Insights The Vacuum Fluctuation Myth - Comments

[vanhees71]

Math is no replacement for English. These are two totally different things that have different functions.
One of the functions of ordinary language is to name things. Math has no such function.

Besides it's physicists themselves that have messed up English in physics. The usage of word "state" as statistical distribution is totally confusing not only for lay people but for physicists themselves. The word "state" has very important but different meaning as current physical configuration for some potentially changing situation. Historically it was state vector that was understood with the word "state" and there the correspondence is rather intuitive and clear.

It's the other way around: Plane everyday languages (it's not restricted to English of course) are no replacement for math ;-).

The usage of the word "state" in the context of QT is not confusing but the essence of its content. A state is defined operationally as an equivalence class of prepartation procedures and the knowledge about the state implies the knowledge of probababilities (and only probabilities!) for outcomes of measurements, given the preparation of the measured system in this particular (pure or mixed) state.

I don't care about history when it comes to the scientific content of physics. The state never was understood as the state vector but as an equivalence class of state vectors, called rays. There are some textbooks that are imprecise with this, and that leads to a lot of confusion. The most general definition of a quantum state in the formalism is of course the Statistical Operator which includes both pure states (i.e., the Stat. Op. is a projector) and mixed states (describing the situation that one has only incomplete knowledge about the quantum state as is usually the case for macroscopic systems).

 

[zonde]

It's the other way around: Plane everyday languages (it's not restricted to English of course) are no replacement for math ;-).

Certainly. However math depends on ordinary language while ordinary language does not depend on math. ;)

I don't care about history when it comes to the scientific content of physics. The state never was understood as the state vector but as an equivalence class of state vectors, called rays.

Well, it seems you are right. Historically state was associated with energy states of electrons in atoms. At least it seems that way after glancing at Schrodinger's paper (1926).

 

[stevendaryl]

Well, although I'm not a native English speaker I'd formulate it more precisely as: Any quantum state implies uncertainties of position and momentum. This in turn implies fluctuations in the sense of an ensemble interpretation of probabilities. How else would you define fluctuations?

You have the same notion also in classical statistical mechanics: A phase-space distribution function implies an uncertainty in energy or momentum and thus implies ("thermal") flucutations of these quantities.

It's a little more subtle than that. If you have an ensemble of a million human beings, there will be a nonzero standard deviation for the height, but that doesn't imply that anybody's height is fluctuating. On the other hand, if the quantity \frac{d (height)}{dt} has a nonzero standard deviation, as well, then that would support the claim that heights are fluctuating.

 

[stevendaryl]

It's a little more subtle than that. If you have an ensemble of a million human beings, there will be a nonzero standard deviation for the height, but that doesn't imply that anybody's height is fluctuating. On the other hand, if the quantity \frac{d (height)}{dt} has a nonzero standard deviation, as well, then that would support the claim that heights are fluctuating.

In the quantum case, though, it seems interpretation-dependent. According to some interpretations, no physical variable has a value until it is measured, so the fact that \frac{dQ}{dt} has a nonzero standard deviation doesn't imply that Q is fluctuating, only that if you ever happen to measure \frac{dQ}{dt}, you will likely get something nonzero.

 

[atyy]

The other way to see the post is that it justifies the myth - one just has to accept the path integral picture, and an interpretation of the path integral picture. So the myth is not a myth, provided we accept that it describes the path integral picture and not the canonical picture. In other words, it is not a myth, provided we add the words "shouldn’t be taken too literally". Already, in Copenhagen, the wave function is not taken literally. So quantum mechanics is intrinsically mythical. There is nothing wrong with adding the path integral as metamyth.

 

[OCR]

This side of physics needs ordinary language along with mathematical language.

We used to think that if we knew one, we knew two, because one and one are two. We are finding that we must learn a great deal more about 'and'.

An interesting thread, please... carry on.

 

[stevendaryl]

The other way to see the post is that it justifies the myth - one just has to accept the path integral picture, and an interpretation of the path integral picture. So the myth is not a myth, provided we accept that it describes the path integral picture and not the canonical picture. In other words, it is not a myth, provided we add the words "shouldn’t be taken too literally". Already, in Copenhagen, the wave function is not taken literally. So quantum mechanics is intrinsically mythical. There is nothing wrong with adding the path integral as metamyth.

I guess there are quantum fundamentalists, who take it literally, and quantum Unitarians, who take it all metaphorically.

 

[A. Neumaier]

You have the same notion also in classical statistical mechanics: A phase-space distribution function implies an uncertainty in energy or momentum and thus implies ("thermal") fluctuations of these quantities.

Except that therrmal fluctuations in classical statistical mechanics are usually regarded (by invoking the ergodic hypothesis) as happening in time, Thus they are regarded as true fluctuations. While in quantum mechanics such a view is not really well-defined.

