Quantum Interpretation Poll (2011)

[tom.stoer]

I think one should ad the relational interpretation of quantum mechanics introduced by Rovelli http://en.wikipedia.org/wiki/Relational_quantum_mechanics + references therein

 

[Rap]

"Quantum Reality #8. The duplex world of Werner Heisenberg (The world is twofold, consisting of potentials and actualities) Most physicists believe in the Copenhagen interpretation, which states that there is no deep reality (QR #1) and observation creates reality (QR #2). What these two realities have in common is the assertion that only phenomena are real; the world beneath phenomena is not.

You misunderstand the Copenhagen interpretation. The CI does not deny deep reality, it simply does not deal with a reality that cannot be experienced scientifically. By scientifically, I mean by repeatable measurements (but not repeatable results of those measurements). If deep reality were God, CI is not atheistic, it is agnostic. "Atheist" means "there is no God", agnostic means "I don't know".

It follows that CI does not say that "observation creates reality" since it is agnostic on the subject of reality. CI does not say that "the world beneath phenomena is not real", it is simply agnostic on the subject.

CI says that "objective reality" as we experience it is a classical concept, a classical approximation, and to cling to it without justification is improper. If there is an objective reality in the quantum realm, it has yet to be discovered, and it likely will not be found by trying to concoct a theory which attempts to jam the classical concept into the quantum realm.

 

[yoda jedi]

But there is still a possibility that the observer and projection postulate in Copenhagen can be literal, isn't it? (Copenhagen just assumes it's calculation aid... but there is possibility it is real). Or has this idea been totally refuted already? How?

copenhagen without observer/measurements, without collapse ?

all versions of copenhagen involve a collapse.
without observer/measurement no collapse.


projection postulate dropped:
many worlds, bohmian mechanics, modal approach.


.

 

[Rap]

The logic is like this. In the deprived world of Copenhagen where they only know it as calculational tool, collapse may seem strange...

As one who favors the CI, this reminds me of the theory of planetary orbits. Medieval scholars believed the planets were pushed around by angels. Then Newton came up with orbital mechanics - a calculational tool to calculate the orbits of the planets without recourse to angels. The theory was "deprived" of its angels. Trying to recover "reality" from the CI is like trying to recover the angels, wondering what goes on in their minds that causes them to push the planets around according to an inverse square law. I mean, sure, maybe angels really are pushing the planets around according to the equations of orbital mechanics, but most physicists don't bother with the idea, since it is not a concept that can be decided by any repeatable measurement. Its not scientific. In particular, the "many worlds" interpretation is especially aggravating to one who favors the CI. It is a completely untestable theory that makes the most simplistic attempt to recover the angels, because, well, there just HAS to be angels, doesn't there?

 

[rodsika]

As one who favors the CI, this reminds me of the theory of planetary orbits. Medieval scholars believed the planets were pushed around by angels. Then Newton came up with orbital mechanics - a calculational tool to calculate the orbits of the planets without recourse to angels. The theory was "deprived" of its angels. Trying to recover "reality" from the CI is like trying to recover the angels, wondering what goes on in their minds that causes them to push the planets around according to an inverse square law. I mean, sure, maybe angels really are pushing the planets around according to the equations of orbital mechanics, but most physicists don't bother with the idea, since it is not a concept that can be decided by any repeatable measurement. Its not scientific. In particular, the "many worlds" interpretation is especially aggravating to one who favors the CI. It is a completely untestable theory that makes the most simplistic attempt to recover the angels, because, well, there just HAS to be angels, doesn't there?

What I'm simply saying is that we must take Copenhagen literally. Heisenberg himself said in 1958:

"Now, the theoretical interpretation of an experiment starts with the two steps that have been discussed. In the first step we have to describe the arrangement of the experiment, eventually combined with a first observation, in terms of classical physics and translate this description into a probability function. This probability function follows the laws of quantum theory, and its change in the course of time, which is continuous, can be calculated from the initial conditions; this is the second step. The probability function combines objective and subjective elements. It contains statements about possibilities or better tendencies ('potentia' in Aristotelian philosophy), and these statements are completely objective, they do not depend on any observer; and it contains statements about our knowledge of the system, which of course are subjective in so far as they may be different for different observers. In ideal cases the subjective element in the probability function may be practically negligible as compared with the objective one. The physicists then speak of a 'pure case'."

Heisenberg said the probability function or wave function combines objective and subjective elements. Here objective means that before measurement the wave function really exist in some netherworld. And Collapse is real too. In Copenhagen, Bohr is pragmatic of it all (meaning he didn't want to explore the nature of what is beneath it all). Hence the origin of "shut up and calculate". Are you siding with Bohr too in the "shut up and calculate" approach?

