I Fields: Unobservable, Yet Physical?

[dm4b]

I'm currently working through Robert Klauber's Student Friendly Quantum Field Theory, which by the way is much more accessible than other texts like, say, Peskin and Schroeder, for others also coming into QFT via the self-study path.

Anyhow, he mentioned something that never really clicked with me before: fields are un-observable. At least, if they have zero expectation value. Does this apply to all known fields in nature? Or, just scalar fields, where is where I am it in the book.

Also, I got caught up on the notion of how something can be un-observable, yet also physical. I guess the same may apply to virtual particles, though.

Any thoughts?

Thanks

 

[hilbert2]

What do you mean by something being un-observable? All of us observe electromagnetic field excitations of certain wavelengths all the time with our eyes (except those who are blind). A gravitational field is observed by monitoring the trajectory of a mass or curvature of a light beam moving through it. There's no point in making a physical theory about something that can't be observed.

 

[dm4b]

There's no point in making a physical theory about something that can't be observed.

That was my initial thought!

What do you mean by something being un-observable?

Operators that reproduce the original state multiplied by an eigenvalue are observable with the eigenvalue being related to the probability of obtaining a particular eigenstate upon measurement.

Fields are operator-valued fields. These kinds of operators don't reproduce the original state, they change states from one state to another state and are generally not observable.

In addition, he also mentions how A_mu has zero expectation value and is also unobservable, or cannot be directly measured. This is the field behind EnM.

 

[dm4b]

That was my initial thought!



Operators that reproduce the original state multiplied by an eigenvalue are observable with the eigenvalue being related to the probability of obtaining a particular eigenstate upon measurement.

Fields are operator-valued fields. These kinds of operators don't reproduce the original state, they change states from one state to another state and are generally not observable.

In addition, he also mentions how A_mu has zero expectation value and is also unobservable, or cannot be directly measured. This is the field behind EnM.

I can't help but wonder if we're getting into "virtual particles" here, or the virtual bosons involved in intermediating the force and found in the propagator, which are not directly observable, or cannot be measured.

 

[phinds]

I can't help but wonder if we're getting into "virtual particles" here, or the virtual bosons involved in intermediating the force and found in the propagator, which are not directly observable, or cannot be measured.

There is a FAQ here on PF about virtual particles, which are stated to be a mathematical fictions to (help with some computations) not real objects.

 

[dm4b]

There is a FAQ here on PF about virtual particles, which are stated to be a mathematical fictions to (help with some computations) not real objects.

I've read that and been involved in the discussion. Damn good argument, but I didn't think it was %100 air tight, either. However, could the same thing be said about the unobservable fields in question?

 

[phinds]

I've read that and been involved in the discussion. Damn good argument, but I didn't think it was %100 air tight, either. However, could the same thing be said about the unobservable fields in question?

I'm not expert at all in this stuff, but I don't think so. It seems to me that virtual particles and fields are just not the same thing. I'm pretty sure I've never had a virtual particle hit my retina

 

[MrRobotoToo]

Operators that reproduce the original state multiplied by an eigenvalue are observable with the eigenvalue being related to the probability of obtaining a particular eigenstate upon measurement.

Fields are operator-valued fields. These kinds of operators don't reproduce the original state, they change states from one state to another state and are generally not observable.

In addition, he also mentions how A_mu has zero expectation value and is also unobservable, or cannot be directly measured. This is the field behind EnM.

It's somewhat subtle since all of the observables in a field theory (i.e., energy, momentum, angular momentum, etc.) can be expressed in terms of the fields and their derivatives, but in such a way that gauge transformations of the fields leave the observables unchanged.

 

[mikeyork]

What do you mean by something being un-observable? All of us observe electromagnetic field excitations of certain wavelengths all the time with our eyes (except those who are blind).

Non sequitur. What we observe is stimulation of the retina. Could just as equally well be described by a photon of a given energy.

A gravitational field is observed by monitoring the trajectory of a mass or curvature of a light beam moving through it.

No. We observe only a trajectory and we theorize about it's route.

