Does the Observer have anything to do with conscious choice

[IamQ]

I have seen many programs on Quantum Physics suggesting that we are the Observer that causes the collapse of the wave function. When I look into the experiments it seems more like the thing that causes the collapse of the wave function is other waves smashing into them. For instance in the double slit experiment the observer is not just watching the electrons as we would with our eye. It is a machine that throws photons at the electrons to figure out where they are. It seems stupid to me to assume that the observer is what is causing the collapse of the wave function when the observer is throwing rocks at the wave. Hence the collapse of the wave function is a physical action not just a conscious measurement.

Thoughts?

 

[arkajad]

The question is "when" the collapse really occurs (if it occurs at all). If you study mathematical models of the collapse, you see that there are different possibilities, and that there is no logical contradiction if you assume that the collapse occurs only after information interacts with consciousness realizing that "an event happened". Another question is is such a prolongation of the chain necessary? This is an unsolved problem. One possible hypothesis is that there is an inherent irreversibility already present at a micro level. If so, then consciousness may not be really necessary. But then the question is where it is exactly? Some physicists assume that the crucial element are creation-annihilation processes (as for instance in "The Transactional Interpretation of Quantum Mechanics: The Reality of Possibility" by Ruth Kastner). So, all possibilities are still open. The fact that something sounds "silly" is not a proof that it is wrong. It may happen to be true as well.

 

[bhobba]

I have seen many programs on Quantum Physics suggesting that we are the Observer that causes the collapse of the wave function.

Its a semantic difficulty some people get into because of words used. Its interpretation dependent but I will give you the standard Copenhagen view and its modern take. You can read about other views like the MWI but I believe this is the simplest, easiest way of looking at it that avoids the most difficulties. I would strongly suggest coming to grips with it before branching out and investigating others.

In Copenhagen an observation is when a quantum system leaves a mark here in the commonsense macro world of everyday experience. It is assumed it exists out there regardless of if anyone is looking at it etc etc. The answer to does a sound get produced when a tree falls in a forest is assumed to be yes - everything is the usual view of the world we take for granted in science. Note any object capable of registering, leaving a mark or whatever is an observer - it does not have to be organic, conscious or anything like that. This is the key semantic difficulty people get confused with.

Schrodinger's Cat, which you have probably heard of, is utterly trivial. The quantum system leaves its mark here in the macro world at the particle detector. Everything is commonsense classical from that point on - the cat is alive or dead - not in some weird superposition until the lid is lifted - it makes no difference if the lid is lifted or not - the observation has occurred at the particle detector.

Now we are faced with the real central difficulty of QM, and what the real import of Schrodinger's Cat is - not the junk you read in populist QM is magic accounts. We have assumed a classical world exists out there but since everything is quantum how does this happen? What we really want is a fully quantum theory of measurement without this assumption. Great progress has been made to that end with the understanding we now have of decoherence. What is now known is that observation is a form of entanglement and the reason the everyday classical world exists is because it is constantly being observed and entangled with its environment. Einstein asked Bohr do you really believe the moon is not there when it's not being observed - the answer is its never not being observed - even the cosmic background radiation is enough to give it the classical properties of definite location, momentum etc etc - and calculations show this occurs very very quickly - even a few stray photons is enough to decohere a dust particle and give it classical properties.

You can get the detail of this modern view from Leonard Susskind's lectures on entanglement:
http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall

Lest I attract the ire of some of the very knowledgeable people that post on this forum issues do remain such as the so called factoring problem, and you can read about them by doing a search. But the above view is much clearer and cleaner than the usual rubbish you get elsewhere.

Thanks
Bill

 

[arkajad]

Great progress has been made to that end with the understanding we now have of decoherence.

The problem with decoherence is that it introduces subjectivity into physics from the very start, when an arbitrary and to a large extent totally subjective decision about what exactly is "environment" is being made.

 

[bhobba]

The problem with decoherence is that it introduces subjectivity into physics from the very start, when an arbitrary and to a large extent totally subjective decision about what exactly is "environment" is being made.

