How can Schrödinger's Cat be both alive and dead?

[Teslanumber1]

Can some one explain to me how the cat can be both alive and dead, I get the principle that since we don't know it is both at the same time because we don't know but in reality it's either alive or alive we just don't know, it's the human factor of sentience just because we don't know doesn't make neither its one we just don't know. Following by that the universe exists because we observe it if we're not alive it's not real, just like the billions of other people you haven't seen aren't real. I'm not claiming anything i just want an explanation for this.

 

[Charles Wilson]

What does it mean to make an "observation"? A "measurement"? From the time Bohr and Heisenberg proclaimed that QM was "complete", this issue has been at the forefront of "What QM means". Bohr stated that there must be a "Classical" component of the determination process of a QM "observation".

The "shift" or "cut" from QM to Classical was never defined. (BTW, a very pleasant description of the History here is found in Louisa Gilder's _The Age of Entanglement_.) Einstein, for one, was never happy with this. Neither was Schrödinger. Suppose an electron travels to a screen that registers the impact as a glowing dot. If the Schrödinger Formalism describes a wave that the Bornian Probability Posit controls, Einstein asks, How do we only see a single dot on the screen when the Probabilistic Waveform encompasses the entire screen? If there is only one dot HERE and there is never another dot THERE, that presupposes a very peculiar action at a distance which violates Relativity.

Find the interview with John F Clauser on the Neils Bohr Institute site. In it, Clauser states, "We have no idea how we got from Schrödinger's waves to Born's dots on the screen".

SO!: The Story of the Cat. Remember, it is the "Radioactive Atom" that may or may not have have decayed. It exists in a Superposition of having decayed or not having decayed over the course of an hour AND the decay based on the Schrödinger equation is reflected in a Bornian Probability. But what constitutes the "observation" that the atom decayed? A dial or a pointer moving to a new position? A screen that displays a colored text? If there MUST be a Classical component to the System, then why not a cat in a box with a vial of poison, etc.?

It is not the cat that is the problem (Well, it is to the cat...), it is the requirement that a Probability accrues to the decay of the atom and this "State" transfers its probability values to the Classical realm.

Thus, the cat MUST be dead and alive until the lid of the box is opened.
Thus the absurdity.

The absurdity can be extended. Suppose that the cat is your cat, your most precious cat and you are gravely ill. Doctors have stated that any traumatic experience might cause your heart to stop. The cruel Doctors give the Box apparatus to you and tell you that you may open it and check the status of your cat or not within the next two hours. Then the doctors go out and play nine holes of golf.

As they return to your room they stand outside for a moment. Are you alive or dead? OR ARE YOU ALIVE AND DEAD? Do you feel alive AND dead? How would you know?

This has played out since QM began and the ramifications of this argument is as lively - no pun intended - today as it has been since first contemplated.

CW

 

[Crazymechanic]

Well the question is have you read the thought experiment itself and the description of it?
Now if you have then I can tell you that it's not that the cat is dead or alive by itself all the time , the uncertainty about it's state of being comes from a different place not the box itself nor any other element.
Well classical objects like a piece of metal of paper usually don't suddenly change by themselves you open a box with a piece of metal in it and it will be a piece of metal in it everytime you open it.
Now in the Schrödinger's cat there is a source of radiation in the box which if decays sends a impulse we could say to a device that upon receiving this impulse let's a tied up hammer go and he hammer breaks a glass of poison of some sort which kills the cat.
Now a radiation source is also usually some kind of metal which is radioactive aka decays.
Now the problem for us arises because we don't know when exactly an atom will decay it can happen now it can happen after 10 minutes and it ca happen upon some sort of interaction.
Now the fact that one opens the box is an interaction by itself and so it can change things.this is where we go quantum , it's not like the piece of metal isn't made out of quantum particles also it is but those are atoms that are stable and don't undergo decay under normal conditions so the experiment is interested in the metals that are radioactive and whose atoms are unstable so decay happens.

