A Are quantum fields real objects in space?

[DarMM]

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 $\lambda$.

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 $\lambda$.

$\psi$ itself being the only hidden variable is the subset of psi-ontic interpretations known as psi-complete interpretations. The usual phrasing of the PBR theorem is that if $\Lambda$ is the space of hidden variables and

1. $\Lambda$ is measureable
2. Experiments have one outcome
3. There is no retrocausality
4. There is no superdeterminism
5. $\Lambda$ has a product structure on two systems, i.e. $\Lambda_{AB} = \Lambda_A \times \Lambda_B$

Then at the very least $\Lambda = \mathcal{H} \times \mathcal{A}$, with $\mathcal{H}$ the quantum mechanical Hilbert space.

Axioms 1-4 are called the ontological framework axioms, with interpretational models obeying it called ontological models. So the theorem may be stated as any ontological model whose state space has a product structure for two systems (i.e. two systems can be prepared independently) must have the wavefunction as a subset of its hidden variables.

One can reject this by rejecting axioms 1-4 of course (I've seen no serious attempts at 5) or one can take the AntiRealist view and reject the existence of $\Lambda$.
This basically views quantum probabilities as not arising from mathematical properties of the quantum system itself. As mad as it sounds one must acknowledge that all realist views (i.e. ones with a $\Lambda$) suffer from fine-tuning problems such as those mentioned in the Pusey-Leifer theorem (a development of Price's theorem).

 

[DarMM]

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.

It's standard in modern quantum foundations to view $\psi$ as a special case of hidden variables.

The best review article on all this stuff is Matt Leifer's:
https://arxiv.org/abs/1409.1570

 

[rrogers]

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?

Is it generally possible to get a copy? I would be interested in other opinions (I think I disagree with a lot of them) on what is "real"; but I am always interested in the reasoning behind various viewpoints.
Not dogmatic: just critical :)

 

[drpi]

“…I don't demand that a theory correspond to reality because I don't know what it is. Reality is not a quality you can test with litmus paper. All I'm concerned with is that the theory should predict the results of measurements….” -- Stephen Hawking

https://en.wikiquote.org/wiki/Stephen_Hawking. This was taken from a debate with Roger Penrose in 1994 at the Isaac Newton Institute for Mathematical Sciences at the University of Cambridge, transcribed in The Nature of Space and Time (1996) by Stephen Hawking and Roger Penrose, p. 121.

 

[Demystifier]

fine-tuning problems such as those mentioned in the Pusey-Leifer theorem

I don't know what is the Pusey-Leifer theorem. Can you gave a reference and/or a brief overview?

 

[Demystifier]

Is it generally possible to get a copy?

At some point it will be on arXiv. At the moment, I can tell that the title will be: Bohmian mechanics for instrumentalists

 

[DarMM]

I don't know what is the Pusey-Leifer theorem. Can you gave a reference and/or a brief overview?

https://arxiv.org/abs/1607.07871

In essence Quantum Mechanics has an observed symmetry, Operational Time Symmetry. They basically show that the only way for this symmetry to also hold at the level of the hidden variables is if the hidden variables have retrocausality. If you don't have retrocausality the symmetry can only emerge effectively through some kind of fine tuning or (currently undemonstrated in any model) thermalisation in the early universe. However if you go with the latter version, then the underlying physics (faster-than-light effects in Bohmian Mechanics, evidence of the other Worlds in MWI) should be visible in the early universe and thus one has divergent predictions from QM.

If you do have retrocausality, you get the symmetry, but you run into the same fine-tuning/thermalisation issues with experiments having not yet seen retrocausal signals.

It's an extension of Price's theorem. Price's original proof only covered theories which are Psi-Ontic ($\Lambda = \mathcal{H} \times \mathcal{A}$, i.e. the wavefunction is one of the hidden variables). Leifer and Pusey extended this to Epistemic models of the wavefunction as well.

Of interest to yourself they explicitly discuss Bohmian Mechanics.

