Does Schrodinger's Cat Paradox Suck?

[alan white]

Interesting thread. I'm new here but I find the forum fascinating.

 

[alan white]

Is Schrodinger's cat paradox a poor pedagogical example for students of QM? In the traditional formulation of the paradox, the cat is declared to be both alive and dead at the same time, but no proof is offered of how the paradox can not be resolved by simply assuming the cat is either dead or alive but not both, before a human observer opens the box. This might give a new student to QM the (incorrect) impression that the non classical properties of quantum states is simply a non standard interpretation of results that can be perfectly explained by classical assumptions. Secondly, the cat paradox implies the whole radioactive source, detector, amplifier, poison capsule and cat system is in a superposition of states, until a human opens the box and makes an observation. This (in my opinion) is misleading, because the superposition breakdown probably occurs much earlier. For example the cat observing the poison capsule breaking is an observation (although brief) by a sentient being, but the Schrodinger cat paradox implies that a living creature such as a cat is not sufficiently sentient to qualify as an observer. Even this is misleading, because an observation does not have to be an observation by a sentient being and can simply be a measurement by a machine such as the detection and amplification of the decay particle by a Geiger counter. In my opinion, even observation or detection is not totally necessary for the superposition to collapse. For example some experiments seem to suggest that passing a photon through a special kind of prism that deflects the photon one way or another according to its polarisation, is sufficient to collapse the superposition because there is potential to make a measurement of the polarisation based on "which way" information, even if there is no detection equipment present. All in all, I get the impression that the Schrodinger's cat paradox does nothing other than potentially confuse newcomers to QM.

P.S. I am just a beginner in QM and these are just my initial impressions for discussion and corrections are welcome.

Perhaps the experiment would be more understandable if you saw each sentient being as a single observer in a personal universe. You are assuming a single objective universe involving all participants including a cat and a number of humans.
It is pointless to talk about the cat's timeline as being identical to the one carrying out the experiment.
It's possible to conceive that proof is belief fulfillment. In that case, experiments only appear to show consistent results. The results can be thought of as a lowest energy outcome for all involved, but the only consistency is what individuals experience, while groups only experience what amounts to a perceived consistency.

 

[soothsayer]

I personally dislike the Schrodinger's cat "paradox" because it fails to take into account that said cat has 9 lives! ;)

For me, the most obvious problem with the thought experiment lies in the fact that the Geiger counter is perfectly sufficient in collapsing the wave function of the decay particle, and superposition ends there, does it not?

Also, the idea was not intended to be a serious experiment relevant to physics, but rather a paradox explaining problems in the Copenhagen Interpretation of QM, to which Dr. Schrodinger was opposed. I think it's educational value is similar to the value of the Twin Paradox in relativity in that they are both interesting paradoxes that help students of physics understand important subtleties in relativity/QM/what have you.

 

[Rap]

For me, the most obvious problem with the thought experiment lies in the fact that the Geiger counter is perfectly sufficient in collapsing the wave function of the decay particle, and superposition ends there, does it not?

Not unless you read or hear the Geiger counter. Until then, it too is in a state of superposition (clicked/not clicked). And you don't read or hear the Geiger counter. The only measurement you make is to open the box, and then the state of the Geiger counter and cat and whatever else collapses. From that you can determine whether the Geiger counter has clicked or has not.

 

[yuiop]

Not unless you read or hear the Geiger counter. Until then, it too is in a state of superposition (clicked/not clicked). And you don't read or hear the Geiger counter. The only measurement you make is to open the box, and then the state of the Geiger counter and cat and whatever else collapses. From that you can determine whether the Geiger counter has clicked or has not.

When you hear your door bell ring, you wonder who it is. Is ti the postman with the parcel you have been waiting for, or is it the pretty girl next door wanting a date, or is a mad axe man, or is it the council wanting to give you a notice to knock down your house to put a highway through, or is it some escaped convicts looking to take you hostage and hide out in your place, or is some passer by wanting to tell you your dog has has escaped and has been run over, or is it someone that turns out to be your mother even though you were not aware you were adopted? According to the interpretation that a lack of knowledge constitutes a state of superposition, the person at the door is a superposition of girl next door and mad axe man and whatever else you imagine until you open the door. Does anyone really believe that the person at the door does not have an objective reality independent of their imagination?

 

[Q-reeus]

Originally Posted by soothsayer View Post
"For me, the most obvious problem with the thought experiment lies in the fact that the Geiger counter is perfectly sufficient in collapsing the wave function of the decay particle, and superposition ends there, does it not?"

Not unless you read or hear the Geiger counter. Until then, it too is in a state of superposition (clicked/not clicked). And you don't read or hear the Geiger counter. The only measurement you make is to open the box, and then the state of the Geiger counter and cat and whatever else collapses. From that you can determine whether the Geiger counter has clicked or has not.

And all this assumes what? Well, if you accept the lesson Decoherence is telling us, basically at least two prerequisites:
1: That the cat is perfectly isolated from the box (despite standing/lying in it), which in turn is perfectly isolated from the table or floor it is lying on, and so on ad infinitum. In other words, essentially perfect isolation from the environment - contrary to how the setup is described.
2: 'Cat' can be taken as a single coherent wavefunction that responds instantly and as a whole to the quantum initiated event 'geiger counter click'. Let's please get real here. In order for the trillions of complex cells, and thense blood vessels, musles, sinews, nerves etc etc to respond as a coherent whole, 'snap freezing' of said cat to a zillionth of a zillionth of a degree above absolute zero is needed (an accomplishment that even in theory would probably take longer than the age of the universe). Is the cat now already dead or alive? Correct!

High time Schrodinger's cat was consigned to a historical footnote, and not continued on as an example of what is actually possible in QM. Let's come up with a worthy and realistic successor that, molecular dimensions wise, presents a realistic scenario. But you disagree?

 

[Q-reeus]

When you hear your door bell ring, you wonder who it is. Is ti the postman with the parcel you have been waiting for, or is it the pretty girl next door wanting a date, or is a mad axe man, or is it the council wanting to give you a notice to knock down your house to put a highway through, or is it some escaped convicts looking to take you hostage and hide out in your place, or is some passer by wanting to tell you your dog has has escaped and has been run over, or is it someone that turns out to be your mother even though you were not aware you were adopted? According to the interpretation that a lack of knowledge constitutes a state of superposition, the person at the door is a superposition of girl next door and mad axe man and whatever else you imagine until you open the door. Does anyone really believe that the person at the door does not have an objective reality independent of their imagination?

Given the whole SC scenario is imho 'Alice-in-Wonderland', I will use 'mind powers' and will into existence an obvious choice!

 

[Rap]

Thanks for the responses. I have a couple of options.

There's two types of trolls. Type 1 argues some point, any point, constantly shifting around, no consistency, just arguing because they want attention, you know, its better than being alone. I dislike those types of trolls.

Type 2 argues some point that they are not totally sure of in hopes that someone will teach them something they don't know and don't believe it will happen unless they aggravate someone.

Then, I could say, hey, I've got an opinion here, but I'm not omniscient, let's discuss this, maybe I'm wrong.

Oh, hell, I'll go with type 2.

First of all, to yuiop, the Schroedinger cat paradox is not like wondering who is at the door. Like Q-reeus implies, SCP is a very idealized system where the cat and everything else in the box implies a totally isolated system, not likely to happen, but is there some fundamental contradiction in assuming such a thing as a thought experiment? I don't think so. Science has to do with isolated systems and repeatable measurements, so the doorbell ringing is not a scientific problem and certainly not a quantum problem.

