Does Schrodinger's Cat Paradox Suck?

[yuiop]

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.

 

[ThomasT]

Is Schrodinger's cat paradox a poor pedagogical example for students of QM?

Imho, yes.

 

[Delta2]

Well in my opinion it shouldn't be labeled as a paradox but as an example how things can be in a superposition of states UNTIL someone or something makes an observation that breaks down the superposition.

For me all the cats of planet Earth that i don't or can't observe are in a superposition of states, i know that for any cat there is probably someone else (different for each cat) other than me observing that cat and he knows the state of the cat, but since he doesn't communicate with me to inform me, for me the cat is in superposition. I think when we intuitevely conclude that a cat can't be in superposition of dead and alive we kind of think of an invisible universal observer that knows the state of the cat, but that kind of observer doesn't necessarily exists.

 

[Q-reeus]

I assume the 'anchor point' around which yuiop has built the argument in #1 seems to exactly reflect an entry by A. Neumaier I saw a while ago (basically that Decoherence 'rules'), but can't find. At any rate, suppose we follow the CI (Copenhagen Interpretation) rationale, and opening the box and viewing/recording the cat collapses the cat's wavefunction - 'causing' it to be dead or alive. What is actually being observed? The entire cat, or the minute fraction of photons reflecting off the surface fur of the cat? Obvious answer surely. And there is some clear theory explaining how such an extremely partial observation propagates instantly to effect the entire cat? What's more, even before opening the box, we note the cat is standing on the box, which is placed on a table or the floor etc. So what rule says 'cat wavefunction' is isolated from the box, and so on in and almost endless chain? There are so many loose ends to the whole notion of collapsing the wavefunction of some cleanly separate and internally coherent entity like 'cat' - imho anyway.

 

[JDude13]

I think that it is a good way of representing quantum mechanics using familiar, macroscopical entities.
My interpretation of the thought experiment is that the superposition of quantum states has only broken down for the cat. From our perspective the events are still in superposition.

Imagine there is a detector which measures the spin of a photon and displays it to a human observer which is concealed in a box. If the photon has one spin, the human raises his right hand, if it has the other, he raises his left.
Once the spin is revealed to the human observer, the superposition breaks down for him. For any outside observers, unaware of the outcome, he, too, is in superposition of both states. Also; if the apparatus is removed, free from any observation by outside observers, the two events are entangled. If you look at which hand the human observer has up the superposition breaks down for the apparatus and the human and vice-versa.

That's how I understand it, anyway.
The breakdown of superposition will be independant and unique for each observer.

 

[Q-reeus]

..Imagine there is a detector which measures the spin of a photon and displays it to a human observer which is concealed in a box. If the photon has one spin, the human raises his right hand, if it has the other, he raises his left.
Once the spin is revealed to the human observer, the superposition breaks down for him. For any outside observers, unaware of the outcome, he, too, is in superposition of both states. Also; if the apparatus is removed, free from any observation by outside observers, the two events are entangled. If you look at which hand the human observer has up the superposition breaks down for the apparatus and the human and vice-versa...

And here's where it really gets hazy and ambiguous imo. Raising a right hand or left hand will materially effect the momentum balance (as a temporal 'jerk' - overall momentum balance is preserved of course), and so 'which hand' is in principle easily inferred (not directly measured) by an outside observer. But if superposition of states applies, as you suggest, there will be no momentum imbalance from a clearly macroscopic act as 'raise hand', since as I understand it, superposition here implies a time averaged mix that leaves the box + person inside in a stable momnentum state - ie no shifts can be inferred. Is that really the case?

 

[DmplnJeff]

Yes, unless you hate cats.

On a more pedological note: I don't think the observer need be sentient. The cat would begin to decay. Or perhaps bacteria are sentient?

But it is often the way of the world that teachers pull the leg of students. It teaches character.

 

[JDude13]

Imagine there is a detector which measures the spin of a photon and displays it to a human observer which is concealed in a box. If the photon has one spin, the human raises his right hand, if it has the other, he raises his left.
Once the spin is revealed to the human observer, the superposition breaks down for him. For any outside observers, unaware of the outcome, he, too, is in superposition of both states. Also; if the apparatus is removed, free from any observation by outside observers, the two events are entangled. If you look at which hand the human observer has up the superposition breaks down for the apparatus and the human and vice-versa.

That's how I understand it, anyway.
The breakdown of superposition will be independant and unique for each observer.

Something I forgot to mention:

Seeing the human observer's hand raised can't tell you with certainty what spin the photon had. People lie, right?

The cat thought experiment is a little different.
The cat cannot fake death, but it may die of another cause.

So the two events are not completely entangled... but they are close.
Whether or not this is enough to preserve the superposition, I don't know.

But, hey! I'm only 16!