 

[Rap]

Heisenberg said the probability function or wave function combines objective and subjective elements. Here objective means that before measurement the wave function really exist in some netherworld. And Collapse is real too. In Copenhagen, Bohr is pragmatic of it all (meaning he didn't want to explore the nature of what is beneath it all). Hence the origin of "shut up and calculate". Are you siding with Bohr too in the "shut up and calculate" approach?

Again, I interpret "objective" as a classical concept. If you must use probability to deal with a classical problem, and then go searching for "hidden variables", you will always find them. This is not true in QM. The wave function is a calculational tool, just as Newtons law of gravitation is a calculational tool, it exists in the classical world and correctly predicts the outcome of future measurements, therefore it is objective. The idea that it "really exists" in some netherworld is not a scientific statement, it is untestable. The collapse is objective in the same sense. Regarding "shut up and calculate", the question arises "shut up about what?" Shut up about untestable, and therefore unscientific, ponderings? Yes, shut up. Shut up about searching for the next insight, looking at QM from every available perspective, trying to understand what the **** is going on, no, never.

In what sense is relativity not a case of "shut up and calculate"? Relativity has similar problems. Relativistic physics exists in an unchanging four-dimensional spacetime. There is no way to calculate what is "now". You cannot use relativity to prove that the present time on Earth is April 22, 2011, some 14 billion years after the birth of the universe. That fact is outside the theory, much like the wavefunction collapse. Like the wavefunction collapse, it is where words like "consciousness" start to pop up.

 

[rodsika]

Again, I interpret "objective" as a classical concept. If you must use probability to deal with a classical problem, and then go searching for "hidden variables", you will always find them. This is not true in QM. The wave function is a calculational tool, just as Newtons law of gravitation is a calculational tool, it exists in the classical world and correctly predicts the outcome of future measurements, therefore it is objective. The idea that it "really exists" in some netherworld is not a scientific statement, it is untestable. The collapse is objective in the same sense. Regarding "shut up and calculate", the question arises "shut up about what?" Shut up about untestable, and therefore unscientific, ponderings? Yes, shut up. Shut up about searching for the next insight, looking at QM from every available perspective, trying to understand what the **** is going on, no, never.

In what sense is relativity not a case of "shut up and calculate"? Relativity has similar problems. Relativistic physics exists in an unchanging four-dimensional spacetime. There is no way to calculate what is "now". You cannot use relativity to prove that the present time on Earth is April 22, 2011, some 14 billion years after the birth of the universe. That fact is outside the theory, much like the wavefunction collapse. Like the wavefunction collapse, it is where words like "consciousness" start to pop up.

　
Then you need to hear the words of the master, Nick Herbert who in stated in
Quantum Reality:

"Legendary King Midas never knew the feel of silk or a human hand after
everything he touched turned to gold. Humans are stuck in a similar Midas-like
predicament: we can't directly experience the true texture of reality because
everything we touch turns to matter"
<snip>

"Heisenberg's potential represents a novel kind of physical existence standing
"halfway between the idea of the event and the actual event itself." Until it's
actually observed, a quantum entity must be considered "less real" than the same
entity observed. On the other hand, an unobserved quantum entity possesses "more
reality" than that available to ordinary objects because it can entertain in
potentia a multitude of contradictory attributes which would be impossible for
any fully actualized entity."

<end>

What are the experiment consequence if Heisenberg was right. Well . If one can
totally isolate a cat inside a 100% hypothetical isolation box that can even
shield against the geometry of spacetime. Then the cat would literally turn into
ghost of potentia or possibilities. That's right. Instead of splitting into many
worlds with own independent histories. That cat would be literally in a
superposition. Jesse argued what if the cat was shown drawing of George
Washington and a duck in the 2 quantum choices. Then it would be in a potentia
or literal superposition with both knowledge of them. Here it is possible
becaues the brain won't exist in literal form, but as ghost-like smeared
superposition of brain cells which can belong to both histories that weren't
there.

Many Worlds has a severe problem in the "measure" mentioned by Everett. For
example, if we make a photon hit an angled sheet of glass, we can make the
probability of reflection anything we like just by adjusting the angle, say,
1/5. But in many worlds, splitting is equal. So how is it distributed. This is
the severe "measure" problem in Many Worlds that is even harder than the
collapse postulate. One can treat the collapse postulate arbitrariness just like
the constants of nature values being arbitrary. They are simply part of the
world. Here Many Worlds is thus refuted.

 

[Rap]

　
That cat would be literally in a
superposition. Jesse argued what if the cat was shown drawing of George
Washington and a duck in the 2 quantum choices. Then it would be in a potentia
or literal superposition with both knowledge of them. Here it is possible
becaues the brain won't exist in literal form, but as ghost-like smeared
superposition of brain cells which can belong to both histories that weren't
there.

