Copenhagen and Decoherence

[jlcd]

Something puzzling and inconsistent. Decoherence was supposed to remove the need for observers in Copenhagen. But how come Lee Smolin didn't discuss anything about decoherence with regards to the Copenhagenists but only the Everettians in his book "Einstein Unfinished Revolution: The Search for what Lies Beyond the Quantum".

Maximilian Schlosshauer made this important point:
https://arxiv.org/pdf/quant-ph/0312059.pdf page 21

"In this sense, the decoherence program has embedded the rather formal concept of measurement as proposed by the standard and Copenhagen interpretations—with its vague notion of observables that are seemingly freely chosen by the observer—in a more realistic and physical framework. This is accomplished via the specification of observer-free criteria for the selection of the measured observable through the physical structure of the measuring."

But in Smolin book. He described "Some anti-realists believe that the properties we ascribe to atoms and elementary particles are not inherent in those objects, but are created only by our interactions with them, and exist only at the time when we measure them. We can call these radical anti-realists. The most influential of these was Niels Bohr. He was the first to apply quantum theory to the atom, after which he became the leader and mentor to the next generation of quantum revolutionaries. His radical anti-realism colored much of how quantum theory came to be understood."

Smolin never discussed about Decoherence except as solution to Everettian as when he described:

"In recent years some rather radical solutions have been offered to the two big puzzles—the preferred splitting problem and the question of where the probabilities come from. The preferred splitting problem is widely thought to have been solved by an idea called decoherence, which I will explain shortly."

He ended it by: "So it seems that decoherence cannot alone be the key to how probabilities appear in the Everett quantum theory, because it is based solely on Rule 1.:"

Again he didn't discuss decoherence as it relates to the anti-realists Copenhagenists.

If you include the concept of decoherence. Copenhagen becomes observer free so doesn't this remove the need for the anti-realist instance since no observer is needed?

Maybe Lee Smolin was not even aware decoherence can work in Copenhagen? But given he is one of the fathers of quantum gravity, it's unimaginable he was not even aware of this very basic fact.

Or maybe Decoherence as introduced by Maximilian etc is not a widespread mainstream topic that all physicists agree? Some don't, like Smolin? How about you?

 

[charters]

If you include the concept of decoherence. Copenhagen becomes observer free so doesn't this remove the need for the anti-realist instance since no observer is needed?

No, because just as decoherence doesn't solve the "probability problem" in MWI, for similar reasons it doesn't solve the "definite outcomes problem" in other interpretations. So in Copenhagen, definite outcomes still must be associated with some notion of external, non-quantum observers making special interventions into the otherwise unitary evolution of quantum systems.

 

[jlcd]

No, because just as decoherence doesn't solve the "probability problem" in MWI, for similar reasons it doesn't solve the "definite outcomes problem" in other interpretations. So in Copenhagen, definite outcomes still must be associated with some notion of external, non-quantum observers making special interventions into the otherwise unitary evolution of quantum systems.

I have thought of this when reading Maximilian paper but there is one big difference. In old Copenhagen. You need observer to collapse the wave function. With decoherence added. The environment can cause decoherence to the system. Here observer is no longer needed to choose the observable or basis. It is the environment that chooses the preferred basis. And many Worlds or bohmian trajectories is what is needed to create the mixed state or change the improper mixture to proper mixture. Observers are no longer needed or required to choose one of the outcomes. Did you see the subtle distinction?

 

[charters]

If you are talking about many worlds or Bohmian trajectories, it's not Copenhagen any more. Copenhagen is defined by using observers to collapse states, which generates definite outcomes.

 

[jlcd]

If you are talking about many worlds or Bohmian trajectories, it's not Copenhagen any more. Copenhagen is defined by using observers to collapse states, which generates definite outcomes.

But it is the environment that can cause decoherences into branches (all definite outcomes). In original Copenhagen, observers were required to collapse the wave function of the system without regards to the any environment that it treated classicaly (not quantum). Is there any modern statement that Copenhagen observers were required to choose one of the outcomes?