There's no point in making a physical theory about something that can't be observed.

Just about all theories in physics are built on stuff that cannot be observed, but predicts stuff that can.

 

[bhobba]

Well the limit of QED is standard EM.

The fields of standard EM carry energy and momentum so physicists like to think of them as very real even though, as Feynman showed by devising a theory where they don't exist (the absorber theory is a direct action at a distance theory) they are obviously not directly observable. Feynman though was never able to come up with a quantum version but I read somewhere Penrose may have. It would be rather difficult to come up with a point in taking that limit where they all of a sudden change from real to not real - but I suppose you could hold to such a view.

That said they are fields of operators - how real and observable is a QM operator?

Not an easy question to answer - but I think of them as real for the reason I gave at the start.

Thanks
Bill

 

[atyy]

As far as I know, fields are not physical in any interpretation of quantum mechanics. In the Schroedinger picture, the wave function may be real. In QFT, one usually uses the Heisenberg picture with time evolution of field operators.

If you look at bhobba's post above, you see he says essentially the same thing.

A way to simplify the question is to go to non-relativistic quantum mechanics. Use the Heisenberg picture. There in the formalism, the position and momentum observables simultaneously exist and evolve with time. However, position and momentum do not commute.

 

[hilbert2]

Non sequitur. What we observe is stimulation of the retina. Could just as equally well be described by a photon of a given energy.

Difficult to say how much neural processing the visual stimulus has to go through before it becomes an "observation" that one can intellectually work on...

Something unobservable would be if I defined a field called "ghost field", denoted $G(x,y,z,t)$ and stated that it doesn't interact with anything else and has a field equation

$\frac{\partial^2 G}{\partial t^2}=a \nabla^2 G - b|G|^2$,

where a and b are some constants. Then someone else could say "no, you're wrong, its field equation is actually $\frac{\partial^2 G}{\partial t^2}=a \nabla^2 G - b|G|^4$", and there would be absolutely no way to show that either of us would be more correct in our claims.

 

[dm4b]

I guess my next question would be ... if fields are indeed un-observable and non-physical, it seems like we can't say they are (more) fundamental, can we? This seems to make fields more akin to a convenient calculational model used to make predictions for (physical) things we really can observe/measure (like momentum, spin, etc), which again doesn't really seem to make fields more fundamental than the physical entities (particles and/or waves) that actually possesses (and physically exhibit) these observable properties.

At the same time, I always found it pleasing how QFT makes sense of wave/particle duality by unifying what appeared to be separate entities (waves and particles) into a single, encompassing underlying (and more fundamental) field. The magic of this seems to fade away if fields are un-observable, un-physical and perhaps, by logical extension, un-real?

Seems like we're still missing something here ...

 

[Karolus]

There is a FAQ here on PF about virtual particles, which are stated to be a mathematical fictions to (help with some computations) not real objects.

who established that virtual particles are "mathematical fictions" ?? What kind of nonsense! Then even a black hole is a "mathematical fiction"! Have you ever seen a black hole?

 

[phinds]

who established that virtual particles are "mathematical fictions" ?? What kind of nonsense! Then even a black hole is a "mathematical fiction"! Have you ever seen a black hole?

Hey, look up the FAQ. I'm just saying there IS one, and telling you what it says.

 

[hilbert2]

If I take a Hamiltonian operator $\mathbf{H} = \frac{\mathbf{p}^2}{2m} + \frac{1}{2}k\mathbf{x}^2 + \lambda\mathbf{x}^4$ and write its ground state as

$|\psi_0 > = |\phi_0 > + \lambda |\phi_1 > + \lambda^2 |\phi_2> + \dots$ ,

where $|\phi_0 >$ is the harmonic oscillator ground state, I can with good reason say that the terms $\lambda^n |\phi_n >$ are "mathematical fictions", but they can still be useful when calculating something.

