1. ## Schrodinger's cat

Here's something I've never understood about that frigging cat.

So let's say the observer has opened the box and discovered the cat is either alive or dead.

Now let's assume that the observer, the box and the cat are also sealed off from observers on the outside. From the point of view of the outside observers, the outcome has yet to be determined, and so the cat would still be both alive and dead.

So, the question I have is, how can the cat be both both alive and dead and either alive or dead? Seems to me that logic would dictate that you're either one or the other.
Last edited by parallaxicality; 2009-Oct-30 at 01:21 PM.

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That's the thing, you've got to take that logic and throw it away. The world at the quantum mechanical level does not work in a way that what we call "logic" would lead us to believe at first.

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Originally Posted by parallaxicality
Here's something I've never understood about that frigging cat.

So let's say the observer has opened the box and discovered the cat is either alive or dead.

Now let's assume that the observer, the box and the cat are also sealed off from observers on the outside. From the point of view of the outside observers, the outcome has yet to be determined, and so the cat would still be both alive and dead.

So, the question I have is, how can the cat be both both alive and dead and either alive or dead? Seems to me that logic would dictate that you're either one or the other.
Also, don't think observer means a human eyeball. An observer is simply anything that will force the cat to be one or the other. It doesn't have to be a human eyeball.

I'm not sure how to properly explain what an observer is. But once the cat is "observed". the wave-function collapses, and the cat is either dead or alive. And it doesn't matter if the observer(again, not necessarily human eyeball) is himself within another closed system/box. The wave-function has already collapsed.

With THAT said, Schrödingers cat is just a thought experiment. And, as far as I understand it, it wouldn't actually work with a cat(or anything else that massive/heavy).

But on smaller scales, it has been tested properly.. again, iirc.

Someone else correct me if i'm wrong.

Sincerely
Me

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The scenario you describe is called "Wigner's Friend".
The external observer imagines that inside the unobserved room there is a superposition of [live cat + observer who has seen live cat] and [dead cat + observer who has seen dead cat]. So although there's an increase in the personnel involved, there's no worsening of the superposition "paradox".

It's best not to get too exercised about that cat: Schrödinger invented it in order to point out how the macroscopic world doesn't operate according to the wavefunctions of quantum mechanics. And we now know that in reality the cat's quantum superposition will decohere to one or the other classical probability extremely quickly. It will be either dead or alive, and the observer will simply be ignorant of which has occurred.

Grant Hutchison

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Erwin's story about the cat was a sort of a joke, a facetious tale to illustrate a point, like Maxwell's demon.

I saw another joke somewhere which I think answers it. We see a cartoon of Pandora opening a box, finding a dead cat in it, and Schrodinger saying "Now look what you've done."

The quantum cat in a box is dead-and-alive. "Observation" (eg by Pandora) causes decoherence, or collapse, of the wave function, and so the dead-and-alive wave function becomes dead-or-alive.

As an ensemble of matter grows larger, it becomes much harder to avoid decoherence, so this cannot really apply. In real life, Schrodinger's (mammalian) cat-in-a-box-with-probabilistic-death-machine is dead-or-alive, not dead-and-alive. In real life, decoherence occurs without the necessity for observation by sentient beings.

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Originally Posted by Ivan Viehoff
As an ensemble of matter grows larger, it becomes much harder to avoid decoherence, so this cannot really apply. In real life, Schrodinger's (mammalian) cat-in-a-box-with-probabilistic-death-machine is dead-or-alive, not dead-and-alive. In real life, decoherence occurs without the necessity for observation by sentient beings.
Yes, the cat itself is more than enough to decohere the quantum superposition of the original decaying radioactive nucleus which precipitates the scenario.
Hence the extreme measures that "quantum computer" experimenters have to take in order to maintain the quantum superpositions of their equipment. That includes extreme cooling, to minimize the number of thermal photons the superpositions interact with. Even then, decoherence typically occurs after only a small number of quantum computations have taken place.