 

[Peter Morgan]

FWIW, I think of mathematical models of quantum field theory as like to an experiment rather as a map is like to the landscape to which the map acts as a guide. It takes experience to learn how to use a map that is much less than isomorphically like to the landscape, with some things perhaps rendered almost photorealistically but with other things located on the map only as a symbol, with much variation of style from one map to another.
What the mathematics of QFT says depends on your choice of axioms. Almost universally, there are states over an algebra of operators that is freely constructed using an operator-valued distribution $\hat F(x)$. The operators are of the form $\hat F_{\!f}=\int\hat F(x)f(x)\mathrm{d}^4x$ (where the "test function" $f(x)$ constructs an average of the operator-valued distribution that is differently weighted in different regions of space-time), and satisfy commutation relations $[\hat F_{\!f},\hat F_{\!g}]=(f^*,g)-(g^*,f)$. The standard vacuum state of the free field is $$\langle 0|\mathrm{e}^{\mathrm{i}\lambda\hat F_{\!f}}|0\rangle=\mathrm{e}^{-\lambda^2(f^*,f)/2},$$ which, if $f=f^*$ is real, is the characteristic function of a normal distribution. The inner product $(f,g)$ fixes both the variance of a normal distribution associated with any given operator $\hat F_{\!f}$ and also the commutation relations that determines where Bell inequalities are violated, where Wigner functions take negative values, et cetera. The form of the inner product makes no difference to the algebraic structure, however, only to the geometrical structure. For interacting fields the characteristic function $\langle 0|\mathrm{e}^{\mathrm{i}\lambda\hat F_{\!f}}|0\rangle$ must be deformed to correspond to a different probability distribution, but still there are, in principle, the field operators $\hat F_{\!f}$. [All the above will seem quite distant to anyone who has learned only the path integral approach and works with time-ordered products of operators, but such constructions can be regarded as derived from the construction above.]
How does this structure relate to an experiment? The best approach, again FWIW, is to work with QFT as a signal analysis formalism, where a test function is called a "window function" (I've also seen Chris Fewster call it a "sampling function"), particularly because all the experimental raw data certainly comes to us as voltages on signal lines (think of CERN, with many thousands of measurements of voltages on signal lines being compressed by hardware and software into times and geometries of many events per second, which are then grouped together so that statistics can be compared to the probability distributions that a given theory proposes; but even a very small experiment does the same kind of signal analysis for far fewer voltages on signal lines).
To get back to the original question, the voltages on signal lines can be said to be "real", if you like, although it makes little difference to the world whether you call it names or not, then we might call the operators that are used to describe the signal analysis we do of the records we have of the voltages on the signal lines as not "real" in quite the same sense, because their relationship to the computing and experimental hardware is somewhat more remote (but this is just to say that we have in this a different kind of map of the experiment, which uses different symbols, a little more like an electron microscope and less like a telescope for the sophistication and directness of the transforms that are being applied, say.)
Let me add that a signal analysis approach allows us to say of states other than the vacuum state that they are modulations of the vacuum state. There is a difference from ordinary field theories, because here the modulation is a modulation of probability distributions associated with a signal line voltage, not of a single value for a signal line voltage. Finally, classical signal analysis is very much about Hilbert spaces, because Fourier transforms are very naturally associated with Hilbert spaces, and classical time-frequency analysis is very much used to working with Wigner functions, so that QFT is very close indeed to classical signal analysis.
I imagine no-one will read to here, but to anyone who does, good luck with your own engagement with QFT.

 

[A. Neumaier]

classical time-frequency analysis is very much used to working with Wigner functions, so that QFT is very close indeed to classical signal analysis.

Only QFT in a space consisting of a single point only.

 

[Peter Morgan]

Only QFT in a space consisting of a single point only.

How right you are, , although a four-dimensional version of classical signal analysis, which I am won't to call a random field when it is generalized to a probabilistic formalism and presented in a Hilbert space formalism, is rather close to QFT. Close enough that one can in some cases construct isomorphisms between Hilbert spaces, and, with more construction required, between the respective algebras of operators of the quantum and random fields.

 

[A. Neumaier]

a random field when it is generalized to a probabilistic formalism

Random fields, though describable in terms of Fock spaces, correspond to Euclidean space-time field theories only. Analytic continuation to real (Minkowski) space-time drastically changes the operator structure.

 

[martinbn]

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.

Can you give a reference for one that is not.

 

[Peter Morgan]

Random fields, though describable in terms of Fock spaces, correspond to Euclidean space-time field theories only. Analytic continuation to real (Minkowski) space-time drastically changes the operator structure.