To Q-reeus, the cat in the box is a way of saying that yes, the cat and the box and everything else is a perfectly isolated system. 1) I don't understand why you bring in the table, why the cat is isolated from the box, etc, etc. and have this infinite chain of succesively isolated systems. 2) No, I am not saying that the collapse occurs at the Geiger counter click. I am saying that it occurs at the "instant" you open the box and see the cat dead or alive. Sure, its much more complicated, you can observe an infinite number of possibilities, cat alive, cat alive but barely, due to poison, cat dead for 1 second, cat dead for a year... But is there some fundamental reason we HAVE to deal with these complications? Do these complications render the whole scenario meaningless, or are they just irrelevant to the point of the whole thought experiment? I think they are irrelevant.

To be very clear about it, I am a Copenhagen sympathizer but not a Copenhagen soldier. There is a thought experiment called "Wigners friend" in which there is a cat etc. in a box observed by a scientist called Wigner's friend, who, along with the cat in the box, is enclosed in a larger box, and Wigner is outside of it. The friend opens the box, sees the result, but Wigner does not. Is the friend in a superposition of states? What does that feel like? Or did the wave function collapse when the friend opened the box? If so, why can't the wave function collapse before the friend opened the box? It is my opinion that Wigner uses one wave function to describe the situation, the friend uses another. The friend's wave function collapses when he/she opens the SC box, Wigner's wave function collapses when he opens his box and gets a report from his friend. Yes, that means that the wave function is not an absolute, objective entity. It is a particular type of encoding of the information availiable to the person using it, involving only quantum probabilities, not classical probabilities. Standard quantum mechanics assumes that all observers are equivalent, sharing common knowledge, and therefore agree on the wave function. SCP is a way of showing that the wave function is not a completely objective physical entity, but rather a tool in the scientists toolbox. Along with the theory and techniques of QM, calculations may be made on the wave function to predict the probability of outcomes of measurements for the particular scientist using them. I would be very interested to hear an objection to this idea OTHER than repulsion or distaste for a wave function that does not have absolute universal objective meaning: i.e. a logical contradiction.

 

[soothsayer]

Not unless you read or hear the Geiger counter. Until then, it too is in a state of superposition (clicked/not clicked). And you don't read or hear the Geiger counter. The only measurement you make is to open the box, and then the state of the Geiger counter and cat and whatever else collapses. From that you can determine whether the Geiger counter has clicked or has not.

Then let's take a look at the Double Slit experiment. We set up our double slit and electron beam and include a measuring device that is turned on, functioning, and recording information, but no one is looking at the data. You would argue that the pattern discerned by the measuring device is in a state of superposition, as is the pattern left by the electrons on the screen behind the slits, this much seems possible. But now imagine that someone was watching the screen and the pattern that is left; the measuring device is on, but relaying information to another room, so the observer has no way of knowing which individual slit each electron is passing through. This observer should see an interference pattern on the screen, this is what is observed without the use of a measuring device,and according to your statement, this measuring device is not in use, since no one is around to determine which slit any of the electrons are traveling through. The screen observer comes to the conclusion that the electrons are in superposition as it passes through the slits.

Now we have someone come into the room where the measuring device is relaying information and determine the results. They would have to conclude from the results that the electrons were in superposition, their wave functions had not collapsed and that there is an interference pattern on the screen, so as to be consistent with the findings of the screen observer. However, this conclusion is nonsensical, since the device would then have detected the electrons actually passing through both slits at the same time, every time and moreover, this would suggest that the measuring device and the person reading it were unable to collapse the wave function.

This thought experiment probably has holes in it, so I'm wondering if someone could address it. I feel like the resolution would be that the screen observer would notice two bands of electrons hitting the screen, since the measuring device collapsed the wave function into one slit even if the data is not being read.

 

[Dadface]

Before proceeding any further let's consider what is meant by a theory:
Theories are attempts to explain certain observations,being informed by those observations and by necessity conforming to those observations.Theories should be tested and testable by obervations and if a theory predicts new observations then it shouild be possible ,even if just in principle,to make those observations.In short,without the observations a theory is nothing.
Now Mr Schrodinger devised his thought experiment in such a way that all relevant observations are forbidden during the time that the box is closed.It follows that any theory,or hypothesis or even guess about what happens in the closed box cannot be proven.The domain of applicability of quantum or any other relevant theory does not extend into the box during the time that it's closed.
Speculation is of course allowed and personally I prefer the common sense view about the happenings in the box but things such as a dead and alive cat?Absolutely not and I think the time is long overdue to demote such pointless metaphysical speculations to the dustbin of scientific curiosities.


(A metaphysician is a man who goes into a dark cellar at midnight,without a light looking for a black cat that isn't there...anon)

 

[Q-reeus]

Thanks for the responses. I have a couple of options.

And thanks in turn for your response, Rap.

...Oh, hell, I'll go with type 2.

Guess the best I can hope is to be classified here is as type 2 - but hell I'm not worthy of self-classification!

...Like Q-reeus implies, SCP is a very idealized system where the cat and everything else in the box implies a totally isolated system, not likely to happen, but is there some fundamental contradiction in assuming such a thing as a thought experiment? I don't think so...

I do think so, but more below.

...To Q-reeus, the cat in the box is a way of saying that yes, the cat and the box and everything else is a perfectly isolated system. 1) I don't understand why you bring in the table, why the cat is isolated from the box, etc, etc. and have this infinite chain of succesively isolated systems...

Is that not what isolation is all about? In order to be truly isolated, such a chain is obligatory I would think. And, given the extreme QM delicacy of the actual system, that implies an essentially 'magical' process of separation at each instance (maybe one could 'practically' have the cat isolated from the box if both are weightless on an orbiting space-station, but hardly the original setup. And that still ignores coupling from thermal radiation, gravitational fluctuations etc.) In #6 I attempted to highlight repercussions when incorrectly assuming that 'opening box' and viewing defined observation - no, whether or not superposition has in fact occurred has repercussions effecting eg momentum 'imbalances' of the box (an isolation issue) which is quite distinct from 'viewing the cat' as per original scenario. If Decoherence is accepted, environmental coupling ensures superposition is not there - period. Hence 'jerks' will tell us whether the cat is for sure dead or alive. If no 'jerks', well Decoherence is 'dead', but I would bet otherwise!

No, I am not saying that the collapse occurs at the Geiger counter click. I am saying that it occurs at the "instant" you open the box and see the cat dead or alive. Sure, its much more complicated, you can observe an infinite number of possibilities, cat alive, cat alive but barely, due to poison, cat dead for 1 second, cat dead for a year...

Point there taken - but please note my response was overall not only to your remarks but also inclusive of Soothsayer you were responding to. Guess I should have delineated better.

To be very clear about it, I am a Copenhagen sympathizer but not a Copenhagen soldier.

Nice to know which interpretation you prefer.

There is a thought experiment called "Wigners friend" in which there is a cat etc. in a box observed by a scientist called Wigner's friend, who, along with the cat in the box, is enclosed in a larger box, and Wigner is outside of it. The friend opens the box, sees the result, but Wigner does not. Is the friend in a superposition of states? What does that feel like? Or did the wave function collapse when the friend opened the box? If so, why can't the wave function collapse before the friend opened the box? It is my opinion that Wigner uses one wave function to describe the situation, the friend uses another. The friend's wave function collapses when he/she opens the SC box, Wigner's wave function collapses when he opens his box and gets a report from his friend. Yes, that means that the wave function is not an absolute, objective entity. It is a particular type of encoding of the information availiable to the person using it, involving only quantum probabilities, not classical probabilities. Standard quantum mechanics assumes that all observers are equivalent, sharing common knowledge, and therefore agree on the wave function. SCP is a way of showing that the wave function is not a completely objective physical entity, but rather a tool in the scientists toolbox. Along with the theory and techniques of QM, calculations may be made on the wave function to predict the probability of outcomes of measurements for the particular scientist using them.

