A Are quantum fields real objects in space?

[JG11]

I always had a tough time understanding how QFT relates to reality. Are these quantum fields (electron field, ect) physically real? Are they things that exist in space or are they just mathematical abstractions that help use calculate things?

 

[David Lewis]

There might not be a satisfying answer because "real" is (at best) ambiguously defined in physics. If you have a mathematical abstraction that successfully predicts experimental outcomes, then it might be considered real in some sense.

 

[ZapperZ]

I always had a tough time understanding how QFT relates to reality. Are these quantum fields (electron field, ect) physically real? Are they things that exist in space or are they just mathematical abstractions that help use calculate things?

They are as "real" as the classical field, which presumably, since you don't have the same issue with, you accept as being related "... to reality..."

Zz.

 

[kith]

The terminology in quantum field theory is a bit tricky because the term "field" is used both for the physical system as well as for an operator which acts on the corresponding Hilbert space. Some time ago, I opened a thread on this which may be of interest to you.

 

[Demystifier]

I always had a tough time understanding how QFT relates to reality. Are these quantum fields (electron field, ect) physically real? Are they things that exist in space or are they just mathematical abstractions that help use calculate things?

The electron field is certainly not real, because it is not even an observable. Observables are hermitian operators, which the electron field (by being fermionic rather than bosonic) is not. Bosonic fields (like electromagnetic field) could potentially be real, but I think all fields are just mathematical abstractions that help us calculate things.

 

[kith]

[...] but I think all fields are just mathematical abstractions that help us calculate things.

Is this a result of your Bohmian particle ontology or would you argue for it already in classical electrodynamics?

 

[atyy]

Is this a result of your Bohmian particle ontology or would you argue for it already in classical electrodynamics?

One can also view fields as not real from the point of view of Copenhagen. Neither observables nor quantum states are real, only measured outcomes and their probabilities are real

 

[kith]

Would you consider the quantum system itself to be real in Copenhagen?

 

[martinbn]

The electron field is certainly not real, because it is not even an observable. Observables are hermitian operators, which the electron field (by being fermionic rather than bosonic) is not. Bosonic fields (like electromagnetic field) could potentially be real, but I think all fields are just mathematical abstractions that help us calculate things.

I don't think that's what he is asking. If he asks whether particles, in classical physics, are real. You are going to say "yes". Possible something more elaborate, but you are not going to say "well, mathematically they are described as curves on a six dimensional symplectic manifold, so they are not real".

 

[martinbn]

One can also view fields as not real from the point of view of Copenhagen. Neither observables nor quantum states are real, only measured outcomes and their probabilities are real

I don't thing that is true. Why wouldn't fields be real according to Copenhagen?

 

[vanhees71]

As all theoretical physics quantum fields are mathematical descriptions of reality. On top of these mathematical descriptions you need a minimal interpretation (which for sure is not Copenhagen for QFT). So what's described by relativistic QFT (or the Standard Model of elementary particle physics)? It in fact (as far as we know today accurately) describes the behavior of the constituents of all observed matter in scattering experiments and (naturally less accurately with many more approximations and less completely) also how the known matter, as a many-body quantum system, behaves as observed (even down to our everyday experience with everyday matter). That's it, no more no less, and that's about the best you can expect from the natural sciences to provide: Objective mathematical quantitative descriptions of how Nature behaves.

 

[martinbn]

As all theoretical physics quantum fields are mathematical descriptions of reality.

I don't think that is well phrased. Quantum fields is the name of what exists objectively (i.e. is real), operator-valued distributions (or some such thing) is the mathematical description.

 

[Demystifier]

Is this a result of your Bohmian particle ontology or would you argue for it already in classical electrodynamics?

It is influenced by my Bohmian way of thinking, but note that in orthodox quantum theory neither particles not fields are "real". In orthodox quantum theory only the macroscopic readings of scientific instruments are real.

 

[Demystifier]

Why wouldn't fields be real according to Copenhagen?

Because we don't measure them.

 

[vanhees71]

Hm, but we "measure"/"observe" at least the electromagnetic field (e.g., by our eyes since light is nothing else than electromagnetic fields oscillating at frequencies, our eyes are sensitive to).

 

[atyy]

Would you consider the quantum system itself to be real in Copenhagen?