Right, it does drastically change the operator structure. One can construct the following for the quantized Maxwell field on Minkowski space in terms of annihilation and creation operators, $\hat F_{\!f}=a_{f^*}+a^\dagger_f$:

• Introduce a nonlocal involution $\widetilde{f^\bullet}(k)=\frac{1}{2}(1+\mathrm{i}\star)\tilde f(k)+\frac{1}{2}(1-\mathrm{i}\star)\tilde f(-k)$, with $\star$ being the Hodge dual applied to the bivector-valued test function $f$, thereby reversing the frequency of one helicity while leaving the other helicity unchanged.
• construct the field $\hat{\sf F}\hspace{-0.35em}{\sf F}_{\!f}=a_{f^{*\bullet}}+a^\dagger_{f^\bullet}$, which satisfies the trivial commutator $[\hat{\sf F}\hspace{-0.35em}{\sf F}_{\!f},\hat{\sf F}\hspace{-0.35em}{\sf F}_{\!g}]=0$, which I therefore take to be a random field.
  
• Then: The Hilbert space generated by the free action of $\hat{\sf F}\hspace{-0.35em}{\sf F}_{\!f}$ on the vacuum vector $|0\rangle$ is isomorphic to the Hilbert space generated by the free action of $\hat F_{\!f}$ on $|0\rangle$.
• And: The algebra of operators generated by the $\hat{\sf F}\hspace{-0.35em}{\sf F}_{\!f}$ and the vacuum projection operator $|0\rangle\langle 0|$ is isomorphic to the algebra of operators generated by the $\hat F_{\!f}$ and the vacuum projection operator $|0\rangle\langle 0|$.

The last bullet point obviously requires the introduction of the vacuum projection operator, which is nonlocal, however the vacuum projection operator is used implicitly in pragmatic physics whenever a transition probability is computed. All the above and more is spelled out in arXiv:1709.06711, of which a current version, in which the math is unchanged but the text has evolved, but not yet enough to be posted to arXiv, can be found on Dropbox here.

 

[DarMM]

Can you give a reference for one that is not.

I'm not aware of a variant that says the objects don't exist before measurement, most posit "something" is there.

However for the properties we measure not existing in any sense before the observation look to Wheeler's writings or QBism.

 

[Demystifier]

Can you give a reference for one that is not.

See refs. [5,6,7] in my http://de.arxiv.org/abs/1112.2034 .

 

[DarMM]

See refs. [5,6,7] in my http://de.arxiv.org/abs/1112.2034 .

For anybody reading these are:

1. Zeilinger-Brukner information interpretation. In this a quantum system has at most one bit of information and if it uses it on one property, it doesn't have a determined value for others. For example a photon entangled with another will use its bit to store the fact that it is entangled (i.e. the form of its relationship with another system), but then the actual polarization bits aren't stored and are thus random. QM's weirdness results from a mismatch from how much information a quantum system has vs how much we expect it logically to have.
2. Mermin's view is QBism. QM's weirdness results because Bayesian reasoning must be modified to take into account the fact that values for certain observables are new elements of the universe introduced at measurements that don't follow from anything previous
3. Rovelli's relational interpretation. Quantum objects don't possesses objective properties, just properties relative to another system, e.g. this electron has spin up for me.

All share the idea that the quantum probabilities are not determined by hidden properties of the object itself in any way. In fact in all of them the probabilities come from the fact that the object doesn't possesses objective properties. Some drop properties existing altogether, others the objective element.

Note that, from having read them, all elements of each interpretation come up in the others. Brukner has recently found his interpretation needs some facts to be relational. In light of the Frauchiger-Renner theorem probably all anti-realist views require this. Their differences are more how primary those elements are, e.g. in the Relational Interpretation the fact that properties are relational is the reason for QM, in the Information interpretation it's just in some cases measurement results have to be relational as the bit of information can't be "read" in a unique way across all environments.

 

[martinbn]

See refs. [5,6,7] in my http://de.arxiv.org/abs/1112.2034 .

Ok, I have them now. Can you point me to the exact part of the text or do I have to read them? I have looked at [5] and [7] before and I don't remember anything about the quantum objects not existing (the moon not being), so I'll have to look again.

 

[vanhees71]

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?