No objection in principle accept that it ignores completely point 2 in #41: both cat and Wigner's friend are not coherent wavefunctions - unless not only perfectly isolated but also in 'ultra deep-freeze', in which case they cannot function as per thought experiment. Can you have it otherwise?
In summary, as a layperson re QM I look for a consistent and believable picture, and SC just doesn't cut it for me. Decoherence does. And that strongly suggests ditching SC and replacing it with a thought experiment consistent with modern understanding - to put it crudely, that 'tiny objects' (barring say superconducting circuits) in superposition are realistic, otherwise not.
Finally, I should thank you for helping me to realize 'trolls aint just trolls' - there's a whole sub-class to be explored! Is this all wrong headed (trolly leading question of course)?

 

[yuiop]

First of all, to yuiop, the Schroedinger cat paradox is not like wondering who is at the door. Like Q-reeus implies, SCP is a very idealized system where the cat and everything else in the box implies a totally isolated system, not likely to happen, but is there some fundamental contradiction in assuming such a thing as a thought experiment? I don't think so. Science has to do with isolated systems and repeatable measurements, so the doorbell ringing is not a scientific problem and certainly not a quantum problem.

Obviously I have hit a raw nerve as you are resorting to thinly veiled personal attacks. I do not see any problem with assuming an idealised isolated system for the cat in a box. We could for example put the box in space and immobilise the cat so that it can not move while it is alive. What I object to is that you seem to think you can prove that the cat is in a superposition of states before the box is opened when there is nothing in the experiment that excludes the possibility that the cat is either dead or alive but not both. By strongly adhering to the claim that the cat is in a superposed state before the box is opened you are making a statement about the state of the cat about which have made no knowledge or measurement. I agree with Q-reeus that we need a new thought experiment or paradox to replace the SCP that clearly demonstrates the superposed state of the cat and eliminates any possibility of the cat being in a definite state that we just happen to have no knowledge of due to lack of information. I contend that if you are in a windowless soundproof building then you as an observer are isolated from the system outside the building and you have a equal lack of knowledge as the observer outside the cat box, the only difference being greater number of possibilities for what is outside the building than inside the cat box.

... 2) No, I am not saying that the collapse occurs at the Geiger counter click. I am saying that it occurs at the "instant" you open the box and see the cat dead or alive.

From what I have seen, it takes very little to to decohere a quantum system. Merely placing 1/4 waveplates in front of two slits can prevent the interference pattern forming, even if the which way information is not analysed. The mere potential to be able to determine the which way path breaks down the interference pattern, so it seems to that a Geiger counter and and a mechanical device to break the poison capsule and living cat are more than enough to decohere the system long before the observer outside the box opens the box. All the observer outside the box has, is a lack of knowledge of what is going on inside the box. We have a reasonable idea of what happens if we do the experiment with the box open throughout, but when we close the box we have no idea what is going inside the box and pixies could be playing in there for all we know, but disappear the moment we open the box. No one can prove that pixies were not in the box while it was closed, but because it beyond our normal expectation, we would ask for proof that pixies are inside the box when it closed and we would ask for proof that the cat is in a superposed state of dead and alive when it closed.

There is a thought experiment called "Wigners friend" in which there is a cat etc. in a box observed by a scientist called Wigner's friend, who, along with the cat in the box, is enclosed in a larger box, and Wigner is outside of it. The friend opens the box, sees the result, but Wigner does not. Is the friend in a superposition of states? What does that feel like? Or did the wave function collapse when the friend opened the box? If so, why can't the wave function collapse before the friend opened the box? It is my opinion that Wigner uses one wave function to describe the situation, the friend uses another. The friend's wave function collapses when he/she opens the SC box, Wigner's wave function collapses when he opens his box and gets a report from his friend. Yes, that means that the wave function is not an absolute, objective entity.

Wigner's friend sealed inside his box, is not much different to my "who is at the door" example. Until he opens his box he has no idea what is happening outside his box. He imagines the cat is in a superposition of dead or alive, but he finds when he opens his box and looks out to find that Wigner has replaced the cat with a dead mongoose so Wigner's friend is wrong about what he thinks is outside his box, but this is not a QM phenomenon, but just a lack of knowledge on Wigner's friends behalf. I find your concept of multiple wave functions (one for every observer) for a given system, difficult to swallow when a system can be described by a single wave function. Why do jump to the conclusion of a wave function with no objective existence when there is an objective alternative?

 

[John232]

It would be like someone that works at a nuclear waste facility would be dying of cancer and not dying of cancer at the same time from being exposed to a leak at the same time until he gets checked out by a doctor by an EKG. It wouldn't be until he got diagnosed that he would actually be able to have it for certain. Would make you think twice before getting checked out for haveing cancer...

 

[alan white]

It would be like someone that works at a nuclear waste facility would be dying of cancer and not dying of cancer at the same time from being exposed to a leak at the same time until he gets checked out by a doctor by an EKG. It wouldn't be until he got diagnosed that he would actually be able to have it for certain. Would make you think twice before getting checked out for haveing cancer...

Very true. And by the same token, quantum collapse occurs each time a new observer observes the same thing. This is a departure from a lot of people who believe that once collapsed, the works been done.
Renegade thinking, because it implies that the objective reality is not really objective, but only appears to be.

 

[John232]

Very true. And by the same token, quantum collapse occurs each time a new observer observes the same thing. This is a departure from a lot of people who believe that once collapsed, the works been done.
Renegade thinking, because it implies that the objective reality is not really objective, but only appears to be.

I kind of ment it more as a joke. I don't think people would never die of cancer if they never got checked out for it, and never noticed any evidence of haveing it themselves. If you could it would be the cure for cancer since you could never have it from radiation if you just never got checked out.

I think there is a difference between the microscopic and macroscopic worlds. The matter we observer on a daily basis isn't in a state where they can exist in multiple states at the same time. I think if there was a reaction that leaked into the macroscopic world it would only happen and not happen relative to the original particle that was also in a state that it didn't decay. The cat may only be alive and dead at the same time relative to the original particle that was also existing in a state where it didn't decay.

 

[Rap]

(maybe one could 'practically' have the cat isolated from the box if both are weightless on an orbiting space-station, but hardly the original setup. And that still ignores coupling from thermal radiation, gravitational fluctuations etc.)

Isolation is when the box is isolated from any outside influence - gravitation, table, the outside scientist, etc.

In #6 I attempted to highlight repercussions when incorrectly assuming that 'opening box' and viewing defined observation - no, whether or not superposition has in fact occurred has repercussions effecting eg momentum 'imbalances' of the box (an isolation issue) which is quite distinct from 'viewing the cat' as per original scenario.

Hmm - I don't understand that, so I won't respond.

If Decoherence is accepted, environmental coupling ensures superposition is not there - period. Hence 'jerks' will tell us whether the cat is for sure dead or alive. If no 'jerks', well Decoherence is 'dead', but I would bet otherwise!