I don't know. But let's say we just consider quantum mechanics, and the simple harmonic oscillator. Is the Hamiltonian real? The Hamiltonian has 2 roles. First, the Hamiltonian governs the time evolution of the quantum state (Schroedinger picture) or the observables (Heisenberg picture) - the quantum state is not real (Schroedinger picture), and neither are the observables (Heisenberg picture, since all observables, including those that don't commute, evolve simultaneously). Secondly, the Hamiltonian can be an observable - maybe it is real when measured, but what if it is not measured? And even if it is measured, only the probabilities in the Born rule (ie. the full scalar product involving both the observable and the quantunm state) describe real events, so maybe the observable or the quantum state individually are not real.

 

[Demystifier]

Hm, but we "measure"/"observe" at least the electromagnetic field (e.g., by our eyes since light is nothing else than electromagnetic fields oscillating at frequencies, our eyes are sensitive to).

I have a more elaborated answer to this in a paper I currently write. Would you like me to send you a draft of the paper?

 

[martinbn]

Because we don't measure them.

So?!

 

[A. Neumaier]

Because we don't measure them.

So the Moon (considered as a many-particle quantum object) is not real when nobody looks at it?

 

[Lord Jestocost]

So the Moon (considered as a many-particle quantum object) is not real when nobody looks at it?

This is a question which “Physics” cannot answer seriously. One should not extend speculations beyond the range of experiments/observations. The reality is in the "observation" of the moon, not in the moon itself.

 

[A. Neumaier]

The reality is in the "observation" of the moon, not in the moon itself.

No. Observations are accidental, reality is not.

Before anyone was able to observe anything, the universe must have existed for a very long time. At least this is considered physical consensus.

 

[Demystifier]

So?!

So the Moon (considered as a many-particle quantum object) is not real when nobody looks at it?

According to some versions of Copenhagen interpretation, the Moon does not exist when nobody looks at it. For instance, Wheeler said that “no phenomenon is a real phenomenon until it is an observed phenomenon.”
I am not defending that interpretation, I am just saying what that interpretation claims.

 

[atyy]

According to some versions of Copenhagen interpretation, the Moon does not exist when nobody looks at it. For instance, Wheeler said that “no phenomenon is a real phenomenon until it is an observed phenomenon.”
I am not defending that interpretation, I am just saying what that interpretation claims.

This interpretation does not exist until discussed.

 

[A. Neumaier]

According to some versions of Copenhagen interpretation, the Moon does not exist when nobody looks at it. For instance, Wheeler said that “no phenomenon is a real phenomenon until it is an observed phenomenon.”
I am not defending that interpretation, I am just saying what that interpretation claims.

So how does this interpretation cope with the quantum physics of the early universe, before there were observers? It cannot.

Thus the Copenhagen interpretation is only a partial interpretation appropriate to the early Copenhagen days of quantum mechanics, where the latter was applied to the study of lab objects only. We have progressed a lot since these days.

 

[martinbn]

According to some versions of Copenhagen interpretation, the Moon does not exist when nobody looks at it. For instance, Wheeler said that “no phenomenon is a real phenomenon until it is an observed phenomenon.”
I am not defending that interpretation, I am just saying what that interpretation claims.

I think you are missing the point. "The moon is not there if no one is looking at it." I understand this as stressing "not there" i.e. position variables(and any other observable) have no value unless measured. But it doesn't mean "is not" i.e. the object doesn't exist. Copenhagen is open on that part.

So, the statement according to Copenhagen is "The moon is not there if no one is looking at it."
The statement as often said in pop sci text is "The moon is not there if no one is looking at it."

I believe it is the first one that Bohr would have agreed with. The second is just poor philosophy or at best poor language use.

 

[A. Neumaier]

So, the statement according to Copenhagen is "The moon is not there if no one is looking at it."

This does not rescue the situation - to send an unmanned spacecraft to the Moon we calculate the position of the Moon at the ultimate meeting time quite in advance - i.e., before anyone has seen it there. And the Moon is indeed there at that time, since we know that the spacecraft has landed there - even though it may send its first observed pictures only minutes or hours later. Thus we know where the Moon is at any time, not to infinite accuracy (which is meaningless anyway for an extended object like the Moon) but well within the limits of the Heisenberg uncertainty relation.

 

[Haelfix]

Copenhagen might have questioned the ‘reality’ (loaded word) of a non measured macroscopic object like the moon at one time, but surely everyone agrees nowadays that it objectively exists as a classical object and is safely and perpetually decohered (the environment of the moon acts as a perpetual measuring device).