 

[charters]

Copenhagen is and has always been about collapsing the state to a single definite outcome (which of course gives you the basis for free anyway).

 

[jlcd]

Copenhagen is and has always been about collapsing the state to a single definite outcome (which of course gives you the basis for free anyway).

I'll show why it is attainable. Page 20

https://arxiv.org/pdf/quant-ph/0312059.pdf
"In brief, the entanglement brought about by interaction with the environment could even be considered as making the measurement problem worse. Bacciagaluppi (2003a, Sec. 3.2) puts it like this:
Intuitively, if the environment is carrying out, without our intervention, lots of approximate position measurements, then the measurement problem ought to apply more widely, also to these spontaneously occurring measurements. (...) The state of the object and the environment could be a superposition of zillions of very well localised terms, each with slightly different positions, and which are collectively spread over a macroscopic distance, even in the case of everyday objects. (...) If everything is in interaction with everything else, everything is entangled with everything else, and that is a worse problem than the entanglement of measuring apparatuses with the measured probes."

How can observers manually distinguish all the branches? They can't.

Only the automatic unitary of Many worlds or Bohmian trajectories can do it without any observers.

Comments?

 

[charters]

I have no idea what you are trying to argue now. Sounds like you may just not believe Copenhagen works well, and are trying to re-define "Copenhagen" to mean "many worlds or Bohm" which would be extremely unnecessarily confusing.

 

[jlcd]

I have no idea what you are trying to argue now. Sounds like you may just not believe Copenhagen works well, and are trying to re-define "Copenhagen" to mean "many worlds or Bohm" which would be extremely unnecessarily confusing.

I mean let's take the case of everyday objects in the room or city. There are milions of superpositions. How can observers manually distinguish them to arrive at one outcome? At least in Many worlds or Bohmians. It's automatic and doesn't need human observers. By observers you mean human?

 

[PeroK]

I mean let's take the case of everyday objects in the room or city. There are milions of superpositions. How can observers manually distinguish them to arrive at one outcome? At least in Many worlds or Bohmians. It's automatic and doesn't need human observers. By observers you mean human?

In most cases you don't need to distinguish. QM does not predict that you might find a table in one room or another room. The uncertainty washes out at the macroscopic level to become practically indistinguishable. The specific position of every particle in a table does not effect what it looks like or where it is.

Even in the classical model, for example, the table is mostly empty space and the atoms are moving due to thermal energy. But that's not what you see.

 

[jlcd]

In most cases you don't need to distinguish. QM does not predict that you might find a table in one room or another room. The uncertainty washes out at the macroscopic level to become practically indistinguishable. The specific position of every particle in a table does not effect what it looks like or where it is.

Even in the classical model, for example, the table is mostly empty space and the atoms are moving due to thermal energy. But that's not what you see.

The point is. Human observers were not required in post Copenhagen with decoherence as you can't observe every processes and the billions of superpositions in the room alone.. so how come Smolin still argued that Born Copenhagen is about "Some anti-realists believe that the properties we ascribe to atoms and elementary particles are not inherent in those objects, but are created only by our interactions with them, and exist only at the time when we measure them."?

 

[PeroK]

The point is. Human observers were not required in post Copenhagen with decoherence as you can't observe every processes and the billions of superpositions in the room alone.. so how come Smolin still argued that Born Copenhagen is about "Some anti-realists believe that the properties we ascribe to atoms and elementary particles are not inherent in those objects, but are created only by our interactions with them, and exist only at the time when we measure them."?

Human observations were never needed in Copenhagen.

I have no insight into what Smolin believes or why.

 

[jlcd]

Human observations were never needed in Copenhagen.

I have no insight into what Smolin believes or why.

If humans were not needed in Copenhagen. Then what caused the selection of the single outcome in the superposition? This is why I said with decoherence presence, I think what Maximilian meant was we need other interpretations like Many Worlds, Objective Collapse or Bohmian. Meaning Copenhagen no longer has any mechanism to cause the collapse to single outcome after the post decoherence era.

