Preferred Basis and Superposition

[rkastner]

...So in both cases there is some justification to say that in the big picture, nothing happens at all. Also in both cases, if we apply the theory to an experimental situation, we get non-trivial dynamics of the system of interest relative to the observer and the environment. ...

This sounds a lot like Rovelli's approach, which I call 'radical relationalism'. It's explicitly observer-dependent and in that sense anti-realist. I think my approach is less radical: things really do happen; what exists isn't just relative to who's looking. And you get Von Neumann's 'process 1' as well as the Born Rule out of it. And it's readily extendable into the relativistic domain, which helps to define emission and absorption and thereby the measurement process itself. I'll be dealing with the latter issues in part 2 of my blog post next week at http://fmoldove.blogspot.com/

 

[bohm2]

If you want to pursue it further the book to get is Schlosshauer's:

This is the most recent paper I've seen by Schlosshauer discussing his views on decoherence:

...Thus if we understand the quantum measurement problem as the question of how to reconcile the linear, deterministic evolution described by the Schrodinger equation with the occurrence of random, definite measurement outcomes, then decoherence has certainly not solved this problem, as is now widely recognized. What decoherence rather solves is a consistency problem: the problem of explaining why and when quantum probability distributions approach the classically expected distributions. But this is a purely practical problem, not a game-changer for quantum foundations. To be sure, the picture associated with the decoherence process has sometimes been claimed to be suggestive of particular interpretations of quantum mechanics or to pinpoint internal consistency issues. But it might be safer to say that certain interpretations (such as the Everett interpretation) are simply more in need of decoherence to define their structure...

Entanglement, scaling, and the meaning of the wave function in protective measurement
http://arxiv.org/pdf/1402.1217.pdf

 

[batmanandjoker]

I can quite follow your reasoning here - I suspect its showing that outcomes occur at all which is a really big problem for the decoherehnce approach.

Thanks
Bill

Bhobba is decoherance still a valid explanation?

 

[rkastner]

To clarify, I'm arguing that nobody has really 'derived' decoherence because they are assuming it at the beginning by the so-called 'factoring' in system vs environment. This problem would not be solved by a theorem to the effect that any factorization gives you the right decoherence properties. This is because it would still presume decoherence in order to 'derive' decoherrence. Decoherence cannot happen without collapse, which is what actually decoheres quantum correlations. It is collapse that actally does the decohering, so 'decoherence' without collapse is not a valid explanation of the appearance of definite outcomes.

 

[kith]

This sounds a lot like Rovelli's approach, which I call 'radical relationalism'. It's explicitly observer-dependent and in that sense anti-realist.

Actually, I was talking about the MWI where all final states are equally real. Relative to a certain observer state, only one outcome occurs.

Yes, basically. In decoherence 'derivations', the only interaction between system and environment is assumed to be due to a given interaction Hamiltonian and not due to any pre-existing quantum correlations. So that begs the question.

Well, decoherence is primarily a physical process which is independent of the measurement process. You can make experiments with two state systems and depending on what you do with them, a state tomography either yields a decohered (mixed) state or not. So there's nothing fishy about decoherence or its derivation in the framework of open quantum systems but only about the claim that it solves the measurement problem.

I fear that the discussion might lead to semantics. Ignoring correlations doesn't imply that these correlations are absent in an ontological sense. It might simply be a practical matter, like when we use a thermal state. Such a state might "really" be a reduced state while the whole (larger) system is in a pure entangled state but since we don't check these correlations it doesn't matter. And because of this, many people don't worry about ontology at all.

Having said this, I don't think that deriving decoherence by assuming certain special initial conditions doesn't tell us something. It shifts the question from "how does decoherence happen?" to "why do we have special initial conditions?" which is a well-known -albeit controversial- question from other branches of physics.

 

[rkastner]

It's not a question of semantics. Deriving decoherence by assuming that a system is decohered from its environment, and environmental systems are decohered from one another, is circular. The 'physical process of decoherence' by way of large numbers of degrees of freedom only achieves the appearance of outcomes because the necessary 'decoherence' has been put in at the beginning. Regarding practicality: if all you want is a 'FAPP' -style account, then it's not necessary to purport to 'derive' decoherence in any case, which is an attempt at an ontological explanation.