 

[jimgraber]

Of course Schrodinger's cat sucks. Schrodinger knew that right from the beginning. In fact it's pretty much the whole point. It illuminates two troubling issues in quantum mechanics, usually called the problem of measurement and the quantum classical transition (or Heisenberg's boundary).
You are certainly right that the cat will in fact be either alive or dead, but not both.
The problem is how to get there without violating the laws of quantum mechanics.
The most popular modern answer to that question is called decoherence.
But not everyone agrees that decoherence really solves the problem.
Jim Graber

 

[jimgraber]

P.S.
Schrodinger's cat was originally invented to puzzle experts, not to instruct newbies.

 

[Dickfore]

Actually, the amplifier, capsule and cat form the classical part of the measuring equipment and the quantum system is the radioactive source. I think the "collapse" of the wavefunction of the nucleus happens just when we turn on the amplifier.

Besides, the collection of radioactive isotopes should be regarded as a system of non-interacting (as far as radioactivity is concerned, the decay constant does not depend on the concentration of atoms) identical particles. A particle that decays is best described by making the energy of the particles with a negative imaginary part:

<br /> E \rightarrow E - i \frac{\lambda}{2}<br />

which makes the evolution of the single particle wave function:

<br /> \Psi(t) = \Psi(0) \exp\left[-\frac{i}{\hbar} (E - i \lambda/2) t\right] = \psi(0) \exp{\left[-<br /> \frac{\lambda t}{2}\right]} \, \exp{\left(-\frac{i}{\hbar} E t\right)}<br />

and the square modulus is:

<br /> |\Psi(t)|^{2} = |\Psi(0)|^{2} e^{-\lambda t}<br />

 

[yuiop]

Yes, unless you hate cats.

On a more pedological note: I don't think the observer need be sentient. The cat would begin to decay. Or perhaps bacteria are sentient?

This is a good point. Let us say the cat has plentiful supply of food, water and air in the box and we leave it for a year before opening the box. Now if when we open the box we observe that the cat is a rotting corpse that is almost a skeleton with billions of bacteria feeding on it. Any reasonable observer would conclude that the cat died a long time before the box was opened and did not collapse into this state of decomposing corpse at the moment the box was opened. The QM interpretation is that not only is the cat in a superposition of living healthy state and decomposing corpse, but some of the bacteria are in a superposition of being alive and not yet born until the box is opened!

Also, does physics have a sufficiently clear definition of sentient? As far as physics is concerned, a sentient being is a computing device above a not clearly defined level of complexity and performance.

 

[DmplnJeff]

Yuiop, there are plenty of ways to make the thought experiment fit. But they are not simple and clear. Thus it sucks as a teaching tool.

It's still useful for other things like pulling the legs of students or dead cats.

 

[torquil]

To Delta2 & JDude13:

Cool, you guys are thinking along the same lines as I. This inspired me to think about this some more this evening. And JDude13, I think your understanding is very impressive for a 16 year old!

 

[Rap]

Yup, I agree with JDude13. The wavefunction is not a totally objective entity, it a tool which encodes one's information about a process, and quantum mechanics then provides the means to estimate probabilities of the future, given that information. If two people have different information, the wave functions they assign to a situation will be different. When the human in the box opens the cat in the box, his infomation set changes, and his wavefunction for the cat collapses, but to the scientist outside the double box, it does not. The scientist outside the box assigns a wave function for the human/cat combination which is a superposition of states, which then collapses when HE opens the box. Most quantum experiments assume multiple equivalent observers, so that the process of measurement by one affects the information of all. This extension to Schroedingers cat (I think its called Schroedinger's friend) is an example where this is not so. Every objection that I have heard to this idea finally boils down to distaste for the idea that the wave function is not absolute. But I am open to suggestion.

 

[JDude13]

Seeing the human observer's hand raised can't tell you with certainty what spin the photon had. People lie, right?

The cat thought experiment is a little different.
The cat cannot fake death, but it may die of another cause.

So the two events are not completely entangled... but they are close.
Whether or not this is enough to preserve the superposition, I don't know.

But, hey! I'm only 16!

Another observation:
If we see the human with his left had raised does it increase the likelyhood of the associated outcome? As in... He may lie and make the information invalid but then again, he may not. The wave-function becomes a measure of his trustworthy-ness or his ability to read the measurement made by the apparatus (without telling us). What if another scientist tells him to only raise his left hand? Does that mean that our model of superposition in the box has no value as the other scientist has already determined the outcome. Perhaps there are subtle differences in the two different outcomes of the tainted experiment. Maybe the human has a different facial expression, depending on what he sees. Our simplification of the two out comes (left hand raised/ right hand raised) doesn't cover the wide range of things which would change.
Left for a longer and longer period of time, the outcomes will become more and more dis-simmilar.