Decoherence does not ensure that superposition is not there. When a simple system (SS) interacts with a measuring device (MD), decoherence assures that the wave function for SS is no longer coherent, but the SS-MD wave function (assuming it is isolated) will still be coherent. Maybe I said that wrong, but what I mean is the SS-MD wave function will still be a purely QM wave function with a huge number of degrees of freedom, which may still be thought of as a superposition of all the dead ones and the live ones, in the case of SC. Only when the SS-MD system is observed does collapse occur.

No objection in principle accept that it ignores completely point 2 in #41: both cat and Wigner's friend are not coherent wavefunctions - unless not only perfectly isolated but also in 'ultra deep-freeze', in which case they cannot function as per thought experiment.

Yes, its perfectly isolated but why the requirement of absolute zero? If I can have a coherent wave function for one particle bouncing around, or two, or three, why not 10^23. In principle, I mean.

What I object to is that you seem to think you can prove that the cat is in a superposition of states before the box is opened when there is nothing in the experiment that excludes the possibility that the cat is either dead or alive but not both.

Let me substitute "electron" for cat and spin up/spin down for dead/alive. This reads: "What I object to is that you seem to think you can prove that the electron is in a superposition of states before a measurement is made when there is nothing in the experiment that excludes the possibility that the electron is either spin up or spin down but not both."

This is a "hidden variables" approach to QM which has been shown to be wrong.

By strongly adhering to the claim that the cat is in a superimposed state before the box is opened you are making a statement about the state of the cat about which have made no knowledge or measurement.

We do have prior knowledge - we know that when the box was closed, the cat was alive and the geiger counter had not clicked. In principle, we therefore have a wave function for the situation at time zero. Then we use Schroedingers equation (or whatever) and calculate the change in the wave function over time. After a time, the wave function will be a superposition of dead states and live states.

I contend that if you are in a windowless soundproof building then you as an observer are isolated from the system outside the building and you have a equal lack of knowledge as the observer outside the cat box, the only difference being greater number of possibilities for what is outside the building than inside the cat box.

As long as you can assign a wave function for the entire universe outside the box, I agree.

so it seems to that a Geiger counter and and a mechanical device to break the poison capsule and living cat are more than enough to decohere the system long before the observer outside the box opens the box.

Yes, but decoherence is not the same as wave function collapse. (see above).

but when we close the box we have no idea what is going inside the box and pixies could be playing in there for all we know, but disappear the moment we open the box.

No, we have the wave function when the box was closed, and we know its isolated, so no pixies unless Schroedinger's equation says so.

I find your concept of multiple wave functions (one for every observer) for a given system difficult to swallow when a system can be described by a single wave function. Why do jump to the conclusion of a wave function with no objective existence when everything can be described by a wave function with an objective existence?

When none of the observers are part of the system being observed, then all observers agree on the wave function, and it feels very absolute. I never said it had no objective existence. It is objective in the sense that encodes your MEASUREMENTS, which are objective, but subjective in the sense that it encodes YOUR measurements. When YOUR measurements are OUR measurements, then we only need one wave function. When one of the observers is part of a system being observed by others, then one wave function for all won't be right. (I'm not yelling, just emphasizing).

 

[Q-reeus]

Isolation is when the box is isolated from any outside influence - gravitation, table, the outside scientist, etc.

We agree on the definition, good. But can cat-in-a-box ever be so, unless totally unrealistic assumptions are made? That's my point - pick a system for which the necessary degree of isolation (both external and 'internal') is realistically achievable, not ridiculously implausible. Nano-scale seems about right to me.

Hmm - I don't understand that, so I won't respond.

Well, a cat that's alive will at minimum have a beating heart, giving off 'tremors' that in principle can be detected without opening the box and viewing, whereas when dead... So I guess theoretical total isolation answers that one - but 'in practice' it highlights that opening the box and viewing is not really needed. The catch-22 here is that if cat+detector is genuinely in superposition - no tremors should be present to detect (all 'cat states' being equally present), which in turn hinges on whether superposition is truly realizable and realized.

Decoherence does not ensure that superposition is not there. When a simple system (SS) interacts with a measuring device (MD), decoherence assures that the wave function for SS is no longer coherent, but the SS-MD wave function (assuming it is isolated) will still be coherent. Maybe I said that wrong, but what I mean is the SS-MD wave function will still be a purely QM wave function with a huge number of degrees of freedom, which may still be thought of as a superposition of all the dead ones and the live ones, in the case of SC. Only when the SS-MD system is observed does collapse occur.

And the key here surely is SS - 'simple system'. One for which a coherent wavefunction makes sense. Why do you imagine the experimentalists in that micro-cantilever (not mirror as I originally said) setup mentioned in #22 had to cool such a tiny object down to very near absolute zero? Isn't it because even such a simple entity is in effect an incoherent jumble of wavefunctions otherwise. How much more so a room-temperature cat, that basically is trillions of environments interacting and continually decohering with each other? My take on decoherence applied here - The cat is it's own very non-isolated environment!

Yes, its perfectly isolated but why the requirement of absolute zero? If I can have a coherent wave function for one particle bouncing around, or two, or three, why not 10^23. In principle, I mean.

My understanding, admittedly as very much a non-expert, is as per previous comment. But I will defer to your greater knowledge on these matters, if you can explain the above. OK - way past bed time for me!

 

[alan white]

I kind of ment it more as a joke. I don't think people would never die of cancer if they never got checked out for it, and never noticed any evidence of haveing it themselves. If you could it would be the cure for cancer since you could never have it from radiation if you just never got checked out.

I think there is a difference between the microscopic and macroscopic worlds. The matter we observer on a daily basis isn't in a state where they can exist in multiple states at the same time. I think if there was a reaction that leaked into the macroscopic world it would only happen and not happen relative to the original particle that was also in a state that it didn't decay. The cat may only be alive and dead at the same time relative to the original particle that was also existing in a state where it didn't decay.

No joke. At least not to me. In Japan, old folks are often not told by the sons/daughters and doctor that they are terminally ill. The ones who are told die much sooner.
Anyway, I don't see any difference at all between macro or micro worlds. The only difference is our beliefs, which are far more limited when it comes to macro realities. Seen from a certain perspective, quantum erasure occurs all the time in everyday (macro) life.
Doesn't it seem strange to you that we consider ourselves so macro when we are aware that the universe at large is so much more enormous compared us?

 

[Rap]

To Q-reeus:

I think we agree that the SC scenario is not practically realizable (at present). But a fundamental aspect is the use of the box as a way of saying the cat/radioactive material/prussic acid/Geiger counter system is totally isolated. That means nothing in, nothing out. The cat's beating heart cannot be detected by the scientist, nothing. Any lack of isolation denies the assumption that the scientist cannot in any way know what is going on inside the box. Isolation implies that since you know the wave function when the box is closed, you can, in principle, use Schroedinger's equation to calculate its evolution in time without any parameters other than those of the box itself. At some later time, the wave function will consist of "dead" states superposed with "live" states. (a vast oversimplification, but the simplification does not strike at the core of the problem).

As I understand decoherence (and I don't understand it enough to do the math which means I don't understand it enough), it deals with how a small system with few degrees of freedom interacts with a larger system (i.e. a measuring device) with many degrees of freedom, and how the small system can cause the measuring device to be very closely approximated as a classical system which can take on a number of different classical states. I think its fair to say that we can treat the SC box, after a certain amount of time, as a set of classical states, each with their own classical probability, and when the scientist opens the box, he/she observes one of those classical states. If we ever run into trouble, we have to remember that QM is the truth, and the classical states idea is an approximation. This whole development does not deny the full wave function treatment, in principle. Applied to the SC problem, decoherence says that after a certain amount of time, the situation inside the box can be practically divided into dead cat/live cat in the classical sense. Many people wrongly assume that this means the true coherent wave function has collapsed. It has not. Only when the box is opened does the true wave function collapse to a single state (not practically observable but there in principle) and one of the classical states is observed, and all of these classical states are potentially observable to the scientist.