 

[vortextor]

If the condition (in the boundary of QM) for "the Moon is there" is an obsevation, a single
photon coming from nowhere absorbed or reflected on the surface of the Moon can be
considered THE observation.
A step outside of QM: considering spacetime curvature produced by by the mass of the
Moon is not observator dependent, as I know.
And what about self-observation? This is a question, not a personal theory.

 

[A. Neumaier]

a single
photon coming from nowhere absorbed or reflected on the surface of the Moon can be
considered THE observation.

Who observes it? An arbitrary interaction is surely not an observation, as the experiments with entangled photons amply demonstrate.

 

[atyy]

Copenhagen might have questioned the ‘reality’ (loaded word) of a non measured macroscopic object like the moon at one time, but surely everyone agrees nowadays that it objectively exists as a classical object and is safely and perpetually decohered (the environment of the moon acts as a perpetual measuring device).

Decoherence is not a measurement. It is only a premeasurement. Neither the environment nor the moon are classical objects in the Copenhagen sense, if they are part of the unitarily evolving quantum state.

 

[vortextor]

@Mr.Neumaier,
I was thinking about the elementary observer as a black box with a single
cell of "memory" or state, whatever, the elementary observation meaning interaction
or input, flipping a bit from 0 to 1. Changing the state of the observer. In this case
the state of the photon.
No preparation procedure, no coffee break,no noisy dot matrix printer.
I am sure that my idea needs correction, so please teach me.

 

[DarMM]

I think Bohr's position might have been that certain objects (such as microscopic systems) don't seem to possesses any of the humanly effable quantities we normally work with in physics, such as Energy, Position, Momentum, etc. The best one can hope for is probabilistic estimates on imprints relating to these quantities on our experimental devices (or anything that possesses them) and of course even then one cannot attempt to compose conclusions from different devices (uncertainty principle) in an attempt to get an objective picture of the subatomic system.

So Bohr and most AntiRealist views don't say the moon isn't there where nobody looks, it's more that for some systems it's only the "looking" that can be given a mathematical description, not the system itself.

Hence it doesn't make sense to speak of the system itself in mathematical terms, only the observations. Note though this is not a denial of the reality of the system.

If you've read Kant (which Bohr is very difficult to read without I think) it is obviously influenced by his ideas of objective and transcendental reality.

 

[A. Neumaier]

I was thinking about the elementary observer as a black box with a single cell of "memory" or state

In the Copenhagen interpretation, an observer is someone or something that collapses the wave function into an eigenstate of the operator observed. How do you ensure that your elementary observer does this?
Surely the interaction of the photon with the Moon does not do this by itself, it just entangles the state of the photon with the state of the Moon.

 

[vortextor]

In the Copenhagen interpretation, an observer is someone or something that collapses the wave function into an eigenstate of the operator observed. How do you ensure that your elementary observer does this?
Surely the interaction of the photon with the Moon does not do this by itself, it just entangles the state of the photon with the state of the Moon.

@Mr.Neumaier, I'm afraid that I am not a big fan of the Copenhagen interpretation, for me
the wave function is just a bookkeeping device, and collapse in the physics lab can happen from too
much coffee. The only physical thing is the interaction with a microscopic (read:subatomic,atomic etc)
part of the measuring instrument, which, thorough an amplification chain, moves the pointer in the x position.
Is this is heresy, it must be a name for it. I don't now what is.

 

[DarMM]

@Mr.Neumaier, I'm afraid that I am not a big fan of the Copenhagen interpretation, for me
the wave function is just a bookkeeping device, and collapse in the physics lab can happen from too
much coffee. The only physical thing is the interaction with a microscopic (read:subatomic,atomic etc)
part of the measuring instrument, which, thorough an amplification chain, moves the pointer in the x position.
Is this is heresy, it must be a name for it. I don't now what is.

Experimental devices being an amplification of subatomic effects:

irreversible amplification in devices

and the wavefunction being just an epistemic bookkeeping device?

That's literally the Copenhagen interpretation.

 

[bhobba]

This is a question which “Physics” cannot answer seriously. One should not extend speculations beyond the range of experiments/observations. The reality is in the "observation" of the moon, not in the moon itself.