I would say that the superposition of states or in this case of the cat alive or dead is not something that is a property of nature itself it's rather our lack and inability of calculating and understanding at which exact moment an atom can and will undergo decay and the cat will be dead or alive.And that's why we have to assume that he could be both but in reality it's never the case he can't be both he is either one but just because we don't know we have to assume that.If we could somehow probe or monitor the inside of the box all the time then we would know precisely the state of the cat but every probe is some sort of device which has some quantum properties and can influence the radioactive source so probing the box with a device could disturb the state of atoms in the radioactive element just as much as a human opening the box so you would get the same result - a great chance of an atom decaying and the poison releasing and the cat being dead before you ever get the chance to record the measurement of your probe.
Ofcourse you could when opening the box and seeing that the cat is dead you could investigate how long has the cat been dead and tell exactly when the atom decayed but it would be too late, the key part here is knowing when the atom will decay in the exact moment of before it happens but we can do that only after it has happened judging from the results in the box when it is opened.

Now there may be a boy called "XXA" in China right now, but you haven't been there and you don't know so you just assume that there is a chance that there indeed is such a guy called like that and also you assume that you could just be wrong and there is no such man in China right now.But the truth is you don't know and there is no way of you knowing that because you are limited by your physical body in the place where you are so you can only assume based on some kind of logic that you have.
One more thing in QM an atom can change it's state upon interaction and by interaction it also means opening the box in quantum case because when opening it more light can go in and etc and there can be a chance of sudden change in state so even though the cat was perfectly fine all the time and could have stayed like that maybe forever when you wanted to check upon him suddenly all that changed and now he is dead.
This could be somewhat similar to building a house of cards , it's very unstable and the smallest wind could collapse it apart.
Now when you don't approach it it stays like that forever and then you decide that you want to see it and no matter how slowly you approach it some sort if vibration in the floor or some sort of little air breeze and the whole thing collapses.

 

[capcom1983]

Shroedingers cat is a fine example of the uncertainity principle. There is no evidence that the cat is alive or dead and kicking or hitting the box my cause the vial to break. Thus making the experiment void. You can't say the cat is dead because you don't know. You can't say its alive either. But it can't be neither it has to one of the states. The only logical conclusion is both.

 

[Omega0]

It is stupid to say the cat is dead or she is not in a sense she is both. We simply don't know if the cat is dead or not. If we have no measure for her being alive than we just don't know it.
Physics is so simple: See what is happening in the universe and describe this with physical terms.
In my eyes it is simple nonsense to think about such metaphysics. We are all living in a universe, we have forces around us. And now? Just because we have a box around this cat, what makes this special?
We apply simple quantum mechanics and now we physicists are going to be philosophers?
About what?
That I am currently living in a multiverse because I have no box? Or does my apartment mean that I am half dead?

Come on...

 

[jambaugh]

Shrodinger's Cat experiment was an attempt to make clear that the superpositon of "states" was not a superposition of states of reality (via the absurdity of a cat both alive an dead) but rather the superposition of states of knowledge about the actual events.

One point, the life process of the cat (et al) is thermodynamic in nature. To actually observe a quantum superposition you need to cool the system down to absolute zero. This is not compatable with observing living vs dead kitties. Further to see a superpositon you need to observe enough systems to form an interference pattern of some type in the "particle" counts. As my thesis advisor pointed out, to see a superposition of Schrödinger's cat requires a beam of frozen identically prepared cats. A highly intractable experiment to carry out. Its hard enough to see superpositions of larger atomic nucleii.

 

[DrChinese]

It is stupid to say the cat is dead or she is not in a sense she is both. We simply don't know if the cat is dead or not. If we have no measure for her being alive than we just don't know it.
Physics is so simple: See what is happening in the universe and describe this with physical terms.
In my eyes it is simple nonsense to think about such metaphysics. We are all living in a universe, we have forces around us. And now? Just because we have a box around this cat, what makes this special?
We apply simple quantum mechanics and now we physicists are going to be philosophers?
About what?
That I am currently living in a multiverse because I have no box? Or does my apartment mean that I am half dead?