Now apply this to Wigner's friend. Wigner will say that, to a high degree of (but not perfect) accuracy, his friend may see the cat dead, may see it alive, and these are two classical possibilites with classical probabilities. We can just as well say that, according to Wigner, his friend is in a superposition of quantum states some of which involve him assigning a dead state to the opened box, some of which involve him assigning a live state to the opened box, and that this superposed state collapses when Wigner's friend tells him which one he is using. Decoherence says you can treat the problem classically, but it does not say that you cannot treat it quantum mechanically (in principle). If Wigner & friend think of it in pure QM terms, the question of which quantum state Wigners friend is assigning to the cat in the box is contained in Wigners wave function for the cat in the box-friend system. The wave function in this case is not absolute, it is a tool used by both Wigner and friend to do quantum mechanics, and I cannot think of any way to prove that this viewpoint leads to a logical contradiction. Objections always amount to distaste for a less than objective wave function, or nit picking about the impracticability (but not impossibility!) of the thought experiment.

 

[G01]

Not unless you read or hear the Geiger counter. Until then, it too is in a state of superposition (clicked/not clicked). And you don't read or hear the Geiger counter. The only measurement you make is to open the box, and then the state of the Geiger counter and cat and whatever else collapses. From that you can determine whether the Geiger counter has clicked or has not.

No. A human observer does not have to be present for a measurement to take place.

The Geiger counter is a classical system. That is the key. The measurement occurs when the quantum system interacts with a classical system. It is this interaction that causes the decoherence phenomenon which collapses the entire composite system, the cat+Geiger counter+ decaying atom into a classical, non superimposed state.

No human intervention or reading of the Geiger counter is necessary to collapse the system.

The human observers uncertainty about the measurement is classical, and does not correspond to a quantum superposition.

 

[Rap]

I think there is a classical analog to this - Classical mechanics will deal exactly with a one or two or 10^23 (i.e. many) particle system. Statistical mechanics does not deal with a few particles, it only deals with averages taken on many particle systems, yielding approximate results. The results are extremely accurate, but still, strictly speaking, approximate. Statistical mechanics does not deny classical mechanics. There is no clear line between the two cases. There are cases in which its a toss up as to whether to use statistical mechanics or the more accurate and detailed results of classical mechanics of particles.

Quantum mechanics will deal with a one or two or a many particle system using the wave function. Decoherence theory deals with a few particle system interacting with a many particle system. It shows that to a very good approximation, the measuring device will behave classically. The results are extremely accurate, but still, strictly speaking, approximate. Decoherence theory does not deny quantum mechanics. The wave function approach is still valid. There is no clear line between the two cases. There are cases in which its a toss up as to whether to use decoherence theory or the more accurate and detailed results of the quantum mechanics of particles.

No. A human observer does not have to be present for a measurement to take place.

If you wish to define "measurement" as a case where a macroscopic system becomes approximatable as a set of classical possibilities with classical probabilities, then fine. I think of measurement as the removal of the probability aspect. To say the cat might be dead and might not be dead and the probabilities are approximately classical does not constitute a measurement to the scientist who has not yet opened the box.

The Geiger counter is a classical system. That is the key. The measurement occurs when the quantum system interacts with a classical system. It is this interaction that causes the decoherence phenomenon which collapses the entire composite system, the cat+Geiger counter+ decaying atom into a classical, non superimposed state.

Yes, approximately. To the scientist who has not opened the box, the system can still in principle, and more exactly, be represented by a quantum wave function. Or, if the scientist chooses, use the very accurate approximation of decoherence theory as a set of classical possibilities with associated classical probabilities.

No human intervention or reading of the Geiger counter is necessary to collapse the system.

Thats like saying that no human intervention is required for statistical mechanics to be valid for a many particle system. Yes, no human intervention is required for the decoherence approximation to become valid. If you want to define this event as a measurement, fine, I choose to define a measurement by X as that point at which the probablity aspect vanishes for X.

The human observers uncertainty about the measurement is classical, and does not correspond to a quantum superposition.

Yes, if they have chosen to use the decoherence approximation. If they choose to conceptualize it as a purely quantum phenomenon, the uncertainty is quantum mechanical and does correspond to a quantum superposition.

 

[soothsayer]

No. A human observer does not have to be present for a measurement to take place.

The Geiger counter is a classical system. That is the key. The measurement occurs when the quantum system interacts with a classical system. It is this interaction that causes the decoherence phenomenon which collapses the entire composite system, the cat+Geiger counter+ decaying atom into a classical, non superimposed state.

No human intervention or reading of the Geiger counter is necessary to collapse the system.

The human observers uncertainty about the measurement is classical, and does not correspond to a quantum superposition.

Thank you, yes, exactly the point I was trying to make a while back. I think paradoxes arise if you posit that human observation must happen for a measurement to take place.

At the same time, in the double slit experiment, human observation alone is not enough to collapse the wavefunction, precise measurement must take place so as to induce momentum uncertainty.

And also, if we assert that the Geiger counter is in a state of superposition until heard or viewed by a human, we would have to explain why the cat, upon hearing the Geiger counter and viewing the hammer fall and glass shatter (or not) would not be able to collapse the system, while a human could, as though only an intelligent enough being could collapse a wavefunction...

 

[Q-reeus]

To Q-reeus:
I think we agree that the SC scenario is not practically realizable (at present). But a fundamental aspect is the use of the box as a way of saying the cat/radioactive material/prussic acid/Geiger counter system is totally isolated. That means nothing in, nothing out. The cat's beating heart cannot be detected by the scientist, nothing. Any lack of isolation denies the assumption that the scientist cannot in any way know what is going on inside the box. Isolation implies that since you know the wave function when the box is closed, you can, in principle, use Schroedinger's equation to calculate its evolution in time without any parameters other than those of the box itself...

Well firstly Rap I should say thanks for taking the time for a detailed and thoughtful response - something I'm not much accustomed to here at PF.

OK I accept that as as dabbler in QM have probably misunderstood the didactic usefulness of SCP, as opposed to it's 'impracticality'. So in essence the lesson is decoherence is a useful approximation - a tool for dealing with real world situations but nothing more. Pure QM tells us superposition is inevitable for a truly isolated system, and a la Copenhagen Interpretation reality only exists post measurement.
I suppose what repulsed me from that view was the popularized image/quote of an 'eye' looking back on the 'universe' with the caption (originally attributed to Freeman Dyson but seized on by J.A. Wheeler) "In a sense, the universe knew we were coming." Ultimate chicken-or-egg scenario (the universe only exists because it is , way way post initial event, observed), but then if I have it right Max Tegmark came along and declared "that's bunk - the universe 'observes itself' and evolved perfectly well without any need of sentient observation/measurement". A welcome sanity check at the time, and from that I loosely assumed 'cat' can be taken to be a mini universe of sorts - 'self observing' in effect. Seems not so.
Tentatively accepting that, there's one aspect that still doesn't quite match up in my mind. Taking the position that pre measurement everything in the box is in a superposed 'ghostly' state, center-of-mass change post measurement seems problematic. Specifically, that say the averaged state of 'alive and standing cat' + 'dead and lying cat' is different from either actualized/measured state. So is there a sudden jump in momentum as a result of the act of measurement, or is that somehow formally taken care of 'observer+observed' = constant?
If it's any comfort Rap, have thought about joining a local chapter of T...s Anonymous!