 

[PeterDonis]

If humans were not needed in Copenhagen. Then what caused the selection of the single outcome in the superposition?

We don't know. But whatever it is, it can't be decoherence, because decoherence does not select a single outcome out of a superposition of outcomes; it just ensures that the different outcomes in the superposition don't interfere with each other.

Copenhagen no longer has any mechanism to cause the collapse to single outcome after the post decoherence era.

Copenhagen never had a "mechanism" to cause the collapse. It just said that when a measurement takes place, and you observe the outcome, you change the wave function you are using to describe the system to the one corresponding to the outcome that was observed. Decoherence does not change any of that.

 

[jlcd]

We don't know. But whatever it is, it can't be decoherence, because decoherence does not select a single outcome out of a superposition of outcomes; it just ensures that the different outcomes in the superposition don't interfere with each other.
Copenhagen never had a "mechanism" to cause the collapse. It just said that when a measurement takes place, and you observe the outcome, you change the wave function you are using to describe the system to the one corresponding to the outcome that was observed. Decoherence does not change any of that.

Before the discovery of decoherence, what chose the basis was the classical apparatus and it collapsed the wave function. The outcomes were only few because one was focusing on the system only so tractable. After the decoherence era. The outcomes were zilliions, so it's no longer the classical apparaturs or observer and the outcomes no longer tractable. Therefore Copenhagen is already outdated. But why do people still treat Copenhagen as valid? It has no mechanism to choose one of the outcomes in post Decoherence era. Many worlds and Bohmians have mechanisms.

If Copenhagen has no mechanism and the classical apparatus or divide is no longer true. Then physicists should declare Copenhagen is already outdated. Without this declaration. Even experts like Smolin think it's still valid.

-----------------------------
Writers must write more accurate facts now. Writers can't earn much now because most are available at Library Genesis (including Smolin's). So the books now should be more about accurate fact and not just exagerrations to make it sell.

 

[Nugatory]

Even experts like Smolin think it's still valid.

Yes, but in this context "valid" means "very useful for thinking about results without getting bogged down in interpretational issues". I'm invoking Copenhagen when I say that the particles deflected upwards by a Stern-Gerlach experiment are spin-up, and that's all that I need to prepare a beam of particles in that state. Only a total masochist would reject this analysis when they care only about the final state.

 

[PeterDonis]

After the decoherence era. The outcomes were zilliions

Not for the same experiments as above. What possible outcomes there are depends on the experiment. Decoherence doesn't change any of that. If you had an experiment like passing a photon through a polarizer, where there are only two possible outcomes (goes through, doesn't go through), decoherence doesn't add any more. And if you have an experiment like diffraction, where the individual particle could end up at any of a huge number of places, that was just as true before decoherence as after.

so it's no longer the classical apparaturs

Sure it is. Decoherence doesn't change that either. If the apparatus is a polarizer, for example, the direction it's oriented determines the eigenbasis of the experiment. That's just as true after decoherence was discovered as before.

Copenhagen is already outdated

If it is, it's not for any of the reasons you give, since those reasons aren't valid. See above.

 

[PeterDonis]

It is the environment that chooses the preferred basis

This is not correct. It's the apparatus that chooses the preferred basis--as in the example of the polarizer in my last post. The environment just facilitates decoherence by providing a huge number of degrees of freedom that cannot be individually tracked in any practical sense.

 

[jlcd]

Not for the same experiments as above. What possible outcomes there are depends on the experiment. Decoherence doesn't change any of that. If you had an experiment like passing a photon through a polarizer, where there are only two possible outcomes (goes through, doesn't go through), decoherence doesn't add any more. And if you have an experiment like diffraction, where the individual particle could end up at any of a huge number of places, that was just as true before decoherence as after.

Can you please give an example where Bohr would give different analysis by not using decoherence and one where modern decoherence analysis would be used?

