Question about Virtual Particles

[PeterDonis]

A thermal bath can't just appear, either it's there or it isn't.

As the Unruh effect shows, this is false, however plausible it seems. The key point is that the notion of "a particle being present" turns out to be observer-dependent when quantum effects are taken into account; a quantum field state that looks like a vacuum to one observer (a freely falling one) can look like a thermal state with particles in it to another observer (an accelerated one). But it's the same underlying quantum field state in both cases. So your intuition here is simply wrong, however plausible it seems.

Whether it is possible to change the order of cause and effect just by viewing two events from a different frame.

In QFT, that's not quite how to put it. The way to put it is this: quantum field operators must commute at spacelike separated events. In other words, the results of quantum measurements at spacelike separated events cannot depend on which order the measurements are made in. The results can only depend on the order of measurements if the events are timelike or null separated--i.e., if the order of events is invariant.

 

[Jilang]

@ JVM Are there not many ways to skin a cat? Would it not be conceivable that each mode did contribute in the Many Worlds, so each mode is a real occurrence in a specific world?

 

[yoron]

Thanks Peter. Although the first citation wasn't mine it caught something I wondered about when it came to causality. The other citation I used came from Causality in special relativity

 

[Vanadium 50]

The key point is that the notion of "a particle being present" turns out to be observer-dependent when quantum effects are taken into account

Yes, but the notion of "a particle is registered in a detector" turns out not to be observer-dependent. Much of this confusion comes from asking questions that are not directly connected to experiment.

 

[PeterDonis]

the notion of "a particle is registered in a detector" turns out not to be observer-dependent.

More precisely, the notion of "a state transition took place" turns out not to be observer-dependent. But observers can still disagree on how to describe the state transition. The accelerated observer describes it as the absorption of a particle by the detector, changing the detector's state, and interprets this as the detection of a particle. But the inertial observer describes it as the emission of a particle by the detector, changing the detector's state.

 

[king vitamin]

In addition to what's been said about the anomalous/off-shell dispersions of virtual particles, I'd like to add that the virtual particle content is gauge dependent. That is, depending on the your gauge choice you can have extra virtual particles (ghosts or spurious Golstones) which are not seen in any physical setting. You're free to choose whether these virtual particles appear in your perturbative expansion or not. Certainly at least these are uncontroversially considered unphysical?

 

[haushofer]

Actually I do think you have a excellent point in lifting up 'thermal baths' ddd, " A thermal bath can't just appear, either it's there or it isn't.

A thermal bath consists of particles. These particles can be counted by a number operator as for the harmonic oscillator. However, this operator is only invariant with respect to inertial observers. Hence, accelerating observers will count a different amount of particles. See e.g. Nikolic's "myths and facts"-paper, which has an excellent treatment on this.

 

[ddd123]

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.

A thermal bath consists of particles. These particles can be counted by a number operator as for the harmonic oscillator. However, this operator is only invariant with respect to inertial observers. Hence, accelerating observers will count a different amount of particles. See e.g. Nikolic's "myths and facts"-paper, which has an excellent treatment on this.

Which of these is correct? Maybe rather than strict number of particles, the invariant is the total energy that comes to the detector? Otherwise i'd be tempted to start taking the vacuum energy more seriously.

 

[Vanadium 50]

The number of particles that the detector registers is invariant. Different observers will disagree on the source and history of these particles, but the number is constant. This is exactly analogous to the relativity of simultanity, where different observers disagree about the time ordering of distant events.

 

[ddd123]

I take it to mean that also the kind of particles is invariant?

 

[PeterDonis]

The number of particles that the detector registers is invariant.

More precisely, the number of state transitions is invariant. But, as I noted before, different observers will have different descriptions of what these state transitions consist of (detection of particles vs. emission of particles).

 

[carllooper]

What can be observed of the physical world (and said about such) is far more important than the physical world in itself. Basically because there is no such thing as the physical world in itself. The physical world is always related to what is observed (and said about such). And vice versa. Otherwise how would we know that what was observed (and said) was even related to the physical world, let alone being of the physical world.

Now the moment you sever this connection between observations and the physical world, one or the other disappears. One is left with either an observation (in search of an explanation) or an explanation (in search of an observation).