What if instead of a cat, it was a person. Upon opening the box, how would the person describe the experience?

 

[rodsika]

What if instead of a cat, it was a person. Upon opening the box, how would the person describe the experience?

Even the cat example is not possible because no macroscopic object can be
shielded from the geometry of spacetime. Supposed it could be shield. Well. We
would be in ghost smeared state too. This is obvious because there is no
geometry of spacetime inside the box, to define positions, so no positions would
exist. Hence the person would describe the experience as living in limbo or
ghost-like like in dreamy state where no definite things occur. This is possible isn't it.
Refute this.

 

[exponent137]

I think that QM is not finished, so its interpretation is also not yet finished.
This option was not in the poll.
1. Interpretation for a microscopic (quantum) observer is not known.
2. Quantum gravity is not known, but it is a BASE theory of quantum mechanics. Space and time are outer parameters for QM, what is not correct.
3. Consciousness is not yet explained (and quantum consciousness is not yet rejected by Tegmark).

It is not necessary to introduce hidden variables, but known paramaters should be included and explained, not only as outer parameters.

 

[Rap]

Even the cat example is not possible because no macroscopic object can be
shielded from the geometry of spacetime. Supposed it could be shield. Well. We
would be in ghost smeared state too. This is obvious because there is no
geometry of spacetime inside the box, to define positions, so no positions would
exist. Hence the person would describe the experience as living in limbo or
ghost-like like in dreamy state where no definite things occur. This is possible isn't it.
Refute this.

Please explain "shielded from the geometry of spacetime". I don't understand that. By explain, I mean, describe a process by which an object can be shielded from the geometry of spacetime, or describe an experimental procedure to determine whether an object is shielded from the geometry of spacetime.

 

[rodsika]

Please explain "shielded from the geometry of spacetime". I don't understand that. By explain, I mean, describe a process by which an object can be shielded from the geometry of spacetime, or describe an experimental procedure to determine whether an object is shielded from the geometry of spacetime.

Ignore the above for now while I ask the relativity forum whether spacetime geometry is produced by matter or it is there even without matter.

I've been reading an interesting thread in the archive called "Does Schrodinger's Cat Paradox Suck?" where you participated and exchanged ideas with someone called Ken G.

Ken G. wrote:

The state of the cat must be viewed as a substate of the whole system, it is a projection that does not obey the Schroedinger equation. That equation applies to the closed system on the Hilbert space, not open substates that are projections onto subspaces of the Hilbert space. The subspaces do not preserve the postulates of quantum mechanics (in particular, they evolve into mixed states under decoherence, not superposition states), and this is the source of a lot of misunderstanding about the cat paradox.

Indeed, that is perhaps the key difference between a micro system and a macro system, it is the meaning of the Heisenberg divide: a micro system, as a substate, can recover its status as a pure state by measuring it and isolating it-- even though it remains a substate of something larger, it can be treated as a pure state going forward (and exhibit interference and so on). But a macro system, once evolved into a mixed state via external interactions, can never recover its pure state status, it is forever a substate of something larger, and will never exhibit interference. It is just wrong to say that baseballs don't give two-slit patterns because their wavelengths are too small, they are simply not in pure states period.

You believe that a cat can never be in pure state by principle? But if one used the Heisenberg Interpretation where there is actual ontology. Can the cat exist as pure state? In Copenhagen, maybe it's not possible because they believe the wave function as just calculational tool. Without tracking the cat body down to the atoms and particles. You don't know the wave function to enter into calculation. In this sense. One can't treat the cat as in pure state when completely isolated in 100% hypothetical isolation box. But in Heisenberg Interpretation where everything actually happens. Maybe we can say that the cat is in pure state even if we didn't measure or prepare it to be in pure state from the beginning?? Or is the analysis the same in both Copenhagen and Heisenberg version where the cat can never be in pure state because beside not able to prepare it in pure state, the cat complex body parts can never in principle be in pure state? (Jesse, if you are reading this, pls comment too as we have discussed at length about pure state, mixed state but you are using Copenhagen version. What happens if we use the Heisenberg version where everything *actually* happens (like superposition, collapse) as described).

 

[unusualname]

This poll doesn't include consistent histories, which is by far the most sensible interpretation.

I believe most of the interpretations will be at least partially consistent with the final model of reality, since you can bend an "ontological probability" into all sorts of nonsense, but if your puny evolved mind can accept an "ontological probability" then you might find the whole shebang has a surprisingly simple description.

 

[yoda jedi]

.-observer dependent.


if wave function is regarded as ontologically real, then, there is not need of observer, if the wave function is epistemic, less yet.

If I understand the terms "ontology" and "epistemology", I believe Copenhagen says they are neither. What is the objection?

ontological real is be independent of observer/measurement, a complete description of reality itself.
epistemic is a representation of an observer’s knowledge of reality rather than reality itself.




heisenberg:

a system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system.