 

[yuiop]

If you wish to define "measurement" as a case where a macroscopic system becomes approximatable as a set of classical possibilities with classical probabilities, then fine. I think of measurement as the removal of the probability aspect. To say the cat might be dead and might not be dead and the probabilities are approximately classical does not constitute a measurement to the scientist who has not yet opened the box.

I agree that it does not constitute a measurement to the scientist outside the closed box, but the interaction with the quantum particle with the Geiger particle does constitute a measurement. In the quantum erasure experiment here http://grad.physics.sunysb.edu/~amarch/Walborn.pdf and here http://grad.physics.sunysb.edu/~amarch/ the placement of two quarter wave plates in front of the dual slits is sufficient to destroy the interference pattern because as far as the quantum particles are concerned a measurement of polarity has been made. This is despite the fact that there is no device to detect the angular deflection of the polariser as the photon passes through and despite the fact there no second polariser before or after the slits to measure the circular polarisation to confirm which slit the photon went through. "Measurement" of which slit the photon went through does not even require the human observer to actually make the measurement. All that is necessary, is that devices are in the wave path that could potentially be analysed later, to constitute "a measurement" as far as the quantum system is concerned. This is what the experiments appear to be telling us. The measurements you are talking about are the best probabilistic guesses of human observers with insufficient information about the system.

In a different experiment using entangled photons, passing a photon through a polariser near the source is sufficient "measurement" of the entangled photon system to destroy the entanglement https://www.physicsforums.com/showthread.php?t=469014.

 

[DrChinese]

In the quantum erasure experiment here http://grad.physics.sunysb.edu/~amarch/Walborn.pdf and here http://grad.physics.sunysb.edu/~amarch/ the placement of two quarter wave plates in front of the dual slits is sufficient to destroy the interference pattern because as far as the quantum particles are concerned a measurement of polarity has been made. ...

The quarter wave plates do NOT, in and of themselves, affect the interference pattern. Further, in a quantum measurement system, it is possible to erase the results of a measurement and thereby restore a prior superposition state.

You are correct that the result of a measurement does not need to be recorded or otherwise observed to achieve decoherence. Again, it depends on the complete setup.

 

[Rap]

There seems to be two definitions of the word "measurement".
1. The establishment of decoherence - more precisely, the establishment of the validity of the decoherence approximation.
2. The collapse of the wave function into a set of wave functions corresponding to a classical measurement device yielding a unique answer.

So I will not use the word "measurement" but rather "decoherence" and "collapse of the wave function".

First, decoherence is not the same as the collapse of the wave function. Decoherence is the realization that the quantum description of a system in terms of wave functions with interfering probability amplitudes can be very accurately, but not perfectly, replaced by a classical "ensemble" with additive probabilities. A classical ensemble is just a bunch of classical possibilities each with their own probability, and the probabilities all add up to one, with no interference effects. In the SCP, the probability of cat dead equals P, probability of cat alive = 1-P, where 0<P<1 so that P + (1-P) = 1

Collapse of the wave function is the act of a scientist replacing his/her information set prior to observing a system with a new information set that reflects the new information gained by the observation. In a microscopic case, this could be choosing one wave function from the set of wave functions that were previously superposed. In the macroscopic case, it could be choosing one of the classical outcomes from a bunch (ensemble) of classically possible outcomes. Note that this corresponds to choosing a wave function which is a superposition of all the wave functions that give the same observation. In the SCP, the scientist opens the box and sees that the cat is dead (probability of cat dead=1, probability of cat alive=0) or perhaps sees the cat alive (probability of cat dead=0, probability of cat alive=1)

The quarter wave plates do NOT, in and of themselves, affect the interference pattern.

I need to study this experiment, but this statement makes sense, decoherence is not the same as wave function collapse.

Further, in a quantum measurement system, it is possible to erase the results of a measurement and thereby restore a prior superposition state.

This bothers me, because I thought that decoherence was irreversible. I assume this means that if you are on the ragged edge of decoherence, it is possible for a system to "re-cohere"?

You are correct that the result of a measurement does not need to be recorded or otherwise observed to achieve decoherence. Again, it depends on the complete setup.

Yes, wave function collapse is not the same as decoherence.

 

[DrChinese]

This bothers me, because I thought that decoherence was irreversible. I assume this means that if you are on the ragged edge of decoherence, it is possible for a system to "re-cohere"?

Yes, it is strange, but you can do some weird things with recombining outputs of things like beam splitters and restoring a previous state. There are some theoretical treatments of things like this (of course this is based on standard application of QM), including one I wrote:

Entangled "Frankenstein" Photons

There are not a lot of good experiments to cite in this particular vein. There are a few but the basic concept itself - erasure after an apparent splitting - is pretty well established.

 

[yuiop]

The quarter wave plates do NOT, in and of themselves, affect the interference pattern. Further, in a quantum measurement system, it is possible to erase the results of a measurement and thereby restore a prior superposition state.

Could you elaborate on this statement? For example in fig 2 of http://grad.physics.sunysb.edu/~amarch/Walborn.pdf when the quarter wave plates are not present there is an interfence pattern that looks like this:

[PLAIN]http://grad.physics.sunysb.edu/~amarch/PHY5653.gif

but when the quarter wave plates are put in front of the slits the pattern looks like figure 3 of the same paper:

[PLAIN]http://grad.physics.sunysb.edu/~amarch/PHY5658.gif

Now while it is debatable whether or not the second image is or is not an interference pattern*, we can certainly say the quarter wave plates do "affect the interference pattern".

Bear in mind that the above effect happens even before a polariser is placed in the path of the primary entangled photon that is used for coincidence counting.

Another example can be seen in figure 5 of this document http://www.fsc.ufsc.br/~lucio/2003-07WalbornF.pdf by the same authors, of the effect of the quarter wave plates on the interference pattern, in a situation that does not involve entanglement. *I plan on starting a new thread to go into the details of the erasure experiment so I will not digress here.

 

[Rap]

Tentatively accepting that, there's one aspect that still doesn't quite match up in my mind. Taking the position that pre measurement everything in the box is in a superposed 'ghostly' state, center-of-mass change post measurement seems problematic. Specifically, that say the averaged state of 'alive and standing cat' + 'dead and lying cat' is different from either actualized/measured state. So is there a sudden jump in momentum as a result of the act of measurement, or is that somehow formally taken care of 'observer+observed' = constant?

Well, the momentum of the whole system would not change, but the position of the box would change if the cat died and fell down. If the box is truly isolated, then you cannot monitor its position, that would require de-isolation, if only to bounce a photon off the box every once in a while. When you close the box, you could measure its position, but to measure its position at some later point in time is tantamount to opening the box. One single measurement of the box's position would tell you whether the cat was standing (alive) or lying (dead), and you might as well just open the box. Until then the position of the box is in a superposition of moved and not-moved.

 

[Q-reeus]

Well, the momentum of the whole system would not change, but the position of the box would change if the cat died and fell down... Until then the position of the box is in a superposition of moved and not-moved.