Sure it is. Decoherence doesn't change that either. If the apparatus is a polarizer, for example, the direction it's oriented determines the eigenbasis of the experiment. That's just as true after decoherence was discovered as before.
If it is, it's not for any of the reasons you give, since those reasons aren't valid. See above.

 

[PeterDonis]

Can you please give an example where Bohr would give different analysis by not using decoherence and one where modern decoherence analysis would be used?

I think you're missing my point; I don't think the emergence of decoherence changes the Copenhagen interpretation. So I would not expect Bohr's analysis of a particular experiment to be any different from a modern Copenhagen proponent's analysis.

 

[jlcd]

I think you're missing my point; I don't think the emergence of decoherence changes the Copenhagen interpretation. So I would not expect Bohr's analysis of a particular experiment to be any different from a modern Copenhagen proponent's analysis.

Thanks for this thought. I have read Maximilian articles over and over again to get his points. I understood his arguments but just want to look at it from different point of views.

In Tegmark 100 years of the quantum. The purpose of Decoherence was put in context.. which is:

"The above-mentioned experimental progress of the last few decades was paralleled by a new breakthrough in theoretical understanding. Everett’s work had left two crucial question unanswered: first of all, if the world actually contains bizarre macrosuperpositions, then why don’t we perceive them? The answer came in 1970 with a seminal paper by Dieter Zeh of the University of Heidelberg, who showed that the Schroedinger equation itself gives rise to a type of censorship. This effect became known as decoherence, because an ideal pristine superposition is said to be coherent."

So decoherence is very useful for pondering why objects don't show macroscopic superposition.
I'd like to ask something in Stephen Hawking book: in “The Nature of Space and Time”.

“Roger is worried about Schrodinger’s poor cat. Such a thought experiment would not be politically correct nowadays. Roger is concerned because a density matrix that has |cat alive⟩|cat alive⟩ and |cat dead⟩|cat dead⟩ with equal probabilities also has |cat alive⟩+|cat dead⟩|cat alive⟩+|cat dead⟩ and |cat alive⟩–|cat dead⟩|cat alive⟩–|cat dead⟩ with equal probabilities. So why do we observe either cat alivecat alive or cat deadcat dead? Why don’t we observe either cat alive+cat deadcat alive+cat dead or cat alive−cat deadcat alive−cat dead? What is it that picks the alive and dead axes for our observations rather than alive + dead and alive - dead. The first point I would make is that one gets this ambiguity in the eigenstates of the density matrix only when the eigenvalues are exactly equal. If the probabilities of being alive or dead were slightly different, there would be no ambiguity in the eigenstates. One basis would be distinguished by being eigenvectors of the density matrix. So why does nature choose to make the density matrix diagonal in the alive/dead basis rather than in the alive + dead / alive – dead basis? The answer is that the |cat alive⟩|cat alive⟩ and |cat dead⟩|cat dead⟩ states differ on a macroscopic level by things like the position of the bullet or the wound on the cat. When you trace out over the things you don’t observe, like the disturbance in the air molecules, the matrix element of any observable between |cat alive⟩|cat alive⟩ and |cat dead⟩|cat dead⟩ states average out to zero. This is why one observes the cat either dead or alive and not a linear combination of the two. This is just ordinary quantum mechanics. One doesn’t need a new theory of measurement, and one certainly doesn’t need quantum gravity”

Hawking reasoned it was just ordinary quantum mechanics. But in Many Worlds. We don't know why the dead and alive basis was chosen in the overall decomposition of the state vectors. Remember Demystifier wrote this in one of his paper detailing that the dead and alive decomposition was not unique. What is the reason why it is problematic in many worlds but not in ordinary quantum mechanics which Hawking was reasoning above?

 

[PeterDonis]

In Tegmark 100 years of the quantum

in Stephen Hawking book: in “The Nature of Space and Time”

Pop science books are not valid sources. If you want to have a PF discussion, please find an actual peer-reviewed paper.

Thread closed.