 

[Karolus]

you can apply similar reasoning to all physical. The black hole turns out to be purely theoretical calculations, not to mention the event horizon, and other abstract entities. The virtual particles are considered to explain the Casimir effect, that is anything but "virtual", or the Hawking radiation. If one day a hypothetical future, but not too hypothetical, could extract energy from the quantum vacuum, and the mechanism of virtual particles, they become quite "real", especially when you have to pay us taxes ...

 

[PeterDonis]

how something can be un-observable, yet also physical

What does "physical" mean? Be very careful of using terms that don't have precise definitions.

you can apply similar reasoning to all physical

It seems like you're conflating models with observations. Virtual particles are models. So are black holes. So are quantum fields. The Casimir effect is an observation.

Any model can be considered a "mathematical fiction". But observations can't.

 

[mikeyork]

if fields are indeed un-observable and non-physical, it seems like we can't say they are (more) fundamental, can we? This seems to make fields more akin to a convenient calculational model used to make predictions for (physical) things we really can observe/measure (like momentum, spin, etc), which again doesn't really seem to make fields more fundamental than the physical entities (particles and/or waves) that actually possesses (and physically exhibit) these observable properties.

Unobservable is not the same as unphysical. Something can be unobservable but if it accurately predicts the observable, then that would suggest it is at least a candidate as physical in the sense of reflecting reality.

 

[bhobba]

I guess my next question would be ... if fields are indeed un-observable and non-physical, it seems like we can't say they are (more) fundamental, can we? This seems to make fields more akin to a convenient calculational model used to make predictions for (physical) things we really can observe/measure (like momentum, spin, etc), which again doesn't really seem to make fields more fundamental than the physical entities (particles and/or waves) that actually possesses (and physically exhibit) these observable properties.

We aren't missing anything.

Its purely a matter of what you consider real and physical which is a philosophical issue. They have energy and momentum - for me that makes them real - but its purely a matter of semantics. One of the silliest things you can argue about is semantics - it gets you no-where fast.

Here are the facts:

1. The fields themselves can never be observed - only their effects.
2, They do carry momentum and energy from Noether, but also from the simple observation since EM effects travel at the speed of light when an object radiates if momentum/energy conservation is to occur, and according again to Noether it must, then it must carry it away.
3, It can be formulated in a direct action at a distance way without fields - but no one really does it that way - it just seems a curiosity Feynman discovered.
4. In QED the field consists of a field of quantum operators.

These are the facts. Philosophers will argue if that implies they are real or not. We don't discuss philosophy here, so make up your own mind - it makes no difference. I think they are real, but as you can see from the above a decent argument could be made they are not. Its one of those things that you can't really answer because it depends on what you mean by the terms you are using like real etc. Observations are real - no VERY real - beyond that -.

Thanks
Bill

 

[bhobba]

Unobservable is not the same as unphysical. Something can be unobservable but if it accurately predicts the observable, then that would suggest it is at least a candidate as physical in the sense of reflecting reality.

Its a philosophical issue of zero physical importance. We don't discuss such things here by forum rules, or at least when we do its kept on a very tight leash, correctly IMHO, so if you want to pursue it go to a philosophy forum. Look at the terms you are using - physical, reality etc. All of them are very imprecise and require careful elucidation - the type of elucidation philosophers do - and generally get no-where. Physicists adopt a more common-sense view. Observations are real, the rest - who cares. What is reality - what our models describe - physics is very simple and common-sense like that. Maybe that's why it has made such spectacular progress, and discovered something very very deep - symmetry is at rock bottom what much of physics is about - see Noethers Theorem I have already mentioned. If physicists and mathematicians became bogged down in this other irrelevant stuff I doubt such a startling discovery would have been made - but people can make up their own mind on that.

If you discuss it with philosophers you can report back here in the unlikely they all reach a consensus .

Thanks
Bill

 

[dm4b]

We aren't missing anything.

Its purely a matter of what you consider real and physical which is a philosophical issue. They have energy and momentum - for me that makes them real - but its purely a matter of semantics. One of the silliest things you can argue about is semantics - it gets you no-where fast.