"The laws of nature which we formulate mathematically in quantum theory deal no longer with the particles themselves but with our knowledge of the elementary particles. ... The conception of objective reality ... evaporated into the ... mathematics that represents no longer the behavior of elementary particles but rather our knowledge of this behavior."



.

 

[Gordon Watson]

ontological real is be independent of observer/measurement, a complete description of reality itself.
epistemic is a representation of an observer’s knowledge of reality rather than reality itself.




heisenberg:

a system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system.

"The laws of nature which we formulate mathematically in quantum theory deal no longer with the particles themselves but with our knowledge of the elementary particles. ... The conception of objective reality ... evaporated into the ... mathematics that represents no longer the behavior of elementary particles but rather our knowledge of this behavior."

.

Dear yoda, please quote your sources. Good info is better info when sources are given.

 

[Rap]

You believe that a cat can never be in pure state by principle? But if one used the Heisenberg Interpretation where there is actual ontology. Can the cat exist as pure state? In Copenhagen, maybe it's not possible because they believe the wave function as just calculational tool. Without tracking the cat body down to the atoms and particles. You don't know the wave function to enter into calculation. In this sense. One can't treat the cat as in pure state when completely isolated in 100% hypothetical isolation box. But in Heisenberg Interpretation where everything actually happens. Maybe we can say that the cat is in pure state even if we didn't measure or prepare it to be in pure state from the beginning?? Or is the analysis the same in both Copenhagen and Heisenberg version where the cat can never be in pure state because beside not able to prepare it in pure state, the cat complex body parts can never in principle be in pure state? (Jesse, if you are reading this, pls comment too as we have discussed at length about pure state, mixed state but you are using Copenhagen version. What happens if we use the Heisenberg version where everything *actually* happens (like superposition, collapse) as described).

Well, I don't "believe" any scientific theory. I support the CI until I find something better. The thread you refer to is very good, but we never reached a final conclusion. Ken G. introduced the possibility that the cat is composed of entangled particles and I could never sort that out to our satisfaction. I don't think this is a problem to worry about, but I can't prove it. At this point, I think a cat could be in a pure state in principle, with the position and momenta of the particles narrowly defined, but not more than allowed by the Heisenberg uncertainty principle. This is notwithstanding the fact that to find the wave function of a cat would destroy the cat. This is true classically as well - to determine the position and momenta of every particle of the cat would destroy it. The thing that interests me is the "Wigner's friend" variation of the SC paradox. It is an excellent thought experiment concerning the "reality" of the wave function. If a scientist (Wigner's friend) is locked in a box which contains a box containing the cat, all of which is in a pure state, then Wigner's friend may use a pure wave function to describe the cat, while the scientist outside may use a pure wave function to describe Wigner's friend and the box. If Wigner's friend agrees to open the box and observe the cat at a particular time, then, to the scientist outside the box, Wigner's friend will go into a superposition state of seeing a dead cat and a live cat. The scientist outside the box opens the box, and Wigner's friend reports that the cat is alive. I am quite sure that when Wigner's friend is questioned about the experience, he will report nothing out of the ordinary, there will be no weird experience of having been in a superposed state as described by the scientist outside the box. In other words, I am quite sure that the state of superposition that the outside scientist ascribes to Wigner's friend is not ontological reality, but rather a calculational tool. It encodes his knowledge, just as Wigner's friend's wave function encodes his. When Wigner's friend opens the box, his knowledge changes, his wave function collapses (actually it changes to a mixed state, since he cannot measure the state of the cat without destroying it). The scientist outside the box knows to a high probability that Wigner's friend has opened the box, but there will be no discontinuous change the outside scientist's wave function. When the outside scientist opens the box, his knowledge will change, and his wavefunction will change to a mixed state, for the same reason.

I never really understood all of Ken G's objections to this scenario, but every time I did come to understand what he was saying, he seemed to be correct. He is a CI supporter, so there was never any introduction of untestable assumptions, such as "ontological reality", many worlds, etc., which was a major reason why we could go as far as we did.