For example: in the two outcomes, 1) & 2),
1)The observer raises his right hand.
2)The observer raises his left hand.

1)The observer is right-handed and so he is able to keep his arm up for a substantial amount of time.
2)The observer is right-handed so he is less able to keep his hand up for a substantial amount of time.

1)The observer's right hand is still raised.
2)The observer's left arm becomes tired and he lowers his hand. He becomes frustrated.

1)The observer's right arm becomes tired and he lowers it.
2)The observer feels he has been left in the box and begins beating on the sides trying to gain the attention of the scientists.

This is a rather rough and not-well-thought-out scenario of the two outcomes which shows us that time affects the differences in scenarios.

Now pretend that the observer doesn't need to do anything but look at the screen which reveals an up arrow or a down arrow when confronted with one of the two spin directions.

1)The arrow points down.
2)The arrow points up.

1)The observer's eyes are subconciously drawn down.
2)The observer's eyes are subconciously drawn up.

And on and on and on... Changing more wildly with each step.

This shows that superposition will be achieved macroscopically with any obsever, whether or not you give conditions and instructions to the observer.

 

[torquil]

Another observation:
If we see the human with his left had raised does it increase the likelyhood of the associated outcome? As in... He may lie and make the information invalid but then again, he may not. The wave-function becomes a measure of his trustworthy-ness or his ability to read the measurement made by the apparatus (without telling us). What if another scientist tells him to only raise his left hand? Does that mean that our model of superposition in the box has no value as the other scientist has already determined the outcome. Perhaps there are subtle differences in the two different outcomes of the tainted experiment. Maybe the human has a different facial expression, depending on what he sees. Our simplification of the two out comes (left hand raised/ right hand raised) doesn't cover the wide range of things which would change.
Left for a longer and longer period of time, the outcomes will become more and more dis-simmilar.

[shortened]

This shows that superposition will be achieved macroscopically with any obsever, whether or not you give conditions and instructions to the observer.

But remember that your lack of knowledge about a system does not itself mean that quantum superpositions between states representing different outcomes appear. Some of those you mention are uncertainties due to lack of information, not uncertainties due to quantum behaviour.

E.g. as you said, the observer might lie. But it might be the case that the microscopic quantum wavefunction description of the observer inside the box will not develop to make that possible, i.e. like a wavefunction that cannot develop into "spin up". This can be the case even though we as outside observer arent' aware of it. In that case, the observer will not be in a superposition of states "lie + no-lie", so it would be incorrect of us to describe him/her in that way.

If you knew the full quantum description of the intenal observer, you would know exactly which superposition of states the composite system (internal observer + experimental quantum system) was in after their mutual interaction occured. Otherwise you must describe the composite system usinga density matrix which includes classical uncertainties, not only a wavefunction.

 

[Phrak]

The best that can be said of measurement in postulatory quantum mechanics is that the information obtained or ignorance removed upon measurement (partially determined according to prescription) is not the same http://en.wikipedia.org/wiki/Information_theory" obtainable before a measurement is made.

There are not dead cats and live cats existing in some odd superposition except by interpretation of the measure of subjective knowledge, nor multiple worlds, nor pilot waves, etc. except as interpretations to make sense of it.

 

[Synetos]

I think that its a great example, eventhough I didn't have the actual subject yet. (so maybe I don't really am entitled to have a say on this :P) But I really like to have imaginable examples for theories, things that i can visualise. And Schrodingers cat does just that.

 

[Phrak]

I think that its a great example, eventhough I didn't have the actual subject yet. (so maybe I don't really am entitled to have a say on this :P) But I really like to have imaginable examples for theories, things that i can visualise. And Schrodingers cat does just that.

Yes. I don't disagree at all. These things are indispensable to some form of mental imagery or deductive chain.

 

[DmplnJeff]

Part of the purpose of the cat experiment is to seek enlightenment about what superposition really is. While this might not be useful to the introductory student, it does reveal a deeper truth about the universe:

There are facts in the universe that are unknowable by experiment. One of these seems to be the nature of superposition. There are several prominent interpretations of superposition that are experimentally identical but show wildly differing views of the universe. http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics"

Initially these serve as distractions to learning the math because each interpretation seems to lead students to err. Later on, contrasting them provides insights. Perhaps some of these might even be disproved by clever scientists. But that's why quantum physics isn't easy like rocket science.

 

[Q-reeus]

Came across the following link which in laymen's terms seems to set the right perspective:
http://factoidz.com/understanding-schrodingers-cat-and-quantum-uncertainty/
Makes it plain why Schrodinger's cat or hand waving humans etc inside boxes with detectors can never in reality be in a superposed state. Experimentalists understand what's really required - even for a ~10um sized mirror, cooling to millikelvin levels and delicate suspension in a vacuum are absolute prerequisites. Compare that to a room-temperature cat/human-in-a-box! Hence the OP's premise is more than confirmed - a very poor and misleading thought experiment, imho.

