More evidence that the wavefunction is ontologically real?

[Ilja]

Sorry - but I don't get that at all eg why even the concept of 'prepared' is applicable to wave-function needs detailing before you can even introduce it in that context. In modern times state and preparation procedure are pretty much synonymous but that requires detailing of what it is in the first place - which is very interpretation dependant.

The point is that in some interpretations, like dBB, the measurement process is covered by the theory. And the essential point of preparing the wave function of a system is to put the trajectory of the measurement device into the wave function of system + device. (In this sense, the argument is indeed interpretation-dependent - which is natural, because I don't know how to make arguments which would make sense in interpretations I reject as nonsensical, like many worlds.)

But in this case, if you want to apply this to the preparation itself, and not just to a measurement, which, then, leads to a subsequent preparation of the system after the measurment, you have to start with some assumptions about the unprepared wave function.

Fortunately, this is not as problematic as it looks like, because in this case you can use epistemic considerations to define the initial wave function. You know nothing, thus, can try to find out which wave function or state accurately describes this knowledge.

The point of the argument is that after the preparation measurement, the effective wave function depends on the trajectory of the measurement device, which is something really existing even in the Copenhagen interpretation.

 

[Ilja]

But, to what do the values correspond? What is it that actually "exists" that the values describe?

In dBB theory it is simply the trajectory of the measurement device used during the preparation procedure.

 

[bhobba]

So, if this paper is claiming that it has demonstrated that the wave function is as equally "real" as is the "pure" quantum state of a physical system... as "real" as any underlying reality that exists... is it arguing that "reality" in general is fundamentally informational?

No. Its quite simple really. All its doing, assuming its valid of course, is, similar to PBR, showing that if its real in a weak sort of wishy washy sense it must be real in a stronger sense.

If its not real in any sense at all, such as for example its simply a state of knowledge like in Copenhagen, then it says nothing.

Thanks
Bill

 

[carllooper]

No. Its quite simple really. All its doing, assuming its valid of course, is, similar to PBR, showing that if its real in a weak sort of wishy washy sense it must be real in a stronger sense.

If its not real in any sense at all, such as for example its simply a state of knowledge like in Copenhagen, then it says nothing.

Thanks
Bill

Why is that a "state of knowledge" be considered as not real? Or why would such a state (whether its be called real or not) not say anything?

Copenhagen can be regarded as a kind of user interface to quantum physics, but for users with an otherwise classical disposition. If one is not classically predisposed to begin with (as many now are), the Copenhagen user interface may not work.

Every now and then Bohr let's out a little frustrated squeal in his otherwise measured responses to Einstein. For Bohr, it is obvious that one can not entertain any classical interpretation of the experimental data. But almost as immediately as this is uttered it is withdrawn. Bohr's main approach is to frame the situation from the point of view of a classical user. Bohr adopts what is called a "quasi-classical" interpretation.

In this approach, the reality or otherwise of what is happening (that which classical thought entertains as comprehensible), is put to one side. Instead of talking in terms of an invisible reality behind observations, Bohr will talk in terms of what is visible instead - the actual detections taking place. Bohr is an empiricist in this regard.

Bohr's effort seems to be aimed at a kind of an awareness that should become obvious - that the more you look at those dots in an interference pattern - that whatever "reality" is behind such a pattern must be a "reality" impossible to reconcile with notions of reality as entertained by classical physics. Bohr is often genuinely perplexed as to why Einstein isn't getting with the program.

But instead of proposing some alternative reality to classical notions of reality Bohr will stick with empericism. He sticks with the user (the classical user) and how the situation can be managed from that perspective. Bohr introduces a dividing line between the classical user and whatever "reality" (be it classical or quantum) might be entertained as behind such. From the point of view of the classical user there are classical assumptions in operation, many of which can actually be employed. For example, in classical physics there is nothing wrong with collecting data and doing a statistical analysis on such. There's nothing wrong with using probability theory to determine where a classical particle might be. He knows full well that the particle in question can't possibly be a classical particle but he allows the classical user to entertain that fiction.