Here are the facts:

1. The fields themselves can never be observed - only their effects.
2, They do carry momentum and energy from Noether, but also from the simple observation since EM effects travel at the speed of light when an object radiates if momentum/energy conservation is to occur, and according again to Noether it must, then it must carry it away.
3, It can be formulated in a direct action at a distance way without fields - but no one really does it that way - it just seems a curiosity Feynman discovered.
4. In QED the field consists of a field of quantum operators.

These are the facts. Philosophers will argue if that implies they are real or not. We don't discuss philosophy here, so make up your own mind - it makes no difference. I think they are real, but as you can see from the above a decent argument could be made they are not. Its one of those things that you can't really answer because it depends on what you mean by the terms you are using like real etc. Observations are real - no VERY real - beyond that -.

Thanks
Bill

You are missing something. The ontic status of a quantum field, which is what I was after (as much as is currently possible, anyhow)

I was looking more for scientific facts, rather than opinions, or what folks "think" a field is. If we don't have the facts yet, that's fine (in which case I don't really mind hearing the opinions of others). But, to say "it makes no difference" or that it's not right to ask questions that probe the deeper nature of reality seems foolish to me.

By the way, your (1) implies fields are real and physical, sort of a requirement for something to be the cause of subsequent "effects". But, I don't think it has to be phrased that way. Another possibility seems to be that that fields may have no effects, meaning they're just a mathematical method for making predictions. They don't even really have to correspond to physical reality to make accurate predictions - sort of like a Ptolemy's Epicycles on steroids. That's not my view on things. It's just another way of looking at (1), which should be acknowledged. (Of course, if everyone one thought things like this make no difference, maybe we'd still be stuck with a geocentric model of the solar system!)

Also, just comparing Heisenberg Picture to Shrodinger Picture - are states evolving in time (for real), or not. Likewise operators. Two pictures, two different "realities", both mathematically correct predictions. One picture more useful in one arena, the other in another. I think we have to take with skepticism how any of our mathematical models actually match with "reality" for these reasons. Likewise with Time Symmetric QM and Standard QM. One with a forward evolving state vector only. The other with two state vectors - one forward evolving in time, the other retrocausally. Seemingly very different models of reality being painted, exact same mathematical predictions. Mathematically equivalent in their predictions right down the line.

Anyhow, with the contradictory answers coming up in this thread, it's pretty obvious there is no clear answer to my question in the OP, which is fine. I half-way as expected as much, which is why I asked - just hoping for as much insight/clarification that was possible.

 

[dm4b]

Unobservable is not the same as unphysical. Something can be unobservable but if it accurately predicts the observable, then that would suggest it is at least a candidate as physical in the sense of reflecting reality.

Well, if it's un-observable in principle, by which I mean one can never, ever, observe it, regardless of technological know-how, how can one ever verify if it is un-physical, or not, for sure? How can one ever know if we're just engaging in mathematical masturbation (as Feynman liked to put it), or actually learning something about the nature of reality. Sounds like we left empirical science behind at that point?

Although, I think you have the best response yet and I tend to agree, in that I think quantum fields have a shot at candidacy as something real or, at the very least, fields give a partial appearance of what reality is truly like (unlike my earlier example of Ptolemy's Epicycles). Alas, in the end, all our theories, even the best ones, are appearance, I suppose.

 

[dm4b]

What does "physical" mean? Be very careful of using terms that don't have precise definitions.

That's kind of what I was getting at. I would like to equate physical with observable, but ... like I said in my other post just prior to this one, how can we know if something is "real" if it's impossible in principle to observe? Or, are there some things empirical science can't answer?

 

[bhobba]

The ontic status of a quantum field, which is what I was after (as much as is currently possible, anyhow) I was looking more for scientific facts, rather than opinions, or what folks "think" a field is. If we don't have the facts yet, that's fine (in which case I don't really mind hearing the opinions of others). But, to say "it makes no difference" or that it's not right to ask questions that probe the deeper nature of reality seems foolish to me.

You simply don't get it. Reality is what our theories describe. There is nothing deeper. Think otherwise. First define reality and get everyone to agree on it - good luck with that. And that is only the first step.

