at higher energies, quarks and gluons are no longer confined.

 

[Jilang]

You misunderstand me. If all the paths contribute, but none except one are not observed, are not the unobserved paths virtual by this criterion?