He will speak in terms of (quasi) classical particles. He will frame an interpretation that, as far as possible, maintains classical assumptions. It will be up to the user to realize that there really is a limit on how far you can actually push such assumptions. He wants the user to discover for themselves the truly astonishing nature of the physical world.

Is this a trick?

No. It is simply respect for the history of physics - and the current framework of such (at that time) - that physics progresses in a very conservative way - ie. by not introducing radical changes if there is not any need to do so. But he and Heisenberg were many times tempted to do just that. They refrained.

It is often said that Bohr is not a realist - but it entirely depends on what one means by realism. In Bohr there is an emphasis on what is visible (empiricism) and in empiricism, what is visible is real - it is the primary reality. Bohr is a realist. Just not in the sense that others mean by such.

This is not subjectivism. It is not some mind over matter model. It would only be subjectivism if you maintained that the visible was subjective. But in empiricism the visible is not subjective. On the contrary the visible is treated as an objective thing. It is photography, invented 50 odd years earlier, that makes this understandable. The visible as an objective thing. As a real thing. Realism.

C

 

[bhobba]

Why is that a "state of knowledge" be considered as not real?

BTW that's 'subjective knowledge'. Because it resides in the head of a theorist.

If you think that's real - go ahead - we don't argue philosophy here - but I doubt most would agree.

Thanks
Bill

 

[jfizzix]

So, this leaves me with the question... Is the information content of the quantum state what is objectively "real". Is it, in fact, all that is "real"?

Caution: Metaphysics is not my forte'.

I don't know what is truly objectively "real"; nobody does.
I do know what over time has become a set of concepts useful for reckoning with what I experience through my senses.
Over time, I have come to believe that there are aspects of the physical world that I experience through my senses that are independent of anyone's particular sensory experience (i.e., objectively determined).
In that sense, I believe the Moon is objectively "real", though it's not something that can truly be "proven" in the mathematical/logical sense.

With that in mind, I think that if there is an objective "reality",
then the information determining physical "reality" is objectively defined,
and anything uniquely defined from that information is objectively "real" as well.

That's really all I'm prepared to say without being forced to define what "reality" is.

 

[Feeble Wonk]

No. Its quite simple really. All its doing, assuming its valid of course, is, similar to PBR, showing that if its real in a weak sort of wishy washy sense it must be real in a stronger sense.

Thanks for the direction to PBR as a reference. I hadn't read much about it previously. It's giving me a different way to think about things. Very interesting.

 

[craigi]

So let's assume the wave function is real. does that mean that qm needs no further interpretations? (consciousness, many worlds, etc)

No.

If the wavefunction is real then we exclude the Copenhagen, Ensemble and Rovelli's Relational interpretation. The differences in other interpretations still address other foundational issues.

The MWI would have to be considered the most natural interpretation, but I think most (about half) of the leading theorists and cosmologists have already come round to that anyway.

Those who still cling to the Copenhagen or Ensemble interpretations, because they find the parallel universes conclusion of the MWI too troubling, would probably seek solace in Consistent Histories.

 

[carllooper]

BTW that's 'subjective knowledge'. Because it resides in the head of a theorist.

If you think that's real - go ahead - we don't argue philosophy here - but I doubt most would agree.

Thanks
Bill

Hmmm, where did I suggest knowledge is subjective?

Oh, that's right. I didn't.

Quite the contrary.

While knowledge can certainly occupy the head of a theorist, that doesn't mean it necessarily originates there. And there is nothing in Copenhagen to suggest it does.

But otherwise, if what we're talking about is "subjective knowledge" then yes, of course, by definition it would not be real (and somewhat irrelevant in a physics forum).