What our theories say about the ontic status of the quantum state is non-committal, although the PBR theorem has recently shed some light on it:
https://arxiv.org/abs/1111.3328

Note however what it says:
Here we present a no-go theorem: if the quantum state merely represents information about the real physical state of a system, then experimental predictions are obtained which contradict those of quantum theory. The argument depends on few assumptions. One is that a system has a “real physical state” – not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes. The other main assumption is that systems that are prepared independently have independent physical states.

So the answer is the same - we don't know nor really does it matter as far as using QM is concerned.

Thanks
Bill

 

[PeterDonis]

That's kind of what I was getting at.

Maybe you didn't quite understand the point of my question. The meaning of "physical" is not something you find out by experiment. It's something you decide when you use the word. Since there is no standard definition of that word in science, you have to explain what you mean by it. What do you mean by it?

how can we know if something is "real" if it's impossible in principle to observe?

Same point here. The word "real" does not have a standard meaning in science, so you have to explain what you mean by it. What do you mean by it?

If your answer to these questions is "I don't know", then perhaps the answer is to stop using these words that don't have precise meanings, and switch to words that do (or switch to math that does).

 

[bhobba]

Maybe you didn't quite understand the point of my question. The meaning of "physical" is not something you find out by experiment. It's something you decide when you use the word. Since there is no standard definition of that word in science, you have to explain what you mean by it. What do you mean by it? Same point here. The word "real" does not have a standard meaning in science, so you have to explain what you mean by it. What do you mean by it? If your answer to these questions is "I don't know", then perhaps the answer is to stop using these words that don't have precise meanings, and switch to words that do (or switch to math that does).

Thanks
Bill

 

[Vanadium 50]

First, the classic treatment on what is real is Margery Williams' The Velveteen Rabbit.

Second, is wind real? You can't see it - you can only see its effects. I maintain that it is logically inconsistent to hold that wind is real and an electric field is not.

 

[dm4b]

Maybe you didn't quite understand the point of my question. The meaning of "physical" is not something you find out by experiment. It's something you decide when you use the word. Since there is no standard definition of that word in science, you have to explain what you mean by it. What do you mean by it?

Look at the way I phrased the title to the OP, with a question mark. Once again, what you are saying is my whole point. I here folks loosely speak of how fields are "physical" or how they are "fundamental". What I'm saying is if fields are un-observable those terms become even slipperier than they were otherwise.

To repeat what I said earlier, I would like to equate physical with things that can be measured, or observed. However, I am open to the possibility that this is wrong. However, if it is wrong, that means facets of reality may be inaccessible to empirical science, even in principle.

Same point here. The word "real" does not have a standard meaning in science, so you have to explain what you mean by it. What do you mean by it?

C'mon, it's not real hard to get the idea of what anyone means by "real". For the sake of this thread, equate un-real with things like Ptolemy's Epicycles, as that will suffice for this discussion.

If your answer to these questions is "I don't know", then perhaps the answer is to stop using these words that don't have precise meanings, and switch to words that do (or switch to math that does).

What do you mean by "precise", especially when it comes to words? No words have complete precision, as language is ultimately lacking in complete precision.

If you think you're on a firm foundation with math, than I would say even there one would be kidding themselves to some extent. One, only a subset of math even corresponds to physical reality, or physics. Two, many physical theories can be explained with multiple mathematical models that seem to tell different stories about reality. Three, confirming the consistent predictions these models give ultimately rely on the measurement process, and the measurement process is something we don't fully understand. Four, there are aspects of reality that may not even be conducive to mathematical modeling, or algorithmic description.

Lastly, if I find myself saying "I don't know", that's where I prefer to be. That's where the mystery and excitement is. That's where revolutionary physics happen. When did the physics field get so afraid of mystery? Not too long ago a famous physicists said, "The most beautiful thing one can experience is the mysterious" (Yep, Einstein). The other thing he said is "Imagination is more important than knowledge". Try and be precise with imagination.

 

[dm4b]

You simply don't get it. Reality is what our theories describe. There is nothing deeper. Think otherwise. First define reality and get everyone to agree on it - good luck with that. And that is only the first step.