Sure, although it'll take some time, until I can have a close look at it, because the semester started, and I've to give two lectures to my teachers students (including one about quantum mechanics ;-)).

 

[vanhees71]

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.

It's only one more hint at how far idiosyncratic completely overrated philosophical ideas by otherwise ingenious physicists lead, particularly Heisenberg, to a somewhat lesser extent Bohr, although he was even more enigmatic in his writings than Heisenberg, but usually (i.e., at least as far as I can make sense of his mostly non-mathematical writings about the foundations of QT) he had had a sensible physical intuition...

 

[A. Neumaier]

Bohr [...] had a sensible physical intuition...

So had Heisenberg.

 

[Demystifier]

Sure, although it'll take some time, until I can have a close look at it, because the semester started, and I've to give two lectures to my teachers students (including one about quantum mechanics ;-)).

I have sent you the draft by e-mail.

 

[martinbn]

See refs. [5,6,7] in my http://de.arxiv.org/abs/1112.2034 .

Ok, I looked through them quickly, it is possible that I missed it, but I found nothing that supports your (and atyy) statement.

 

[Demystifier]

Ok, I looked through them quickly, it is possible that I missed it, but I found nothing that supports your (and atyy) statement.

Let us take, for instance, some quotes from "Message of the quantum" by Zeilinger:

But I think that the concept of reality itself is at stake ...
... it is not always possible to assign definite measurement outcomes, independently of and prior to the selection of specific measurement apparatus in the specific experiment. ...
Rather, the assumption that a particle possesses both position and momentum, before the measurement is made, is wrong. Our choice of measurement apparatus decides which of these quantities can become reality in the experiment.
... the distinction between reality and our knowledge of reality, between reality and information, cannot be made. There is no way to refer to reality without using the information we have about it. ...
Maybe this suggests that reality and information are two sides of the same coin, that they are in a deep sense indistinguishable. If that is true, then what can be said in a given situation must, in some way, define, or at least put serious limitations on what can exist. ...

Now you may say that Zeilinger does not say explicitly that things do not exist until measured, but I think that he says it implicitly. Or at least, I do not see any other coherent interpretation of his words compatible with all those sentences. If you see another coherent interpretation of his words, I would like if you could spell it out in a concise and unambiguous form.

Or take e.g. the Bell theorem, which is often presented as a statement that QM is incompatible with local reality. So to save QM and locality, one must give up reality. How would you interpret "giving up reality" if not by saying that things do not exist without observations?

 

[martinbn]

Let us take, for instance, some quotes from "Message of the quantum" by Zeilinger:

But I think that the concept of reality itself is at stake ...
... it is not always possible to assign definite measurement outcomes, independently of and prior to the selection of specific measurement apparatus in the specific experiment. ...
Rather, the assumption that a particle possesses both position and momentum, before the measurement is made, is wrong. Our choice of measurement apparatus decides which of these quantities can become reality in the experiment.
... the distinction between reality and our knowledge of reality, between reality and information, cannot be made. There is no way to refer to reality without using the information we have about it. ...
Maybe this suggests that reality and information are two sides of the same coin, that they are in a deep sense indistinguishable. If that is true, then what can be said in a given situation must, in some way, define, or at least put serious limitations on what can exist. ...

Now you may say that Zeilinger does not say explicitly that things do not exist until measured, but I think that he says it implicitly. Or at least, I do not see any other coherent interpretation of his words compatible with all those sentences. If you see another coherent interpretation of his words, I would like if you could spell it out in a concise and unambiguous form.

But he doesn't say anything that implies that the particle doesn't exists. He only says that the values of some observables cannot exists, which is very different.

 

[A. Neumaier]

If you see another coherent interpretation of his words, I would like if you could spell it out in a concise and unambiguous form.

He says:

There is no way to refer to reality without using the information we have about it. ...

This is quite unambiguous, and is the core of his statement. The subsequent conclusion begins with maybe, hence is not a claim made.

His statement is true of anything we refer to in place of reality, so of knowledge and information itself. So if we do not give the status of reality to a particle because we do not have precise information about its position and momentum, we can neither give the status of reality to knowledge because we do not have precise information about knowledge - it is conceptually more vague than a particle.

 

[Fra]

Are they things that exist in space or are they just mathematical abstractions that help use calculate things?

Is there an actual difference between the options?