However I wasn't addressing subjective knowledge, even if Bhobba was. I was addressing the suggestion that Copenhagen would be proposing such a thing. It doesn't. Nor do I.

Indeed, in what I said, I explicitly rejected a subjectivist interpretation of knowledge, and I quote what I said:

This is not subjectivism. It is not some mind over matter model. It would only be subjectivism if you maintained that the visible was subjective. But in empiricism the visible is not subjective. On the contrary the visible is treated as an objective thing. It is photography, invented 50 odd years earlier, that makes this understandable. The visible as an objective thing. As a real thing. Realism.

So when discussing a photograph, for example, the emphasis from a physics point of view would not be on what a photograph might represent (be it classical reality or some other reality) but on the photograph in itself (as a visible material reality thing in itself). And one can describe this visible material thing in terms of concepts such as particles of light. And one can elaborate those concepts using the photograph (the visible/material thing) as a constraint on such elaborations. If we remove this constraint the elaborations could very well become that 'dreaded' thing we otherwise call "philosophy" (I'd suggest).

C

 

[bhobba]

Hmmm, where did I suggest knowledge is subjective?

I never claimed you did. I claimed I did in reply to the assertion:

Why is that a "state of knowledge" be considered as not real?

Thanks
Bill

 

[carllooper]

I never claimed you did. I claimed I did in reply to the assertion

I don't quite follow. Here is the post in question:

BTW that's 'subjective knowledge'. Because it resides in the head of a theorist.
If you think that's real - go ahead - we don't argue philosophy here - but I doubt most would agree.

On the one hand this post is a clarification of what Bhobba meant by 'knowledge', ie. he meant subjective knowledge:

BTW that's 'subjective knowledge'. Because it resides in the head of a theorist..

And that's fair enough. So the clarification means we're really talking about two different things. Subjective knowledge on the one hand (Bhobba) and objective knowledge on the other (me).

But then Bhobba goes on to clearly suggest that I might like to think 'subjective knowledge' (what he is talking about) is real, and challenges me to go ahead and argue that case:

If you think that's real - go ahead - we don't argue philosophy here - but I doubt most would agree.

In response to this obvious barb I am merely saying that there is no indication, whatsoever, in my post, that I would hold such a position, or would be even remotely interested in arguing such a position. If we're talking about two different things, then that is what we're doing. There is no need to re-conflate the two.

What I go on to pursue in my post is an objection to the populist idea that Copenhagen be understood as a subjectivist interpretation (regardless of who may, or may not, hold such an idea).

C

 

[zonde]

He will speak in terms of (quasi) classical particles. He will frame an interpretation that, as far as possible, maintains classical assumptions. It will be up to the user to realize that there really is a limit on how far you can actually push such assumptions.

Copenhagen attributes wavefunction to single particle. And Einstein's answer was ensemble interpretation.

He wants the user to discover for themselves the truly astonishing nature of the physical world.

So you have discovered "the truly astonishing nature of the physical world" and think that that was as intended by Bohr? Maybe cool down?

 

[carllooper]

Copenhagen attributes wavefunction to single particle. And Einstein's answer was ensemble interpretation.

So you have discovered "the truly astonishing nature of the physical world" and think that that was as intended by Bohr? Maybe cool down?

I entirely agree with what is said about Copenhagen and Einstein.

As for the second part, Bohr's intentions come nowhere near what I think.

C

 

[carllooper]

Einstein treats the wave function (or psi-function as he calls it) as an "ensemble" description of something more detailed to be found or inferred. By an ensemble description, Einstein means by such a description that such a description is incomplete, meaning that the description is more like some sort of average than characteristic of any reality underlying such.

Bohr treats the wave function as describing a single particle. But this does not in any way stop an ensemble of such particles producing a composite wave function no different from a single one. If the experimental setup remains a static one there will be no difference between a wave function representing individual particles and one representing heaps of them. The obvious benefit with associating the wave function with a single particle is that it becomes a lot easier to model dynamic experimental setups, where there is only a trickle of detections.