No, our theories are an appearance. Take Newtonian Physics. Pretty much everything there relies on the assumption of absolute spacetime. But, absolute spacetime does not exist anywhere in reality any more than a perfect circle does. In other words, Newtonian Physics is an approximation to reality at best, or an appearance reality takes within a certain domain. Now, if one insists that the theory IS reality, I would say we would have morphed Newtonian Physics even into a false picture of reality!

What our theories say about the ontic status of the quantum state is non-committal, although the PBR theorem has recently shed some light on it:
https://arxiv.org/abs/1111.3328

Note however what it says:
Here we present a no-go theorem: if the quantum state merely represents information about the real physical state of a system, then experimental predictions are obtained which contradict those of quantum theory. The argument depends on few assumptions. One is that a system has a “real physical state” – not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes. The other main assumption is that systems that are prepared independently have independent physical states.

So the answer is the same - we don't know nor really does it matter as far as using QM is concerned.

Thanks
Bill

I'm familiar with all the above in relation to QM. What I wasn't familiar with was the non-committal state that apparently exists in QFT, as well. Like I said, we're missing something ...

 

[dm4b]

Thanks
Bill

First, the classic treatment on what is real is Margery Williams' The Velveteen Rabbit.

Seriously guys, don't you find this kind of childish? The Velveteen Rabbit? A reply solely to put up no less than 10 emoticons? Is this the kind of discussion physicsforums likes to promote?

The ontic nature of fields is a serious physics question. Why this forum seems to feel threatened in some way by questions like these is beyond me.

 

[PeterDonis]

I here folks loosely speak

Yes, people often do speak loosely. That's a major reason why here at PF we have rules about acceptable sources: because only in those sources (textbooks and peer-reviewed papers, or the equivalent) are people forced to not speak loosely, to actually precisely define what they are talking about, almost always with math.

So, for example, in the textbook you mention in your OP, where it says "fields are unobservable", what is the context? What actual math does that statement correspond to?

Also, you have used the words "physical" and "real". Do those specific words appear in the textbook?

 

[dm4b]

So, for example, in the textbook you mention in your OP, where it says "fields are unobservable", what is the context? What actual math does that statement correspond to?

I already specified in a post above.

 

[PeterDonis]

I would like to equate physical with things that can be measured, or observed

Why do you want to equate "physical" with anything at all? Why is that word so important?

it's not real hard to get the idea of what anyone means by "real"

Yes, it is. That's why there have been interminable philosophical discussions about it, for centuries if not millennia. And that's why, here, we say that unless you can precisely define what you mean by "real", it's not worth talking about. As with "physical" above, why is that word so important? If you already have a theory that makes correct predictions, what additional value is there in pasting the word "real" on certain things?

What do you mean by "precise", especially when it comes to words?

In physics, "precise" almost always means you're using math, not words. In cases where words are used, there will be precise math somewhere that those words refer to. And the reason I keep asking you what you mean by "physical" and "real" is that I am not aware of any precise math that those words correspond to.

only a subset of math even corresponds to physical reality, or physics

Yes, that's true. We find out which pieces of math actually describe reality by doing experiments and comparing them with what the math says. Yes, there is no automatic, cookie-cutter process that decides which math we use to make the predictions; building physical theories requires much human ingenuity and insight. That doesn't change the fact that in the end the test is perfectly objective.

many physical theories can be explained with multiple mathematical models

In cases where the models all make the same predictions, they are mathematically equivalent; in all such cases that I'm aware of (the main one I'm thinking of right now is the Schrodinger vs. Heisenberg formulations of non-relativistic QM), the mathematical equivalence has been proven.