Come on...

Welcome to PhysicsForums, Omega0!

The superposition is fundamentally different from being in an unknown but already determined state. There are a variety of ways to interpret that statement. When a single localized particle is in such a state, there aren't a lot of ways to demonstrate this difference.

However, the superposition behavior is much more clearly seen when you have a pair of entangled particles that are in different places. By Bell's Theorem, you end up with situations that are incompatible with classical descriptions. There are various interpretations of this as well.

 

[tensor33]

In my eyes it is simple nonsense to think about such metaphysics. We are all living in a universe, we have forces around us. And now? Just because we have a box around this cat, what makes this special?

It's not the fact that the cat is in a box that makes it both dead and alive, it is the fact that the box contains a vile of poison that is a tracked to a Geiger counter that measures a single radioactive atom. If the atom decays (which is a random quantum mechanical process) then the Geiger counter goes off and the cat dies. Since (unless we measure the atom or look at the cat) we can't know which state it is in, we say it's both dead and alive.

 

[BruceW]

I think my present opinion of Schrödinger's cat is that superposition of states is a proper physical phenomenon for particles and molecules. And there is nothing to say that it couldn't work for cats. But, of course, it would be f*** difficult to do such an experiment with cats, so we don't know really. They have made superpositions of currents, going clockwise and anticlockwise, which are fairly 'large-scale', but again, they involve particles and molecules, not cats.

 

[Omega0]

Welcome to PhysicsForums, Omega0!
The superposition is fundamentally different from being in an unknown but already determined state. There are a variety of ways to interpret that statement. When a single localized particle is in such a state, there aren't a lot of ways to demonstrate this difference.

I don't see any sense in this picture. We just don't know about the state, that's it. If I have a dice and you know that I will kill the cat if I play the dice and have a 6 then I am with a chance of 1/6 a cat killer and the cat is with a chance of 1/6 dead.
That's all. This does not produce any superpositions of killers or not-killers and cats being dead or alive. It's only quantum mechanics where we don't know about the outcome of a situation, it is nothing but that we don't know when something will happen but we know from experience that it will happen earlier or later (not me, I won't kill a cat).
We see quantum entanglement but this has nothing to do with superstates of a cat or whatever. Einstein wrote, in translation more or less "God does not play dices". He might have been wrong but it might be the same way wrong to overstress something simple given: Uncertainty.

 

[tensor33]

I don't see any sense in this picture. We just don't know about the state, that's it. If I have a dice and you know that I will kill the cat if I play the dice and have a 6 then I am with a chance of 1/6 a cat killer and the cat is with a chance of 1/6 dead.
That's all. This does not produce any superpositions of killers or not-killers and cats being dead or alive. It's only quantum mechanics where we don't know about the outcome of a situation, it is nothing but that we don't know when something will happen but we know from experience that it will happen earlier or later (not me, I won't kill a cat).
We see quantum entanglement but this has nothing to do with superstates of a cat or whatever. Einstein wrote, in translation more or less "God does not play dices". He might have been wrong but it might be the same way wrong to overstress something simple given: Uncertainty.

You're thinking from a purely classical point of view. It seems logical to just say that we don't know about the state. But many experiments have proven that's not the case.
The things you are saying are correct (at least in the macroscopic world) It's when you get down to the very small quantum level that things start working differently.

 

[Bill_K]

The things you are saying are correct (at least in the macroscopic world) It's when you get down to the very small quantum level that things start working differently.

Can you be a bit more specific - how are they different? E.g. is there any way we can tell whether the cat died as the result of a decaying atom or a dice roll.

 

[Omega0]

You're thinking from a purely classical point of view. It seems logical to just say that we don't know about the state. But many experiments have proven that's not the case.