 

[A. Neumaier]

There are symbols in physics theories that correspond to physically measurable things. Mathematical statements however do not depend on the correspondence we attach to mathematical objects. In that sense symbols that are used as placeholders for mathematical objects are not names for anything.

Most definitions in mathematics define language naming things. The concept of group, of multiplication, of a field, a vector space, a vector, a set ..., the symbol + * / etc. All are creating descriptive language.

 

[PeterDonis]

Mathematical statements however do not depend on the correspondence we attach to mathematical objects.

Um, what? A mathematical symbol refers to a mathematical object. That's why we use it.

my statement assumes that there is "wrong" way to describe physics in English.

My response still applies with this interpretation.

don't forget that there is experimental side to physics. This side of physics needs ordinary language along with mathematical language.

Experimental apparatus can be described mathematically; in fact it has to be in order to compare experimental results with theoretical predictions. One does need a correspondence between mathematical symbols and actual objects in the laboratory (e.g., this 4-vector corresponds with this measuring device sitting in the lab).

any physics theory has to establish correspondence with physical reality.

No, any physics theory has to establish correspondence with the experimental evidence we use to test it. What, if any, correspondence it has with "physical reality" is a question of philosophy or metaphysics, not physics.

 

[RockyMarciano]

No, any physics theory has to establish correspondence with the experimental evidence we use to test it. What, if any, correspondence it has with "physical reality" is a question of philosophy or metaphysics, not physics.

I don't know about metaphysics but experimental(observational in general) evidence IS "physical reality" in physics by definition.

 

[clarkvangilder]

So...for the novice here...is it safe to say that the summary of all of this wrangling is that quantum fluctuations are a useful fiction? Useful in the sense that its metaphorical import is useful in describing some process or set of processes? Could the same be said of the probability waves that come with the overall game in QM?

NOTE: I freely admit that I may have missed what some are saying in this thread. Just trying to glean as much as I can with some direct questions. Feel free, however, to recommend further reading. I am not opposed to doing homework. ;-)

 

[PeterDonis]

.is it safe to say that the summary of all of this wrangling is that quantum fluctuations are a useful fiction? Useful in the sense that its metaphorical import is useful in describing some process or set of processes? Could the same be said of the probability waves that come with the overall game in QM?

I would say that it is important to keep in mind that terms like "quantum fluctuations", "probability waves", etc. are not the actual theory. They are attempts to describe some aspect of the actual theory in ordinary language. But ordinary language is vague and imprecise, and often there is no way to describe the theory in ordinary language without distortion. So you have to be very, very, very careful in trying to reason about the theory using ordinary language descriptions. That is why physicists themselves don't use these descriptions in their work; they use math. The mathematical description of the theory, and the concrete predictions derived from the math, are the actual theory, and to be sure you are reasoning correctly about what the theory says, the math is what you need to use.

 

[ftr]

NOTE: I freely admit that I may have missed what some are saying in this thread. Just trying to glean as much as I can with some direct questions. Feel free, however, to recommend further reading. I am not opposed to doing homework. ;-)

The best reference I could find for you with reasonable explanation in English is this

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

 

[clarkvangilder]

The best reference I could find for you with reasonable explanation in English is this

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Perhaps a quick comparison of these virtual particles to the point-particle model in classical mechanics? It seems that virtual particles are much much more than just a model? (Not that you or anyone else said otherwise). The title of this article below sort of captures the spirit of misconception relative to this topic. Scientific American is not a great journal of physics; but the person who wrote it ought to be an expert.

https://www.scientificamerican.com/article/are-virtual-particles-rea/

 

[ftr]

Perhaps a quick comparison of these virtual particles to the point-particle model in classical mechanics? It seems that virtual particles are much much more than just a model? (Not that you or anyone else said otherwise). The title of this article below sort of captures the spirit of misconception relative to this topic. Scientific American is not a great journal of physics; but the person who wrote it ought to be an expert.

https://www.scientificamerican.com/article/are-virtual-particles-rea/

I don't have a scientific survey but I would say(10 years of watching) the majority here and elsewhere do not agree with that point of view.

 

[clarkvangilder]

I don't have a scientific survey but I would say(10 years of watching) the majority here and elsewhere do not agree with that point of view.

Thanks for that insight ... I was hoping/thinking that must be true.