 

[rodsika]

Well, I don't "believe" any scientific theory. I support the CI until I find something better. The thread you refer to is very good, but we never reached a final conclusion. Ken G. introduced the possibility that the cat is composed of entangled particles and I could never sort that out to our satisfaction. I don't think this is a problem to worry about, but I can't prove it. At this point, I think a cat could be in a pure state in principle, with the position and momenta of the particles narrowly defined, but not more than allowed by the Heisenberg uncertainty principle. This is notwithstanding the fact that to find the wave function of a cat would destroy the cat. This is true classically as well - to determine the position and momenta of every particle of the cat would destroy it. The thing that interests me is the "Wigner's friend" variation of the SC paradox. It is an excellent thought experiment concerning the "reality" of the wave function. If a scientist (Wigner's friend) is locked in a box which contains a box containing the cat, all of which is in a pure state, then Wigner's friend may use a pure wave function to describe the cat, while the scientist outside may use a pure wave function to describe Wigner's friend and the box. If Wigner's friend agrees to open the box and observe the cat at a particular time, then, to the scientist outside the box, Wigner's friend will go into a superposition state of seeing a dead cat and a live cat. The scientist outside the box opens the box, and Wigner's friend reports that the cat is alive. I am quite sure that when Wigner's friend is questioned about the experience, he will report nothing out of the ordinary, there will be no weird experience of having been in a superposed state as described by the scientist outside the box. In other words, I am quite sure that the state of superposition that the outside scientist ascribes to Wigner's friend is not ontological reality, but rather a calculational tool. It encodes his knowledge, just as Wigner's friend's wave function encodes his. When Wigner's friend opens the box, his knowledge changes, his wave function collapses (actually it changes to a mixed state, since he cannot measure the state of the cat without destroying it). The scientist outside the box knows to a high probability that Wigner's friend has opened the box, but there will be no discontinuous change the outside scientist's wave function. When the outside scientist opens the box, his knowledge will change, and his wavefunction will change to a mixed state, for the same reason.

I never really understood all of Ken G's objections to this scenario, but every time I did come to understand what he was saying, he seemed to be correct. He is a CI supporter, so there was never any introduction of untestable assumptions, such as "ontological reality", many worlds, etc., which was a major reason why we could go as far as we did.

What Ken G was saying was simply that when the cat has other things beside him in the box such as Wigner friend or geiger counter or radioactive source, the cat becomes a subsystem, and in a subsystem, it is no longer in pure state. Hence the cat can't go into superposition.

I don't know if this is really true.

Anyway. Supposed the cat is left alone in the box and completely isolated without any thing beside him. Then he is a complete system, here I wonder if it can be in pure state.

Also what if one uses the Heisenberg Interpretation where everything *actually* happens. Here the wave function exists independently in the cat and geiger counter without the existence of the observer outside which doesn't need any encoding of knowledge or the concept of wave function as mere calculational tool. In such a case, can we say the cat is in pure state using Heisenberg ontological interpretation. Hope Jesse or Ken or other experts can assist here.

 

[yoda jedi]

Dear yoda, please quote your sources. Good info is better info when sources are given.

heisenberg:

a system is completely described by a wave function ψ, representing an observer's subjective knowledge of the system.



Heisenberg, W., 1958, Daedalus 87, 95
"The laws of nature which we formulate mathematically in quantum theory deal no longer with the particles themselves but with our knowledge of the elementary particles. ... The conception of objective reality ... evaporated into the ... mathematics that represents no longer the behavior of elementary particles but rather our knowledge of this behavior."



.

thanks gordon, you are welcome.

Heinsenberg, February 2, 1960 ..."The act of recording, on the other hand, which leads to the reduction of the state, is not a physical, but rather, so to say, a mathematical process. With the sudden change of our knowledge also the mathematical presentation of our knowledge undergoes of course a sudden change."...

Jammer, M., 1974,


.

 

[Rap]

Also what if one uses the Heisenberg Interpretation where everything *actually* happens. Here the wave function exists independently in the cat and geiger counter without the existence of the observer outside which doesn't need any encoding of knowledge or the concept of wave function as mere calculational tool. In such a case, can we say the cat is in pure state using Heisenberg ontological interpretation. Hope Jesse or Ken or other experts can assist here.

Again, you have to deal with Wigner's friend, where two different scientists have different wave functions for the same object. If the wave function is ontologically real, how do you resolve the Wigner's friend problem?

Heinsenberg, February 2, 1960 ..."The act of recording, on the other hand, which leads to the reduction of the state, is not a physical, but rather, so to say, a mathematical process. With the sudden change of our knowledge also the mathematical presentation of our knowledge undergoes of course a sudden change."...

Jammer, M., 1974,

I agree with that, completely. The wave function collapse is a collapse in our uncertainty, not a collapse in something physical.

 

[Einstein Mcfly]

Ensemble interpretation ftw!

 

[Fyzix]

The ensemble interpretation doesn't really interpret anything tho.
Your still left with the same questons, why does the cat die or live?

 

[rodsika]

Again, you have to deal with Wigner's friend, where two different scientists have different wave functions for the same object. If the wave function is ontologically real, how do you resolve the Wigner's friend problem?



I agree with that, completely. The wave function collapse is a collapse in our uncertainty, not a collapse in something physical.

