Are quantum fields real objects in space?

[stevendaryl]

The wavefunctions in QFT are of the form (in the Heisenberg picture):
$$\Psi(\phi), \quad \phi \in \mathcal{S}^{'}\left(\mathbb{R}^{d-1}\right), \quad \Psi \in \mathcal{H} = \mathcal{L}^{2}\left(\mathcal{S}^{'}\left(\mathbb{R}^{d-1}\right),d\nu\right)$$
with $\mathcal{S}^{'}\left(\mathbb{R}^{d-1}\right)$ the space of tempered Schwarz distributions on a spacelike slice. Depending on the measure $d\nu$ the Hilbert space has a Fock decomposition.

Yes. In going from non-relativistic quantum mechanics to quantum field theory, I originally found it confusing because I thought of $\phi$ as the analogy for the nonrelativistic wave function, when it's actually $\Psi$. The confusion is made worse by the fact that the Heisenberg equations for $\phi$ look (at least for free fields) like the Schrodinger equation for the single-particle wave function.

 

[ZMacZ]

My personal view on this it's either field XOR particle..
The only time this appears to be untrue is when the two combined cause effect at the same time..
But in that case it's a field separate from the particle effect..
I think a lot of people mistake a field for a particle effect that they can't explain given current physics knowledge..

But in any case, when particles are involved it's not a field, and when it's a true field, it's not particles..
That's the annoying part..the particles do something we can only explain using field theories..
Hence it's called particle field, which in and by itself is a misnomer of sorts..

Anyways, that's my 10 cents..

 

[bhobba]

I think a lot of people mistake a field for a particle effect that they can't explain given current physics knowledge..

In QFT its all explained by the mathematics of the field - it has a form similar to the so called second quantization formulation, and hence quantum particles
https://pdfs.semanticscholar.org/2fb0/4475228ff385a44a16e3ba42b432d3bf5b17.pdf

That's how particles emerge from fields in QFT,

Thanks
Bill

 

[Elroch]

It is safe to say that both wave particle duality and the collapse of the wave function are not present when quantum mechanics is modeled precisely (particles are just waves which are localised, "collapsing" is the strong dependence of the state on the information that is available.
Roughly speaking a quantum state which is defined over both space and time expresses a state of knowledge. It is this state of knowledge which is affected by things such as the detection of a particle at a specific location. With a two slit experiment, the Schroedinger equation for a particle without any knowledge of a detector consists of a field which diffracts outwards from the two slits and exhibits interference. But once detected, the past locations of the particle are highly constrained to the two paths from the slits to the detection. If we knew in advance where the array of detectors were and the physics indicated that it was highly likely that one of the detectors would detect the particle, the correct wave function to express our state of knowledge before detection would be the sum of the ones after each of the individual possible detections (with some appropriate weights). [This notion is not part of the Copenhagen interpretation, and I have not seen it in an introductory text - it owes more to Feynman's viewpoint. The Copenhagen viewpoint seems to be based on a desire to retain causality in the evolution of the wave function as a physical field. An argument against this is that the wave function is a state of belief depending on some set of relevant information, and this remains true when the information is associated with things in the future of the wave function (such as the location of a set of detectors].
By contrast with the two wave functions associated with passing through one or other of the slits, these wave functions associated with distinct detections interfere with each other very little and add classically, like probabilities. The thing that makes the detections have this dramatic effect on the wave function is that the paths that end in a detection have very high probability for reasons which may be best expressed in terms of them being high entropy. This makes the contribution from other paths very small, unlike when the detectors are not there.

 

[stevendaryl]

It is safe to say that both wave particle duality and the collapse of the wave function are not present when quantum mechanics is modeled precisely (particles are just waves which are localised, "collapsing" is the strong dependence of the state on the information that is available.

I don't see how that interpretation of quantum probabilities is consistent with what we know about quantum mechanics.

If you pass unpolarized light through a polarizing filter, it comes out polarized in the direction corresponding to the filter orientation. Viewing this as a matter of acquiring information about the photons is certainly not complete. If you have three filters oriented at angle $0^o$, $45^o$ and $90^o$, 12.5% of the unpolarized photons (1/8 of them) will pass through all three filters. That can't possibly be a matter of just learning the pre-existing polarization of the photons, because it is impossible to have a photon that will pass through a filter at both 0 and 90 degrees. Passing through the filter changes the polarization of the photons. So it's not simply a matter of updating information.

 

[TonyP0927]

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

The Copenhagen interpretation is more "real" because it's observable: Upon observation, all quantum possibilities collapse into one outcome. This has been tested with the double-slit experiment.
There's a problem though, and it has to do with the "quantum information paradox". Upon waveform collapse, the information about all of the other possible quantum states is lost, thus violating the conservation of information.
Everett's MWI offers a solution to this: All outcomes are equally real but exist in different branches of the multiverse, which we can not observe. The conservation of information states that information can not be destroyed but it does not mean it has to be accessible, so the MWI satisfies this.

