# Why is Schrodinger's Cat only a thought experiment?

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## Main Question or Discussion Point

The explanation I've heard is that quantum superposition is an atomic-scale phenomenon, that it makes no sense to apply it to a macro-scale object such as a cat.

But that's not what the experiment is doing. The radioactive isotope is subject to quantum effects, and it seems to follow that the result can be detected (and thus acted upon) at a macro-level.

So, why would the cat not exist in two states?

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Well, it would according to the many-worlds interpretation. The worlds would split as soon as the cat interacted with anything else in a theremodynamically irreversible way.

In any other interpretation, you'd have to define what it means for the cat to "exist" in two states. The cat certainly knows whether it's alive or not, so from his perspective there's only one state. From the outside world's perspective, the state is one of ignorance. Mathematically, it's in superposition, and so the wave function as calculated by the outside observer would indeed reflect both states. So in a sense, the cat *does* exist in both states as far as the outside world is concerned. But are there literally two cats, or is the superposition of states nothing more than a fancy way of saying that we don't know the cat's state? It's a matter of interpretation.

turbo
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Schrodinger was posing what he considered an absurdity to point out what he felt was a disconnect between deterministic classical physics and quantum theory. Clearly a cat cannot be both alive and dead at the same time, and opening the box after an hour to observe whether the cat is dead or alive does not collapse the wave-form of the cat (a macroscopic being), it simply tells us that the radioactive material either did or did not decay during the previous hour. Penrose uses this example in his popular lectures on the possibility of unifying quantum physics with classical physics.

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Schrodinger was posing what he considered an absurdity to point out what he felt was a disconnect between deterministic classical physics and quantum theory. Clearly a cat cannot be both alive and dead at the same time, and opening the box after an hour to observe whether the cat is dead or alive does not collapse the wave-form of the cat (a macroscopic being), it simply tells us that the radioactive material either did or did not decay during the previous hour.
I know, but why does it not work? Where does it fall apart?

Does it indicate that the isotope particle is not really in a superpsoed state?

I know, but why does it not work? Where does it fall apart?
It falls apart when you try to assign physical meaning to the wave function. Only then do you have to ask the question whether the cat is "really" alive or dead. The wave function tells you the probability amplitudes of the cat's various possible states, and works perfectly well as long as you don't try to use it for anything other than calculating probabilities.

And again, if you believe in MWI, then there literally ARE two cats, with the universes splitting as the cats interact with the rest of the world.

Does it indicate that the isotope particle is not really in a superpsoed state?
The isotope is absolutely in a superposed state as far as the ignorant observer is concerned. As far as the cat is concerned, it's not. Now, you can start getting really philosophical and ask if the cat is intelligent enough to be considered an observer - that's really stretching the bounds of meaningful science if you ask me. Again, as long as you treat the wavefunction as nothing more than probability amplitudes, there is no paradox. If you try to assign deeper meaning to it, then you run into trouble, and I believe MWI is the only thing you can use to make any sense of it.

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Now, you can start getting really philosophical and ask if the cat is intelligent enough to be considered an observer
Well, observation has a specific meaning in this context, and it has nothing to do with intelligence. A camera could be an observer.

Schrodinger was posing what he considered an absurdity to point out what he felt was a disconnect between deterministic classical physics and quantum theory. Clearly a cat cannot be both alive and dead at the same time, and opening the box after an hour to observe whether the cat is dead or alive does not collapse the wave-form of the cat (a macroscopic being), it simply tells us that the radioactive material either did or did not decay during the previous hour. Penrose uses this example in his popular lectures on the possibility of unifying quantum physics with classical physics.
Clearly, according to the quantum mechanics, the cat will be in superposition of being alive and dead at the same time. Combining this with your claim, we arrive at the source of the paradox. Two claims, both clearly true, both contradictory. That's why we call it a paradox! IMO ignoring the paradox only tells about lack of understanding of quantum mechanics.

DaveC426913 said:
Why is Schrodinger's Cat only a thought experiment?
I believe I can answer this: Because a physical experiment would be useless. There is no interferences for a macroscopic objects, so you cannot prove the existence of the superposition.

Classical theory: The cat will be either dead or alive in the closed box. When you open the box, you see the cat being dead or alive.

Quantum theory: The cat will be in superposition of being dead and alive in the closed box. When you open the box, the cat's wave function collapses, and you see the cat being dead or alive.

The experimental result: When you open the box, the cat is dead or alive.