Hmm... no problem with following that sequence, but it's the dynamics at the point of wavefunction collapse/measurement that has me still wondering. Collapse is supposed to be essentially instantaneous, I gather this means also the transition in cat centre-of-mass - from superposed to either standing & alive, or lying & dead. Whether or not the box is taken to act as an exact counterpoise momentum wise, seems kind of potentially violent - we all know what a really rapid dp/dt implies. I don't for a moment believe such could happen, but it does suggest maybe collapse has to be in fact a somewhat leisurely affair? Interesting to speculate about possible additional restrictions on the wavefunction/collapse process if we were to say have a rotating cat-n-box setup (conservation of angular momentum). Bed time again!

 

[Rap]

Hmm... no problem with following that sequence, but it's the dynamics at the point of wavefunction collapse/measurement that has me still wondering. Collapse is supposed to be essentially instantaneous, I gather this means also the transition in cat centre-of-mass - from superposed to either standing & alive, or lying & dead. Whether or not the box is taken to act as an exact counterpoise momentum wise, seems kind of potentially violent - we all know what a really rapid dp/dt implies. I don't for a moment believe such could happen, but it does suggest maybe collapse has to be in fact a somewhat leisurely affair? Interesting to speculate about possible additional restrictions on the wavefunction/collapse process if we were to say have a rotating cat-n-box setup (conservation of angular momentum). Bed time again!

Well, no, we have to keep separate the event of the cat dying and the event of opening the box. The cat dying will be not give infinite dp/dt, that can be a "leisurely affair", but observing whether the cat is dead or alive can be considered instantaneous.

 

[Rap]

Thank you, yes, exactly the point I was trying to make a while back. I think paradoxes arise if you posit that human observation must happen for a measurement to take place

I am not positing that human observation must happen. Any entity making symbolic quantum mechanical calculations and capable of modifying those calculations in light of new information (i.e. a measurement) will respond to this new information in the quantum mechanical way - collapsing the wave function that it is using to describe the system.

At the same time, in the double slit experiment, human observation alone is not enough to collapse the wavefunction, precise measurement must take place so as to induce momentum uncertainty.

Human observation (or its equivalent - see above) is sufficient to collapse the wave function. If the human is observing a screen responding to individual photons, the human will collapse the wave function to one in which position is well defined, momentum is not. Its a question of what the device is set up to measure. If it were set up to measure momentum, then it would induce position uncertainty and the human would collapse the wave function to something different.

And also, if we assert that the Geiger counter is in a state of superposition until heard or viewed by a human, we would have to explain why the cat, upon hearing the Geiger counter and viewing the hammer fall and glass shatter (or not) would not be able to collapse the system, while a human could, as though only an intelligent enough being could collapse a wavefunction...

The cat would collapse the wave function of a system it is observing (Geiger counter), the scientist outside would not collapse the wave function of a system he is not observing (Geiger counter). If the cat is capable of performing symbolic quantum calculations, then it would collapse the wave function it was using to describe the geiger counter. It would then be like "Wigner's friend". As and object of study by Wigner, who is outside the box, the wave function Wigner uses to describe the cat etc will be a superposition of states, some of which describe the cat as alive and making quantum calculations and collapsing the wave function it is using, some as dead and not.

The resolution to the SC paradox is the realization that the wave function is objective in the sense that it encodes your MEASUREMENTS, but subjective in the sense that it encodes YOUR measurements. Realizing that you know more after you have made a measurement (i.e. collapsing the wave function you use to describe a system) is not an act that modifies the system you are studying. Collapsing the wave function does not affect the system being observed. Acquiring the information you use to collapse the wave function does. If we have a group of scientists, none of which are part of the system being studied, then they will all make the same measurement, and all will use the same wave function, and it will seem very objective. But if one of the scientists is part of the system being studied, then the wave function that scientist uses to describe e.g. the Geiger counter, will not be the same as the one used by the scientists who are not part of the system being studied. The wave function is a very special way of encoding what you have measured, and the formalism of QM spells out in detail what you can then infer. For a community of scientists, what one knows, all know (in principle), and the wave function is the same. If one scientist knows more, by being an integral part of an isolated system that others are studying, then there is trouble. The SC paradox and in particular, Wigner's extension, is a brilliant way to bring this issue to the front.

 

[Q-reeus]

Rap, you've done a sterling job explaining your position but let's see if I have understood one key aspect right. That the time evolving wavefunction of the totally isolated system cat+Geiger-counter+poison+box etc. is basically just a 'knowledge probability function' tied entirely to the radioactive decay process within, and quite distinct from the 'actual physics' going on in said system (ie. - cat at any moment is alive, or is dead/dying, we just don't know). In #64 you wrote "...Until then the position of the box is in a superposition of moved and not-moved." I will in light of your subsequent statements take that as just a semantic slip-up - that you really meant "knowledge of the position...". Yes? Otherwise, the bang-crash problem posed in #65 remains imho acute and real [or maybe not, see below] (That dirty 'r' word many deny has real meaning). If so then quite honestly I cannot see any practical difference to an entirely classical system - Sir Isaac N opens his box and observes, but that has no influence on what's already happened - merely his awareness. Indeed why bother with unattainable but theoretically possible total isolation. Isn't it all governed by chance radioactive decay, which is quite insensitive to isolation or not? For that matter, dispense with the sole quantum aspect that matters - radioactive decay, and substitute a purely classical random event generator that over say the life of the universe will generate randomness indistinguishable from the real McCoy.

My understanding from various sources has been quite different - superposition means an actual indeterminacy of the physical system - it is, really and truly, in all possible states at once, but the weighting of the 'blurred system' evolves smoothly and deterministically in time - more and more favoring 'dead cat' as time goes on. And that observation suddenly crystallizes the actual physical state. Not merely revealing what already is, but forcing into existence at that point - hence the concerns in #65. [EDIT: Given that the Schrodinger eq'n evolves in a Hamiltonian manner, that should guarantee all superposed states are equivalent re energy & momentum? Severely restricting the cat states - cannot be standing/lying at the same time - maybe 'floating' at all times. Then there is no bang crash crisis upon wavefunction collapse, right? On further thought, that still doesn't restrict changes in center of mass - looks like back to square one.] Seems to be the lesson from say double-slit; observed or not effects the interference pattern in a very physical way. It's not just an alteration of our which-way knowledge, the pattern of screen hits alters tangibly - energy/momentum density has altered for real. So is there even one clear Copenhagen interpretation of SCP? From: http://en.wikipedia.org/wiki/Schrodinger's_cat, under Copenhagen Interpretation:

"In the Copenhagen interpretation of quantum mechanics, a system stops being a superposition of states and becomes either one or the other when an observation takes place. This experiment makes apparent the fact that the nature of measurement, or observation, is not well-defined in this interpretation. The experiment can be interpreted to mean that while the box is closed, the system simultaneously exists in a superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat", and that only when the box is opened and an observation performed does the wave function collapse into one of the two states.

However, one of the main scientists associated with the Copenhagen interpretation, Niels Bohr, never had in mind the observer-induced collapse of the wave function, so that Schrodinger's Cat did not pose any riddle to him. The cat would be either dead or alive long before the box is opened by a conscious observer.[5] Analysis of an actual experiment found that measurement alone (for example by a Geiger counter) is sufficient to collapse a quantum wave function before there is any conscious observation of the measurement.[6] The view that the "observation" is taken when a particle from the nucleus hits the detector can be developed into objective collapse theories. In contrast, the many worlds approach denies that collapse ever occurs."

So dissent from the beginning even within the CI camp it would seem. From #65 I consider it unphysical that observer initiated sudden collapse of a physically superposed SC system occurs, but the 'knowledge only' interpretation seems airy-fairy and essentially classical in all but name.

