Understanding Schrödinger's Cat: A Non-Physicist's Perspective

[JRodriguez]

This may have been posted or questioned before but I have yet to see it anywhere. I must also warn you that I am not a physicist... I just find the subject interesting.

The Schrödinger's cat experiment has never felt right to me not because I had trouble grasping the concept of something existing in two states until observed it was something else that I couldn't quite put my finger on until recently.

It hit me a couple nights ago that the cat is its own observer therefore preventing itself from ever existing in both the dead and alive state at once. This sounds to simple to be true but I cannot see another way around the fact that the cat knows its alive based on its senses and if it doesn't then its dead.

It might be argued that the cat is incapable of knowing whether its dead or alive but it doesn't need to comprehend its state. The cat may not see or hear anything but its aware of its own weight on its paws its aware of the temperature in the box etc etc

I would appreciate any input on this and I will be posting this on other forums as well to see what others think.

- Jeffrey Rodriguez

 

[Dmitry67]

Cats its own observer - it is a part of an "advanced" version of this experiment:
http://en.wikipedia.org/wiki/Wigner's_friend

 

[dmtr]

From the Wikipedia Schrödinger's cat article:

The mainstream view (without necessarily endorsing many-worlds) is that decoherence is the mechanism that forbids such simultaneous perception.

 

[JRodriguez]

Thanks for pointing that out

Here is the other post I made http://www.physforum.com/index.php?showtopic=27092

 

[DrChinese]

I don't know if this makes it more clear or not:

When a particle is in a superposition, that is fundamentally different from saying it is one state or another. If you *DIRECTLY* observe the property of "is cat alive" you will always get the answer: it is alive OR it is dead. But if you *INDIRECTLY* observe that same property (using inference about its state without actually observing it), you will always get the answer that it is HALF ALIVE. (You could also say that it is both DEAD and ALIVE.) Obviously, that is a completely different answer. That is because they are completely different states. The first is an eigenstate and the second is not.

If you have a hard time seeing this difference, consider the two slit experiement. Does the particle go through one slit or the other? If you DON'T see interference, then you know the particle was NOT in a superposition when it went through the slits (we can know which slit). If you DO see interference, then you know the particle WAS in a superposition when it went through the slits (and we don't know which slit). Same with the cat.

 

[JRodriguez]

I don't know if this makes it more clear or not:

When a particle is in a superposition, that is fundamentally different from saying it is one state or another. If you *DIRECTLY* observe the property of "is cat alive" you will always get the answer: it is alive OR it is dead. But if you *INDIRECTLY* observe that same property (using inference about its state without actually observing it), you will always get the answer that it is HALF ALIVE. (You could also say that it is both DEAD and ALIVE.) Obviously, that is a completely different answer. That is because they are completely different states. The first is an eigenstate and the second is not.

If you have a hard time seeing this difference, consider the two slit experiement. Does the particle go through one slit or the other? If you DON'T see interference, then you know the particle was NOT in a superposition when it went through the slits (we can know which slit). If you DO see interference, then you know the particle WAS in a superposition when it went through the slits (and we don't know which slit). Same with the cat.

This is what I have a problem with... you state that if you directly observe the state you get the answer but indirectly observing causing it to have the answer of "HALF ALIVE". The problem I have is that it is already being "*DIRECTLY*" observed by the cat itself... kind of like the two slit experiment being measured preventing the particle from being in the superposition forcing it to go through one slit or the other not both.

Once again I am not a physicist :)

 

[DrChinese]

This is what I have a problem with... you state that if you directly observe the state you get the answer but indirectly observing causing it to have the answer of "HALF ALIVE". The problem I have is that it is already being "*DIRECTLY*" observed by the cat itself... kind of like the two slit experiment being measured preventing the particle from being in the superposition forcing it to go through one slit or the other not both.

Once again I am not a physicist :)

It doesn't act the same. That is the issue. If it did, we wouldn't need to express the state differently.

Now, you must recall that the cat analogy is just an analogy. No one is actually trying to say that an entire cat is half alive. But if we talk about a quantum particle, then it really is "half alive" (i.e. in a quantum superposition) and it cannot observe itself.

 

[Dmitry67]

For those who believe in Copenhagen Interpretation, "wavefunction is not an objective reality, but just observer's knowledge of a system", so different observers (experimenter and a cat) should not always agree on if wavefunction collapsed or not.

