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What's wrong in taking the cat as an observer in Schrodinger's experiment? Plz kindly elaborate! And if possible also describe about possible logics if the question bears the answer No.

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If we take cat as an observor,then the rest of the world will be in a state of superposition of dead or alive. –  AaKASH Mar 4 '13 at 4:03
No, because in Quantum MEch, "observer" has a very precise meaning: someone looking at a measurement apparatus which has sensed, using amplification, a quantum object's observable. See below. The cat is not using the apparatus to measure the universe or itself, it is using the apparatus to measure the radioactive decay event. –  joseph f. johnson Jun 5 '13 at 17:00
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5 Answers

In Schrodinger’s cat experiment there is a mixture of a macroscopic object, the cat, and a quantum mechanical object, the radioactive atom, which if decayed it would activate a hammer to break the flask, with the poisonous gas in it, and therefore killing the cat.

This description is very straight forward and not very hard to understand: the cat in the box with the flask containing the poisonous gas, and the radioactive element which upon decay would activate the hammer.

If the box containing all these things was made of glass, we would be able to watch the whole drama until the sad final eventuality (no need to describe, we don’t even want to think about it!)

The problem, however, begins when the box is not transparent, and we want to make some prediction of the fate of the cat inside the box. The first prediction we would have to make is whether and when the atom will decay. This is clearly a quantum mechanical event and we can only assign a probability to it. We do this by writing down the quantum state of the atom having decayed and atom not having decayed. But according to quantum mechanics, before we actually perform an observation, the atom is in a superposition of these two states, decayed and un-decayed at the same time as shown below:


If this is the situation with the atom, what can be said about the cat? The presence of the cat in the problem requires the enlargement of the quantum state so that to describe the whole cat-atom system. According to quantum mechanics, the enlarged state must have the cat in a superposition of cat alive and cat dead in the same way the atom is in the above superposition

|Atom-cat>=$D_1$|Atom Decayed-Cat Dead>+$D_2$|Atom Undecayed-cat Alive>.

This is where things go pare shape and play havoc with our logic. I think experiments have been undertaken, without a real cat of course. Instead, the experimenters used sensitive electric quantum systems on which they performed very short time duration observations, so that they did not destroy the superposition! See this link below.


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This is generally correct. But in fact it also demonstrates that the wave function constructed mirrors what the experimenter knows. Consider a second set of observers watching the cat through a glass wall. Their wave function will not be a superposition at all. It will be either |alive> or |dead> (in terms of the cat). If the cat dies there will be a transition between the two states. That transition between the states is equivalent to the decay of an electron from one atomic orbital to another, a simple transition. –  Paul J. Gans Mar 4 '13 at 1:06
This is generally correct except for calling the cat a macroscopic object. Schroedinger wanted to have it both ways. And Quantum mechanics allows him to have it both ways. If we are the observer, the cat etc. is a quantum object until we observe it. Then it is treated as a macroscopic object. If the cat is an observer, it is always a macroscopic object and never a quantum object. In the decoherence viewpoint, there is no difference in theory between macroscopic and quantum, only a practical distinction that macroscopic objects have a tendency to decohere much faster than quantum objects. –  joseph f. johnson Jun 5 '13 at 19:41
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This is a thought-provoking question, but... the concept of observer is only important if one takes the Copenhagen interpretation. So let us stick to that for the purpose of answering the OP.

The cat cannot be an observer of itself. In the logic of the Copenhagen set-up, there is a quantum mechanical object being observed, a classical measurement apparatus with pointers pointing to values obtained by the measurement, and us observing the pointers or dials or whatever.

In Schroedinger's thought experiment... please do not do this to an actual cat, it would be illegal in New York, and immoral anywhere else... the cat plus the radioactive atom plus the poison gas etc. is the quantum mechanical system. Our naked eye observation of the cat's dead or alive state is the classical measurement apparatus and we are also the observer.

You see that Schroedinger was deliberately stretching the Copenhagen framework way beyond the kinds of situations it was designed for. It was only designed for situations where the radioactive atom is the quantum mechanical object, and the cat is the observer, and the poison gas apparatus is the classical measurement apparatus which measures the decay of the radioactive atom: here the dial is the poison gas, the two things the dial can point to are: release of the gas or non-release of the gas.