 

[G01]

Of course Schrodinger's cat sucks. Schrodinger knew that right from the beginning. In fact it's pretty much the whole point. It illuminates two troubling issues in quantum mechanics, usually called the problem of measurement and the quantum classical transition (or Heisenberg's boundary).
You are certainly right that the cat will in fact be either alive or dead, but not both.
The problem is how to get there without violating the laws of quantum mechanics.
The most popular modern answer to that question is called decoherence.
But not everyone agrees that decoherence really solves the problem.
Jim Graber

Exactly. The point of Schrodinger's cat is not to say that the can can indeed be both dead and alive, but instead to illustrate the connection between, measurement, decoherence, and the emergence of classical laws.

There was a recent thread whose discussion was along these lines. Check out my post, as well as Tiny Tim's:

https://www.physicsforums.com/showthread.php?t=467362

 

[Q-reeus]

No doubt that is true for serious students of QM who are taught the right perspective(s), but do a web search using "Schrodinger's cat" and just see how many links present the 'classical interpretation' of alive+dead cat! It's the perpetuation of a popular myth that the non-specialist/general public accept as 'weird fact' that bugs me.

 

[G01]

It's the perpetuation of a popular myth that the non-specialist/general public accept as 'weird fact' that bugs me.

Add it to the list. It's not the first time public perception has misunderstood science.

 

[JDude13]

But remember that your lack of knowledge about a system does not itself mean that quantum superpositions between states representing different outcomes appear. Some of those you mention are uncertainties due to lack of information, not uncertainties due to quantum behaviour.

E.g. as you said, the observer might lie. But it might be the case that the microscopic quantum wavefunction description of the observer inside the box will not develop to make that possible, i.e. like a wavefunction that cannot develop into "spin up". This can be the case even though we as outside observer arent' aware of it. In that case, the observer will not be in a superposition of states "lie + no-lie", so it would be incorrect of us to describe him/her in that way.

If you knew the full quantum description of the intenal observer, you would know exactly which superposition of states the composite system (internal observer + experimental quantum system) was in after their mutual interaction occured. Otherwise you must describe the composite system usinga density matrix which includes classical uncertainties, not only a wavefunction.

I realize that. I was just exploring whether or not seeing the observer with one hand up was enough to break the superposition given that he may put that hand up regardless of the outcome. I then went on to say that even seeing him, regardless of his decision, shows slight differences between each state which is in superposition and that the entanglement will break down apon seeing the observer regardless of whether or not you told him to do something different for each outcome. There will be subtle differences in the behaviour of the observer in each outcome. Such is the nature of the human brain.

 

[kote]

It's been touched on already... but Schrodinger's paradox was originally formulated by Einstein as a reductio ad absurdum argument against the completeness of QM.

Einstein, in a letter to Schrodinger:

The system is a substance in chemically unstable equilibrium, perhaps a charge of gunpowder that, by means of intrinsic forces, can spontaneously combust, and where the average life span of the whole setup is a year. In principle this can quite easily be represented quantum-mechanically. In the beginning the psi-function characterizes a reasonably well-defined macroscopic state. But, according to your equation [i.e., the Schrödinger equation], after the course of a year this is no longer the case. Rather, the psi-function then describes a sort of blend of not-yet and already-exploded systems. Through no art of interpretation can this psi-function be turned into an adequate description of a real state of affairs; in reality there is just no intermediary between exploded and not-exploded.

And Schrodinger's version of the paradox begins, by the way, with:

One can even set up quite ridiculous cases. A cat is penned up...

Even Schrodinger didn't buy one bit of the the alive+dead cat thing... it was exactly the opposite.

If the paradox is used to teach anything it should be the inadequacy of many explanations of QM.

See http://plato.stanford.edu/entries/qt-epr/#1.3 for the Einstein quote. http://www.tu-harburg.de/rzt/rzt/it/QM/cat.html is Schrodinger's paper. You'll have to excuse me for bringing history and philosophy into a physics discussion :).

 

[kote]

The point of Schrodinger's cat is not to say that the can can indeed be both dead and alive, but instead to illustrate the connection between, measurement, decoherence, and the emergence of classical laws.

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:

Reality resists imitation through a model. So one let's go of niave realism and leans directly on the indubitable proposition that actually (for the physicist) after all is said and done there is only observation, measurement. Then all our physical thinking thenceforth has as sole basis and as sole object the results of measurements which can in principle be carried out, for we must now explicitly not relate our thinking any longer to any other kind of reality or to a model.

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.

 

[Rap]

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.

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.

 

[nmondal]

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.

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.

 

[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.