But you should not focus on the collapse theories, I recommend checking non-collapse interpretations, where this problem is solved in much more elegant way.

 

[Codexus]

This is what I have a problem with... you state that if you directly observe the state you get the answer but indirectly observing causing it to have the answer of "HALF ALIVE". The problem I have is that it is already being "*DIRECTLY*" observed by the cat itself... kind of like the two slit experiment being measured preventing the particle from being in the superposition forcing it to go through one slit or the other not both.

The problem with any interpretation that requires an 'observer' is that you have to define what an observer is. Why would some physical entities be considered observers and not others? Humans, cats, worms, bacterias, computers, measuring equipments? How do you tell the difference? You'd have to introduce some notion of 'consciousness' which, in my opinion, has no place in physics.

There has to be an objective and rational explanation to explain the decoherence in a way that doesn't require 'conscious observers' and prevents the cat from being in a superposition of states.

 

[Dmitry67]

No one is actually trying to say that an entire cat is half alive.

Cat can be quite big, like billions of electrons in a superconductive ring - in superposition.

 

[Dmitry67]

There has to be an objective and rational explanation to explain the decoherence in a way that doesn't require 'conscious observers' and prevents the cat from being in a superposition of states.

Yes, and there is such explanation, called 'Quantum Decoherence' :)

 

[JRodriguez]

It doesn't act the same. That is the issue. If it did, we wouldn't need to express the state differently.

Now, you must recall that the cat analogy is just an analogy. No one is actually trying to say that an entire cat is half alive. But if we talk about a quantum particle, then it really is "half alive" (i.e. in a quantum superposition) and it cannot observe itself.

If you take the cat out of the equation then it makes perfect sense.

The problem with any interpretation that requires an 'observer' is that you have to define what an observer is. Why would some physical entities be considered observers and not others? Humans, cats, worms, bacterias, computers, measuring equipments? How do you tell the difference? You'd have to introduce some notion of 'consciousness' which, in my opinion, has no place in physics.

There has to be an objective and rational explanation to explain the decoherence in a way that doesn't require 'conscious observers' and prevents the cat from being in a superposition of states.

As far as I am concerned the cat is an observer because the thing to be observed is it being alive or dead and what better to observe that then the cat itself...

Edit: So maybe what qualifies something as an observer is what is to be observed...

 

[DrChinese]

... and prevents the cat from being in a superposition of states.

That is not accurate, in my opinion. It is in a superposition, and that is different that an "unknown but definite" state (which is what you get when you perform an observation but don't look at the results). If you have the possibility of knowing the results, there is collapse.

Note: I agree there are some interpretational issues with Copenhagen and collapse (it is sometimes called the measurement problem). However, it does not make any incorrect predictions. MWI was created in part to resolve the issue that different observers might see collapse differently, and many consider this a good solution to the measurement problem. Of course, you end up trading one thing for another in each interpretation. The key is that a superposition acts differently when you put all the facts together. That won't be the case if you take a classical perspective (in which there are no superpositions).

 

[JRodriguez]

Cat can be quite big, like billions of electrons in a superconductive ring - in superposition.

Could see this if the cat was in a coma lol

 

[DaveC426913]

If you take the cat out of the equation then it makes perfect sense.



As far as I am concerned the cat is an observer because the thing to be observed is it being alive or dead and what better to observe that then the cat itself...

Edit: So maybe what qualifies something as an observer is what is to be observed...

Yes, the cat is an observer. The cat observes that the poison did not release and thus it lives.
The cat also observes that the poison did release and thus it will die.

Both those states exist within that box until the box is opened by you.

But it doesn't have to end there. You too are in a larger box, and you too are in superposition until the box you are in is opened and observed. In one superposed state, you open your Schrodinger's box and the cat is alive. In the other superposed state, you open the box and the cat is dead. (You only experience one of these.)

Like a russian doll, this can continue. The observer who opened the box that you are in is likewise in his own box and he too is superposed.

 

[Cryxic]

This may have been posted or questioned before but I have yet to see it anywhere. I must also warn you that I am not a physicist... I just find the subject interesting.

The Schrödinger's cat experiment has never felt right to me not because I had trouble grasping the concept of something existing in two states until observed it was something else that I couldn't quite put my finger on until recently.