There is no way to "see" reality except through a physical observer.

The only difference lies in how you may a build a theory, to relate observers. By assuming that there is a structure somewhere, that is not subject to the constraints of having to be inferred, things get easier of course. But there is no empirical or instrumental justification for that beyond that it seems like the least complicated thing to try, rather than making everything fluid at once.

/Fredrik

 

[Demystifier]

But he doesn't say anything that implies that the particle doesn't exists. He only says that the values of some observables cannot exists, which is very different.

But it's not clear what it means. What exactly are the quantities (or qualities) of the particle that do exist without measurement?

 

[Demystifier]

His statement is true of anything we refer to in place of reality, so of knowledge and information itself. So if we do not give the status of reality to a particle because we do not have precise information about its position and momentum, we can neither give the status of reality to knowledge because we do not have precise information about knowledge - it is conceptually more vague than a particle.

Is it a critique or a defense of the Zeilinger claims?

 

[A. Neumaier]

What exactly are the quantities (or qualities) that do exist without measurement?

Approximate position and approximate momentum. Just like with every classical object in the world.

If we don't know approximately where a particle is, it may not be in the lab - so how can we do an experiment with it?
And if we do not know approximately its momentum, how can we tell whether it will move along a ray where it is supposed
to move, according to a planned experiment?

Thus approximate position and approximate momentum must exist before measurement, or all of our experiments do not make sense.

Interpretational problems appear only if we want to pretend that values exist to an arbitrary precision.

 

[A. Neumaier]

Is it a critique or a defense of the Zeilinger claims?

Both.

 

[Fra]

So if we do not give the status of reality to a particle because we do not have precise information about its position and momentum, we can neither give the status of reality to knowledge because we do not have precise information about knowledge - it is conceptually more vague than a particle.

They way i see this from an inference perpective: I see one distinction here, the information an observer has about something, can not be rated by himself. It's just the defining part in a conditional part of subjective probability.

This is the conceptual trick i personally use to justify statistics, in cases where its obvious that the empirical basis for it does not exist. The statistical basis is rather than possibly only the information the observer has (wether its right or wrong as per somone else is not relevant). In this sense the illusion is always more "first hand" than the real thing. But this does not imply that we have agreement among observer. So if real i supposed to mean "all observers agree" we just transformed the problem into asking - how come the subjective inferences from different interacting observers, tend to magically agree, at least in the case of classical observers?

/Fredrik

 

[A. Neumaier]

to justify statistics, in cases where its obvious that the empirical basis for it does not exist.

In these cases it cannot be justified.

if real is supposed to mean "all observers agree"

I don't think it is supposed to mean that. Real means independent of observers and observations, no matter whether or not they agree. One needs the latter only to check what precisely is real!

 

[Peter Morgan]

There is no way to refer to reality without using the information we have about it. ...

The information we have recorded about an experiment has to be unambiguous and uncontentious, with a trail that an experimenter could follow and reproduce, otherwise Nature or Science will retract the article that depends on the raw data record. We can't get away with saying that our raw data hasn't been reviewed by Wigner's friend yet, so the editors and referees can't look at it.
The trail in a characteristically QM experiment will be from a thermodynamic transition of an engineered device embedded in the apparatus and driven by its surrounding environment, to a change of voltage on a signal line driven by that engineered device, to hardware that identifies that there has been a change of voltage definite enough to qualify as being called a measurement event, then the time of the event has to be obtained from a piece of hardware that has been engineered to act as a clock (characterized as unaffected by its surrounding environment, as far as possible, in contrast to the responsiveness of the measurement hardware), and finally a record will be made in computer memory (used to be, the record would be in a lab book, of the movement of a pointer, but modern data rates make that impossible). Note that the computer record could be less compressed: we could have stored the signal line voltage as a 12-bit number after analog-digital conversion, every nanosecond or every picosecond, allowing significantly more sophisticated post-analysis, but that would increase the data rate too much.
All of the records stored in computer memory are classical raw data, but a process of careful selection from that data and the computation of statistics for the various curated ensembles might allow us to demonstrate that, for example, Bell-CHSH inequalities are violated. This has usually been said to be because the classical raw data was not caused by a classical process, however the violation of some inequality or another as much depends on what selection and computation of statistics is done; certainly the computation from the ensemble is modeled by the operator we use (if we square values that we use an operator $\hat A$ to model, we use $\hat A^2$ to model the different computation); it seems that whether or not the raw data is caused by a nonclassical process, the choice of one operator or another must also model the different possible selection processes from the raw data that allow the computation of statistics.
I see all the above as agreeing with Arnold's #82 and inserting more detail, but perhaps he will disagree.

