More evidence that the wavefunction is ontologically real?

[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