Does Schrodinger's Cat Paradox Suck?

[Delta2]

I would gracefully decline this. Define a dead system. You would see what I mean.
Life is chemistry, and when some critical functionalities stop in the brain, we call it brain dead or dead for short. That is the only definition of dead. As a result of micro state change - the macro state of dead appears. For smaller organisms like a virus, there is no *dead*.

Are prions dead or Alive?
Are viruses dead or alive? The dead body and alive body might be interchangeable from a pure chemistry perspective. Like a dead virus and an alive virus is.

There is no paradox here, never was.
I think ERP is a better one, at that.

I don't think the problem is whether the macro states of dead/alive are well defined but that when we measure(observe the cat in this case) we can only have one of these states and not a superposition. To say that the cat is in superposition of dead and alive is , for me, another way to say that i don't know in which of these states the cat is until i do a measurement.

 

[torquil]

I don't think the problem is whether the macro states of dead/alive are well defined but that when we measure(observe the cat in this case) we can only have one of these states and not a superposition. To say that the cat is in superposition of dead and alive is , for me, another way to say that i don't know in which of these states the cat is until i do a measurement.

Classical uncertainty is not the same as quantum superposition. Lack of knowledge about quantum state is not equivalent to a quantum superposition.

Also, everyone agrees that the measurement cannot tell us that the cat is both alive and dead simulaneously. This problem is about which quantum wavefunction describes the cat prior to the measurement. I though everybody agreed that all physical objects are described by wavefunctions, since QM encompasses classical mechanics. QM does not have restricted domain of validity according to current physics.

It is a matter of principle: Even if e.g. interactions with the environment causes the cat to be in a superposed state for only 10^-10000000 seconds, it is still a matter of principle that QM predicts that at some point it is a superposition. At least for an "idealized cat" that can be correctly described by two such quantum states.

 

[torquil]

Exactly. The superposition of dead cat/alive cat does not mean the cat is both dead and alive. It means there is a certain probability of opening the box and finding the cat dead and one minus that probability of finding it alive. Before we open the box, we just don't know and cannot know. To be very strict about it, even the statement that the cat is either dead or alive before we open the box is improper, because "is" implies a reality that cannot be measured or accessed.

Well, in the two-slit experiment we often say that one particle is two places at once. That's is another example of classical terminology used in quantum situations. I have no problem with that, and think it is a natural description due to the wave nature of the particle. Also, the path-integral QM formalism ties in nicely with this as well.

Feynman was critizied for using such terminology by Bohr when he presented his path-integral formalism (particle paths is classical terminology). But there is no problem with it if it is used correctly.

 

[G01]

The point of Schrodinger's cat is also not to illustrate decoherence (a concept developed 50 years after the paradox) or the emergence of classical laws from quantum things. Schrodinger is very explicitly an instrumentalist. From his cat paradox paper:For Schrodinger and his cat, there is no emergence of the classical from the quantum, because there are no such things as quantum states. Again, the whole point of the paradox is to show the impossibility of quantum states. It's an argument for instrumentalism in QM.

Let me reword what I said above and focus it on the OP's original question:

The modern day pedagogical usefulness of the Schrodinger's cat experiment is that it helps to illustrate the connection between measurement, decoherence, and the emergence of classical laws.

This is my answer to the OP original question. It may not be the original purpose of the thought experiment, but I think this is why Schrodinger cat is still a useful pedagogical tool.

 

[Rap]

I don't think the problem is whether the macro states of dead/alive are well defined but that when we measure(observe the cat in this case) we can only have one of these states and not a superposition. To say that the cat is in superposition of dead and alive is , for me, another way to say that i don't know in which of these states the cat is until i do a measurement.

No, its not a question of which state the cat is in before the measurement. You know what state it is in before the measurement, and that state is a superposition of dead and alive, which means that upon measurement, it will be found either dead or alive. To say that it is either dead or alive before the measurement is equivalent to a "hidden variables" approach to QM which has been proven wrong.

The fact that classically it makes sense to think of the cat as dead or alive before the measurement is because the act of measurement has a ridiculously small probability of affecting the outcome of the measurement. In the microscopic regime this is not always the case, and then it DEFINITELY makes no sense to think of what a superposed microscopic system is "really" like before measurement, beyond that given by the superposed wave function itself. Strictly speaking, it makes no sense in either case.

 

[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.