 

[martinbn]

But it's not clear what it means. What exactly are the quantities (or qualities) of the particle that do exist without measurement?

Well, if the particle doesn't exist, then what do you measure? So, there is no particle, I guess it is vacuum. Then you measure the spin(of what, the vacuum?) and pop the particle exists, but only for a moment, only when measure, then it doesn't exist until the next measurement. That seems a very strange way of describing the situation. And I don't see that, nor anything that would suggest it, in the papers you cited.

 

[Peter Morgan]

Well, if the particle doesn't exist, then what do you measure? So, there is no particle, I guess it is vacuum. Then you measure the spin(of what, the vacuum?) and pop the particle exists, but only for a moment, only when measure, then it doesn't exist until the next measurement. That seems a very strange way of describing the situation. And I don't see that, nor anything that would suggest it, in the papers you cited.

Suppose there are no particles, but the event devices we set up cause identifiable changes of voltage, pops, on signal lines every so often. In a dark room, there is still the "dark rate", but when we turn on the power to a source device in the room the pops happen at a different rate, indeed perhaps with quite different statistics. Suppose we have two event devices in the room, now we can ask about correlations on the signal lines and about whether identifiable pops happen at the same time, and so on.
Now, we know that how many source devices there are and where the source devices and the event devices are in the room changes the event statistics, so we can think of the sources as causing the pop statistics. If we want to say more than that, we have to introduce some new idea: we know that thinking that particles cause the pops is only sustainable if we're willing to adopt something like a de Broglie-Bohm interpretation (which we might or might not be willing to do, or perhaps only sometimes), so we ask whether we might think of the events being caused by a field in the immediate surroundings of each event device. Now there's a lot to do, but the distance between an appropriate random field and a quantum field can be shown to be not very great, so that we can think of QFT as just one way (a useful way) of doing signal analysis in the presence of noise sophisticatedly. Classical noise is correlated across time and space, and we effectively engineer those correlations when we introduce elaborate sources of the kind that cause violations of Bell inequalities, et cetera. Furthermore, noncommutative operators are commonplace in classical signal analysis, so the violation of Bell-CHSH-type inequalities is not a surprise.
I don't expect many people will read this far, and likely no-one will feel compelled by the discussion above, but I have found it very helpful to think that events pop, not particles. Events are correlated because the vacuum is already correlated and because we further engineer events to be correlated in a variety of ways. FWIW, a slogan I've been playing with of late has been to speak of field/event duality instead of wave/particle duality, though that seems sometimes too trite and sometimes just right.
Of course this leaves questions as to why the source and event devices exist as distinguishable objects with trajectories, but for me it's one thing at a time, walk before you try to run, ...

 

[Fra]

In these cases it cannot be justified.

In my understanding it can likely be justified, but it depends on modifying the theory of course, so its not just about interpretations (and its an open question).

The justification is to consider let's say a bayesian kind o observer depedent frequentist intepretation. This is what you get when all observer can not agree on pointer states and classical counters. The event counters themselves are necessarily of the nature that can not be copied. The observer "consumes" the information. Ie. the "evidence" that makes up the probabilistic foundation is observer dependent.

The exception is where we have a classical observer, but this must be a special case of something more general.

I don't think it is supposed to mean that. Real means independent of observers and observations, no matter whether or not they agree. One needs the latter only to check what precisely is real!

Its how i think of it. I can't see another meaniful definition?

To just by real mean independent of observers of observations honestly make no sense to me, because how are you to infer or verify something that is detached from observation? Like the colour of gods underwear? Such a thing would have no implications for experiments, because it would contradict the fact that it was assume to be independent of anything we can observe.

/Fredrik

 

[Fra]

Approximate position and approximate momentum. Just like with every classical object in the world.

If we don't know approximately where a particle is, it may not be in the lab - so how can we do an experiment with it?
And if we do not know approximately its momentum, how can we tell whether it will move along a ray where it is supposed
to move, according to a planned experiment?

