Quantum Interpretation Poll (2011)

[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?

 

[Rap]

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?

If the wave function is "real", having existence independent of an observer, then the very existence of the cat implies a pure state. In the CI, the wave function is the most complete encoding of our knowledge of the cat possible, and is not "real" in the same sense. In classical physics, you cannot know the position and momentum of every particle of the cat without destroying the cat. Nevertheless, you can carry out thought experiments, saying "if I did know the position and momentum of every particle in an isolated system, then I can draw the following conclusions". In QM, I don't see any problem in saying "if I did know the wave function of an isolated many particle system, then I can draw the following conclusions". So in that sense, the cat can be in a pure state in the CI.

 

[rodsika]

If the wave function is "real", having existence independent of an observer, then the very existence of the cat implies a pure state. In the CI, the wave function is the most complete encoding of our knowledge of the cat possible, and is not "real" in the same sense. In classical physics, you cannot know the position and momentum of every particle of the cat without destroying the cat. Nevertheless, you can carry out thought experiments, saying "if I did know the position and momentum of every particle in an isolated system, then I can draw the following conclusions". In QM, I don't see any problem in saying "if I did know the wave function of an isolated many particle system, then I can draw the following conclusions". So in that sense, the cat can be in a pure state in the CI.

Why, is knowing the position and momentum enough to define the pure state in the cat? Or must one know every history of every particle in the body like knowing how it got there since the birth of the cat? Maybe this is why it is difficult even in essence to think a cat can be in pure state because you don't know all the history of every particle. To know it you have to measure, and measuring it entangled the cat to all your particles making the problem worse. I think it's similar to Bohr and Einstein debate where Einstein gave thought experiment about HUP being violated. But if you have to take the complete information, it can't be violated as reasoned by Bohr.

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

 

[camboy]

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

De Broglie-Bohm. None of these things is even an issue in that interpretation - it simply doesn't have a measurement problem. And it gives a very precise picture of what goes on in a double-slit experiment, because it is a dynamical theory of motion, rather than a theory of observation.

Wave goes through both slits, particle goes through one, wave influences particles to clump in regions of constructive interference (19 words - it's not so hard, is it?).

 

[rodsika]

De Broglie-Bohm. None of these things is even an issue in that interpretation - it simply doesn't have a measurement problem. And it gives a very precise picture of what goes on in a double-slit experiment, because it is a dynamical theory of motion, rather than a theory of observation.

Wave goes through both slits, particle goes through one, wave influences particles to clump in regions of constructive interference (19 words - it's not so hard, is it?).

　

De Broglie-Bohm is almost refuted already. We know that particle accelerators like LHC can precipitate new particles upon collision of the original particles. Since in Bohmian mechanics, a particle is always a particle, then any particle accelerator experiments refute it. We also know particles quantum tunnel. Bohmian mechanics forbid it. This means Bohmain mechanics is already dead on arrival. There is no particle acceleration when Bohm first suggested. Now everyone knows particles are just temporary. Since Bohmian is all about the permanence of particles. It's obvious it doesn't tally with reality. Can anyone here defend the Bohmians?

 

[Fyzix]

At first I thought you were genuinly trying to understand the quantum world, now you look more and more arrogant.
Hopefully Demystifier will shut you down...

 

[camboy]

　
De Broglie-Bohm is almost refuted already. .. It's obvious it doesn't tally with reality.

You sound so serious, yet I'm afraid it's quite obvious that you simply don't understand anything about it.

We also know particles quantum tunnel. Bohmian mechanics forbid it.

On the contrary, unlike any other interpretation, it explains tunnelling.

Since in Bohmian mechanics, a particle is always a particle, then any particle accelerator experiments refute it.

You're talking about the non-relativistic Schroedinger level of theory, I take it? Well, the orthodox interpretation of non-relativistic QM doesn't explain particle creation and destruction either. Why require that of de Broglie-Bohm and not of orthodox QM? (incidentally, 'Bohmian mechanics' is an outdated term used by a single research group which Bohm himself strongly objected to - the current usage is de Broglie-Bohm theory or pilot-wave theory - see e.g. Wikipedia).

If you're talking about quantum-field theoretic generalizations of de Broglie-Bohm, they make the same predictions as the regular QFT.

I would give references, but since you clearly haven't read anything on the subject you should try something something simpler first. I'm sorry to be so harsh, but if you're going to violently attack other people's work then it is common politeness to at least try to understand it first.