 

[vanhees71]

Perhaps a quick comparison of these virtual particles to the point-particle model in classical mechanics? It seems that virtual particles are much much more than just a model? (Not that you or anyone else said otherwise). The title of this article below sort of captures the spirit of misconception relative to this topic. Scientific American is not a great journal of physics; but the person who wrote it ought to be an expert.

https://www.scientificamerican.com/article/are-virtual-particles-rea/

Hm, I'm a bit puzzled how an expert in particle physics can write such an article :-(. As an practicioner of QFT, I'm sure he knows very well that a particle interpretation of relativistic QFT is possible in clear terms only for asymptotic free states, and for vacuum QFT (i.e., the theory describing scattering events) the only observable outcomes are S-matrix elements, i.e., transition rates for going from an asymptotic free in state (usually two particles) to an asymptotic free out state (which can be any many-particle state, that is only restricted by the conservation laws like energy-momentum, angular momentum conservation and the conservation of various charges like electric charge etc.) or, equivalently, cross sections. All this is discussed already at length in this thread!

 

[tzimie]

I have a question.

How that usual claim (virtual particles are not real, they are just math) can be interpreted in a framework of MUH (Mathematical Universe Hypotesis) - as obviously in MUH there is no distinction between "actually happening" and "being just math".

 

[vanhees71]

How does Bohmian mechanics solve "the measurement problem" (I assume you mean the question, why you find sharp values when measuring an observable on a system at a state which is not an eigenstate of the observable)? It assumes unobservable, i.e., ficticious, trajectories, but it doesn't claim that all observables are determined before the measurment, right?

 

[Demystifier]

I have a question.

How that usual claim (virtual particles are not real, they are just math) can be interpreted in a framework of MUH (Mathematical Universe Hypotesis) - as obviously in MUH there is no distinction between "actually happening" and "being just math".

According to MUH, any self-consistent mathematical theory, even a theory which directly contradicts observations, is true.

 

[tzimie]

According to MUH, any self-consistent mathematical theory, even a theory which directly contradicts observations, is true.

1. It is not "true", it "exists" in some methaphysical way.
2. And only some of these universes are "observed"

Anyway, I think the "insights" are not interpretation-neutral (actually they are Copenhagen-biased) hence not universally valid. (Am I wrong?)

 

[vanhees71]

Hm, if you call the minimal interpretation Copenhagen, then of course the Insights are biased towards these, since this is a science and not a philosophy forum!

 

[tzimie]

Hm, if you call the minimal interpretation Copenhagen, then of course the Insights are biased towards these, since this is a science and not a philosophy forum!

It is perfectly fine to say that you don't want to talk about interpretation wars because it is philosophy, not physics.
If there is a big period after that claim.
But after the point you added that Copenhagen is the best/minimal/etc - then your position is inconsistent.

I don't want to start Interpretation Wars. It is science forum and not a philosophy one. Ah, and BTW, Copenhagen doesn't make any sense and MWI is the best )))
Happy New Year! )))

 

[vanhees71]

Well, it should be Copenhagen without collapse, i.e., the minimal statistical interpretation. Some people think that's alreayd MWI, but I don't need unobservable branches of the universe where something else happens than what's observed in the branch I'm living in ;-)). Happy New Year!

 

[OmCheeto]

I still don't know what "the myth" is, and I read the article 3 times.
Perhaps, some of us were not meant to know.

 

[haushofer]

Hm, I'm a bit puzzled how an expert in particle physics can write such an article :-(. As an practicioner of QFT, I'm sure he knows very well that a particle interpretation of relativistic QFT is possible in clear terms only for asymptotic free states, and for vacuum QFT (i.e., the theory describing scattering events) the only observable outcomes are S-matrix elements, i.e., transition rates for going from an asymptotic free in state (usually two particles) to an asymptotic free out state (which can be any many-particle state, that is only restricted by the conservation laws like energy-momentum, angular momentum conservation and the conservation of various charges like electric charge etc.) or, equivalently, cross sections. All this is discussed already at length in this thread!

I''m also puzzeled. I can't make anything of that article.

 

[mattt]

Perhaps a quick comparison of these virtual particles to the point-particle model in classical mechanics? It seems that virtual particles are much much more than just a model? (Not that you or anyone else said otherwise). The title of this article below sort of captures the spirit of misconception relative to this topic. Scientific American is not a great journal of physics; but the person who wrote it ought to be an expert.

https://www.scientificamerican.com/article/are-virtual-particles-rea/

I am more than puzzled, I am seriously dissapointed.

 

[mikeyork]

I am curious as to how those who say that virtual particles are a myth would describe "resonances" such as the $\Delta(1232)$ that appears in pion-nucleon scattering. This is usually called a particle as its sits in the baryon decouplet of SU(3). Yet it seems to have all the characteristics of a "virtual particle" in that it is extremely shortlived and because it is seen as a peak in the scattering cross-section and can be viewed as having uncertain mass -- suggesting temporary violation of energy conservation over an interval inversely proportional to the width of the bump. (In order to allocate a precise mass, one has to allow the mass to be complex, the imaginary part given by the width of the bump and representing the uncertainty in real energy.)

 

[mfb]

See the previous pages, we discussed this (mainly with the Z as example) in detail. It depends on your point of view.