It hit me a couple nights ago that the cat is its own observer therefore preventing itself from ever existing in both the dead and alive state at once. This sounds to simple to be true but I cannot see another way around the fact that the cat knows its alive based on its senses and if it doesn't then its dead.

It might be argued that the cat is incapable of knowing whether its dead or alive but it doesn't need to comprehend its state. The cat may not see or hear anything but its aware of its own weight on its paws its aware of the temperature in the box etc etc

I would appreciate any input on this and I will be posting this on other forums as well to see what others think.

- Jeffrey Rodriguez

I think part of the confusion may stem from how you understand the thought experiment. A cat is a macroscopic entity and does not exist in a superposition of states like, say, electrons can. You can think of the cat as being in one state at all times. What actually exists in a superposition of states is the radioactive substance that decays and (possibly) sets off the Geiger counter, which eventually kills the cat. The "observation" is not watching the cat, but watching the radioactive substance that holds the cat's ultimate fate. It's a very common misconception in quantum theory that the human mind is what determines measurement. You get lots of crackpot ideas from that premise, but always be careful to watch out for it.

 

[JRodriguez]

Yes, the cat is an observer. The cat observes that the poison did not release and thus it lives.
The cat also observes that the poison did release and thus it will die.

Both those states exist within that box until the box is opened by you.

I thought that the act of observing knocked it into existing in only one state like the double slit experiment.

 

[JRodriguez]

I think part of the confusion may stem from how you understand the thought experiment. A cat is a macroscopic entity and does not exist in a superposition of states like, say, electrons can. You can think of the cat as being in one state at all times. What actually exists in a superposition of states is the radioactive substance that decays and (possibly) sets off the Geiger counter, which eventually kills the cat. The "observation" is not watching the cat, but watching the radioactive substance that holds the cat's ultimate fate. It's a very common misconception in quantum theory that the human mind is what determines measurement. You get lots of crackpot ideas from that premise, but always be careful to watch out for it.

Wouldn't the cat existing in one state or the other force the decaying radioactive substance out of superposition and into one state?

 

[Cryxic]

Wouldn't the cat existing in one state or the other force the decaying radioactive substance out of superposition and into one state?

The cat does not exist in linear superposition of states. It's the superposition of the radioactive substance that eventually forces the cat to be dead or alive. When the substance decays and triggers the Geiger counter, then it transitions into a new state.

 

[JRodriguez]

The cat does not exist in linear superposition of states. It's the superposition of the radioactive substance that eventually forces the cat to be dead or alive. When the substance decays and triggers the Geiger counter, then it transitions into a new state.

So the experiment is to show that the radioactive substance is in superposition before its detected by the counter and that's all?

 

[DrChinese]

So the experiment is to show that the radioactive substance is in superposition before its detected by the counter and that's all?

Just keep in mind that you are talking about quantum systems with probabilities of appearing in one state or another (or another etc.). Generally there is not much to see in the superposition unless the probabilities are something like 50-50. If it is 99-1 then you will see some effects but they may not be pronounced. Once it is possible that the state is known, then there is decoherence and the superposition is no longer.

 

[JRodriguez]

The purpose of the thought experiment is to illustrate this apparent paradox. Our intuition says that no observer can be in a mixture of states; yet the cat, it seems from the thought experiment, can be such a mixture.

Just read the wiki entry and I guess you are suppose to ignore the cat as an observer... :\

 

[Fredrik]

Wouldn't the cat existing in one state or the other force the decaying radioactive substance out of superposition and into one state?

Yes. The state of the system evolves into a state with correlations between the eigenstates of the subsystems, like this:

|cat>|atom> → |dead>|decayed> + |alive>|not decayed>

So a measurement that tells you that the cat is dead also tells you that the atom has decayed.

So the experiment is to show that the radioactive substance is
in superposition before its detected by the counter and that's all?

I think the point is to show that if microscopic objects can be in superpositions, the linearity of the Schrödinger equation implies that macroscopic objects can also be in superpositions. Ballentine (the author of a very good advanced QM book called "Quantum mechanics: a modern development") uses this as an argument against the idea that a wavefunction can be thought of as a representation of all the properties of a system. (The alternative is that it represents the properties of an ensemble of identically prepared systems).

