Graduate 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.