No, I disagree. The contrary: The Laplace's demon is dead. QM teaches that there is nothing like an unstable equilibrium which will be "stable" over time. Try to place a ball on another ball when you have gravitation, do you believe the ball will stay in this position? Say we have vacuum and no other forces but gravitation? I guess you know what I mean. The ball will fall down. If you try to place it exactly it will be nevertheless an unstable equilibrium and QM will deserve for a falling ball.
Please let me know about any experiment or theory which tells me the direction the ball is falling down.

 

[tensor33]

Can you be a bit more specific - how are they different? E.g. is there any way we can tell whether the cat died as the result of a decaying atom or a dice roll.

The way I understand it is that when you roll a die to decide if a cat dies, the die will always be in one of the six states it can be in, whereas with a decaying atom, it is in a superposition of the different states before it is measured.
Also, with the die, the randomness is only due to our lack of knowledge of the system (eg. how high the die was thrown) whereas with the atom it is a purely random process.

 

[tensor33]

No, I disagree. The contrary: The Laplace's demon is dead. QM teaches that there is nothing like an unstable equilibrium which will be "stable" over time. Try to place a ball on another ball when you have gravitation, do you believe the ball will stay in this position? Say we have vacuum and no other forces but gravitation? I guess you know what I mean. The ball will fall down. If you try to place it exactly it will be nevertheless an unstable equilibrium and QM will deserve for a falling ball.
Please let me know about any experiment or theory which tells me the direction the ball is falling down.

I'm not sure I understand what you are saying.
But as for the experiments I talked about in my earlier post, this Wikipedia page has a few examples
http://en.m.wikipedia.org/wiki/Quantum_superposition#section_3

 

[Omega0]

Can you be a bit more specific - how are they different? E.g. is there any way we can tell whether the cat died as the result of a decaying atom or a dice roll.

No, we cannot. If we measure the result, if we open the box we simply see a state. The cat is dead or not. I shot the cat or not. Again: This is no 1/6 dead cat if I play the dice. It is just a cat or my main light switch which I activate, there is no difference. The light is on or off.

 

[Maui]

Can some one explain to me how the cat csn be both alive and dead, I get the principle that since we don't know it is both at th same time because we don't know but in reality it's either alive or alive we just don't know, it's the human factor of sentience just because we don't know doesn't make neither its one we just don't know. Following by that the universe exists because we observe it if we're not alive it's not real, just like the billions of other people you haven't seen aren't real. I'm not claiming anything i just want an explanation for this.

Contrary to popular opinion, the 'cat' is quantum mechanical as well .


The question as posed may seem deep at first sight, but at a second glance is just scratching the surface of an even deeper problem.

 

[DrChinese]

I don't see any sense in this picture. We just don't know about the state, that's it...

That is not the end of the story, else no one would be saying that quantum theory is so strange!

And it certainly does not make classical sense. However, superpositions lead to results that are qualitatively different than what you would get from an ensemble of unknown but certain states.

For example, entangled particle pairs are in a superposition, and that superposition remains in effect even after they are separated. Because of that, you get statistics that are incompatible with what you describe. Are you familiar with Bell's Theorem?

 

[Omega0]

I'm not sure I understand what you are saying.
But as for the experiments I talked about in my earlier post, this Wikipedia page has a few examples
http://en.m.wikipedia.org/wiki/Quantum_superposition#section_3

Okay, again... Quantum superposition is existing, I wrote about quantum entanglement above. The cat will be killed by an decaying atom and now? This is nothing but likelihood. This does not mean that the cat is in a superposition state or something. For sure, this makes physics perhaps interesting for someone outside: OH, are we all just states, is this ruled by quantum mechanics?
Do we have multiple universes? So cool... but this has nothing to do with real physics.
Light is on or not if you look if there is light, so simple.

 

[tensor33]

Okay, again... Quantum superposition is existing, I wrote about quantum entanglement above. The cat will be killed by an decaying atom and now? This is nothing but likelihood. This does not mean that the cat is in a superposition state or something. For sure, this makes physics perhaps interesting for someone outside: OH, are we all just states, is this ruled by quantum mechanics?
Do we have multiple universes? So cool... but this has nothing to do with real physics.
Light is on or not if you look if there is light, so simple.