 

[Rap]

See, you and everybody else who attack my position by looking for logical inconsistencies and absurdities are the people my inner type-2 troll is looking for. People who quote this and that authority are not. Was it someone on this thread who included the quote from Einstein who, when confronted by a book entitled "100 scientists against Einstein", responded "Why 100? If I were wrong, one would be enough". Excellent.

... the time evolving wavefunction of the totally isolated system cat+Geiger-counter+poison+box etc. is basically just a 'knowledge probability function' tied entirely to the radioactive decay process within, and quite distinct from the 'actual physics' going on in said system (ie. - cat at any moment is alive, or is dead/dying, we just don't know).

As a Copenhagen sympathizer, I would say that there is no "actual physics" going on in said system, just like there is no "actual spin" of an electron, just what you measure given the measurement device you set up, which may measure along z axis, or maybe x or y. "actual physics" would be an appeal to a hidden variable approach to QM, which has been shown to be false. But, this produces a problem/learning opportunity in the case of Wigner's friend. To the friend inside the friend+SC box (F+SC), there is no "actual physics" inside the SC box, but to Wigner outside the F+SC box, the is no "actual physics" going on inside it?!? Grrrr. Let me think about it.

In #64 you wrote "...Until then the position of the box is in a superposition of moved and not-moved." I will in light of your subsequent statements take that as just a semantic slip-up - that you really meant "knowledge of the position...". Yes? Otherwise, the bang-crash problem posed in #65 remains imho acute and real

A more accurate statement would be "until then the wave function for the box and contents will consist of a superposition of moved and not moved regarding the position of the box".

If so then quite honestly I cannot see any practical difference to an entirely classical system...

If a system is described by a QM wave function, then there can be interference, causing a zero amplitude where + and - valued wave functions add their amplitudes. The probability is the absolute value of the amplitude, which can yield a null result from two non-null amplitudes. Classical probability is >0 and simply additive. The only way you get zero is the sum of zeroes. The probabilities inside the box must be calculated quantum mechanically, so no, it is still a quantum system.

My understanding from various sources has been quite different - superposition means an actual indeterminacy of the physical system - it is, really and truly, in all possible states at once, but the weighting of the 'blurred system' evolves smoothly and deterministically in time - more and more favoring 'dead cat' as time goes on. And that observation suddenly crystallizes the actual physical state. Not merely revealing what already is, but forcing into existence at that point - hence the concerns in #65.

I would not say that it is in all states at once. I would simply say that our bookkeeping regarding our knowledge of the system is encoded in a wave function which is represented as a vector in an N-dimensional space. Opening the box will collapse the state vector to one of many possible other vectors in the space that correspond to an observation (eigenstates). The state vector before opening the box can be expressed as a weighted sum of these eigenstates: i.e it is a "superposition" of these eigenstates. And that's all I would say. When we closed the box, the state vector was in an eigenstate corresponding to "cat alive, Geiger counter not clicked". Schroedingers equation expresses the evolution of what we may say about the system as time goes on: the state vector, which we knew at time zero, then moves around in the vector space in a particular way, described by Schroedinger's equation, and when we express it as a weighted sum of eigenstates, those weights change smoothly in time. When we open the box, our new information changes the state vector from the one calculated to the one observed. It is pointless to think of the state vector as "really" being in some eigenstate, while "apparently" being the vector we calculate using Schroedinger's equation, before we open the box. Its a bookkeeping problem, but not a classical one, because the evolution of the system uses quantum wave functions yielding interference effects which is not the way classical systems behave.

Given that the Schrodinger eq'n evolves in a Hamiltonian manner, that should guarantee all superposed states are equivalent re energy & momentum? Severely restricting the cat states - cannot be standing/lying at the same time - maybe 'floating' at all times. Then there is no bang crash crisis upon wavefunction collapse, right? On further thought, that still doesn't restrict changes in center of mass - looks like back to square one.

Energy, momentum, center of mass will all remain constant, to within Heisenberg uncertainty, which is negligible in this huge system. But when the cat drops dead, the energy, momentum, and center of mass do not change. I guess we have to say the box is not in a gravitational field, to insure isolation, so the cat is weightless, it does not drop, but it still might change its configuration. But even if there were a gravitational field, the position of the box would go up as the cat went down, and the center of mass would remain the same. Same for momentum and energy: no change.

Seems to be the lesson from say double-slit; observed or not effects the interference pattern in a very physical way. It's not just an alteration of our which-way knowledge, the pattern of screen hits alters tangibly - energy/momentum density has altered for real. So is there even one clear Copenhagen interpretation of SCP? From: http://en.wikipedia.org/wiki/Schrodinger's_cat, under Copenhagen Interpretation: (quote)

I pretty much agree with the quote. But its not "observed or not" that affects the interference pattern, its what instrument you choose as your measuring device. If you choose to measure position, you get interference. If you choose to measure momentum, no interference pattern. (Thats a bit glib, but I think its correct)

However, one of the main scientists associated with the Copenhagen interpretation, Niels Bohr, never had in mind the observer-induced collapse of the wave function, so that Schrodinger's Cat did not pose any riddle to him. The cat would be either dead or alive long before the box is opened by a conscious observer.[5]

This surprises me. I would expect Bohr to deny any physical reality to something that was not yet measured. Applied to an electron, I would not expect Bohr to say that an electron was spin up or spin down long before the electron spin is measured.

Analysis of an actual experiment found that measurement alone (for example by a Geiger counter) is sufficient to collapse a quantum wave function before there is any conscious observation of the measurement.[6] The view that the "observation" is taken when a particle from the nucleus hits the detector can be developed into objective collapse theories.

This sounds like decoherence theory to me, and decoherence theory is derived from QM, and therefore cannot deny it. The collapse is a collapse to a bunch of classical possibilities, and is an approximation. When the box is opened, if you are working in pure QM, the wave function collapses to a superposition of all the many possible "dead" eigenstates, or a superposition of all of the many possible "alive" eigenstates. In decoherence, you "collapse" the wave function to a sum of the two possibilities, and the probabilities are very accurately, but not perfectly additive. Then you open the box and one or the other becomes apparent.

In contrast, the many worlds approach denies that collapse ever occurs."

I object to the many worlds idea because it is unverifiable, you can make no measurement to verify or refute it, and therefore it is not scientific.

 

[Q-reeus]

See, you and everybody else who attack my position...

Hey Rap, sorry if you feel that way, but honestly not my intent - just a genuinely confused QM dummy. Really appreciate all the effort you go to.

As a Copenhagen sympathizer, I would say that there is no "actual physics" going on in said system, just like there is no "actual spin" of an electron, just what you measure given the measurement device you set up, which may measure along z axis, or maybe x or y. "actual physics" would be an appeal to a hidden variable approach to QM, which has been shown to be false...

Fine, I'm thinking that is basically Schrodinger's Instrumentalist approach someone mentioned earlier. Have to say though that the following impresses me still re 'actual physics' - refers to that micro-cantilever experiment mentioned in #22:
"Scientists supersize quantum mechanics": http://www.nature.com/news/2010/100317/full/news.2010.130.html
The way it's presented, comes across to me as achieving a 'real' superposed state.

Energy, momentum, center of mass will all remain constant...

I worded that badly. Basically was suggesting that cat center of mass could be anywhere within the box, without in any way violating the constancy of energy/momentum, and the total system COM.
As for the rest, well I really need to bone up in order to have serious dialogue on this sort of thing. So no more pestering you on this topic - honest. Have a nice stress-free day!