In cases where the models don't make the same predictions, then they are not the same theory, they're different theories; for example, Newtonian gravity vs. GR. In such cases, we can test the theories by experiment, as experiment has shown us that GR is more accurate than Newtonian gravity.

even the consistent predictions these models give ultimately rely on the measurement process for confirmation

Yes.

this is something we don't fundamentally understand

You're conflating two different things here. We know how to make observations and compare them with theoretical predictions. What we don't fully understand is how to model that process using quantum mechanics (in classical theories, like Newtonian gravity and GR, we do understand how to do that). But not knowing fully how to model the process is not the same as not knowing how to execute the process. Humans were able to accurately throw spears long before they discovered the physical laws that govern the process.

there are aspects of reality that may not even be conducive to mathematical modeling, or algorithmic description

If that is the case, such aspects of reality will never be part of what is modeled by physics, so they're off topic here.

The ontic nature of fields is a serious physics question

It is? What different experimental predictions are made by the different physical theories of the ontic nature of fields? (Hint: AFAIK the answer to that is "none, since there aren't different physical theories of the ontic nature of fields, there are just different stories people tell about the same underlying theory which makes the same predictions regardless of the story".)

 

[PeterDonis]

if I find myself saying "I don't know", that's where I prefer to be

Then I am really, really confused about why you are posting here.

 

[PeterDonis]

I already specified in a post above.

Do you mean post #3? All you said there was what you mean by fields being unobservable. I'm asking what the textbook meant by that, i.e., what specific, explicit math does that statement in the textbook correspond to?

 

[dm4b]

Then I am really, really confused about why you are posting here.

Since you took what I said and quoted it out of the context it was written in, I'm not surprised by your confusion.

But, I believe this will be my last post at physicsforums regardless, as I see no reason to post here anymore. This was a waste of my time as usual, and always seems to be lacking in discussion of any real depth. Parroting back papers everyone can read for themselves seems to be the only thing allowed. That and acting more like a calculator, just focusing on computations and never asking "why". Definitely not the kind of physics I fell in love with.

Do you mean post #3? All you said there was what you mean by fields being unobservable. I'm asking what the textbook meant by that, i.e., what specific, explicit math does that statement in the textbook correspond to?

That was me quickly summarizing what the textbook said, as well as the math used. I thought that would have been obvious since I did ultimately say "In addition, he also mentions ..." , i.e.these were not what I necessarily mean by fields being unobservable.

I love how I am told "I don't get it" when people aren't even reading my posts very closely at all ...

Good luck and Peace out

 

[PeterDonis]

Since you took what I said and quoted it out of the context it was written in

The question I asked was taking the context into account. My point is that the question you have posed in this thread has nothing to do with "where the mystery and excitement is" in physics; it's all about definitions of words. Definitions of words are not physics. What difference does it make to any actual prediction, any actual observation, whether fields are "physical" or not? None at all, as far as I can see. Certainly no such difference is conveyed by anything you have posted or any reference you have given. But different predictions, different possible observations we might make that go beyond the ones we've already made, and how to make correct predictions of them, are "where the mystery and excitement is". Just saying "I don't know" doesn't do anything; neither does arguing over what "physical" should mean. Coming up with new models, extracting predictions from them, and comparing them with experiment, is how physics advances.

 

[PeterDonis]

Coming up with new models, extracting predictions from them, and comparing them with experiment, is how physics advances.

And to go back to the OP, if the question was "how could we model fields in some future theory that goes beyond current QFT, such that we would have a better understanding of why they are unobservable in our current models?", that question belongs, if anywhere at all on PF, in the Beyond the Standard Model forum. This thread only belongs here, in the Quantum Physics forum, if the question in the OP was "what does 'fields are unobservable' mean in the context of our current QFT?" But based on the last few posts, it doesn't seem like that was the question the OP actually meant to ask.

 

[Vanadium 50]

The ontic nature of fields is a serious physics question

I think it's navel-gazing philosophy, and not very good philosophy at that. Furthermore, the word "ontic" appears in exactly 4 Inspire records, 1 published, and a whopping 3 citations, none of which are to the published paper and 2 of which are self-cites. I think that's pretty good evidence that the field is more aligned with my opinion than yours.

I also noticed you completely ignored my wind argument. If we can only observe fields by their effects, the same can be said for wind. That forces you to either explain why electric fields are fundamentally different than wind (a difficult task, as wind is a velocity field of air), or why wind isn't real, which means you are using a different definition of "real" than 99.9999% of the population.