But why did Heisenberg also state that the wave function (probability function) combines objective element. What does he meant by it in the following:

"The probability function combines objective and subjective elements. It contains statements about possibilities or better tendencies ('potentia' in Aristotelian philosophy), and these statements are completely objective, they do not depend on any observer; and it contains statements about our knowledge of the system, which of course are subjective in so far as they may be different for different observers."

Complete context in:

http://www.marxists.org/reference/subject/philosophy/works/ge/heisenb3.htm

 

[rodsika]

Well, I don't "believe" any scientific theory. I support the CI until I find something better. The thread you refer to is very good, but we never reached a final conclusion. Ken G. introduced the possibility that the cat is composed of entangled particles and I could never sort that out to our satisfaction. I don't think this is a problem to worry about, but I can't prove it. At this point, I think a cat could be in a pure state in principle, with the position and momenta of the particles narrowly defined, but not more than allowed by the Heisenberg uncertainty principle. This is notwithstanding the fact that to find the wave function of a cat would destroy the cat. This is true classically as well - to determine the position and momenta of every particle of the cat would destroy it. The thing that interests me is the "Wigner's friend" variation of the SC paradox. It is an excellent thought experiment concerning the "reality" of the wave function. If a scientist (Wigner's friend) is locked in a box which contains a box containing the cat, all of which is in a pure state, then Wigner's friend may use a pure wave function to describe the cat, while the scientist outside may use a pure wave function to describe Wigner's friend and the box. If Wigner's friend agrees to open the box and observe the cat at a particular time, then, to the scientist outside the box, Wigner's friend will go into a superposition state of seeing a dead cat and a live cat. The scientist outside the box opens the box, and Wigner's friend reports that the cat is alive. I am quite sure that when Wigner's friend is questioned about the experience, he will report nothing out of the ordinary, there will be no weird experience of having been in a superposed state as described by the scientist outside the box. In other words, I am quite sure that the state of superposition that the outside scientist ascribes to Wigner's friend is not ontological reality, but rather a calculational tool. It encodes his knowledge, just as Wigner's friend's wave function encodes his. When Wigner's friend opens the box, his knowledge changes, his wave function collapses (actually it changes to a mixed state, since he cannot measure the state of the cat without destroying it). The scientist outside the box knows to a high probability that Wigner's friend has opened the box, but there will be no discontinuous change the outside scientist's wave function. When the outside scientist opens the box, his knowledge will change, and his wavefunction will change to a mixed state, for the same reason.

I never really understood all of Ken G's objections to this scenario, but every time I did come to understand what he was saying, he seemed to be correct. He is a CI supporter, so there was never any introduction of untestable assumptions, such as "ontological reality", many worlds, etc., which was a major reason why we could go as far as we did.

This interesting what you said the cat is composed of entangled particles. And you believed that a cat could be in a pure state in principle, with the position and momenta of the particles narrowly defined, but not more than allowed by the Heisenberg uncertainty principle. Well. I think a cat could only be in pure state in principle using the Many Worlds Intepretation. In Copenhagen it may not be possible if what Ken said is right. I read that thread over and over again for hours and the following statement by Ken seemed to say it all ( what he says is being scrutinized in the thread I just made "Substrates don't evolve according to Schrodinger equations?". ):

Ken wrote:

"But that's what I'm saying isn't true-- even if we start with pure states for each component of the system, when we couple them, the only pure state is now a combined system. The cat is now a substate of that system, and substates don't evolve according to the Shroedinger equation, so they don't evolve unitarily and they don't become superposition states. There is really no such thing as the state of a part of a system, but we as physicists can make correct predictions by using the concept of a mixed state to treat such substates, or in some special circumstances, we have enough information to treat a substate as a pure or superposition state. That ability is quickly lost for the cat in the box, even if it starts out in an impossible-to-know pure state."

If true, it means that Wigner friend inside the box with a box of cat is pure state as a combined system. But Wigner friend is a substate of that system and substrates don't evolve according to the Schroedinger equation. so there is no superposition of any kind state. The paradox doesn't happen.

I need confirmation if this is true. If true. Schrodinger cat can never be in superposition of dead and alive even in principle in the boring world of the Copenhagen. But it is all possible in Many worlds because here the cat and universe can be in pure state by principle.

 

[JesseM]

This interesting what you said the cat is composed of entangled particles. And you believed that a cat could be in a pure state in principle, with the position and momenta of the particles narrowly defined, but not more than allowed by the Heisenberg uncertainty principle. Well. I think a cat could only be in pure state in principle using the Many Worlds Intepretation.

If you have an isolated system like the inside of an ideal perfectly shielded box, then if immediately prior to isolating it you could perform a measurement which measured a complete set of commuting observables for the entire isolated system, this would automatically give you a pure state for the isolated system.