Another ensemble approach, which Bohr does not pursue, but is compatible with Bohr's empiricism, is to treat the distribution of particle detections as no less a "phenomena" as each individual particle detection.

This would differ from Einstein's ensemble in the sense that there is no more detailed information to be found by subdividing such up - on the contrary it would lead to a loss of information. By analogy, a digital photograph can be regarded as an ensemble of pixels, but if you subdivide the photograph up you just end up with unrelated pixels. The information we might have otherwise assigned the ensemble is lost. There are relationships between the pixels that are not to be found in any particular pixel - that are not a function of the pixels. On the contrary, the pixels are a function of the ensemble (so called).

Carl

 

[bhobba]

Einstein treats the wave function (or psi-function as he calls it) as an "ensemble" description of something more detailed to be found or inferred.

Its exactly the same as Ballentine and its a standard interpretation - in fact its the one I hold to:
http://en.wikipedia.org/wiki/Ensemble_interpretation

Its actually quite similar to Copenhagen:
http://en.wikipedia.org/wiki/Copenhagen_interpretation
The subjective view, that the wave function is merely a mathematical tool for calculating the probabilities in a specific experiment, has some similarities to the ensemble interpretation in that it takes probabilities to be the essence of the quantum state, but unlike the ensemble interpretation, it takes these probabilities to be perfectly applicable to single experimental outcomes, as it interprets them in terms of subjective probability.

Its basically the difference between Bayesian and Frequentest view of probability.

Einstein's and Bohr's interpretations were in fact not that dissimilar - the key difference was Bohr considered the state a complete description - Einstein most definitely did not.

Thanks
Bill

 

[andresB]

Unfortunately Copenhagen sometimes isn't explained well - the following fixes that:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

Thanks
Bill

From the text"In practice, everyone can use pretty much the same wave function. But in principle, the wave function is subjective. If the observer A looks at a quantum system S in the lab, he will use a wave function where S has a well-defined sharp spin eigenstate as soon as the spin of S is measured by A. However, B who studies the whole system A+S confined in a lab won't "make" any collapse, and he evolves both S and A into linear superpositions until B measures the system. So A and B will have different wave functions during much of the experiment. It's consistent for B to imagine that A had seen a well-defined property of S before it was measured by B - but B won't increase his knowledge in any way by this assumption, so it is useless. If he applied this "collapsed" assumption to purely coherent quantum systems, he would obtain totally wrong predictions."So, where I can read more bout that?

 

[carllooper]

Its exactly the same as Ballentine and its a standard interpretation - in fact its the one I hold to:
http://en.wikipedia.org/wiki/Ensemble_interpretation

Its actually quite similar to Copenhagen:
http://en.wikipedia.org/wiki/Copenhagen_interpretation
The subjective view, that the wave function is merely a mathematical tool for calculating the probabilities in a specific experiment, has some similarities to the ensemble interpretation in that it takes probabilities to be the essence of the quantum state, but unlike the ensemble interpretation, it takes these probabilities to be perfectly applicable to single experimental outcomes, as it interprets them in terms of subjective probability.

Its basically the difference between Bayesian and Frequentest view of probability.

Einstein's and Bohr's interpretations were in fact not that dissimilar - the key difference was Bohr considered the state a complete description - Einstein most definitely did not.

Thanks
Bill

Thanks Bhobba, that's well summarised.

The only point I'd disagree with is the idea that Bohr would hold either the wave function (the mathematical formalism) to be subjective, or what it represents to be subjective. In Copenhagen (as much as other interpretations) the wave function still represents (or encodes) what is understood as physically taking place.

If we otherwise characterise Copenhagen as a "subjectivist" interpretation we would have to demonstrate in what way that would be the case, either independently of any claims Copenhagen makes, or conversely where in Copenhagen it characterises itself in that way.