The atom is in a superposition. And since the cat's life is dependent upon the atom having decayed, it can be said that the cat is also in a superposition. (until it is measured)

 

[Maui]

This does not mean that the cat is in a superposition state or something. For sure, this makes physics perhaps interesting for someone outside: OH, are we all just states, is this ruled by quantum mechanics?
Do we have multiple universes? So cool... but this has nothing to do with real physics.
Light is on or not if you look if there is light, so simple.

Quantum mechanics underlies everything that looks classical and everything always inevitably reduces to it. There are no exceptions to this. This is the real physics.

 

[Bill_K]

However, superpositions lead to results that are qualitatively different than what you would get from an ensemble of unknown but certain states.

This is what I have asked. Does it make any discernible difference in the present case?

 

[Omega0]

That is not the end of the story, else no one would be saying that quantum theory is so strange!
For example, entangled particle pairs are in a superposition, and that superposition remains in effect even after they are separated. Because of that, you get statistics that are incompatible with what you describe. Are you familiar with Bell's Theorem?

I believe I understand it and I believe I know it. I just see a typical risk: To use strange pictures does not help. To think out of the box is always not easy but in physics you have always the situation of an measured emergence. For example what about thermodynamics? Would you like to describe it with single atoms? No, you wouldn't. Statistical mechanics works different. Why? Because we see the emergence of new entities. Would you agree that it is not a good idea to describe a bridge to be created in a quantum mechanical way? Why? Because classical mechanics is sufficient.
This is a typical problem in understanding physics, from my point of view: Who wants to understand the "real nature", the "reason for everything" seem to need a deep understanding of quantum mechanics. Perhaps you have this deep understanding?
In my eyes you to think about emergence.
I really won't attack you. It is just critics about how physics works as it seems "on a quantum level sometimes": The more strange, the more interesting. Light is on or off. That's it.

 

[Omega0]

The atom is in a superposition. And since the cat's life is dependent upon the atom having decayed, it can be said that the cat is also in a superposition. (until it is measured)

Absolutely correct, you can call it that way - but it makes no sense. So here we are in the world of philosophers what I said above. If the cat is measured to be dead she is dead.

 

[DrChinese]

This is what I have asked. Does it make any discernible difference in the present case?

From my perspective (and a lot of folks wouldn't agree with this): If you COULD perform a weak measurement on the system with some kind of magic wand that measures the amount of life inside without collapsing the cat into a certain state (alive or dead), THEN you would register something inside as being HALF alive.

My point is that classically you cannot have a half alive state, but quantum mechanically you do have something analogous to that. Just as classically you say the particle goes through one slit or the other of a double slit setup, but not both. But interference effects say that is not the case.

 

[Boy@n]

Why cannot we simply say that the atom is in a superposition while for cat we just don't know its state (dear OR alive)?

 

[Crazymechanic]

@OmegaO
Well first of all if you have a dice and rolling it on 6 mans you kill the cat you don't get 1/6 of a chance of killing the cat you rather get God knows what kind of a chance of killing him as the dice can fall on 3 or 5 or any other number multiple times.haven't you ever played monopoly? :D
@Boy@n Well that's actually what we are saying for the past decades or so but that doesn't answer the question it only assumes that the question is unanswerable or that maybe there is no deeper answer but are you certain about that in fact is anyone here ?

Second of all I would agree that superposition is rather our lack of knowledge and calculation when a atom will decay than a real physical phenomenon.Now I say this because let's take a uranium atom.
Now when the atom releases a neutron does the neutron is half released half not released ? Can a neutron or any other particle emitted from a nucleus that undergoes decay be released like a few fm from the nucleus and then paused or so?
i don't think so the emission of a particle happens spontaneously.So there is no superposition only a impossible order known only to nature and the atom when one is about to release a particle.
But because nature is not self aware I believe that leaves us to conclude that there is an uncertainty about whether something is about to happen at the lowest scales so we are left with a probability that we can calculate depending on many experiments with the same kind.