This is the so called factoring problem I mentioned previously. Key theorems are lacking showing the preferred basis does not depend on the decomposition. I read a paper a while ago that showed for a simple model it didn't matter - regardless of how it was partitioned it made no difference - the same basis was singled out. What is needed are key theorems on when it does apply and when it doesn't and exactly what the situation is with the world of everyday experience.

Interestingly standard textbooks such as Schlosshauer do not even mention it:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

It really seems to only have traction around here. Why that is I don't know but if someone wanted to contact Schlosshauer about it and post what he says that might prove interesting.

Thanks
Bill

 

[arkajad]

Key theorems are lacking showing the preferred basis does not depend on the decomposition.

One should also distinguish "does not depend" FAPP from "usually does not matter" and from "does not depend at all, exactly and always". Also: "does not matter under which exactly assumptions" and "are they reasonable assumptions"?

 

[bhobba]

One should also distinguish "does not depend" FAPP from "usually does not matter" and from "does not depend at all, exactly and always".

Sure - the key point though is it needs further investigation.

I believe its simply dotting the i's and crossing the t's sort of stuff - but one never knows. They thought similar things about black-body radiation and the failure to detect an aether at the end of the nineteenth and start of the twentieth century - and we know how that turned out.

Thanks
Bill

 

[IamQ]

So, based on the responses it seems that while there is no facts that go against the hypothesis that consciousnesses can collapse the wave function through the simple action of observation, there is also no facts that support the idea that it requires consciousness instead of just physical interaction between quantum waves.

 

[StevieTNZ]

It was suggested by Euan Squires and Eugene Wigner (and von Neumann) that conscious perception is where collapse of the wave function occurs. As David Chalmers has suggested, one can probe where collapse occurs using a Mach-Zehnder interferometer. However, in our email discussion he didn't elaborate on any specific experimental set-up.

 

[bhobba]

Since the time of Von Neumann and Wigner a lot of work has been done on decoherence and it is now pretty obvious that is the place to put the collapse - not in consciousness. Wigner held to the consciousness causes collapse view but when he heard of some early work of Zurek on decoherence realized it was no longer necessary and abandoned it - not that it was necessary before but the evidence against it was now much stronger. The basic reason he held to it was the collapse can be placed anywhere and consciousness was the only place different if you followed that back. Decoherence however shows exactly the most obvious place to put it - and that is well before the conscious observer.

This consciousness causes collapse thing is basically solipsism in another guise. I can't prove you wrong for holding it but you are led to a pretty cockeyed view of the world if you do and most people reject it as being silly - which it is. Its simply the mystical new age touchy feely types that want to use QM as the basis for their views such as we create our reality, the unconscious gaia mind and the other, well to be blunt rot, they go on about that adhere to it. I don't think there is are many physicists or mathematicians that holds to it these days - they would be in a very small minority.

If you want to see the state of idiocy it leads to have look at the movie What The Bleep Do We Know:
http://en.wikipedia.org/wiki/What_the_Bleep_Do_We_Know!?

When watching it keep the view of QM I have presented here in the fore and contrast it to what they are saying.

Thanks
BGill

 

[bhobba]

So, based on the responses it seems that while there is no facts that go against the hypothesis that consciousnesses can collapse the wave function through the simple action of observation

That is a matter of opinion. Like I said Wigner, who was the high priest of consciousness causes collapse, abandoned it when he found out about some early work of Zureck on decoherence. The fact that after decoherence a pure state is converted to an improper mixed state strongly suggests that is where the collapse occurred. Without delving into technicalities it also suggests that actual collapse doesn't really occur - but is only 'apparent':
http://en.wikipedia.org/wiki/Wave_function_collapse#Quantum_decoherence

'Everett's many-worlds interpretation deals with it by discarding the collapse-process, thus reformulating the relation between measurement apparatus and system in such a way that the linear laws of quantum mechanics are universally valid; that is, the only process according to which a quantum system evolves is governed by the Schrödinger equation or some relativistic equivalent. Originating from Everett's theory, but no longer tied to it, is the physical process of decoherence, which causes an apparent collapse. Decoherence is also important for the consistent histories interpretation. A general description of the evolution of quantum mechanical systems is possible by using density operators and quantum operations. In this formalism (which is closely related to the C*-algebraic formalism) the collapse of the wave function corresponds to a non-unitary quantum operation.'