Alleged derivations of decoherence have indeed been presented as explanations for why we perceive a decohered situation, but the whole point is that they arrive at that alleged 'explanation' by assuming a version of it at the beginning. This is a separate issue from special initial conditions, since they are assuming the very sort of initial condition they're alleging to 'derive' as an emergent condition. The whole program is circular, and presumes what it purports to demonstrate, so it does not in fact demonstrate anything.

 

[bhobba]

Bhobba is decoherance still a valid explanation?

By itself it doesn't solve all the issues - but with some further assumptions it does.

In that sense its perfectly valid.

But there is debate on exactly what that further assumption is and should be.

For example I simply assume the improper state is a proper one, MWI simply assumes each outcome is a separate world, Decoherent Histories does it via frameworks.

Rkastner has another process in mind - but he is the one to explain that rather than me.

Thanks
Bill

 

[bhobba]

This is because it would still presume decoherence in order to 'derive' decoherrence.

You have me beat with that one.

Decoherence cannot happen without collapse, which is what actually decoheres quantum correlations. It is collapse that actally does the decohering, so 'decoherence' without collapse is not a valid explanation of the appearance of definite outcomes.

Come again.

Tracing over the environment is a well defined process and converts a pure state to a mixed state.

The issue is its an improper mixed state - it needs to be a proper one to explain collapse. I simply assume it is - no observation can prove me wrong. The issue though is how it accomplishes this wonderful feat and is closely tied up with why we get definite outcomes.

My understanding is you have an explanation for that which is great - but that's the issue - decoherence doesn't assume collapse - making sense of it does - but that where extra interpretive assumptions come in.

Thanks
Bill

 

[bhobba]

Bhobba: that's not a solution though. Currently the factorization problem is fatal for MWI.

We have been through this before.

Its not fatal because a theorem is required to show it depends on the decomposition.

If you have such a theorem get it published. You will probably win a Nobel prize because quite a few interpretations will go out the window.

Thanks
Bill

 

[bhobba]

Well, decoherence is primarily a physical process which is independent of the measurement process.

It is - but a bit of care is required.

For example tracing over the environment is often invoked to explain it, but its validity requires the Born rule (see Lubos's responce):
http://physics.stackexchange.com/qu...ake-the-partial-trace-to-describe-a-subsystem

It can't be used to explain the Born rule, that would be circular. But providing you avoid that it indeed is independent of the measurement process.

Thanks
Bill

 

[bhobba]

The 'physical process of decoherence' by way of large numbers of degrees of freedom only achieves the appearance of outcomes because the necessary 'decoherence' has been put in at the beginning.

How does tracing over the environment put decohrence in there from the start?

Certainly its validity requires the Born rule - but providing you don't use it to justify the Born rule there is no issue.

Thanks
Bill

 

[bohm2]

Rkastner has another process in mind - but he is the one to explain that rather than me.

He's a she.

 

[rkastner]

It's not tracing over the environment that puts decoherence in from the start. It's effectively presuming from the start that the environment is already decohered from the system of interest, by assuming random phases, that allows you to trace over the environment and get a reduced density matrix that is diagonal wrt nice classical properties. That's what is circular. Then of course pretending that the improper mixture can be epistemically interpreted is helping oneself to a big piece of 'FAPP pie' ;) -- but that's another issue.

BTW my full name is Ruth E. Kastner. In case you're interested, Part 2 of my guest post is now up on Florin Moldoveanu's blog: fmoldove.blogspot.com

 

[rkastner]

Bhobba: it is collapse that explains decoherence. The mistake that decoherentists make is in trying to derive decoherence without collapse. As I've noted, they have to implicitly assume a form of decoherence in order to 'derive' it.
The collapse process I have in mind is the transactional process. See my 2-part guest post on FM's blog (as I posted previously, Part 1 discusses collapse) and also this guest post at the SciAm site for some details: http://blogs.scientificamerican.com...by-stepping-out-of-space-and-time-guest-post/

 

[bhobba]

Bhobba: it is collapse that explains decoherence.

That I agree with ie the Born Rule is required to explain tracing over the environment - as I have mentioned a number of times.

But the improper mixed state after decoherence when the Born Rule is applied leads to APPARENT collapse, meaning it can be assumed to be a proper mixed state in which case collapse would have actually occurred.

BTW by collapse I assume you mean the Born rule. There are interpretations like MW that collapse never even occurs. Also most of the time the system is destroyed by observation, its only in filtering type observations its changed and these days such are considered state preparation procedures.