Remember the moon is being observed all the time by its environment. How the classical world emerges is discussed by Hartle and Gell-Mann in a famous paper here:
https://arxiv.org/abs/quant-ph/0609190

To 'decipher' it see here (as well as a link to another paper by Hartle and Gell-Mann about it):
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

That uses normal QM - not QFT - but of course QM is a limiting case of QFT. Are the fields of QFT real? Well first ask are the fields of EM real. Wheeler and Feynman formulated EM in terms of action at a distance so you do not need fields - so why do physicists believe in them? The answer is Noether - we believe in conservation of energy and momentum. If you move a charged particle then another charged particle does not react straight away due to relativity. But we want energy and momentum to be conserved because of Noether. I read somewhere where Wigner even produced some no-go theorems about it. The answer was to have a field which via Noether (again) has energy and momentum. Well with energy it can in principle be converted to mass - most physicists are not too philosophical and think mass real - whatever real is - so fields are real - even though you never directly observe them - just their effects - and as mentioned you can formulate EM without them. Bohr was an exception - there probably are others as well - but most these days seem to be like Feynman because of his experience with philosophy at MIT - the teacher bored him to death and spent his time in the class drilling small holes in his shoes - thought it all hooey. To add insult to injury, he didn't care what his son studied as long as it was not philosophy. Guess what - he studied philosophy. He later switched to computer science and Feynman was happier. Anyway, to recap, because most physicists consider mass real they consider fields real. You could apply the same argument to Quantum Fields - they in a certain limit become the stuff around us. Its a pretty common sense sort of view - but physicists often are like that. For example read Weinberg on Kuhn and his view of what science is:
http://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html

'I remarked in a recent article in The New York Review of Books that for me as a physicist the laws of nature are real in the same sense (whatever that is) as the rocks on the ground. A few months after the publication of my article I was attacked for this remark by Richard Rorty. He accused me of thinking that as a physicist I can easily clear up questions about reality and truth that have engaged philosophers for millennia. But that is not my position. I know that it is terribly hard to say precisely what we mean when we use words like "real" and "true." That is why, when I said that the laws of nature and the rocks on the ground are real in the same sense, I added in parentheses "whatever that is." I respect the efforts of philosophers to clarify these concepts, but I'm sure that even Kuhn and Rorty have used words like "truth" and "reality" in everyday life, and had no trouble with them. I don't see any reason why we cannot also use them in some of our statements about the history of science. Certainly philosophers can do us a great service in their attempts to clarify what we mean by truth and reality. But for Kuhn to say that as a philosopher he has trouble understanding what is meant by truth or reality proves nothing beyond the fact that he has trouble understanding what is meant by truth or reality.'

It's not deep is it? Its pretty common-sense - and that basically is how many (perhaps even most - but probably not all) physicists view it. For a counter view read Penrose - I will say no more - his views are rather unusual - but strangely compelling - one can see why those of a certain 'bent' would gravitate to it - I did at one time.

Thanks
Bill

 

[bhobba]

In the Copenhagen interpretation, an observer is someone or something that collapses the wave function into an eigenstate of the operator observed. How do you ensure that your elementary observer does this?
Surely the interaction of the photon with the Moon does not do this by itself, it just entangles the state of the photon with the state of the Moon.

I think that's one reason Hartle and Gell-Mann developed Decoherent Histories - but they are the first to admit the program is not complete - there are issues remaining that need to be worked out. Maybe they can't in which case - down the gurgler it goes - but as of now they are open questions.

A few of the issues can be found in Omnes book on the subject:
https://www.amazon.com/dp/0691004358/?tag=pfamazon01-20

Thanks
Bill

 

[bhobba]

So how does this interpretation cope with the quantum physics of the early universe, before there were observers? It cannot.

It doesn't, and is a known problem.

Thanks
Bill

 

[Demystifier]

But it doesn't mean "is not" i.e. the object doesn't exist. Copenhagen is open on that part.

As I stressed many times, there are several different versions of "Copenhagen" interpretation. Some versions are, as you say, open to it. Some are not.

More importantly, if one accepts that properties of the object exist even before observation, and that measurement just changes (rather than creates) those properties, then the Bell theorem implies that those changes obey non-local (deterministic or stochastic) laws. It is inconsistent to think that nature is both local and existing without observations. Some Copenhagenians are aware of that, which is why, by insisting on saving locality, they argue that objects don't exist before observation.