But Bohr can logically escape. It is key to the Copenhagen Geist that you have to pick how you analyse the situation, and then abide by the consequences. Just as you have to decide whether you are measuring position or measuring momentum, and cannot illegitimately switch around mid-stream, so here, you have to analyse separately who is the observer.

In Schroedinger's intention, we are the observer, but then the cat is what we are observing and so the cat has to be considered as a quantum mechanical object.

The OP wants to know what if the cat is the observer. If the cat is the observer, you see by the above that what the cat is observing is the radioactive decay. The cat is not observing itself. The cat "looks at the value pointed to on the dial of the measurement apparatus" i.e., breathes in the poison gas or does not breathe it in, those are the ways this inhumane measurement apparatus points to the results of the measurement, decay or non-decay.

The cat is not observing itself.

This "paradox" is then the same as the paradox of "Wigner's friend". (Oh, I am Wigner's (son's) friend too although we are out of touch now..David Wigner proved some real theorems in his day.) Wigner's friend runs an experiment and observes it. But Wigner does not, he waits, and then observes Wigner's friend. Before Wigner observes his friend, his friend must be in a superposition of states of knowledge of the result of the experiment.

There is no logical contradiction in either paradox as long as one adopts the Copenhagen interpretation consistently and brutally....

In daily life, a cat does observe itself, but only classically. A cat never observes itself through a quantum measurement, and so it is never "observation" in the sense of Copenhagen. It is "observation" in our everyday meaning, but that is a classical meaning. In the Schroedinger experiment, the cat surely notices whether it is dying or not, but it notices this in a classical way, not a quantum way. Or, at least, I would notice it if I were the cat. Or, at least, I would notice it if I were paying attention...

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In every description of the experiment on this page, it is a single nucleus that decays and triggers the hammer. Is it even possible to isolate a single nucleus and have the product(s) of the decay trigger a mechanism (1) in principle and (2) is the technology to do so now at our disposal? Could the experiment still be conducted with an ensemble of atoms, say a couple hundred, and then we measure how many have decayed after time X, if fewer than statistically expected, the hammer is triggered, if more, the hammer is not triggered? –  Eugene Seidel Jun 5 '13 at 18:35
In my description too. Yes this experiment could be done today. Also for not a single nucleus but for a nano-tech collection of a few hundred. But none of these issues is the real point. The real point is the state of the cat "after" the trigger or non-trigger but before we open the box and look inside. It is technologically impossible to probe the quantum state of the cat inside the box during that in-between period. Physicists are divided as to whether it will ever become possible to do so, many of them have abandoned the Copenhagen viewpoint. –  joseph f. johnson Jun 5 '13 at 19:37
The decoherence theory would assert that the cat rapidly settles its quantum state to be all dead or all alive even way before we open the box. But in the decoherence theory, the concept of "observer" is unimportant, so this doesn't quite fully address the OP. –  joseph f. johnson Jun 5 '13 at 19:38
The cat is not an observer for all the reasons already given plus the fact that the cat cannot communicate what it observes to us. So the cat can be ignored. But I repeat my statement above involving two sets of observers, one able to see the cat, the other not. For the observers that see the cat, there is no superposition. If the cat dies, a transition between the life and death states is seen. Otherwise the cat continues alive. The conclusion to all this is that the state function reflects the observer's knowledge and NOT necessarily objective reality. –  Paul J. Gans Jun 6 '13 at 2:00
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The experiment is intended to highlight the problem of quantum superposition applied to macroscopic objects.

It's not inconceivable that a radioactive nucleus can be in a superposition of states. When you interact the nucleus with the detector, hammer and glass of poison, the wavefunction that describes these items become entangled with the wavefunction of the nucleus, and they must also enter a superposition of states. The wavefunction that describes the cat must also become entangled, hence the cat must also exist in a superposition of states (dead and alive). Finally when the experimenter looks at the cat the wavefunction that describes the experimenter also becomes entangled and the experimenter must also be in a superposition of states i.e. the states of having observed the cat to be alive and having observed it to be dead.