Many theoretical physicists prefer the latter situation - maintaining the notion of a physical world (it's explanation) without the need to involve observations. For such theorists, observations are just questions (not answers). However, without an observation, the physical world becomes no different from the mathematical concepts that would otherwise "explain" an observation. There's nothing wrong with this idea but it's questionable whether it's really physics. However we can maintain it's at least a rehersal.

For example, one could program a computer, using the developed explanations (the mathematical models), to display an animation of some physical process. But the result is not itself the physical process. Its an animation of the physical process. A representation. A rehersal. Until the explanation is used to manipulate the actual physical world in some way, it remains somewhat removed from the physical world - living in it's own little bubble of sorts. Its not a bad bubble and there's all sorts of problems to be solved in that context.

But it's really important to connect an explanation back to the observations it otherwise "explains" - which means doing physical experiments - not just animations. Seeing if the physical world, when reconnected to observations, behaves in the way that is otherwise modeled or represented (eg. by an animation).

Of course, most of these experiments have already been done - but that's no reason not to do them again - precisely because, when they're not actually done, the explanation ends up back in it's own little bubble again, going nowhere, doing nothing but encouraging the perverse idea that the physical world really is no more than the explanation. For example, the idea that the physical world is fundamentally mathematical, has it's origin in this sort of bubble.

But an explanation only makes sense when there is something to be explained. Of course that's no reason to stop rehersing explanations. We never know when it will be needed. We want to be sure we have our game hat on when a real world problem is to be solved. Rather, it is say that an explanation can be not the entire truth - that there is a world bigger than the explanation - and that this always involves observations (actual experimental observations - not just the idea of such). That the physical world is not just the "physical world" of which the theorist prefers (that which can be modeled on a computer), but that real physical world which actually takes place and is expressed in actual physical experiments.

Anyway all of this is to suggest why explanations, on their own, can't be nailed down in terms of which is correct and which isn't. They are somewhat impervious to criticism - mathematical models particularly so. They will be correct regardless of the real observable physical world.

C

 

[Gerinski]

How you reach that conclusion from this thread beats me.

If they were real all ways of doing the QFT calculations would have them - but they don't.

Thanks
Bill

Well, there are real phenomena which get commonly attributed to virtual particle interactions (Casimir effect, vacuum polarization, the contribution to mass of the vacuum fluctuations etc). If they are real phenomena they must be caused by something real. Mathematical tricks can not produce observable effects can they?
If they all have an uncontroversial explanation in terms of real particles and fields only, kindly provide these. I don't know why would anyone bother with explaining them in terms of virtual particles if there was a more straightforward explanation available. That's why I said: "there must be something real going on causing these effects, perhaps the issue is that whatever it is, it should not be called 'particles', since as the thread seems to confirm, it has little to do with real, on-shell particles".

 

[Gerinski]

Advise:

first: study it all in its mathematical rigorous (if possible) formulation (it will take many many long years of hard study).

second: only after that, read popularizations (pop-science books, famous quotes, whatever...) if you like, but now knowing perfectly well what actually lies beneath.If you ever want to (really) understand some of it, never start with pop-books.

So you are asking all of us laymen to stay away from PF?
That's why some of us come here, to ask and to learn from people who know the business, to get them correcting our misconceptions and to grow in our understanding of physics. If you don't like sharing your knowledge with laymen that's fine, but maybe there are others who don't mind.

 

[bhobba]

Well, there are real phenomena which get commonly attributed to virtual particle interactions (Casimir effect, vacuum polarization, the contribution to mass of the vacuum fluctuations etc).

Haven't you been reading the thread? That they are caused by virtual particles is simply due to the pertubative formalism used. There are other ways of doing the calculations where virtual particles do not appear. Hence the cause can't be virtual particles.

Those other methods are mathematically more difficult requiring some really really advanced math and/or are only solvable on computers. That's why they are not used and pertubative theory used instead.

Here is an example of a reference:
https://books.google.com.au/books?id=bHtpAgAAQBAJ

It was also mentioned that Wienbergs 'bible' on QFT, even though it also uses perturbation theory, doesn't mention virtual particles either:
https://www.amazon.com/dp/0521670535/?tag=pfamazon01-20

Why isn't it done that way? It hard, very very hard. Those texts are known as 'challenging' to say the least, meaning basically for the very advanced - and even then only 'brave' ones.

All this thread is doing is going over the same thing again and again. Doing that will not change anything.