I guess what is really at issue is this term "subjective". What is it's function or purpose? How does it enter the language? If it's sole purpose is to suggest that the wave function itself is a representation, then by that definition all interpretations of the wave function would probably call it "subjective" - not just Copenhagen. It is a representation, be it a complete one, or an incomplete one. But the issue isn't (or shouldn't be) the internal reality or otherwise of the wave function itself, but in what it represents. Is what it represents, subjective? Therein we are on firmer ground. In Copenhagen I'd argue that what it (the wave function) represents is not being interpreted or proposed as subjective - or at least not intended to be interpreted that way.

That all said, I do get Motl's take on this. Heisenberg, if not Bohr, employs the term "subjective". Heisenberg's use of the term is in a way that will be agreeable to classical thought and classical realism. To not buck the system (as one might say). To speak in terms that can be understood (at that time, or even now). But it is certainly not in any way meant to suggest that the wave function represents some sort of fantasy. Bohr is quite clear about this. What becomes a fantasy for Bohr is just classical realism. Not realism full stop.

While Bohr might think the wave function (and/or what it represents) is the last word in the matter, that doesn't (of course) mean it is, but nor does it mean that any other word in the matter would look anything like that which a post-Einstein conception might propose.

Carl

 

[bhobba]

So, where I can read more bout that?

I am not quite sure what you want to read more about, but these days Copenhagen, as indicated in the linked article, is a bit dated. Consistent Histories is the more modern take:
http://quantum.phys.cmu.edu/CHS/histories.html

Its generally accepted decoherent and consistent histories are basically the same - but there are a few differences in approach. Interestingly decoherent histories is rather like many worlds without the many worlds.

Thanks
Bill

 

[bhobba]

I guess what is really at issue is this term "subjective".

You and me both.

It's the same thing as Frequentest vs Bayesian probability - you find heated arguments on both sides. Bayesian is subjective - in fact I can't really tell the difference between Copenhagen and the Bayesian interpretation of QM:
http://math.ucr.edu/home/baez/bayes.html

John Baez, who often penetrates to the heart of issues, says it well:
'It turns out that a lot of arguments about the interpretation of quantum theory are at least partially arguments about the meaning of the probability!'

Here is the actual Bayseayn interpretation of QM:
http://en.wikipedia.org/wiki/Quantum_Bayesianism

Thanks
Bill

 

[andresB]

I am not quite sure what you want to read more about,

I was asking about that example that I quoted above, basically the subjectivity of the wave function proved by comparing the measurement of two different observers.

 

[bhobba]

I was asking about that example that I quoted above, basically the subjectivity of the wave function proved by comparing the measurement of two different observers.

That doesn't prove subjectivity - that would be impossible since there are many interpretations where its anything but subjective.

That said - no I don't know of other sources that go down that path.

Thanks
Bill

 

[carllooper]

You and me both.

It's the same thing as Frequentest vs Bayesian probability - you find heated arguments on both sides. Bayesian is subjective - in fact I can't really tell the difference between Copenhagen and the Bayesian interpretation of QM:
http://math.ucr.edu/home/baez/bayes.html

John Baez, who often penetrates to the heart of issues, says it well:
'It turns out that a lot of arguments about the interpretation of quantum theory are at least partially arguments about the meaning of the probability!'

Here is the actual Bayseayn interpretation of QM:
http://en.wikipedia.org/wiki/Quantum_Bayesianism

Thanks
Bill

I'd agree with the quote from John Baez.

The difference between Bayes and Copenhagen is that Bayes is completely compatible with and assumes classical realism (classical probability) whereas Copenhagen ultimately isn't. One can use Bayes in a coin flip experiment, and one can even use Bayes in a quantum theory experiment. But one can't use Copenhagen in a coin flip experiment, unless one is being particularly selective on what aspects of Copenhagen one uses, such as it's Bayesian aspect.

C