Thus approximate position and approximate momentum must exist before measurement, or all of our experiments do not make sense.

Interpretational problems appear only if we want to pretend that values exist to an arbitrary precision.

hmm i suspect this does related to your termal interpretation? But do i understand this to be a FAPP type of interpretation? or does this interpretation supopse to be valid a the future QG theory? (just checking so i don't attempt to overinterpret this)

An approximate value sound like nothing else than the observer expectation. And this in my view exists encoded in the observers microstructure. Ie. its part of the MAP. So do you agree that what exists is an uncertain (~approximate) map, and can be identify this map with the observer? But this "map" is then effectivelt the same thing as an observers "information" about something. And the "approximation" comes only from the fact that the information is not complete and uncertain.

OTOH, if you remove the observer altogether, i don't understand whose average or expectation you refer to as real?

/Fredrik

 

[bhobba]

Can you give a reference for one that is not.

Copenhagen has evolved and changed since Bohr and Einsteins time:
https://arxiv.org/pdf/1511.01069.pdf

These days modern Copenhagenists seem to be switching to Decoherent Histories they call Copenhagen done right just as Feynman did at the end. Gell-Mann thinks it's basically MW without the many worlds ie just one world.

That seems to be the emerging consensus interpretation - but there are many others about - Copenhagen is still the most generally held view according to Sean Carrol:
http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/

Thanks
Bill

 

[bhobba]

I'm not aware of a variant that says the objects don't exist before measurement, most posit "something" is there.

Trouble is in doing that you have to invoke some other process other than Schrodinger Evolution or you run into Kochen-Specker. My interpretation Ignorance Ensemble holds that as the central issue - how does a mixed state become a proper mixed state. Copenhagen ignores it completely. Decoherent Histories simply redirects the issue to histories ie sequences of projection operators.

Thanks
Bill

 

[DarMM]

Trouble is in doing that you have to invoke some other process other than Schrodinger Evolution or you run into Kochen-Specker. My interpretation Ignorance Ensemble holds that as the central issue - how does a mixed state become a proper mixed state. Copenhagen ignores it completely. Decoherent Histories simply redirects the issue to histories ie sequences of projection operators.

Thanks
Bill

I could be completely wrong here, but does one need a "process" as such. In antirealist one views $\psi$ as simply encoding probabilities to see various values of observables and since there is no $\Lambda$ and thus no assumption of no pre-existent values associated with the system this would make the Kochen-Specker theorem irrelevant I think.

My interpretation Ignorance Ensemble holds that as the central issue - how does a mixed state become a proper mixed state. Copenhagen ignores it completely. Decoherent Histories simply redirects the issue to histories ie sequences of projection operators.

(I know you know most of the below, just laying it out to hear your thoughts)

I think this is the central issue in all antirealist (a term I dislike, better would be "no system variables" or similar) interpretations. QBism, Neo-Copenhagen, Ignorance Ensemble, etc basically all say that when decoherence happens one has a value you don't know.

Another way of phrasing it is, when decoherence occurs we obtain usual (Kolmolgorov) probability theory, which we know to interpret as ignorance. The issue is how to interpret the (non-commutative) probabilities prior to decoherence. QBism for example says that prior to measurement the "fact" of the value isn't created yet and QM is a probability calculus for yet-to-exist quantities. However once the value has been created you get the proper mixed state as you simply don't know the value now.

I think you might find the views of Richard Healey interesting. He basically views decoherence as measuring when classical concepts make sense and one then has the ignorance interpretation once it has occured.

Again I think it comes back to what is the meaning of non-decohered states in QM and why is the probability calculus noncommutative.

 

[martinbn]

Copenhagen has evolved and changed since Bohr and Einsteins time:
https://arxiv.org/pdf/1511.01069.pdf

These days modern Copenhagenists seem to be switching to Decoherent Histories they call Copenhagen done right just as Feynman did at the end. Gell-Mann thinks it's basically MW without the many worlds ie just one world.

That seems to be the emerging consensus interpretation - but there are many others about - Copenhagen is still the most generally held view according to Sean Carrol:
http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/

Thanks
Bill

Ok, there are versions of the interpretation. My question is according to which version the fields are not real objects in space. That was the statement that @atyy made.