Thanks
Bill

 

[StevieTNZ]

Decoherence doesn't solve the measurement problem. Nothing in the current QM formalism can. You mention that it might appear as collapse occurs - but really, has it? QM says no.

I adhere to the consciousness collapses wave function view, and although it may sound silly it can't be outright dismissed on that ground.

 

[rkastner]

The question is "when" the collapse really occurs (if it occurs at all). If you study mathematical models of the collapse, you see that there are different possibilities, and that there is no logical contradiction if you assume that the collapse occurs only after information interacts with consciousness realizing that "an event happened". Another question is is such a prolongation of the chain necessary? This is an unsolved problem. One possible hypothesis is that there is an inherent irreversibility already present at a micro level. If so, then consciousness may not be really necessary. But then the question is where it is exactly? Some physicists assume that the crucial element are creation-annihilation processes (as for instance in "The Transactional Interpretation of Quantum Mechanics: The Reality of Possibility" by Ruth Kastner). So, all possibilities are still open. The fact that something sounds "silly" is not a proof that it is wrong. It may happen to be true as well.

Thank you arkajad; actually I do think the problem is solved if you take into account absorption as a real physical process, in a direct action theory. Then 'measurement' IS just absorption and you don't need 'consciousness' to collapse anything -- absorption causes collapse. A past criticism of TI has been that absorption is not well defined, but I define it very carefully in my Chapter 6 -- and it provides a nice account of the transition from the micro- to macro- regimes. Basically any current capable of emitting another type of field is also capable of absorbing another field (this is just 'coupling'). A large enough number of such currents (together with suitable energy conditions) constitute a macroscopic absorber (or emitter). (This discussion is also in my FoP paper: http://arxiv.org/abs/1204.5227)

 

[rkastner]

I have seen many programs on Quantum Physics suggesting that we are the Observer that causes the collapse of the wave function. ...It seems stupid to me to assume that the observer is what is causing the collapse of the wave function ... the collapse of the wave function is a physical action not just a conscious measurement.

Yes, there is a physical process that causes collapse. It is the process of absorption, which is not taken into account in 'mainstream' approaches to interpreting QM, yet which is a basic and crucial process in relativistic QM. Think about it: whenever there is a measurement, something got absorbed. The reason this has been neglected is because it requires a time-symmetric ('direct action') theory in which the advanced field propagation is included, which some people find not to their taste. However, if we understand both retarded and advanced quantum fields as conveying possibilities, the backwards propagation is 'defanged' -- there isn't really any energy propagating backward in time. I discuss all this in my book; also see an introduction here: http://blogs.scientificamerican.com...by-stepping-out-of-space-and-time-guest-post/

 

[bhobba]

Decoherence doesn't solve the measurement problem. Nothing in the current QM formalism can. You mention that it might appear as collapse occurs - but really, has it? QM says no.

Strictly speaking it doesn't but that is not what the advocates of decoherence are claiming. The key issue (and again lest I attract the ire of the knowledgeable there are other issues - but this seems the main one) with decoherence and if it solves the measurement problem has to do with the difference between a proper and improper mixture. They are observationaly indistinguishable, but if it was a proper mixture the measurement problem would be solved. This is what is meant by apparent wavefunction collapse. Since there is no way to tell the difference the easiest, simplest and most obvious solution is to assume it is a proper mixture. Indeed there are some formulations of QM where this distinction is built right into its foundations eg:
http://arxiv.org/pdf/0911.0695v1.pdf
'Axiom 1. (Information capacity) An elementary system has the information carrying capacity of at most one bit. All systems of the same information carrying capacity are equivalent.'