Added Later:
I dug up the following that hopefully will settle the issue:
https://www.physicsforums.com/threads/non-perturbative-qft-without-virtual-particles.485597/

Thanks
Bill

 

[ddd123]

Bhobba, since the unsettling part of all these posters seems to be the lack of real-ness of mathematical formalisms vs. the real effect they are associated with, why don't you answer them that the real cause of the real effect is simply the quantum field (modes)? Is it simply a coincidence or is there a reason why you don't want to answer in this way?

 

[Gerinski]

Bhobba, since the unsettling part of all these posters seems to be the lack of real-ness of mathematical formalisms vs. the real effect they are associated with, why don't you answer them that the real cause of the real effect is simply the quantum field (modes)? Is it simply a coincidence or is there a reason why you don't want to answer in this way?

Indeed that's just what I was asking. If there are real effects and you say that their cause are not virtual particles since they are nothing real, just kindly explain me what is the real cause of those effects and I will be happy. Again that's why I said: "there's something real some people call virtual particles even if they are not virtual particles. Just explain me what it is". If so the problem is just in the name given to the cause, the cause is still something real, isn't it?

 

[Jilang]

Oscillations of quantum fields that just don't have enough energy to make a real standalone particle that can be detected?

 

[craigi]

Oscillations of quantum fields that just don't have enough energy to make a real standalone particle that can be detected?

The Vacuum Energy is actually large. If we estimate it from the cosmological constant we get 10^-9 Joules/m³. If we calculate it from QFT we get 10¹¹³ Joules/m³. A very large difference, but in either case, there's plenty of energy for real particles, under the right conditions. For comparion, the energy for photons in the visible part of the spectrum is of the order of 10⁻¹⁹ Joules. We can go many orders of magnitude lower as we move towards radio frequencies.

Read the paper which I posted towards the start of the thread, to better understand the conditions in which we can generate real particles from the vacuum.

 

[Gerinski]

Haven't you been reading the thread? That they are caused by virtual particles is simply due to the pertubative formalism used. There are other ways of doing the calculations where virtual particles do not appear. Hence the cause can't be virtual particles.

I do really believe you, don't get me wrong, I am a layman nobody and you clearly know the stuff. I just want to say that this argument is weak. The fact that there are different calculation approaches or interpretations for something does not 'per se' imply that particularly one of them is the wrong one. Most probably they are all wrong in one way or another. The double slit experiment can be explained by Many Worlds, and there are other ways to explain it which do not invoke other worlds. This does not automatically imply that the Many Worlds are not real and the other explanations are the correct ones. Thus the sentence "there are other ways of doing the calculations where virtual particles do not appear hence the cause can't be virtual particles" does not seem a very definitive argument. Still, I do believe you, I just wish you could explain me what is it that causes the effects attributed in the perturbative formalism to virtual particles. Cheers.

 

[Vanadium 50]

I just want to say that this argument is weak

I disagree. The argument posed in favor of the reality of virtual particles is that they are required to explain X, Y and Z. This argument is demolished by the fact that X, Y and Z can be explained without them. This is not physics; it is logic. If we disagree here, we need to address that before we can argue physics.

 

[bhobba]

Bhobba, since the unsettling part of all these posters seems to be the lack of real-ness of mathematical formalisms vs. the real effect they are associated with, why don't you answer them that the real cause of the real effect is simply the quantum field (modes)? Is it simply a coincidence or is there a reason why you don't want to answer in this way?

What do you mean by quantum field modes? Are you referring to something Peter Donas said? If so that wasn't what he was saying. What he was referring to is a handwavey view of quantum fields.

Thanks
Bill

 

[PeterDonis]

The fact that there are different calculation approaches or interpretations for something does not 'per se' imply that particularly one of them is the wrong one.

It's not a question of which approach is "wrong" and which is "right". The point is that, since there are different approaches with different entities in them, you can't use any of the approaches by itself to make ontological claims. You have to base any ontological claims you make on something else.

 

[PeterDonis]

why don't you answer them that the real cause of the real effect is simply the quantum field (modes)?

If that "modes" in parentheses is, as bhobba suspects, a reference to something I said (probably in post #49), he's right that I was being hand-wavey. A more rigorous description would have to involve math. If you're just looking for a short term for reference, "quantum fields" is probably as good as you're going to get; that points in the right general direction, without making any commitment to a particular approach (perturbation theory, lattice QFT, etc.).