The point of the experiment is that common sense tells us we don't see cats that are both alive and dead, so somewhere along the line the argument presented above must break down. The Copenhagen interpretation is that the act of observation collapses the superposition, and insofar as the term observer has a meaning it is the point at which this happens. Since humans attach a great deal of importance to being conscious beings they tend to regard themselves as the observer, and hence the cat would not be considered an observer. The cat would presumably disagree.

The current interpretation (well, "a" current interpretation) of the experiment is described by the theory of decoherence and the Many-Worlds Interpretation and does not involve the concept of an observer. The act of observation does not have any effect, so the point is moot. Neither the cat nor the experimenter are observers in the Copenhagen sense.

The idea behind decoherence is that every object interacts with its environment, and the more complex the object the faster it interacts. The interaction breaks the superposition and the system divides into separate histories that cease to interact. While a nucleus is small and simple enough to remain as a superposition for a long time, the detector, hammer and galss vial are already too complex to remain in a superposition for a measurable time, let alone the cat and the experimenter.

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I don't quite understand what you're asking, but Schrodinger was using reductio ad ridiculum (appeal to ridicule) as an attempt to disprove quantum mechanics, as a living thing can't be both alive and dead as far as we know (unless you get into the realm of parallel universes etc.)

Also, don't get confused about the observation part. E.g. in the double slit experiment, it's not our brains that change reality, but the method used to observe (firing particles at it, for example). Don't think that just humans being aware of it changes the reality; unfortunately we aren't that powerful!

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You are right about the observation part. Actually, it is in nature of nature to be unpredictable. That is the postulate of QM –  Cheeku Mar 3 '13 at 15:24
I meant to ask just as we are observer for the cat and are predicting about cat's superposition state, can the cat also be condidered as an observer so that he may also find us in superposition state of dead or alive???? –  AaKASH Mar 4 '13 at 11:26
The cat knows the outcome... –  DarkLightA Mar 4 '13 at 15:11
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There is also another possible solution to this problem which fits perfectly well, but its a strange solution that turns everything outside in. In the June number of Scientific American there was an article about Quantum Bayesianism where the wave equation of all objects is placed in the mind of the observer. This explains why the act of observing a photon can change it from wave-behavior to a particle. Or how a measurement can establish the position, spin and velocity of an electron. So Qbism propose that the act of subjectively observing establish the outside world and everything is in our mind. This is of course very strange and in many ways like a dream. And we all agree that in the dream-state the wave equation of objects are in our mind, but while we are dreaming we think its real and we think that the world is created from the outside. So if this is a dream, we would refuse the idea that this is a dream.

But if its all in the mind whose mind is it in? It has to be in the mind of every subjective observer. Then we all live in our subjective worlds where what we think becomes the truth. We think seeing is believing, but the stronger mechanism may be this other way around that believing is seeing, and what we believe are based on our experiences and expectations, so it just seems like seeing is believing.

So if you think there is something like a working random generator, there is a working random generator in your subjective world, which will kill the cat in a parallel reality while its perfectly fine in another, and you will unconsciously choose the parallel reality with a result close to what you expect and believe. Different observers may then end up in different realities where the cat can be dead or alive according to their expectations. To add some more strangeness, the observers are also observing each other, and create wave-equations of each other in their mind and define each other, everything everybody see is then a reflection of themselves, and we all live in our individual worlds interacting with versions of other individual worlds which fit with our beliefs.

In one parallel reality the cat dies and its sent to church for burial, then Jesus comes along and he believes he can bring it back to life so he does, and the people in church believe Jesus can do this, so they can witness him do this miracle.

When we give a patient a placebo, its not the saline water or the sugar pill which cures the patient, but the patients belief that he will get well which cures him. So the body of the observer is by the observer looked upon as an object which the subjective mind can define and change by will if you believe you can change it by will.

Personally I don't believe that there is such a thing as beliefs manifesting reality, so then I may manifest a subjective world where it seems like thoughts cannot manifest reality. Then again if I start to believe that reality is in my mind and what's in my mind I can change, then I could start to do some conscious manifestation, as I can do in my dreams when I am aware of that I am dreaming. And in some parallel reality of mine, I may end up as the new Jesus bringing all those cats back to life.

So the answer to the question may be: Every observer, the cat included, chooses consciously or unconsciously a parallel reality aligned with their own beliefs, and who knows, the cat in the cats reality may end up alive and kicking every time, because it believes it can.

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