An actual experiment wouldn't tell us anything. That is why this is a thought experiment. Somebody might claim that the Occam's razor instructs us to abandon the hypothesis of cat being in superposition, since it is so complicated idea, and cannot be checked by experiment, but IMO this is invalid use of Occam's razor. This is a fact: Whenever the predictions of classical and quantum theory differ, the experimental result is that quantum theory is right. So IMO it seems simplest to assume, that the quantum theory is right also when it cannot be checked.

peter0302 said:
In any other interpretation, you'd have to define what it means for the cat to "exist" in two states. The cat certainly knows whether it's alive or not, so from his perspective there's only one state. From the outside world's perspective, the state is one of ignorance. Mathematically, it's in superposition, and so the wave function as calculated by the outside observer would indeed reflect both states. So in a sense, the cat *does* exist in both states as far as the outside world is concerned. But are there literally two cats, or is the superposition of states nothing more than a fancy way of saying that we don't know the cat's state? It's a matter of interpretation.
Wise words! IMO the solution to the paradox lies in the many worlds interpretation, and not in restricting the quantum theory to the microscopic objects only.

I know, but why does it not work? Where does it fall apart?

Does it indicate that the isotope particle is not really in a superpsoed state?
One particular modern viewpoint is that the cat will be both alive or dead, but will unitarily evolve into a definite state because superimposed states tend to be unstable. Read up on Einselection ( http://en.wikipedia.org/wiki/Einselection ) and if you are familiar with density matrices, I invite you to take a look here: http://www.physics.thetangentbundle.net/wiki/Quantum_mechanics/einselection [Broken]

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Well, observation has a specific meaning in this context, and it has nothing to do with intelligence. A camera could be an observer.
That's not exactly right. The pure form of the Copenhagen Interpretation is agnostic as to what qualifies as observation and what doesn't. Some people believe that intelligent observatoin is actually required for wave function collapse. I once accidentally offended someone on here quite seriously by calling those people nuts, but apparently it's not without its supporters.

The subject line of this thread made me think you were asking what's wrong with putting a cat in a box with a vial of cyanide with a random trigger ....:surprised

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An actual experiment wouldn't tell us anything.
I hadn't thought of it that way. The test is not falsifiable.

Although I think that's not the real reason why this is only a thought experiment.

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That's not exactly right. The pure form of the Copenhagen Interpretation is agnostic as to what qualifies as observation and what doesn't. Some people believe that intelligent observatoin is actually required for wave function collapse. I once accidentally offended someone on here quite seriously by calling those people nuts, but apparently it's not without its supporters.
I am of the pure camp.

So, why would the cat not exist in two states?
Decoherence! The isotope needs to interact with the mechanism that breaks the vial. This mechanism is macroscopic. Entanglement with the particles of this mechanism will eliminate superposition, resulting in classical probabilities for the outcome.

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Decoherence! The isotope needs to interact with the mechanism that breaks the vial. This mechanism is macroscopic. Entanglement with the particles of this mechanism will eliminate superposition, resulting in classical probabilities for the outcome.
Decoherence eliminates interference, not superposition.

Decoherence eliminates interference, not superposition.
Well... actually... you can eliminate superposition by changing basis... so decoherence can also eliminate it, for what it's worth.

vanesch
Staff Emeritus
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So, why would the cat not exist in two states?
I think the obvious answer is that we all like cats, and we are not going to do cruel things to them

Seriously, MWI-people claim that the cat DOES exist in two states, of which we are only aware of one of them (and other "we's" are aware of the other state).

Hurkyl
Staff Emeritus
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To my knowledge, only three options are offered by all of the main interpretations of quantum mechanics are:

(1) You see a cat that's dead, or you see a cat that's alive.
(2) You are actually in a superposition of states, some of which see a dead cat, some of which see a live cat, and none of which see any sort of ambiguity.
(3) Both (1) and (2) are valid analyses.

LURCH
I hadn't thought of it that way. The test is not falsifiable.

Although I think that's not the real reason why this is only a thought experiment.

I notice that you posted this last night. Having had a chance to sleep on it, do you still have the same opinion? I myself am quite convinced that this is exactly the reason this test is not performed in reality (using something other than a live cat, of course).

The test is very falsifiable... Take a beam splitter and shine one part of the beam on the alive/dead cat and allow the other to pass straight through. Recombine the beams onto a detector. I believe you should get a different interference pattern based on whether the cat is alive/dead/superposition.

I didn't say the test was practical, though.

The subject line of this thread made me think you were asking what's wrong with putting a cat in a box with a vial of cyanide with a random trigger ....:surprised
vanesch said:
I think the obvious answer is that we all like cats, and we are not going to do cruel things to them
LURCH said:
this is exactly the reason this test is not performed in reality (using something other than a live cat, of course).
You are so excessively ethical! The medical industry already uses animal experiments for scientific purposes. Why not theoretical physics too!?