 

[Neo_Anderson]

I think part of the confusion may stem from how you understand the thought experiment. A cat is a macroscopic entity and does not exist in a superposition of states like, say, electrons can. You can think of the cat as being in one state at all times. What actually exists in a superposition of states is the radioactive substance that decays and (possibly) sets off the Geiger counter, which eventually kills the cat. The "observation" is not watching the cat, but watching the radioactive substance that holds the cat's ultimate fate. It's a very common misconception in quantum theory that the human mind is what determines measurement. You get lots of crackpot ideas from that premise, but always be careful to watch out for it.

I happen to agree with this. Why must the cat be in the superposed state? Why not the atom that releases/doesn't release the radioactivity on the cat?

 

[JRodriguez]

I happen to agree with this. Why must the cat be in the superposed state? Why not the atom that releases/doesn't release the radioactivity on the cat?

This is where I find there to be a problem how can you have the particle in a superposed state while its being observed... The cat is observing the state of the box so how can you say the poison has and hasn't been released? Then again the sensor can count as a measuring device preventing it as well.

So the particle can only exist in the superposed state if you ignore all observation in the box and just act like everything in the box is in a superposed state that's what I was questioning... or you can go with the Multiple worlds interpretation ( This is what I didn't understand was not logical for me to just ignore part of an experiment to make it work.)

 

[DrChinese]

This is where I find there to be a problem how can you have the particle in a superposed state while its being observed... The cat is observing the state of the box so how can you say the poison has and hasn't been released? Then again the sensor can count as a measuring device preventing it as well.

So the particle can only exist in the superposed state if you ignore all observation in the box and just act like everything in the box is in a superposed state that's what I was questioning... or you can go with the Multiple worlds interpretation ( This is what I didn't understand was not logical for me to just ignore part of an experiment to make it work.)

If the state of the system (radioactive particle + larger entity) cannot be known, in principle (whether you ignore it or not), it remains in a superposition. A system in a superposition can demonstrate different behavior than one that is not. So it is not a question of "accepting" it or not, it is a fact.

 

[eaglelake]

The first difficulty with Schrodinger's cat is that it is not a quantum particle, like an electron is, and its behavior is correctly described by classical mechanics, not quantum mechanics. However, we will assume that this cat does obey the rules of quantum theory.

Quantum mechanics (QM) only gives us two kinds of information:
1. The possible results of a measurement and 2. the probability distribution of those results. That's all there is! QM is a purely statistical theory. Classical theory describes the behavior of things. QM does not.

For the cat, QM predicts two possible outcomes; we find that the cat is either alive or we find it dead. These are the only possible results of this experiment. We never find the cat both alive and dead at the same time (because the eigenstates are orthonormal).

The state vector for this experiment is a superposition of the alive eigenstate and the dead eigenstate. This quantum superposition allows us to calculate the probability for finding the cat dead and, also, the probability for finding the cat alive when we open the box. QM tells us that if we repeat the experiment many times, half the time we find the cat dead and half the time we find the cat alive. That is all we know about "Schrodinger's cat"! A quantum superposition does not mean that the cat is in two different states at the same time.

We have no idea what the cat is doing before we open the box. QM does not describe the behavior of the cat. It only gives us probabilities for the possible outcomes when we make the measurement. QM does not suggest that the cat is dead and alive, or half dead and half alive, when the box is closed. We have no information of any kind that tells us the condition of the cat in the closed box. Therefore, there is no way for us to answer any questions about it. Statements that cannot be verified by the experiment are meaningless. We can speculate, but speculation should have no place in physics, which is supposed to be an empirical science.

An 'observer' need not be a living thing. We only need a measuring device to complete the experiment. Once the measurement is made, the experiment is over and done. We can drive ourselves crazy speculating about conscious observers, or what is the condition of the cat in a superposition state. It's not worth the effort. If we can accept QM at face value, it is less confusing.

 

[DaveC426913]

An 'observer' need not be a living thing. We only need a measuring device to complete the experiment. Once the measurement is made, the experiment is over and done.

True but that measuring device is also in superposition: one state where it observed an effect, and one where it did not. The system that includes the cat AND the measuring device does not collapse into one or the other states until that system is observed. It is like a Russian Doll.

 

[Phrak]

Every cat you have had the chance to observe is in a deterministically evolving wave state until you look at it. It's interaction with other wave states only results in another wave state without you direct intervention. In fact this goes for people too. It's your world, I am just living in it.