 

[dextercioby]

I believe he's gone. He's interested in the "why" question. Physics is not about the "why", but about the "how". "How" would I make the model whose mathematical predictions coincide with what can be measured? One more thing, the word "real" is not used, it is abused and voided of any other meaning than "what humanity (as of now still the only intelligent and evil beings) can sense". It is the damned human and conscientious observer unspoken of in classical theories and explicitly mentioned in the quantum ones.

 

[Vanadium 50]

I believe he's gone.

He'll be back. Didn't he say that in his anti-BB screed a while back? They say they won't come back, but they always do.

 

[bhobba]

No, our theories are an appearance.

Really - and everyone, will agree with you on that? Are you acquainted with the writings of Penrose who believes the theories are real and what we experience as reality is some sort of shadow from this Platonic realm where 'reality' is. Before dismissing it read it then form your own view. I used to believe in it until I saw a video by Gell-Mann who has a different view again:
https://www.ted.com/talks/murray_gell_mann_on_beauty_and_truth_in_physics

Take Newtonian Physics. Pretty much everything there relies on the assumption of absolute spacetime. But, absolute spacetime does not exist anywhere in reality any more than a perfect circle does. In other words, Newtonian Physics is an approximation to reality at best, or an appearance reality takes within a certain domain. Now, if one insists that the theory IS reality, I would say we would have morphed Newtonian Physics even into a false picture of reality!

Our theories describe reality. How good a description is it - well we have this thing called experiment.

I'm familiar with all the above in relation to QM. What I wasn't familiar with was the non-committal state that apparently exists in QFT, as well. Like I said, we're missing something ...

Its as committal or non committal as any other theory. Nothing special about it that way.

Thanks
Bill

 

[vanhees71]

The ontic nature of fields is a serious physics question. Why this forum seems to feel threatened in some way by questions like these is beyond me.

No, it's a philosophical discussion, which in my opinion has lead to nothing but confusion. Physics, as a natural science, is about objectively observable and quantifiable phenomena and their mathematical description in terms of natural laws and nothing else.

For a physicist the electromagnetic field exists and is quantitatively defined by its action on test charges (study any serious textbook about classical electrodynamics). That's it. There's no doubt about its existence whatsoever, and any discussion about the ontic vs. epistemic "nature" of the electromagnetic field is rather mute. I don't see any merit in it.

The same holds for quantum theory. There you have endless discussions about "interpretation", and philosophers tend to confuse the subject by using unsharp definitions of notions like "reality" and again "ontology vs. epistemology" and things like that. Within physics, there's a clearly defined probabilistic meaning of what a quantum state and an observable is and how it is mathmatically described, and finally everything is defined through descriptions of concrete measurement protocols in Nature. The success of QT as a physical theory is that its theoretical predictions are in accurate accordance with the observations. Everything else goes beyond physics and is not subject in these forums.

 

[bhobba]

No, it's a philosophical discussion, which in my opinion has lead to nothing but confusion. Physics, as a natural sciencergent Universe can be trusted and are legit e, is about objectively observable and quantifiable phenomena and their mathematical description in terms of natural laws and nothing else.

That's the exact issue - they confuse philosophy and science and end up understanding neither.

We even had one person, who when they found out a respondent had a PhD thought, literally, it was a doctorate in philosophy and was aghast they were 'anti' philosophy. They had no idea it was just a title.

Science and Mathematics divorced themselves from philosophy ages ago. That is not to denigrate philosophy - it's just science has taken a different direction. We have a few people like David Wallace who has a PhD in both particle physics and philosophy. Their science writings like the Emergent Multiverse can be trusted, but by and large most philosophers do not understand science that well - of course the reverse is likely true as well.

Thanks
Bill

 

[bhobba]

A reply solely to put up no less than 10 emoticons?

I can't let this go without comment.

I strongly agree with someones post and reply with a heap of smiles to indicate such and its childish. Amazing.

Thanks
Bill