Well... actually... you can eliminate superposition by changing basis...

$$|\psi\rangle_{\textrm{cat is alive}},\;|\psi\rangle_{\textrm{cat is dead}}$$

we choose a new basis

$$|\psi\rangle_1 = |\psi\rangle_{\textrm{cat is alive}} + |\psi\rangle_{\textrm{cat is dead}}$$

$$|\psi\rangle_2 = |\psi\rangle_{\textrm{cat is alive}} - |\psi\rangle_{\textrm{cat is dead}}?$$

Now the cat won't be in superposition of two basis vectors, but this is not a very convincing trick if you wanted to solve the Schrödinger's paradox!

so decoherence can also eliminate it, for what it's worth.
You put the word "so" there, as if the previous comment somehow explained how decoherence removes superposition

The test is very falsifiable... Take a beam splitter and shine one part of the beam on the alive/dead cat and allow the other to pass straight through. Recombine the beams onto a detector. I believe you should get a different interference pattern based on whether the cat is alive/dead/superposition.
This is only a complicated way of opening the box.

$$|\psi\rangle_{\textrm{cat is alive}},\;|\psi\rangle_{\textrm{cat is dead}}$$

we choose a new basis

$$|\psi\rangle_1 = |\psi\rangle_{\textrm{cat is alive}} + |\psi\rangle_{\textrm{cat is dead}}$$

$$|\psi\rangle_2 = |\psi\rangle_{\textrm{cat is alive}} - |\psi\rangle_{\textrm{cat is dead}}?$$

Now the cat won't be in superposition of two basis vectors, but this is not a very convincing trick if you wanted to solve the Schrödinger's paradox!

You put the word "so" there, as if the previous comment somehow explained how decoherence removes superposition

This is only a complicated way of opening the box.
My first statement elucidates the fact that it isn't the superposition that gets people in knots, but the direction the state is pointing in. So either all states are equally good, and detractors should just shut up, or that some states seem "preferred", and we should figure out why. (Said differently, if you buy the whole wavefunction collapse argument, why can't we simply collapse it into $$\left|\psi\right\rangle_1$$ etc.?) Personally I believe that environmental eigenstate selection gives a decent explanation.

My second statement, a "complicated way of opening the box", eliminates the possibility that the act of measurement will cause a) collapse or b) decoherence, e,g, by a macroscopic observer. Of course, the cat could still observe itself (thereby violating linearity/unitarity of QM), but I digress... See statement 1.

Wise words! IMO the solution to the paradox lies in the many worlds interpretation, and not in restricting the quantum theory to the microscopic objects only.
Doesn't the many worlds interpretation SERIOUSLY violate the first law of thermodynamics though? The only way I think you can get around that is to argue that time doesn't exist (as the common conception) and all possible states ever exist simultaneously.

Ken G
Gold Member
Clearly, according to the quantum mechanics, the cat will be in superposition of being alive and dead at the same time.
Quantum mechanics does not say that because a combined cat/apparatus state does not have to project onto a pure (or "superposition") state for the cat alone. This is a common misconception you see in a lot of places. But as Hurkyl said, the real issue is whether or not the entire system (including you, importantly), can be in a superposition state or not. That is not at all the same as asking if the cat can be.
Combining this with your claim, we arrive at the source of the paradox. Two claims, both clearly true, both contradictory. That's why we call it a paradox!
But it isn't a paradox, there is no difficulty in resolving it-- the real problem is we have multiple choices of how to do it! (Again see Hurkyl's post.)
Quantum theory: The cat will be in superposition of being dead and alive in the closed box. When you open the box, the cat's wave function collapses, and you see the cat being dead or alive.
Again, this is not what quantum theory says. Quantum theory says that "opening the box" means projecting the cat onto a state you can label "alive" or "dead", that's just what you mean by opening the box, however you do it. For simple systems, that involves "collapsing the wave function", but a cat doesn't have a wave function. Even for quantum systems, however (which are "systems usefully treated using wave functions"), collapse occurs for a particular reason-- you intentionally forced them to project onto a certain basis. You collapsed it on purpose, in other words, it wasn't anything mysterious. What is mysterious is why all we can get is a statistical prediction of how that collapse will play out, but there may not be any resolution of that problem-- it may be all we can do (it certainly is so far).
Somebody might claim that the Occam's razor instructs us to abandon the hypothesis of cat being in superposition, since it is so complicated idea, and cannot be checked by experiment, but IMO this is invalid use of Occam's razor.
It can be checked. If the alive cat is standing, and the dead cat is lying down, then you can easily check for interference between the two states. The beamsplitter approach has already been mentioned by lbrits.
This is a fact: Whenever the predictions of classical and quantum theory differ, the experimental result is that quantum theory is right. So IMO it seems simplest to assume, that the quantum theory is right also when it cannot be checked.
But a standing/lying cat that you could send a light beam through is not the prediction of quantum mechanics. To use that language, you would have to be talking about the projected state of the cat, and any such projection will destroy the superposition you are claiming to exist.
Wise words! IMO the solution to the paradox lies in the many worlds interpretation, and not in restricting the quantum theory to the microscopic objects only.
I would go the opposite direction-- restrict quantum theory to the realm of whereever wave functions are actually useful, which excludes the realm of the experimenter and suggests Hurkyl solution #1. Anything else just seems like semi-magical philosophical baggage to me.

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