If that is true then an improper and proper mixture are equivalent and the measurement problem is solved.

You are entitled to put it at consciousness, but precisely why you would do that has me beat. Its like solipsism - I can't prove you wrong but since the world view it engenders leads to the sort of rubbish you find in the movie I mentioned one has to wonder - why? BTW I am not suggesting you or any person actually familiar with what QM says holds to the rather extreme views in that movie but it does tend to lead one down that path.

I have discussed decoherence many times on this forum and I know nothing will be resolved one way or another by rehashing it here again.

To the OP - yes one can hold to views like consciousness causes collapse, and a few do, but you don't have to and you avoid a lot of baggage if you don't.

Thanks
Bill

 

[bhobba]

Yes, there is a physical process that causes collapse. It is the process of absorption, which is not taken into account in 'mainstream' approaches to interpreting QM,

Just a note to the OP. What is being talked about here is the Transactional interpretation of QM. It's perfectly valid interpretation, but a minority one.

I personally am not enamored with it - I hold to the decoherence ensemble interpretation:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf
'Postulating that although the system-apparatus is in an improper mixed state, we can interpret it as a proper mixed state superficially solves the problem of outcomes, but does not explain why this happens, how or when. This kind of interpretation is sometimes called the ensemble, or ignorance interpretation. Although the state is supposed to describe an individual quantum system, one claims that since we can only infer probabilities from multiple measurements, the reduced density operator is supposed to describe an ensemble of quantum systems, of which each member is in a definite state.'

Actually the paper above is quite a good summery on the current state of play with the measurement problem and what it is. Basically my view is since a proper mixture is informationally equivalent to an improper one they are equivalent right at the foundations of QM so there is no issue.

To the OP - just my view - as you may have guessed this sort of stuff creates quite a bit of debate that doesn't resolve anything. The key point though is extreme views like consciousness causes collapse are not required - quite a few alternate positions exist.

Thanks
Bill

 

[bhobba]

I discuss all this in my book

Just to be sure I got the right one - is this it:
https://www.amazon.com/dp/0521764157/?tag=pfamazon01-20

I might get a copy - I do enjoy reading about different interpretations of QM.

I just don't like the QM is magic touchy feely stuff - it really leaves me cold.

Thanks
Bill

 

[rkastner]

Yes that's it. thanks for your interest and I hope you enjoy it.

 

[rkastner]

improper and proper mixtures not interchangeable

An improper mixture can't be given an ignorance interpretation, so improper and proper mixtures are not equivalent. One argument is given in R.I.G. Hughes's book Structure & Interpretation of QM and I also discuss improper mixtures in Chapter 5 of my book, in the context of quantum eraser experiments.

TI gives a nice explanation of how you get a proper mixture upon measurement -- basically it just hands you von Neumann's 'process 1' on a silver platter.

Yes, it's a minority view right now -- many people seem allergic to the advanced component -- but Bohm's theory was a minority view several decades ago and that seems to have become mainstream. I used to be a Bohmian, but I found that TI gives a much better account of the Born Rule and is much better suited to bridge the gap between nonrelativistic and relativistic qm.

 

[bhobba]

An improper mixture can't be given an ignorance interpretation, so improper and proper mixtures are not equivalent.

Observationally how do you tell the difference?

That being the case, if you accept the axiom I gave in the link, they are equivalent.

That's not the issue - every person I know accepts they are observationally equivalent - the issue is if you consider that good enough. I do - you don't. This is the key point it hinges on.

I think the paper I linked to on the measurement problem and decoherence lays the issue bare. How is an impoper mixture made a proper one - that's it in a nutshell. I simply do not consider you need a mechanism for that - if they are observationally equivalent then for me that's good enough - measurement problem solved.