Grant Hutchison

7. Being neither a quantum physicist nor a philosopher, I can only guess but I think "observer" is a poor choice of word, as it implies something capable of making an intelligent conclusion about an event. I suspect that a more appropriate term would be "a sufficiently large collection of particles," but that's a bit unwieldy.

8. I think you are all missing the point of Schrödingers cat as formulated by Hugh Everett in 1957 in his many-worlds interpretation of quantum mechanics. In this interpretation each and every event creates alternative universes in which every possible outcome has occurred. When the observer opens the box and say finds the cat dead, he has entered into a particular universe as a consequence of the wave-function, while at the same time creating another equally real parallel universe in which he, the observer witnesses a cat that is alive. Both universes coincide simultaneously and are inextricably interwoven with one another. Which parallel universe one enters or creates at any given moment is dependent upon which quantum fluctuations are occuring during that particular event. So, in the case of the observer who has opened the box and seeing that the cat is dead, the cat in reality is dead and not. In another parallel universe the cat is very much alive and well. But it doesn't take seeing the event in the literal sense.

It has been suggested that just the quantum fluctuations involved when one merely thinks a thought can determine which parallel universe we enter or create at any given moment. Each and everyone one of us is continually entering, leaving or creating parallel universes at any given moment. For example, there is a parallel universe in which I am not now writing this reply. There is another in which this thread hasn't even been posted....and still another where this forum doesn't even exist and so on, ad infinitum....all equally as real as this universe.

Now in the original example where there are observers on the outside who are sealed off from the opened box, the cat is still dead to those observers in that universe who can't see the cat, because they all are part of that particular parallel universe that has been created once the box has been opened. But they have incalculable counterparts in many of the other alternative universes in which other outcomes have come to pass. We are not aware of the other parallel universes because in Hugh Everett's model they are all decoherent from each other. We are only aware of the universe in which we are currently in. We are not even aware that we are creating or entering or leaving a particular universe at any given moment.

I should point out, and I think that this is from where a lot the confusion is arising, that there is the Copenhagen model of parallel universes in which the wave-function does collapse after the box is opened. But as Nobel Laureate, Steven Weinberg pointed out, this is wrong because the observer and the box containing the cat and the device must be governed by the same quantum mechanical rules that govern everything else in the universe. In the Copenhagen model, however, it is treated as an event governed by classical mechanics. In Hugh Everett's interpretation, the wave-function doesn't collapse and both universes in which the cat is alive and dead persist, thus creating two separate decoherent universes.

I think that the most difficult thing to accept about the many-worlds interpretation of quantum mechanics as formulated by Hugh Everett is the fact that it implies that there are more than one of each of us, perhaps an infinite number and, in some cases, parallel universes in which we don't even exist. It is extremely counter-intuitive and disturbing on a personal level. And, I think that's why of all of the four levels of mutliverse theory out there, this one is the most controversial and not because the mathematics isn't sound and not because there isn't experimental evidence to support the theory. But, how does one reconcile the fact that there are a number of each of us in separate, decoherent and yet inextricably interwoven parallel universes which are us, but not us?

Confusing....isn't it? LOL

Eric
Last edited by EricFD; 2009-Oct-30 at 04:28 PM.

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I would say that it's not just that it's counter-intuitive, but that infinite parallel universes are simply not required to explain events and outcomes.

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Originally Posted by EricFD
I think you are all missing the point of Schrödingers cat as formulated by Hugh Everett in 1957 in his many-worlds interpretation of quantum mechanics.
Not really missing the point.
"Many worlds" is just one way of thinking about what happens to the classical probabilities that aren't observed. That's why it's described as an "interpretation" of quantum mechanics; just one among several.

Originally Posted by EricFD
I should point out, and I think that this is from where a lot the confusion is arising, that there is the Copenhagen model of parallel universes in which the wave-function does collapse after the box is opened.
The Copenhagen interpretation just tells us how to figure out the classical probabilities; as formulated by Bohr, it actually makes no mention of wavefunction collapse. (Since Bohr carefully avoided making any claims for the reality of the wavefunction, it really would make no sense to talk about it "collapsing".)