But that leaves a question atleast to me personally , why doesn't the atoms decay faster or maybe why isn't there a case when an element decays extremely fast.
If I would be on or in a unstable state , like in a building that's on fire i wouldn't wait for my half life I would run to safety immediately :D
Now maybe thee is something that we yet don't fundamentally understand about nature which makes us think that things get random down to qm scales and superpositions allow us to have half dead cats.

 

[Omega0]

From my perspective (and a lot of folks wouldn't agree with this): If you COULD perform a weak measurement on the system with some kind of magic wand that measures the amount of life inside without collapsing the cat into a certain state (alive or dead), THEN you would register something inside as being HALF alive.

So we are here back in QM: What is a "weak measurement"? What kind of wonderwall do you need?
How would you differentiate between a "week measurement" and a strong one? The weak one, which is able to circumvent QM itself? And the strong measurement that shows that QM works. This is a contradiction.

A1: Every system reacts on measurement.
A2: On a quantum level, it not possible to have a measurement not contradicting the Heisenberg uncertainty inequality
Conclusion:
C1:There is no measurement telling you something about the system below the level of influence.
C2: If there is a measurement you will get back the state of the system

My point is that classically you cannot have a half alive state, but quantum mechanically you do have something analogous to that. Just as classically you say the particle goes through one slit or the other of a double slit setup, but not both. But interference effects say that is not the case.

For sure this does not exist in classical mechanics - but this does not change the position that you wouldn't know about this bizarre nature without measurement. The double slit is the perfect example: You just believe to know about the existence of photons and you measure them and so you are sure they exist. There behavior is strange so you think about a non classical theory but this does not change the situation: It is by measurement. I have been growing up with that crap about the double nature of light - but there is none. We just found no picture but is this picture really so strange? Every day you are having decisions to do this or that. Why? Because you are a |DrChinese> and if you now meets <world| and you integrate over time, what is <world|DrChinese>? Do you really believe you are who you are because of quantum effects? Again, I really will not offend you - I am just searching.

 

[BruceW]

Why cannot we simply say that the atom is in a superposition while for cat we just don't know its state (dear OR alive)?

exactly. That is pretty much what we do. The theoretical difficulty is that there is no clear line for when we should apply one rule, and when we should apply the other. For example, in a superconducting quantum interference device, we have a current, made up of millions of electrons. So you might think this is a 'cat' situation. But actually, the current is a quantum superposition of 'all the electrons going one way' plus 'all the electrons going other way'. So it is definitely not a 'cat' situation.

Also, there is no reason theoretically why a cat cannot be in a superposition. It is just that it would be very hard to do this experiment, so we don't know if a whole cat actually could be put into a superposition. The answer is that we just don't know (experimentally).

 

[Charles Wilson]

http://motls.blogspot.com/2011/01/hardys-paradox-kills-all-realistic.html

Edit: Perhaps I should have given the punch line:

"The very Yes/No properties of a particle don't commute with each other. What are the implications of this statement that is self-evident in quantum mechanics but deep and confusing away from quantum mechanics? The statement implies that we can't ever imagine that a particle is ready to react to different kinds of measurements at the same moment. In some sense, it's just another form of the uncertainty principle, optimized for the projection operators and binary properties in this case.

"Different properties of a physical system can't simultaneously "exist" because of the refusal of the operators to commute with each other. And because none of the observables is privileged - experimenters may measure the spin with respect to any axis - it follows that none of the observables can "exist" prior to the measurement."

So, in a sense, "No, the light bulb isn't on or off, that's it...". Bell proved Locality is a No-Go when it comes to deriving QM results. Locality may "appear" to work "completely", in a particular way but QM doesn't work that way.

CW