 

[rodsika]

　

I read last week that a science advisor and mathematician extraordinaire Arnold Neumaier already declared Bohmian mechanics null and void so I thought you guys refuted it already. Neumaier said:

"I lost interest in Bohmian mechanics once I noticed that all electrons in hydrogen atoms in the ground state stand still at arbitrary positions. I wrote up the report as my farefell address."

...

"As my paper cited before shows, Bohmian mechanics is inconsistent with standard QM once time correlations are taken into account."

...

"The reality is much worse. Bohmian mechnaics achieves agreement with orthodox quantum mechnaics _only_ through measurement considerations. Thus they are able to hide any weakness in their approach by burying it in the fuzzyness of measurement problems."

...

"But I don't care anymore about BM, and therefore won't defend my statement in the paper further."

But I learned a while ago another science advisor Demystifier stated (I thought science advisors words here are final that was why when I heard Neumaier stated it, i believed him immediately):

"BM is NOT all particles. BM is particles AND wave function. It is the wave function that causes the particle to move through the barrier. It does not teleport through it, but simply gets more energy to "jump" over it."

　

So Bohmian Mechanics is still alive.

But isn't it that in orthodox QM, a prisoner has non-zero probability of quantum tunneling outside a cement wall? In Bohmian M, no matter how much energy you give the prisoner, you can't quantum tunnel him outside of the cell without accelerating his body to the cement walls shredding it to pieces. Here QM has more flexibility. Isn't it?

 

[Rap]

Why, is knowing the position and momentum enough to define the pure state in the cat? Or must one know every history of every particle in the body like knowing how it got there since the birth of the cat? Maybe this is why it is difficult even in essence to think a cat can be in pure state because you don't know all the history of every particle. To know it you have to measure, and measuring it entangled the cat to all your particles making the problem worse. I think it's similar to Bohr and Einstein debate where Einstein gave thought experiment about HUP being violated. But if you have to take the complete information, it can't be violated as reasoned by Bohr.

"Entangled" to me means, for example, two downshifted photons created from one using nonlinear optics. The spins are certainly opposite, and measuring the spin of one will give you the spin of the other, measured with respect to the same axis. Is this your meaning of entangled? I think that if you prepare a pure state, it will not matter if the particles associated with that state are previously entangled. That would mean there are many different types of photons, each behaving differently depending on their previous history of entanglement, and I don't think this is the case. The same is true of the cat. In any case, you are talking about the state preparation problem, and I agree it would be a massively difficult thing to prepare a cat in a pure state. But I don't see the reason, in principle, that it could not be done, any more than I can see the reason, in classical physics, that a gas could not be prepared in a particular microstate. I can prepare one particle in a pure state, I can prepare two with more difficulty, and I don't see that there is a particular number of particles where it suddenly becomes impossible, unless you want to bring in the size and age of the universe or something like that.

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

The question of what the particle is doing in between is a scientifically improper question. Its like asking what the momentum of a particle is after I have measured its position. That too is an improper question. There is no way to measure the position and momentum simultaneously, and to ask a question for which, in principle, there is no testable answer is improper.

There is no problem assuming the wave function is real until you come to the question of "Wigner's friend". It is the central problem, addressed at the very beginning in Everett's "many-world" thesis (http://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf) because he realized that this is the central problem in the discussion of wavefunction reality. (He did not call it "Wigner's friend" however). I disagree with his conclusions because his many-world resolution to the problem is untestable, but his statement of the problem is excellent.

 

[rodsika]

"Entangled" to me means, for example, two downshifted photons created from one using nonlinear optics. The spins are certainly opposite, and measuring the spin of one will give you the spin of the other, measured with respect to the same axis. Is this your meaning of entangled? I think that if you prepare a pure state, it will not matter if the particles associated with that state are previously entangled. That would mean there are many different types of photons, each behaving differently depending on their previous history of entanglement, and I don't think this is the case. The same is true of the cat. In any case, you are talking about the state preparation problem, and I agree it would be a massively difficult thing to prepare a cat in a pure state. But I don't see the reason, in principle, that it could not be done, any more than I can see the reason, in classical physics, that a gas could not be prepared in a particular microstate. I can prepare one particle in a pure state, I can prepare two with more difficulty, and I don't see that there is a particular number of particles where it suddenly becomes impossible, unless you want to bring in the size and age of the universe or something like.

Are you saying that the moment you can create a pure state cat. You can put it in any superposition you like such as dead/alive, pregnant/not pregnant, cancer/healthy, virgin/used, old/young, etc.? Without any limitations whatsoever? If there is limitation, maybe a pure state cat can only have superposition of positions of being on the left and right side at the same time and not on being dead/alive, pregnant/nonpregnant, etc.?