In Copenhagen it may not be possible if what Ken said is right. I read that thread over and over again for hours and the following statement by Ken seemed to say it all ( what he says is being scrutinized in the thread I just made "Substrates don't evolve according to Schrodinger equations?". ):

Ken wrote:

"But that's what I'm saying isn't true-- even if we start with pure states for each component of the system, when we couple them, the only pure state is now a combined system. The cat is now a substate of that system, and substates don't evolve according to the Shroedinger equation, so they don't evolve unitarily and they don't become superposition states. There is really no such thing as the state of a part of a system, but we as physicists can make correct predictions by using the concept of a mixed state to treat such substates, or in some special circumstances, we have enough information to treat a substate as a pure or superposition state. That ability is quickly lost for the cat in the box, even if it starts out in an impossible-to-know pure state."

I think Ken is just saying the cat subsystem is not in a pure state, but the whole isolated system (everything inside the box, assuming the box can keep everything inside completely isolated from outside influences) can be. If you want to imagine a cat in a space suit in a perfect vacuum, then the isolated system could consist of just the cat and its suit. If the cat has external surroundings within the box, then if you divide things up into a cat subsystem plus the rest of the box as its environment, presumably there will be decoherence and the cat subsystem will be modeled as being in something close to a mixed state.

 

[rodsika]

If you have an isolated system like the inside of an ideal perfectly shielded box, then if immediately prior to isolating it you could perform a measurement which measured a complete set of commuting observables for the entire isolated system, this would automatically give you a pure state for the isolated system.

I think Ken is just saying the cat subsystem is not in a pure state, but the whole isolated system (everything inside the box, assuming the box can keep everything inside completely isolated from outside influences) can be. If you want to imagine a cat in a space suit in a perfect vacuum, then the isolated system could consist of just the cat and its suit. If the cat has external surroundings within the box, then if you divide things up into a cat subsystem plus the rest of the box as its environment, presumably there will be decoherence and the cat subsystem will be modeled as being in something close to a mixed state.

Do you agree with Ken that in the Copenhagen there is no way for the cat to be in superposition of being both dead and alive as when Ken stated:

"So my point is, whether we start with a putative (but impossible) pure-state cat, or if we adopt a mixture of pure states with some statistical distribution, doesn't matter for the cat paradox-- because correct quantum mechanics says that once we couple that cat to the mechanism that can kill it, there is no longer any such thing as the state of the cat in quantum mechanics. There is only a projection of the full state onto the cat degree of freedom, but that isn't a quantum mechanical state, it is a classical treatment of a quantum mechanical state. It makes no difference to the quantum mechanics if we now assert that the cat "really is" alive or dead and we have no way of knowing which, or if we assert that we have chosen to treat it that way in our mathematics-- the correct quantum mechanics is completely moot on the point, there is no cat-state wavefunction so there is no superposition of alive or dead."

So you mean decoherence is the explanation why when we couple the cat to the mechanism that can kill it, there is no longer any such thing as the state of the cat in quantum mechanics?

I'ts been 75 long years since the Schrodinger Cat. Let's settle it once and for all as far as Copenhagen interpretation is concerned. We know in many worlds, cat can be both dead and alive.. but we are just focusing on pure Copenhagen for now.

 

[JesseM]

Do you agree with Ken that in the Copenhagen there is no way for the cat to be in superposition of being both dead and alive as when Ken stated:

If he means there is no pure state for the cat I agree, but my understanding of decoherence is that you could still have a reduced density matrix for the cat subsystem, and that although decoherence would drive this reduced density matrix into something close to a mixed state, the interference terms wouldn't quite go to zero so there is still a superposition of sorts. Ken or some other knowledgeable person can correct me if I've misunderstood this stuff though...

 

[rodsika]

Again, you have to deal with Wigner's friend, where two different scientists have different wave functions for the same object. If the wave function is ontologically real, how do you resolve the Wigner's friend problem?

　

Your Wigner friend example can be modeled with a simple illustration Supposed you have a 430 atom buckyball emitted. Since you believe superposition has no ontological reality. Then you think the buckyball will take classical trajectory to the screen? Copenhagen is pragmatic in that it doesn't want to commit to any picture of what happens. If you believe the 430 atom buckyball has classical trajectory and only after a number of trials can give you the interference patterns, then you follow the belief of ensemble or statistical interpretations where it is only meaningful after a number of runs? This is similar to Wigneg friend and the box of cat and another scientist outside the box. Only the calculations make sense and everything is classical from the beginning. Pure state then is just there is something to calculate about. So you believe in the statistic interpretation instead of pure Copenhagen (where imagining what occurs inside is outlawed) ?

I believe it is possible though that superposition is literal, so the buckyball is literally in superpositon of positions. It's like the 430 atom buckyball lost out the classical form and become an apparison or ghost in between emission and detection. Your Wigner friend example can refute this but it can't happen in the first place because of decoherence which makes the substates unable to use the Schroedinger equation and there is no quantum mechanics at all.