Grant Hutchison

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Originally Posted by grant hutchison
Not really missing the point.
"Many worlds" is just one way of thinking about what happens to the classical probabilities that aren't observed. That's why it's described as an "interpretation" of quantum mechanics; just one among several.

The Copenhagen interpretation just tells us how to figure out the classical probabilities; as formulated by Bohr, it actually makes no mention of wavefunction collapse. (Since Bohr carefully avoided making any claims for the reality of the wavefunction, it really would make no sense to talk about it "collapsing".)

Grant Hutchison
grant. Which immensely simplifies things and saves time talking about it.

12. Originally Posted by grant hutchison
Not really missing the point.
"Many worlds" is just one way of thinking about what happens to the classical probabilities that aren't observed. That's why it's described as an "interpretation" of quantum mechanics; just one among several.

The Copenhagen interpretation just tells us how to figure out the classical probabilities; as formulated by Bohr, it actually makes no mention of wavefunction collapse. (Since Bohr carefully avoided making any claims for the reality of the wavefunction, it really would make no sense to talk about it "collapsing".)

Grant Hutchison
Both very good points, Grant.

Eric

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Correct me if I'm wrong; I may be operating under a delusional misconception here...

But as I understood wave function collapse, the effect is more an illusion created by our measurement using probabilities than because of observer interaction.
The elimination of a probability into a certainty is bound to collapse a wave function into one state. Not because an observer interacted, but because certainty was established.

Am I way off base, here?

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Originally Posted by Neverfly
Correct me if I'm wrong; I may be operating under a delusional misconception here...

But as I understood wave function collapse, the effect is more an illusion created by our measurement using probabilities than because of observer interaction.
The elimination of a probability into a certainty is bound to collapse a wave function into one state. Not because an observer interacted, but because certainty was established.

Am I way off base, here?
I think right on base but perhaps turned backwards.
Quantum mechanics (very successfully) gives us a way of calculating the probabilities of certain outcomes evolving from a given quantum state. For any state, QM gives us a probabilistic mixture. But macroscopically we never see those probabilities: whenever we check what Nature is up to, we find one pure outcome. This is called the "Measurement Problem".
If we want to think of the wavefunction as representing a real thing in Nature (rather than just a formalism that lets us figure probabilities) then we need to imagine that at some point something makes this wavefunction "choose" one probability out of its range of options; it then collapses to be a 100% representation of that state. Does that happen spontaneously, or because different universes split off containing the various probabilities, or because of some "hidden variable" we're not smart enough to have worked out yet, or because of something to do with the macroscopic nature of the measurement process, or because a "consciousness" (whatever that is) becomes involved? Those are the interpretations people lay on top of the Measurement Problem.

So it's not that our measurement of an outcome necessarily makes wavefunctions collapse: it's that treating wavefunctions as if they are real things requires us (or perhaps encourages us) to think about what collapses them into the outcomes we measure.

Grant Hutchison

15. Originally Posted by Neverfly
Correct me if I'm wrong; I may be operating under a delusional misconception here...

But as I understood wave function collapse, the effect is more an illusion created by our measurement using probabilities than because of observer interaction.
The elimination of a probability into a certainty is bound to collapse a wave function into one state. Not because an observer interacted, but because certainty was established.

Am I way off base, here?
No, not really. But if you recall Heisenberg's uncertainty principle, the mere act of making measurements is in effect interacting with and affecting what you are measuring on the quantum level. Now in the case of Schrödingers cat, the cat is already alive and dead, regardless of whether we open the box or not. It's just we won't know which universe we're in until we open the box, i.e., the one in which the cat is dead or the one in which the cat is alive. But then there are other options as well, namely the universe in which we don't open the box, the universe in which we don't exist to open the box, the universe in which there is no box or cat, etc., etc.