 

[bhobba]

I do really believe you, don't get me wrong, I am a layman nobody and you clearly know the stuff. I just want to say that this argument is weak.

There is a theory called LET - Lorentz Ether Theory. It explains relativity by the interaction of rulers and clocks with the ether. In the theory the ether is very real and leads to very real effects. But it is totally undetectable. Then we have special relativity. It explains it as space-time geometry. You can ask in LET how is it something unphysical leads to real effects. You can do all the same arguments I see here. However it doesn't matter how you semantically view it the fact remains an ether is not required. SR explains it perfectly well without it. Because of Occam's razor hardly anyone these days ascribes to LET. Its exactly the same with virtual particles. They are not required. One can use Lattice Field Theory. That's what all physicists would do except for one key difference - it requires a computer and as yet hasn't the same accuracy. The usual way can be done, by, admittedly often mind numbing, calculations. But it can be done. That however changes nothing - virtual particles, like the ether, are not real.

Peter Donas hit the nail on the head:

It's not a question of which approach is "wrong" and which is "right". The point is that, since there are different approaches with different entities in them, you can't use any of the approaches by itself to make ontological claims. You have to base any ontological claims you make on something else.

Its exactly the same as LET. LET has an ether - SR doesn't. Scientifically there is no way to choose - they make exactly the same predictions. But because SR doesn't have an ether you can't say that's what causes things like length shortening.

Thanks
Bill

 

[bhobba]

The Vacuum Energy is actually large.

It's actually infinity. That large value is from a cutoff about the plank scale because we are, correctly, unsure of the physics beyond that scale.
http://math.ucr.edu/home/baez/vacuum.html

It can be made zero by something called normal ordering. But its one of the first indications there is a 'sickness' in QFT, a sickness that fully manifests in the need for renormalisation.

Thanks
Bill

 

[Jilang]

The Vacuum Energy is actually large. If we estimate it from the cosmological constant we get 10^-9 Joules/m³. If we calculate it from QFT we get 10¹¹³ Joules/m³. A very large difference, but in either case, there's plenty of energy for real particles, under the right conditions.

Yes, but you still have to do something to them to change them into real particles. All the ground state modes do add up to a huge amount that's true, but all that energy cannot be tapped into.

 

[Gerinski]

Yes, but you still have to do something to them to change them into real particles. All the ground state modes do add up to a huge amount that's true, but all that energy cannot be tapped into.

Just for me to make sure that I'm not misleaded. This vacuum energy you are talking about is the 'dark energy' right? It is said to be 'negative energy' while dark matter, ordinary matter and radiation are 'positive energy'. Hence the cosmological theory which says that both actually have the same total value and cancel each other, so the Universe could have popped up out of nothing.

In this theory, the negative vacuum energy can turn into positive energy by producing anti-pairs (i.e. a pair of particle + its antiparticle), so positive energy has two signs.
But should not the negative energy (dark energy) also have two signs? I mean, why should positive energy have two signs (matter + antimatter) but negative energy only one sign?
Could we think of the opposite, a bit of positive energy (a particle or antiparticle) being able to turn into negative energy by turning also into an anti-pair, a dark energy bit (causing spacetime to expand) and a 'bright' energy bit (causing spacetime to contract)?

 

[ddd123]

Dark energy is associated with negative pressure, not negative energy. So it has nothing to do with antimatter which anyway doesn't have negative energy, that was a mathematical quirk of the single particle Dirac theory which went away with QFT.

 

[craigi]

Just for me to make sure that I'm not misleaded. This vacuum energy you are talking about is the 'dark energy' right? It is said to be 'negative energy' while dark matter, ordinary matter and radiation are 'positive energy'. Hence the cosmological theory which says that both actually have the same total value and cancel each other, so the Universe could have popped up out of nothing.

In this theory, the negative vacuum energy can turn into positive energy by producing anti-pairs (i.e. a pair of particle + its antiparticle), so positive energy has two signs.
But should not the negative energy (dark energy) also have two signs? I mean, why should positive energy have two signs (matter + antimatter) but negative energy only one sign?
Could we think of the opposite, a bit of positive energy (a particle or antiparticle) being able to turn into negative energy by turning also into an anti-pair, a dark energy bit (causing spacetime to expand) and a 'bright' energy bit (causing spacetime to contract)?

Dark Energy isn't the same thing as Vacuum Energy. Discussion on Dark Energy is better addressed in the Cosmology forum.