Nothing against TI at all - as evidenced by the fact I am interested in getting your book. I think the popularity of interpretations wax and wain. Consciousness causes collapse was fairly popular at one time now its very much a minority. Never liked Bohm - this inherently unobservable pilot wave deliberately cooked up to be so smacks of the aether to me - and just like the aether entirely for the purpose of preserving a particular view of the world. Personally I can live with some the quirkiness of QM without such a crux. Copenhagen seems on the wain right now and MWI on the rise. People are wising up to Copenhagen's problem - namely how the classical world it assumes comes about. I like MWI - very beautiful and elegant - just can't stomach its implications. Also like Consistent Histories - but it strikes me as rather like defining your way out of trouble and, while it claims to be a minimalist interpretation, when you delve into it's anything but - still there is probably some validity to those that say it's Copenhagen done right. I also like the Ensemble Interpretation advocated by Ballentine and detailed in his book - QM - A Modern Development - it's by far the best book I have ever read on QM and is my goto book. Hugh's is pretty good as well but I consider it purely as a warm up for Ballentine who lays exactly what is going on bare with developing it from two axioms. My issue with that lies in Kochen-Specker - namely you can't consider it an ensemble of outcomes - but rather an ensemble of observational apparatus and system combined - reality does not exist at the time of observation but rather only the outcomes of observations exist. That's why I like the ensemble interpretation including decoherence. As a proper mixture it has that property prior to observation.

Thanks
Bill

 

[meBigGuy]

I can't do the math, but it seems the relational interpretations are the most logical. Different observers can see different things to a point. This allows unobserved entities to interact while still remaining "uncollapsed" to the rest of the world.

To take it to the philosophical mumbo-jumbo world, you could say the guy around the corner exists as a probability until you interact with him or something he interacted with.

Can you comment on relational interpretations in a general way?

 

[IamQ]

You Guys rock

Thanks for all the responses, I am one step closer to knowing the secrets of the universe, and I think I understood some of it too.

 

[bhobba]

Can you comment on relational interpretations in a general way?

I am not sure you are up on exactly what the relational interpretation is saying. Wikipedia gives a nice account:
http://en.wikipedia.org/wiki/Relational_quantum_mechanics

For me its a bit redundant - my view of QM includes it anyway, its just not stated explicitly - its more or less understood its like that. And its only of any concern prior to decoherence which generally occurs very very quickly meaning all observers agree on it after that. You undoubtedly can cook up situations where it's not like that but they in general are not what we see in the everyday world around us.

Thanks
Bill

 

[rkastner]

taking the explanandum as explanans

For Bill: the problem is that saying they are 'observationally equivalent' in effect assumes what needs explaining at the theoretical level: namely, how/when/why collapse occurs. That is the measurement problem.

Saying that the two mixtures are "observationally equivalent' is just saying that 'we observe a collapsed situation'. That is the explanandum -- the thing that needs explaining. Invoking observation to 'explain' why we observe a collapsed situation is taking the explanandum as an explanans.

One can't get collapse from the theory itself unless one takes an absorber response into account, which breaks the linearity of the theory. Otherwise, the theory itself will not provide collapse. Bringing in 'consciousness' is of course another tactic, but as others here have noted, it's questionable how scientifically sound that is. As A. Kent has noted, we don't have a good scientific theory of mind, so this opens the door for an 'anything goes' foray into psycho-physical parallelism. Why do this if there is a perfectly good alternative physical formulation (W/F, Cramer, Davies)?

 

[bhobba]

For Bill: the problem is that saying they are 'observationally equivalent' in effect assumes what needs explaining at the theoretical level: namely, how/when/why collapse occurs. That is the measurement problem.

There is no problem. It follows from my favored axiomatic foundation of QM - informationally equivalent systems are equivalent:
http://arxiv.org/pdf/0911.0695v1.pdf

The three axioms which separate classical probability theory and quantum theory from all other probabilistic theories are:

Axiom 1. (Information capacity) An elementary system has
the information carrying capacity of at most one bit. All systems
of the same information carrying capacity are equivalent.

Axiom 2. (Locality) The state of a composite system is completely
determined by local measurements on its subsystems
and their correlations.

Axiom 3. (Reversibility) Between any two pure states there
exists a reversible transformation.