Eric

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Originally Posted by EricFD
No, not really. But if you recall Heisenberg's uncertainty principle, the mere act of making measurements is in effect interacting with and affecting what you are measuring on the quantum level. Now in the case of Schrödingers cat, the cat is already alive and dead, regardless of whether we open the box or not. It's just we won't know which universe we're in until we open the box, i.e., the one in which the cat is dead or the one in which the cat is alive. But then there are other options as well, namely the universe in which we don't open the box, the universe in which we don't exist to open the box, the universe in which there is no box or cat, etc., etc.

Eric
Eric, don't get me wrong, but it almost appears as though you are advocating multi-verse hypothesis, here.

Grant:
I think I understand the controversy now.
Yes, backwards.
I was under the impression that the wave function collapsed because certainty was established. However, if the function collapses, establishing certainty, without the observer knowing why... Well, I can see how that would be baffling.

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Originally Posted by Neverfly
However, if the function collapses, establishing certainty, without the observer knowing why... Well, I can see how that would be baffling.
The Copenhagen interpretation is to treat the wavefunction as a calculation tool which gives the correct probabilities, and not to fret about the "reality" of its unobservable properties. There's then no need for "collapse".
Some see that as a cop-out. Some see it as properly understanding the difference between scientific models and reality.

Grant Hutchison

18. Originally Posted by Neverfly
Eric, don't get me wrong, but it almost appears as though you are advocating multi-verse hypothesis, here.

Grant:
I think I understand the controversy now.
Yes, backwards.
I was under the impression that the wave function collapsed because certainty was established. However, if the function collapses, establishing certainty, without the observer knowing why... Well, I can see how that would be baffling.
Well, actually I'm more of a proponent of 11-dimensional M-theory than of the many-worlds interpretation of quantum mechanics.

And, I think I like Grant's explanation much better than my own! LOL

For me, this is just an intellectual exercise.

Eric

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Originally Posted by grant hutchison
The Copenhagen interpretation is to treat the wavefunction as a calculation tool which gives the correct probabilities, and not to fret about the "reality" of its unobservable properties. There's then no need for "collapse".
Some see that as a cop-out. Some see it as properly understanding the difference between scientific models and reality.

Grant Hutchison
I think scientific models are supposed to try for the greatest accuracy possible.

Aside, my brain wanders to the macro-world.
In which we deal with high degrees of complexity such as human behavior and fluid mechanics. I cannot help but wonder what wave functions would do in these circumstances where it's a measure of probabilities (although different from QM, where QM deals with existences and fluid mechanics deals with motion.)
Either way, it's similar...

Copenhagen interpretation seems to be eliminating confusion about the unknown.

20. This has been a really good discussion.

Eric

21. Originally Posted by grant hutchison
The scenario you describe is called "Wigner's Friend".
The external observer imagines that inside the unobserved room there is a superposition of [live cat + observer who has seen live cat] and [dead cat + observer who has seen dead cat]. So although there's an increase in the personnel involved, there's no worsening of the superposition "paradox".
So if I were the observer, does that mean that until the outside observer sees me, I exist in a superposed state, aware of both consequences simultaneously?

1) the concepts of "collapse" and "decoherence" are closely related, and both have to do with the need for physicists, as they carry out their physics, to construct projections of the wave function. One must understand the concept of a projection in order to understand either collapse or decoherence. Grant said it already, if you take quantum mechanics perfectly literally, then you must get a superposition of (observer sees dead cat + dead cat) + (observer sees alive cat + alive cat). Note this is a complex animal, but when you project it only only the state of the cat, with no reference to the observer, that's where you get a "collapsed" wave function-- the state of the cat alone has to be either alive or dead, never both, but the state of the observer+cat is a superposition.

But is the presence of an "observer" there important? Enter decoherence: decoherence says that the observer is not crucial, and coupling to a macro system will do. You get superpositions like (macro system in state #1 + dead cat) + (macro system in state #2 + alive cat), that's decoherence. Again the projection onto the state of the cat is either alive or dead, no superposition, but the state of the whole universe (if you take quantum mechanics literally), is still a superposition, even in the presence of rapid decoherence. So all decoherence does is clarify why both Many Worlds and Copenhagen both work, and why you don't need to worry too much about "what counts as an observer", but it still does not tell you how literally you should take your quantum mechanics. So we still have both Many Worlds and Copenhagen, alive and well as viable interpretations of a physics theory.