 

[Rap]

Are you saying that the moment you can create a pure state cat. You can put it in any superposition you like such as dead/alive, pregnant/not pregnant, cancer/healthy, virgin/used, old/young, etc.?

I think, in principle, yes.

 

[rodsika]

I think, in principle, yes.

How is that possible. Pure state in the cat body simply meant all the particles are entangled. How can you make this create both old and young cat? Hmm.. are you saying that if you can put the old Einstein in pure state, he can become young again... like the fountain youth? What in pure state makes this possible at all? Hope you can explain a bit more so it can be clear to both of us the limitations and possibilities. Thanks.

 

[tom.stoer]

You have to specify what "create a pure state" and "put it in a superposition" means. In QM this means "preparation of the state" and that's what is done in setting up an experiment with well-defined initial conditions (position, momentum, ...).

You can prepare a superposition "alive + dead" simply by following Schrödinger's Gedankenexperiment: take the cat, the box, ... - put the cat into the box - close the box and wait for some time (defined by half-life) - voila

 

[rodsika]

You have to specify what "create a pure state" and "put it in a superposition" means. In QM this means "preparation of the state" and that's what is done in setting up an experiment with well-defined initial conditions (position, momentum, ...).

You can prepare a superposition "alive + dead" simply by following Schrödinger's Gedankenexperiment: take the cat, the box, ... - put the cat into the box - close the box and wait for some time (defined by half-life) - voila

But according to a physicist, Ken, it is not even possible in principle to know the cat pure state before closing the box. So the Schroedinger Cat thought experiment is impossible in principle. Which of the following part don't you agree:

(Ken stated)

"No, that is just exactly what you could never do, not even in principle. Because the only way to know that would be to do measurements on the cat, but that would involve a measuring device, so immediately the cat becomes a subspace of the thing that is a pure state. Unlike measuring the spin of a single particle, where there is no information being ignored, if you measure an "entire cat", there is vast amounts of information you could never get a handle on, like herding cats (literally). There's no measurement like that which even in principle could result in complete information about the cat's wave function that could be treated as a closed system going forward, too much of the data (all the phase coherences) that would need to be tracked is going to be entangled with the instruments doing the measuring, not to mention the brain processing that information.

This is the key point-- the information that goes into determining a wave function is not in the entity being observed, it is in the environment doing the observing and processing that information. Physics is done by physicists, even if we can imagine the action of hypothetical physicists not actually present in the environment. If that environment does include a real brain, it might be able to treat the entity as having a pure-state wavefunction (as for the spin of a particle), but it could never be empowered to treat a cat in a pure-state wavefunction, there would always have to be too much overlooked information (indeed, judicious overlooking of information is more or less the foundational principle of physics). It is only ever the whole system including the observer that could be treated as a pure state, and only if it started out in a pure state, which brings in the issue of history."

 

[tom.stoer]

The key point is that Ken wants to "measure" the cat whereas I want to "prepare" it. That is something different.

When setting up an experiment with monochromatic, coherent light emitted by a LASER the light is by definition monochromatic and coherent w/o measuring it! You check the properties of the light before starting the experiment in order to ensure the correctness of the setup, but during the experiment you do no longer check them; you simply believe that these properties persists w/o measurement. The same applies to the cat.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

That means that "collapse of the wave function", "split according to MWI interpretation ..." does apply on the level of density matrices. This is technically more involved but causes the same fundamental problems regarding ontology.

 

[rodsika]

The key point is that Ken wants to "measure" the cat whereas I want to "prepare" it. That is something different.

When setting up an experiment with monochromatic, coherent light emitted by a LASER the light is by definition monochromatic and coherent w/o measuring it! You check the properties of the light before starting the experiment in order to ensure the correctness of the setup, but during the experiment you do no longer check them; you simply believe that these properties persists w/o measurement. The same applies to the cat.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

That means that "collapse of the wave function", "split according to MWI interpretation ..." does apply on the level of density matrices. This is technically more involved but causes the same fundamental problems regarding ontology.

But if it's a mixed state (a density matrix) then it's just classical probability of being alive or dead. Not a superposition (which involves pure state or ray). Since pure Quantum Mechanics only deal with pure state evolving unitarily. Then preparing it alive + dead is not quantum mechanics already. Just a classical problem. But I wonder why density matrix must fall under quantum mechanics since there is no longer a ray. Any idea?