On the other hand, those who believe in Many Worlds believe the buckyball gets duplicated in many branches.

Bohmian believes in classical trajectory but they were "push" by the wave function and pilot wave.

 

[Rap]

What does Heisenberg mean by the following:

"The probability function combines objective and subjective elements. It contains statements about possibilities or better tendencies ('potentia' in Aristotelian philosophy), and these statements are completely objective, they do not depend on any observer; and it contains statements about our knowledge of the system, which of course are subjective in so far as they may be different for different observers."

I think he means that, if we agree on a wave function for a system, it encapsulates our knowledge of the system (subjective), but the probabilities that we calculate as we propagate the wave function forward in time, (using e.g. the Schroedinger equation) are objective.

In Copenhagen it may not be possible if what Ken said is right.

If true, it means that Wigner friend inside the box with a box of cat is pure state as a combined system. But Wigner friend is a substate of that system and substrates don't evolve according to the Schroedinger equation.

The thing I objected to was the assumption that the cat and the device were separate systems. This seems arbitrary. I think, in principle, you can have the device and the cat as one pure state, evolving according to the Schroedinger equation. In the case of Wigner's friend, the same applies.

Your Wigner friend example can be modeled with a simple illustration Supposed you have a 430 atom buckyball emitted. Since you believe superposition has no ontological reality. Then you think the buckyball will take classical trajectory to the screen?

No. QM is about measurements and the only reality is that which is revealed by those measurements. If I said the buckyball took a classical trajectory, that would mean I could measure its position and momentum to a high degree of accuracy without appreciably disturbing it, all along its trajectory. Superposition would not be an issue, the wave function would collapse at each measurement. If you say that the position and momentum is unmeasured during its travel, then QM and superposition will apply. But then, the question of whether it was following a classical trajectory is untestable, and is therefore not a proper scientific question.

Your Wigner friend example can refute this but it can't happen in the first place because of decoherence which makes the substates unable to use the Schroedinger equation and there is no quantum mechanics at all.

Decoherence in this case is a mathematical approximation, not a physical occurrence. It says that you can approximately replace a pure wave function by a mixed state: an ensemble of macroscopic observations which do not yield as much information as a wavefunction collapse, and whose probabilities are additive. These observations do not collapse the wave function to an eigenstate, but rather selects one macroscopic observation out of the ensemble of observations. This is what happens when you open the box. You don't collapse the wave function, measuring the position and momenta of every particle of the cat (to within Heisenberg), such a measurement would destroy the cat. You make a much less informative measurement, noting only whether the cat is alive or dead.

 

[A. Neumaier]

But there is no way to describe the system "system + device + observer" unitarily, either.

Why not? The state of the whole universe, including everything, may well have a unitary evolution. It is consistent with everything I know.

 

[rodsika]

I think he means that, if we agree on a wave function for a system, it encapsulates our knowledge of the system (subjective), but the probabilities that we calculate as we propagate the wave function forward in time, (using e.g. the Schroedinger equation) are objective.



The thing I objected to was the assumption that the cat and the device were separate systems. This seems arbitrary. I think, in principle, you can have the device and the cat as one pure state, evolving according to the Schroedinger equation. In the case of Wigner's friend, the same applies.



No. QM is about measurements and the only reality is that which is revealed by those measurements. If I said the buckyball took a classical trajectory, that would mean I could measure its position and momentum to a high degree of accuracy without appreciably disturbing it, all along its trajectory. Superposition would not be an issue, the wave function would collapse at each measurement. If you say that the position and momentum is unmeasured during its travel, then QM and superposition will apply. But then, the question of whether it was following a classical trajectory is untestable, and is therefore not a proper scientific question.



Decoherence in this case is a mathematical approximation, not a physical occurrence. It says that you can approximately replace a pure wave function by a mixed state: an ensemble of macroscopic observations which do not yield as much information as a wavefunction collapse, and whose probabilities are additive. These observations do not collapse the wave function to an eigenstate, but rather selects one macroscopic observation out of the ensemble of observations. This is what happens when you open the box. You don't collapse the wave function, measuring the position and momenta of every particle of the cat (to within Heisenberg), such a measurement would destroy the cat. You make a much less informative measurement, noting only whether the cat is alive or dead.

One can't even put a wooden chair into pure state. So how can you do it with a cat. You have to discard maybe 90% of the information so the cat would become more like a statue and it is very much dead. Lol... Also dead and alive is not like spin of a particle. Dead and alive are already classical.

But what's weird is that in Many World, the cat can be in pure state even without measuring it. Maybe you are trying to imagine a cat in Many world-like setting.

I wonder why a cat can be in pure state in Many worlds while impossible in Copenhagen. Rap or anyone?