 

[DrChinese]

A quantum superposition does not mean that the cat is in two different states at the same time.

This statement shows the problems we have with language and QM. I am not saying it is wrong as written but at the same time: the cat is definitely NOT in ONE particular state. It can be considered as being in NO states OR TWO states. You pick it.

 

[Dmitry67]

This who thread is a good example of how much harm the Copenhagen Interpretation had done. I remember myself struggling with questions like 'did wavefunctions collapse before the first human was born etc'

 

[DrChinese]

This who thread is a good example of how much harm the Copenhagen Interpretation had done.

That might be a little harsh.

You can always shut up and calculate.

 

[eaglelake]

True but that measuring device is also in superposition: one state where it observed an effect, and one where it did not. The system that includes the cat AND the measuring device does not collapse into one or the other states until that system is observed. It is like a Russian Doll.

Almost true! In fact all parts of the experiment, including the cat, the measuring device, and the measured result, are non-separable and must be considered as a whole. We shouldn't speak of the cat and the measuring device as separate and independent objects [1]. The measured results depend on the entire experimental arrangement.

All we know is that half the time the cat is found alive and half the time the cat is found dead. We never observe the superposition state, or an evolving wave state, or the collapse of anything. In fact, we no nothing about any cat that is alive, or dead, or drinking a bud lite, for that matter, while waiting for someone to open the box!

Further, we no nothing about the measuring device before it is triggered. When that happens, the experiment yields a single result - alive or dead. Repeating the experiment many times yields the probability distribution of the two possible results. The experiment tells us nothing about the separate behavior of the cat or the measuring device. If we insist on discussing the cat, or the measuring device, independently, then we only generate confusion, contradictions, weirdness, etc.. This forum attests to that!

As an example: If the device is "collapsing", then surely, we should be able to see it happen. And, what is it collapsing from? We don't see that either. In my opinion, such a discussion is not only unnecessary, but further contributes to the confusion.

I do not mean to sound pompous about this. I struggle with this along with everyone else, but I try to accept quantum mechanics at face value, without trying to 'explain' it. (I don't even know what 'explain' means in this context!)
Best wishes.

[1] Neils Bohr. See Wheeler, J. A. and Wojciech, W. H (eds):
Quantum Theory and Measurement. Princeton University Press, Princeton, New Jersey, (1983) pp. 3-7

 

[eaglelake]

This statement shows the problems we have with language and QM. I am not saying it is wrong as written but at the same time: the cat is definitely NOT in ONE particular state. It can be considered as being in NO states OR TWO states. You pick it.

You are correct. We speak a classical language which becomes a terrible barrier when we try to discuss quantum events. But, here, I am trying to emphasize that we continually introduce concepts which have no basis in quantum theory or in the experiment. It is now commonplace in quantum mechanics to discuss unobservable entities as if they were, to the contrary, real. In this case, we discuss the behavior of Schrodinger's cat before opening the box, even though there is no way to verify it.

We all want to believe that Schrodinger's cat, prior to observation, is like any other cat in a box, which we have all seen many times. But the cats we observe are classical cats who exist somewhere at every instant, just like all other classical objects. Schrodinger's cat, on the other hand is a quantum object which has an existence only at the instant of observation. That is the only time we see the cat! We cannot see the cat, even in principle, prior to measurement. Wheeler [1] goes so far as to call the measurement event "an act of creation" to emphasize that there is no experiment without an experimental result [2]. Hence, there is no Schrodinger's cat until we open the box!

Yet, we insist on discussing the behavior of the cat prior to measurement without any evidence that there is actually a cat in the box. The original EPR experiment [3] assumes that quantum particles do exist prior to measurement. And the predicted results are erroneous! This tells us that we should not be talking about Schrodinger's cat before opening the box.

So, I think this is more than just a language problem.
Best wishes.

[1] John Archibald Wheeler, “Law Without Law” in John Archibald Wheeler and Wojciech Hubert Zurek, eds., Quantum Theory and Measurement (Princeton University Press, Princeton, New Jersey,1983),
pp. 182-213
[2] Niels Bohr in John Archibald Wheeler and Wojciech Hubert Zurek, eds., Quantum Theory and Measurement (Princeton University Press, Princeton, New Jersey,1983), pp. 3-7
[3] A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777 (1935)
or simply Google in "EPR"