Thanks
Bill

 

[rkastner]

...

But the improper mixed state after decoherence when the Born Rule is applied leads to APPARENT collapse, meaning it can be assumed to be a proper mixed state in which case collapse would have actually occurred.

I'm sorry, but this is confused. If one has only 'apparent collapse' meaning that collapse hasn't actually occurred, then it hasn't actually occurred -- period. One can't say that an improper mixed state can be assumed to be a proper mixed state based on a collapse that hasn't really occurred. That is just assuming what one wishes to show. As I've said before, of course we all know that we see collapsed outcomes -- that's the empirical fact that needs explaining. Assuming that there is collapse when one only has apparent collapse, just because we applied the Born Rule in a FAPP way, is not an explanation of anything. (Especially when we got a density matrix diagonal in the right basis because that basis was implicitly assumed from the start.)

BTW by collapse I assume you mean the Born rule. There are interpretations like MW that collapse never even occurs. Also most of the time the system is destroyed by observation, its only in filtering type observations its changed and these days such are considered state preparation procedures.

No, this is why I referred you to my blog posts. I'm talking about actual, physical collapse, which is the reason for the Born Rule. You get this from the transactional interpretation.

Thanks
Bill

 

[bhobba]

It's not tracing over the environment that puts decoherence in from the start. It's effectively presuming from the start that the environment is already decohered from the system of interest, by assuming random phases, that allows you to trace over the environment and get a reduced density matrix that is diagonal wrt nice classical properties. That's what is circular.

Hmmm. Don't agree there. To my mind its simple statistical thinking in that if you randomly pick systems then they will have random phases. Still I suppose just like the factoring problem strictly speaking its an assumption that needs justification.

Then of course pretending that the improper mixture can be epistemically interpreted is helping oneself to a big piece of 'FAPP pie' ;) -- but that's another issue.

Of course - anyone that claims it explains ACTUAL collapse is seriously tampering with the truth, and whenever anyone does that on this forum myself or others will point out their error rather quickly.

What it does is explain APPARENT collapse - by which is meant it explains how an improper mixed state comes about. Improper mixed states, when the Born rule is applied, are observationally indistinguishable from proper mixed states. If it was a proper mixed state then the state that is observed is there prior to observation which is much more common sense view of the world. Another way to look at it is the Von Neumann regress showed the collapse (and I am equating collapse with the Born rule) could be placed anywhere. But since the only place that was different is human consciousness that's where Von Neumann placed it and you get this weird consciousness created world. But now we know a place that's different - just after decoherence. For example a few stray photons from the CBMR are enough to decohere a dust particle and convert it to an improper mixed state. By putting collapse right there you have a much more reasonable, commonsense, and rational world ie you can consider dust particles as having actual positions and the existence of a classical commonsense world we experience everyday is much more reasonable.

That's what is meant by apparent collapse. When that is explained some say they don't think apparent collapse is the correct way of describing it. I think its very apt personally, but that view is fine with me, just as long as it's understood what is meant.

Thanks
Bill

 

[bhobba]

I'm sorry, but this is confused. If one has only 'apparent collapse' meaning that collapse hasn't actually occurred, then it hasn't actually occurred -- period.

This is obviously a semantic issue.

The whole point of calling it apparent collapse is that applying actual collapse (ie the Born Rule) leads to it looking like the system was in that state prior to observation and we have a much more commonsense view of the world. That is what is meant.

You asked before if interpretive issues are on topic in this forum. They are - providing they do not slip on over to philosophy.

Semantic discussion of apparent collapse and if it's appropriate to call it that is most definitely philosophy and will be shut down pretty quick by the mods.

Thanks
Bill

 

[rkastner]

Bill the whole point is that you can't apply 'ordinary statistical thinking' and assume 'random systems' with 'random phases' because quantum theory with only Schrodinger evolution doesn't license that. This is the circularity issue.

In QM with only Sch. evolution there are no 'stray photons' unless you assume they are 'stray' (i.e, already have randomized phases) from the beginning. All the talk of this or that random dust particle or stray photon is circular. Yes these are all stray and decohered but not because of 'decoherence derivation' which assumes that from the beginning. In fact all these stray objects are decohered *because* there is real collapse. 'Apparent collapse' into the preferred basis is circular and doesn't explain anything. OK I think this will have to be my last post on this specific topic/thread. I do hope you will take a loook at the blog posts I mentioned for clarification of these issues and what real physical collapse (as opposed to the circular 'apparent collapse') is all about. Thanks to all.

