Undergrad Fields: Unobservable, Yet Physical?

[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