Preferred Basis and Superposition

[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

 

[rkastner]

I'm glad! For collapse details, please see fmoldove.blogspot.com, Part 1 (that's last week).

 

[rkastner]

Its a general assumption.

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

Thanks
Bill

But it's an assumption you're not allowed to make in order to derive decoherence, because it implicitly injects decoherence from the start. Yes, as I've said, of course things like stray photons are decohered, but that's an empirical observation that can't be used in order to theoretically 'derive' our empirical observation from only the Schrodinger evolution without any actual collapse that could really decohere things. That's why it's circular.

Do you know Boltzmann's H-theorem purporting to derive the 2nd Law of Thermo? Same deal there. He put in a 'hypothesis of molecular disorder' which implicitly assumed irreversibility, in order to 'derive' irreversibility from reversible laws. Decoherence 'derivations' have exactly the same kind of circularity fallacy.

 

[bhobba]

I'm glad! For collapse details, please see fmoldove.blogspot.com, Part 1 (that's last week).

Yea - that looks valid.

But like all such things we need a way to test it experimentally.

I have said it before, and I will say it again, these discussions about the weirdness of QM, Schrodinger's cat etc leave me cold. To me, the real problem with QM, is pick any issue that annoys you and you can find an interpretation where its not a problem. But we don't have any way to experimentally distinguish them.

Just one point though about Kochen-Specker. Its really a simple corollary to Gleason's Theorem which IMHO presents the issue in a much more general light as to why the Born rule is correct. There are a number of assumptions that go into using Gleason to prove Born, but the most important one is basis independence (ie non contextuality) and IMHO is the central issue.

Mathematically non-contextuality is ugly - if the formalism of QM is correct it more or less implies Born.

Thanks
Bill

 

[bhobba]

Do you know Boltzmann's H-theorem purporting to derive the 2nd Law of Thermo?

Don't know that one.

Not deeply into statistical physics right now.

Thanks
Bill

 

[kith]

I agree with Ruth to a certain extent but I don't agree that the derivations of decoherence are necessarily useless wrt to fundamental questions.

If you derive decoherence for the system density matrix, you usually assume things like
a) the initial state of system & environment is a product state
b) the environment is in a thermal (decohered) state and
c) the environment isn't influenced by the system because it is much bigger.

It doesn't seem unreasonable to me to say that it is precisely the fact that the environment is assumed to be already in a time-independent decohered state which leads to permanent decoherence in the system. I also think that there is a strong connection to the H-theorem.

The crucial difference between classical and quantum statistical mechanics is that in QM, a change of coherence / entropy of the system is possible with ordinary Schrödinger dynamics for the combined system. So the big question for me is, what happens if we acknowledge permanent decoherence as an approximation? Does the neglected recoherence forbid the application of decoherence to the measurement process or can it be included?

I'd like to stress again that above all, decoherence is a well-confirmed measureable process in open quantum dynamics. The process and its theoretical description are not controversial at all. What is controversial is the relation of this description to the measurement problem.

 

[Maui]

It doesn't seem unreasonable to me to say that it is precisely the fact that the environment is assumed to be already in a time-independent decohered state which leads to permanent decoherence in the system.

So, the decoherence of system X takes place because the environment is decohered. And the environment is decohered because... err... it's obvious when one looks at walls and tables and desks. And so the gist of the argument is that the existence of chairs and desks explains the occurrence of permanent decoherence. And those same chairs they are surely not composed of the very same quantum stuff. No, they are not! :)
They are the decoherence chairs, the chairs that bring forth the classical world.

 

[Jilang]

Decoherence is neither instantaneous or non-reversible. The interaction with a macroscopic system however makes it extremely unlikely to be reversible. Think of it like a reversal of a diffusion process.

 

[bohm2]

I think this also the reason why the Everett interpretation which is more in need of decoherence to define their structure fails to deliver the goods as outlined in a number of posts.

Demystifier summarized that point when reviewing the Schwindt paper:

To define separate worlds of MWI, one needs a preferred basis, which is an old well-known problem of MWI. In modern literature, one often finds the claim that the basis problem is solved by decoherence. What J-M Schwindt points out is that decoherence is not enough. Namely, decoherence solves the basis problem only if it is already known how to split the system into subsystems (typically, the measured system and the environment). But if the state in the Hilbert space is all what exists, then such a split is not unique. Therefore, MWI claiming that state in the Hilbert space is all what exists cannot resolve the basis problem, and thus cannot define separate worlds. Period! One needs some additional structure not present in the states of the Hilbert space themselves. As reasonable possibilities for the additional structure, he mentions observers of the Copenhagen interpretation, particles of the Bohmian interpretation, and the possibility that quantum mechanics is not fundamental at all.

Many Worlds proved inconsistent?
https://www.physicsforums.com/blog.php?b=4289

Ilja summarizing his papers made the same point:

MWI in it's current form simply becomes invalid, with or without Born rule, because it does not have an additional structure which is necessary to fix the preferred basis: The papers prove that different choices are possible, and lead to different physics. The Copenhagen intepretation solves this problem with its association of the operators p, q with classical experimental arrangements, but this solution is not available in the Everett interpretation. Thus, to make MWI a (viable) intepretation, you not only have to derive the Born rule, but also have to add some new structure to fix the canonical preferred basis.

Why MWI?
http://onqm.blogspot.ca/2009/07/why-mwi.html

 

[kith]

I think this also the reason why the Everett interpretation which is more in need of decoherence to define their structure fails to deliver the goods as outlined in a number of posts.

I disagree with this and I've outlined in post #29 why I think so.

 

[Dale]

Seems like this thread has reached the end of its useful life.