 

[TonyP0927]

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

Oh it's real! The gravitational attraction is obvious, otherwise there would be drastic changes to tides if nobody looked at the moon.

You can think of the probability of a macroscopic object existing is equal to that the ~sum of the probabilities of enough particles being in the position they need to be to form the moon is essentially 100%.

 

[Elroch]

I don't see how that interpretation of quantum probabilities is consistent with what we know about quantum mechanics.

If you pass unpolarized light through a polarizing filter, it comes out polarized in the direction corresponding to the filter orientation. Viewing this as a matter of acquiring information about the photons is certainly not complete. If you have three filters oriented at angle $0^o$, $45^o$ and $90^o$, 12.5% of the unpolarized photons (1/8 of them) will pass through all three filters. That can't possibly be a matter of just learning the pre-existing polarization of the photons, because it is impossible to have a photon that will pass through a filter at both 0 and 90 degrees. Passing through the filter changes the polarization of the photons. So it's not simply a matter of updating information.

I did not use the incorrect notion that there is a "pre-existing polarization of the photons" that stays the same throughout the life of the photon. This is physically inaccurate: the polarisation of a photon changes when it interacts with a polarising filter.
Instead what you have is the following situation. After a photon reaches a polarising filter it either passes through and acquires a specific definite state of polarisation or it does not pass through. The probability of it passing through is determined by its previous (possibly definite, possibly different) state of polarisation.
So when you have detected a photon that has passed through a set of polarising filters, you know the state of polarisation it had in the space between each two polarising filters. This polarisation changes at each filter if the filters are not aligned.
So no problem there.

 

[stevendaryl]

I did not use the incorrect notion that there is a "pre-existing polarization of the photons" that stays the same throughout the life of the photon. This is physically inaccurate: the polarisation of a photon changes when it interacts with a polarising filter.
Instead what you have is the following situation. After a photon reaches a polarising filter it either passes through and acquires a specific definite state of polarisation or it does not pass through.

Okay, but "acquiring a definite state of polarization" is what is meant by "collapse". You were saying that it was about information. I don't see that it has anything to do with information.

 

[Elroch]

It's all about information. :) The information interpretation of quantum mechanics is entirely valid and I can detect no differences between it and my own way of thinking.
A wave function is a description of what is known about a particle which includes its position and momentum, but also all information about polarisation, spin etc., in full generality. (Also eg even flavor, in the case of neutrino oscillation).
In both the most common case of the discovery of the position of a particle by detection which is the typical example of "wave function collapse" and the case of a photon passing through a filter, what happens is an interaction between the particle and another quantum object which in the first case determines the position of the particle and in the second case determines its polarisation afterwards.
Position is an infinite dimensional property expressed in terms of a unique preferred basis of delta functions at every point, while polarisation is a 2-dimensional property which can be expressed equally well in terms of bases based on different chosen orientations: this is the distinct property that makes experiments with polarising filters different.
In both the case of position and polarisation, clearly the properties change as a wave function evolves. As I see it, the difference is that the former changes constantly, the latter changes only with certain interactions (hence astronomers can measure polarisation produced light years away).

 

[zdcyclops]

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

It is not known. Nor can it be known until you receive information from it. Quantum objects do not exist until they interact with other objects and cause a change in objects we are observing. The momentum, spin, charge, are unknown to the entire universe and are shared only through interaction with other particles.

 

[Rohan]

Whilst I realize it is not popular ; I consider it is useful to distinguish between science and philosophy.
Where philosophy is itself divisible as
- values and ideology
-epistemology: what we know and how we know it
-ontology:essential nature of things and definition of the meaning of words.

Personally I also find it useful to restrict the meaning of metaphysics to ;
- relations and correlations between subjective experience and the objective material world and
the meaning of of mysticism to - profounder aspects of subjective experience.
I include these two later words just because some physicists seem to conflate the two as meaning
"just some obscure, irrelevant and speculative matter."

Whilst the vast majority of interpretations of QM (Copenhagen etc) are 'ontic' (eg electron location is actually fuzzy);
there have been 'epistemic' interpretations (eg electron location is fuzzy knowledge), which don't rely on hidden variables.
Eddington's 'Fundamental Theory' seems to be an example of the later.
Whilst this was never successful; an epistemic interpretation of the collapse of the wave function following observation does;
have appeal to me as the " the response of quantified uncertainty to a new observation"

Finally I mention all the above as context to my comment that whilst most of the discussion has been in the grey area between epistemology and theoretical QM; BUT when we start to enquire about 'reality' as distinct from say the shared quality of objective material phenomena; we are likely to encounter deeply held metaphysical differences !

 

[PeterDonis]

Thread is closed. Too much philosophy