 

[DarMM]

Ok, there are versions of the interpretation. My question is according to which version the fields are not real objects in space. That was the statement that @atyy made.

@atyy knows better, but some versions of Copenhagen like that of Rudolf Haag would just posit that an observer working in spacetime region $\mathcal{O}$ can measure quantities associated with the C*-algebra $\mathcal{A}(\mathcal{O})$ and that the C*-algebra has a basis consisting of observables of the form $e^{i\phi(f)}$.

This would just see observables as being formed from fields, i.e. one has a map $f: \phi \rightarrow \mathcal{A}(\mathcal{O})$, but without having fields as real objects in the theory.

 

[Demystifier]

Well, if the particle doesn't exist, then what do you measure? So, there is no particle, I guess it is vacuum. Then you measure the spin(of what, the vacuum?) and pop the particle exists, but only for a moment, only when measure, then it doesn't exist until the next measurement. That seems a very strange way of describing the situation. And I don't see that, nor anything that would suggest it, in the papers you cited.

I don't think you answered my question.

 

[Demystifier]

Thus approximate position and approximate momentum must exist before measurement, or all of our experiments do not make sense.

I disagree. For instance, beauty does not exist until one observes it (beauty is in the eyes of the beholder), yet it doesn't mean that observation of beauty does not make sense. Of course, the shape of the beautiful object exists before the observation, but the shape by itself is not beautiful.

Another, more physical example is the color. The EM wave has its wavelength even without observation, but it has a color only when someone observes it.

 

[martinbn]

@atyy knows better, but some versions of Copenhagen like that of Rudolf Haag would just posit that an observer working in spacetime region O\mathcal{O} can measure quantities associated with the C*-algebra A(O)\mathcal{A}(\mathcal{O}) and that the C*-algebra has a basis consisting of observables of the form eiϕ(f)e^{i\phi(f)}.

This would just see observables as being formed from fields, i.e. one has a map f:ϕ→A(O)f: \phi \rightarrow \mathcal{A}(\mathcal{O}), but without having fields as real objects in the theory.

How is this related to whether the fields are real objects in space or not?

I don't think you answered my question.

Do you mean this question?

What exactly are the quantities (or qualities) of the particle that do exist without measurement?

What do you mean by quantities and qualities? It exists and its state can be fully described by a vector in a certain vector space.

 

[martinbn]

But you didn't answer my question. If the particle doesn't exist, does it mean that we have vacuum i.e. empty space?

 

[Demystifier]

It exists and its state can be fully described by a vector in a certain vector space.

OK, now you answered my question so I can proceed. Essentially, you identify the particle with the corresponding state in the Hilbert space. The particle is the state in the Hilbert space. Fine, now let us see what are the consequences of this statement.

Consider a state which before the measurement is a Gaussian wave function with a very large width $\Delta x$ in the position space. So far so good. But now assume that we perform the measurement of the particle position. This means that the wave function changes to a new wave function with a new width $\delta x$ ,where $\delta x\ll \Delta x$. This change is called the wave function collapse. But the collapse is non-local, it happens faster than light. And that's the problem.

A way out of this problem is to say that collapse is not a real physical event, but only the update of our knowledge. However, you are not allowed to use that argument, because you essentially said that the wave function is the particle. This means that the collapse is a real physical event, and not only an update of knowledge.

Do you agree with my sequence of arguments? Do you accept that measurement involves a nonlocal collapse as a real physical event? Do you find such kind of nonlocality problematic?

 

[DarMM]

How is this related to whether the fields are real objects in space or not?

I don't understand, if the fields are not taken as really existing, but just taken as mathematical method for generating basis elements for an observable algebra, then they're not real objects in space right? It's directly related to it by saying they're not real. I genuinely don't understand, it seems fairly clearly related.

 

[DarMM]

I disagree. For instance, beauty does not exist until one observes it

Does this not ignore those who are objectively handsome/beautiful such as us Advisors?

 

[Demystifier]

But you didn't answer my question. If the particle doesn't exist, does it mean that we have vacuum i.e. empty space?

If you think that it is not easy to make sense of interpretation in which particle does not exist until measured, I perfectly agree with you. One needs to work hard to make sense of it. As a result of such a hard work, I wrote the paper http://de.arxiv.org/abs/1112.2034