 

[DarMM]

Some Copenhagenians are aware of that, which is why, by insisting on saving locality, they argue that objects don't exist before observation.

I think the most common version of this today is denying that the quantum probabilities are set by some hidden variable $\lambda$ of the system in question, though the system itself exists.

 

[Demystifier]

I think the most common version of this today is denying that the quantum probabilities are set by some hidden variable $\lambda$ of the system in question, though the system itself exists.

But probabilities must be determined by something. For example, if that something is the wave function $\psi$, that also qualifies as a special case of $\lambda$.

 

[A. Neumaier]

For example read Weinberg on Kuhn and his view of what science is:
http://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html

Nice essay!

 

[A. Neumaier]

Is this is heresy, there must be a name for it. I don't know what it is.

It is the heresy of superficial speculation.

 

[bhobba]

But probabilities must be determined by something. For example, if that something is the wave function $\psi$, that also qualifies as a special case of $\lambda$.

Hmmm. Interesting view. What do you think of probabilities themselves - do you reach the same conclusion?

Thanks
Bill

 

[bhobba]

Is this is heresy, it must be a name for it. I don't now what is.

As has been correctly pointed out there are different versions of Copenhagen and each version has different issues. These days many Copenhagenists have moved over to Consistent/Decoherent Histories calling it Copenhagen done right - you can read about its basics here:
http://quantum.phys.cmu.edu/CQT/index.html

But there is no free lunch - as I mentioned it is more concise than general Copenhagen, eg no issues with measurement because it is replaced by the concept of history. So far so good - but as I mentioned before there are some issues in principle not yet worked out fully. Only the future will tell us how it fares. If its of any value Murray gave a lecture on it when his friend (the friendship cooled towards the end - but the respect never did - nor did Feynman's respect for Murray - calling him the worlds best living physicist) Feynman was alive and was in the audience. At the end he got up and everyone thought a ding dong was on - but instead said - I agree with everything said. So he was converted to it towards the end.

Thanks
Bill

 

[Demystifier]

What do you think of probilities themdrlves - do you reach the same conclusion?

I'm not sure what do you mean by probabilities themselves, but I guess the PBR theorem is relevant here.

 

[bhobba]

I'm not sure what do you mean by probabilities themselves, but I guess the PBR theorem is relevant here.

What I meant is one view if you are into Gleason etc is it is simply something that helps in calculating probabilities which are something that simply helps in calculating frequencies of observations. In a sense you can look on both as just calculational devices for the reality which is observations. I personally do not believe that - there is more to it eg how a mixed state becomes a proper mixed state - just interested in what you think. BTW I have never thought of the wave-function itself as a hidden variable - but now you have placed the idea in my mind I think it deserves serious thinking about.

And PBR is definitely relevant - if it is not just some abstract calculational device its pretty hard to deny its reality in the sense of the theorem.

Thanks
Bill

 

[Demystifier]

BTW I have never thought of the wave-function itself as a hidden variable - but now you have placed the idea in my mind I think it deserves serious thinking about.

And PBR is definitely relevant - if it is not just some abstract calculational device its pretty hard to deny its reality in the sense of the theorem.

The PBR theorem can be stated as follows. If there is some $\lambda$ at all, then either $\psi$ is $\lambda$, or $\psi$ is uniquely determined by $\lambda$.

 

[bhobba]

The PBR theorem can be stated as follows. If there is some $\lambda$ at all, then either $\psi$ is $\lambda$, or $\psi$ is uniquely determined by $\lambda$.

Yes.

Just for those not aware of it here is the paper:
https://arxiv.org/pdf/1111.3328.pdf

The out is what is mentioned in the paper - some for some reason seem to forget it, even though its clearly stated it in the original paper:
'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.'

You are right - if you do not invoke the out then the wave-function is clearly a hidden variable - I just never thought of it that way.

Thanks
Bill

 

[Demystifier]

Yes.

Just for those not aware of it here is the paper:
https://arxiv.org/pdf/1111.3328.pdf

The out is what is mentioned in the paper - some for some reason seem to forget it, even though its clearly stated it in the original paper:
'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.'

You are right - if you do not invoke the out then the wave-function is clearly a hidden variable - I just never thought of it that way.

Thanks
Bill

Exactly. After all, nobody ever measured $\psi$ by a single measurement, so it is hidden. In fact, Bohmians often say that it is $\psi$, not the particle position, which is really "hidden".