I find those axioms very natural, almost obvious.

If you want to dig deeper than that and explain why - well that's perfectly OK and I have no problem with it. But, as I am sure you are aware, any explanation rests on some assumptions - all you are doing is looking for assumptions you like. Nature may be just like that - who knows. I have found the ones I like and am happy with. That doesn't mean I am not interested in getting your book and reading up on your views - but it does mean that I don't see your approach a prioi necessary in that any of the axioms I use requires explanation.

To put it another way - of course the first axiom sweeps the measurement problem under the rug. But to me that's a good thing.

Think back to SR. Why remove the aether and replace it with the geometry of space-time? Some people detest this saying you need a mechanism for rod shortening etc. But simply accepting the POR obviates the need for this and is very elegant. Same here.

Thanks
Bill

 

[rkastner]

Thanks Bill -- I make a case for a new kind of realism in my book, which I think solves the measurement problem without sweeping it under the rug, and also yields an interesting new picture of quantum reality worthy of this great theory (quantum mechanics). Questions/comments welcome.

 

[bhobba]

Thanks Bill -- I make a case for a new kind of realism in my book, which I think solves the measurement problem without sweeping it under the rug, and also yields an interesting new picture of quantum reality worthy of this great theory (quantum mechanics). Questions/comments welcome.

Really look forward to reading it.

Many people such as Wienberg believe QM is in for a shake up. What you are advocating may just be it.

I am going through David Wallace's Emergent Mulitiverse right now. Interesting but so far it simply doesn't grab me to the point I want to move to the MWI, It's very elegant and beautiful but my gut simply is not saying - this is it - a bit too weird a world view for my liking.

Thanks
Bill

 

[rkastner]

I have to say that MWI does not grab me either.

Looking forward to your thoughts on my book. :)

 

[StevieTNZ]

With regards to decoherence, if an apparatus is in a lab - that is as much as possible isolated from the environment - does the apparatus still experience decoherence due to the 'universal entanglement' (term from http://www.decoherence.de/). Therefore, the apparatus is not isolated from the environment, and never can be if we take Quantum Mechanics seriously?

 

[bhobba]

Therefore, the apparatus is not isolated from the environment, and never can be if we take Quantum Mechanics seriously?

No-one, (to the best of my knowledge anyway) has ever claimed either the system being observed or the apparatus observing the system is isolated from the environment. In fact its the entanglement with it that leads to decoherence and is the modern view of what an observation is.

It is difficult to 'unentangle' systems from the environment - but not impossible. When done some very very strange things happen eg:
http://physicsworld.com/cws/article/news/2010/mar/18/quantum-effect-spotted-in-a-visible-object

Thanks
Bill

 

[rkastner]

Under TI, disentanglement occurs very quickly due to absorption, and resulting collapse. At its root, this is a relativistic process (via field coupling), not taken into account in the usual attempts to interpret QM. This is explained in my book and this paper: http://arxiv.org/abs/1204.5227 We like to think that nonrelativistic QM is a 'stand-alone' theory, but it isn't. There are relativistic processes going on all the time.

 

[Maui]

With regards to decoherence, if an apparatus is in a lab - that is as much as possible isolated from the environment - does the apparatus still experience decoherence due to the 'universal entanglement' (term from http://www.decoherence.de/). Therefore, the apparatus is not isolated from the environment, and never can be if we take Quantum Mechanics seriously?

This explanation assumes that there is an apriori classical environment(made of bullet-like or ball-like particles?) that causes quantum systems to collapse during interaction. It seems decoherence can not go without a theory of measurements(or something like the MWI) and at this point the MWI becomes the only natural sounding option.

 

[rkastner]

This explanation assumes that there is an apriori classical environment(made of bullet-like or ball-like particles?) that causes quantum systems to collapse during interaction. It seems decoherence can not go without a theory of measurements(or something like the MWI) and at this point the MWI becomes the only natural sounding option.