 

[rkastner]

No, not a semantic issue. Applying Born Rule is FAPP collapse, not real physical collapse. Very different things and the distinction is crucial for this issue.

 

[bhobba]

Bill the whole point is that you can't apply 'ordinary statistical thinking' and assume 'random systems' with 'random phases' because quantum theory with only Schrodinger evolution doesn't license that. This is the circularity issue.

You mean to say independent systems prior to interacting and becoming entangled can't be considered to have random phases relative to the system they become entangled with?

 

[bhobba]

No, not a semantic issue. Applying Born Rule is FAPP collapse, not real physical collapse. Very different things and the distinction is crucial for this issue.

The word apparent means exactly that its not real.

Now precisely what have you against placing actual collapse just after decoherence?

Was Von Neumann wrong in saying it can be placed anywhere?

Placing it right there solves many issues. Its not actual collapse - which is one of the issues that actually remain - but a whole heap of others are resolved.

Thanks
Bill

 

[rkastner]

You mean to say independent systems prior to interacting and becoming entangled can't be considered to have random phases relative to the system they become entangled with?

You just said they were independent, which means they have no quantum correlations -- which means they are already presumed as decohered before one starts 'deriving' anything. That's exactly what is circular about the 'derivations'. If there are nothing but quantum correlated systems as implied by only having Schrodinger evolution (with no collapse that could disrupt those correlations) then there are no independent -- as in uncorrelated -- systems, and one can't assume that there are in order to show that things 'decohere' along the desired basis.

 

[Maui]

If decoherence assumes that which it tries to explain, then decoherence is philosophy and just another interpretation(cripped at that).

 

[rkastner]

The word apparent means exactly that its not real.

Now precisely what have you against placing actual collapse just after decoherence?

You can't get decoherence unless you have collapse first -- that's what I've been saying.

Was Von Neumann wrong in saying it can be placed anywhere?

Indeed, and it's because he couldn't figure out why anything physically collapses, so he didn't know where to put it. That's the arbitrary 'Heisenberg Cut'. What I've been arguing is that the transactional picture provides a physical, non-arbitrary place for this 'cut'. And that's what the Born Rule actually applies to.

Placing it right there solves many issues. Its not actual collapse - which is one of the issues that actually remain - but a whole heap of others are resolved.

What I've been arguing is that these 'solutions' are illusory, tendentious, and circular. TI gives a genuine solution. Again I really should end my replies now. I do thank you all for the discussion and I hope you will take a look at the material I've mentioned. Happy to answer questions about it via email, see my blog for contact info: http://transactionalinterpretation.org/

Best wishes, RK

 

[rkastner]

If decoherence assumes that which it tries to explain, then decoherence is philosophy and just another interpretation(cripped at that).

Ouch. No, it's just a bad argument. You've just defined 'philosophy' as a bad argument here. ;) Believe it or not there can be good arguments in philosophy!

 

[bhobba]

You just said they were independent, which means they have no quantum correlations -- which means they are already presumed as decohered before one starts 'deriving' anything.

Come again you lost me there.

You better spell that one out in full detail.

Take two non entangled systems arbitrarily picked, say two photons, its utterly obvious they will have random phase relative to each other. Of course like the factoring problem we need a proof of this but that IMHO is a VERY long bow to pull if you want to disprove the standard textbook treatments of decoherence that way.

I may be wrong, but I can't quite understand why you are arguing this. You have an explanation for actual collapse just prior to decoherence. Explaining actual collapse is a genuine issue and I will one day get your book to check it out. But all this stuff I have been prattling on about is bog standard textbook stuff. You may be like Grete Hermann - a voice crying in the wilderness - but really that's not the most likely situation.

So until you can actually PROVE it to me I think the most reasonable explanation is the textbooks are correct.

Thanks
Bill

 

[rkastner]

How do you know they are nonentangled?

 

[bhobba]

You can't get decoherence unless you have collapse first -- that's what I've been saying.

And on that I agree. No argument.

Can we move on please where you detail your actual mechanism for collapse.

To me that's much more interesting.

Thanks
Bill

 

[bhobba]

How do you know they are nonentangled?

Its a general assumption.

For example do you believe stray photons from the CBMR interacting with a dust particle are entangled?

Thanks
Bill