It's true that decoherence cannot account for collapse. However MWI is certainly not the only natural option (for one thing, it has big trouble accounting for the Born Rule in natural physical terms). Have you considered PTI? Intro material here:

rekastner.wordpress.com

 

[bhobba]

It seems decoherence can not go without a theory of measurements(or something like the MWI) and at this point the MWI becomes the only natural sounding option.

It does not seem that way way at all - there are many different ways of dealing with it and even the option the I take of simply saying, basically, it don't matter.

The key issue is the difference between a proper and an improper mixture. MWI assumes nothing happens - we simply partition the universal wave-function into the states of the mixture. The basic axiom of my approach to QM has all systems of the same information carrying capacity equivalent so there is no issue. The TI has another take and still others like GRW, and Consistent Histories exist.

Thanks
Bill

 

[Maui]

It does not seem that way way at all - there are many different ways of dealing with it and even the option the I take of simply saying, basically, it don't matter.

The key issue is the difference between a proper and an improper mixture. MWI assumes nothing happens - we simply partition the universal wave-function into the states of the mixture. The basic axiom of my approach to QM has all systems of the same information carrying capacity equivalent so there is no issue. The TI has another take and still others like GRW, and Consistent Histories exist.

Thanks
Bill

A system's information capacity has what to do with its probability for a given measurement outcome? I don't see a connection. If it's another no-interpretation interpretation, because supposedly that is what you observe and when you shut up and calcule you get the probabilities - ok, but the role of an interpretation is to provide a framework why we observe what we do.

 

[meBigGuy]

I am not sure you are up on exactly what the relational interpretation is saying. Wikipedia gives a nice account:
http://en.wikipedia.org/wiki/Relational_quantum_mechanics

For me its a bit redundant - my view of QM includes it anyway, its just not stated explicitly - its more or less understood its like that. And its only of any concern prior to decoherence which generally occurs very very quickly meaning all observers agree on it after that. You undoubtedly can cook up situations where it's not like that but they in general are not what we see in the everyday world around us.
l

I have read that paragraph, but maybe I'm not understanding it. If you are saying "its more or less understood that it's like that" (which is what I think), why does it have a separate heading? And the description doesn't seem to say it is "like that". Or, is that just your personal view?

The thing I keep getting back to is the interaction of two particles, like an electron and a proton. That interaction gives rise to the probabilities associated with a subsequent observation. But the two particles are interacting, so must be observing each other. The proton "knows" more about the electron than we do since it is interacting and affecting it. We know about the pair as a probability distribution until we actually observe it. That, to my ignorant mind, seems to drive one to a relational interpretation.

 

[bhobba]

I have read that paragraph, but maybe I'm not understanding it. If you are saying "its more or less understood that it's like that" (which is what I think), why does it have a separate heading? And the description doesn't seem to say it is "like that". Or, is that just your personal view?

Different interpretations emphasize different things. Just because one interpretation chooses not to worry about it doesn't meant it doesn't agree with it. It's just not important IMHO because decoherence occurs very very quickly and all observers agree on the resultant mixed state.

The thing I keep getting back to is the interaction of two particles, like an electron and a proton. That interaction gives rise to the probabilities associated with a subsequent observation. But the two particles are interacting, so must be observing each other.

For some reason you don't seem to get an observation is something that leaves a mark here in the macro world.

Thanks
Bill

 

[bhobba]

A system's information capacity has what to do with its probability for a given measurement outcome?

Measurement outcomes convey information. The actual values of an outcome are arbitrary - its the information it conveys that is important. Think of the Stern-Gerlach experiment. It has spin up or down. That coveys a bit of information - what we call the bit on and off is irrelevant. But to be clearer its equivalent to saying systems that are observationally equivalent are equivalent. Information seems to be the buzzword in physics these days. What it means here is since an improper mixed state is observationally the same as a proper one then by the axiom they are equivalent - measurement problem solved.

Thanks
Bill

 

[arkajad]

But to be clearer its equivalent to saying systems that are observationally equivalent are equivalent.

It would be even more clear if we could specify what we mean by a "system". Different axiomatizations may mean different things. Do we mean a "single object" or an "ensemble of objects"?
And under what conditions? People using the same word "system" but with different meaning or undefined at all will argue without realizing that they are talking about different things.

So, one should define "system", "observationally equivalent", "equivalent".

 

[bhobba]

It would be even more clear if we could specify what we mean by a "system". Different axiomatizations may mean different things. Do we mean a "single object" or an "ensemble of objects"? And under what conditions? People using the same word "system" but with different meaning or undefined at all will argue without realizing that they are talking about different things. So, one should define "system", "observationally equivalent", "equivalent".

See the paper I linked to before where the 3 axioms came from. That paper is a bit sloppy but when read with Hardy's seminal paper its a lot clearer:
http://arxiv.org/pdf/quant-ph/0101012v4.pdf

But the terms you mention seem pretty clear.

Can I ask you a question? Is your background math/physics or philosophy? In my experience people that get hung up on what at least seems to me pretty obvious semantics are more likely into philosophy. Its the same sort of stuff you see when people think observation implies an observer. The better books like Dirac and Ballentine are rather careful to use the term measurement instead of observer but once you understand what's meant its not confusing.

Thanks
Bill

 

[arkajad]

See the paper I linked to before where the 3 axioms came from. That paper is a bit sloppy but when read with Hardy's seminal paper its a lot clearer

Axiom 5 (in Hardy, 3 in yours) involves the term "pure state" that has not been defined.

Can I ask you a question? Is your background math/physics or philosophy?

To get my math/physics degree I had to pass an exam in philosophy as well.

 

[bhobba]

Axiom 5 involves the term "pure state" that has not been defined.

It's a standard term in QM and probability theory.

It means a state that can't be expressed as the convex sum of other states. In probability theory that would be the possible outcomes and in QM a state (here state is the general state defined by Von Neumann being a positive operator of unit trace) with only one eigenvector.

My background is applied math so I am not that sensitive to what others think of as semantic imprecision.

Thanks
Bill

 

[arkajad]

It means a state that can't be expressed as the convex sum of other states.

Where is the proof that the set of states is a convex set? It should precede the use of the term "pure state".

 

[bhobba]

Where is the proof that the set of states is a convex set? It should precede the use of the term "pure state".

Its a definition - no proof required.

These are standard terms.

Thanks
Bill

 

[micromass]

Where is the proof that the set of states is a convex set? It should precede the use of the term "pure state".

Where do you need that the set of state is convex in order to define pure states?

 

[bhobba]

Where do you need that the set of state is convex in order to define pure states?

Its in Ballentine page 50 Section 2.3 - its a very simple theorem from the definition of a pure state proven on p52. A state is pure iff it can't be expressed as the convex sum of other states.

Scratching my head why standard stuff like this needs to be explained.

Thanks
Bill

 

[arkajad]

Where do you need that the set of state is convex in order to define pure states?

This is what boobba wrote:

"It means a state that can't be expressed as the convex sum of other states."

"Its a definition - no proof required."

But I do not see the term "convex" in the definition.

 

[micromass]

Its in Ballentine page 50 Section 2.3 - its a very simple theorem form the definition of a pure state proven on p52. A state is pure iff it can't be expressed as the convex sum of other states.

Scratching my head why standard stuff like this needs to be explained.

Thanks
Bill

Sure, I know perfectly what a pure state is. But I fail to see why you need to prove that the states form a convex set in order to define a pure state. A pure state is a state that can't be expressed as the nontrivial convex combination of other states. Nowhere is needed that the states actually do form a convex set.

Sorry if I'm being pedantic.

 

[arkajad]

Its in Ballentine page 50 Section 2.3

But I am not asking about Belinfante's set of axioms. I am asking about about sets of axioms in the two mentioned papers.

 

[arkajad]

Sorry if I'm being pedantic.

It's OK. One should first define "convex combination of states" starting with the axioms and only with them. Otherwise why would we need axioms?