2.) The question of how troubled we should be about that darn cat is not a physics question, it is a metaphysics question, i.e., a question for the philosophy of science. An age-old debate in the philosophy of science is if physics is just supposed to help us organize, understand, and predict our empirical results (the empiricist standpoint), or if the understanding we achieve is itself the reality we seek, and our empirical data is nothing but constraints on that fundamentally rational pursuit (the rationalist standpoint). I think it is reasonably clear that empiricists would tend to align with the Copenhagen interpretation, because of its adherence to the importance of just predicting what we see and making no ontological claims beyond what we see, whereas rationalists are more free to adopt the Many Worlds view, because of the aesthetic appeal of seeing the mathematics of quantum mechanics as being the fundamental truth that this theory points to. Another way to slice that is, empiricists tend to see the wave function as mere device, whereas rationalists tend to see the wave function as the reality itself.

Personally, I try to see the value in both perspectives, and I think it behooves us all to be aware of both, but I also feel that science is a fundamentally empirical endeavor. So I think it's fine to hold a philosophical adherence to the Many Worlds view, but in so doing, one has left the realm of science. The Copenhagen approach does not require leaving the basic prescriptions and goals of science as an empirical endeavor. So I say, physicists should couch their findings in the framework of the Copenhagen school (which is what initially happened, but more rationalist perspectives seem to be seeping in more and more), but when they go home and night and formulate their own personal philosophies informed by their physics, they are more than welcome to knock themselves out and take the Many Worlds view.

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Originally Posted by EricFD

For me, this is just an intellectual exercise.

Eric
It's about all it can be for most of us

Originally Posted by EricFD
This has been a really good discussion.

Eric
I can fix that for ya, if you like...

KenG
Although I do not disagree with any particular point in your post- I disagree with it as a whole.

The reason being: You almost seem to be taking interpretation to the level of faith or belief.
The line there is fuzzy enough to make it understandable and yet, I think one must take great care on treading that line.

ETA: my perception may just be that I prefer a good solid "I have no clue" to a <Insert wild speculative idea here.>

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Originally Posted by parallaxicality
So if I were the observer, does that mean that until the outside observer sees me, I exist in a superposed state, aware of both consequences simultaneously?
Well, no; because you're an observer. When we observe Nature we detect single outcomes, not superpositions. When we don't observe Nature, we get to think about superpositions.

Grant Hutchison

25. Originally Posted by parallaxicality
So if I were the observer, does that mean that until the outside observer sees me, I exist in a superposed state, aware of both consequences simultaneously?
No, you are never in a superposition state all by yourself, no macro object ever is (because of decoherence). However, if you take QM literally, and imagine that the wave function is a real thing, then no object ever actually has its "own wave function", there's just one wave function-- and that's the wave function of the whole universe. It is always in what is known as a "pure state", which in regard to any particular observable, is what is known as a superposition state. However, there is a projection of that universal wavefunction onto the state of every object, and in the presence of decoherence (which is rampant for any macro object), that projection is not a superposition state, it is what is known as a "mixed state" (an either/or kind of thing).

Which brings us to what is meant by "you". If we take "you" to be "you as seen by others", then you are in a mixed state-- there are things that are true about you but the others just don't know it yet. If we take "you" to mean "you as seen by yourself", then you are in a "collapsed" state, where you know the truth about yourself. However, the mind-bending thing is that if you take the collapsed states of all the objects in the universe, and call that "reality", and put them all together, you will not recover the universal wave function that quantum mechanics literalists (like Many Worlds folks) imagine represents the truth about the universe as a whole. That's because all possible collapsed states, along with their probabilities, would need to be included-- not just the ones we perceive as "actualized." That's how Many Worlds works, and note that I've been talking only in terms of macro objects-- this is about how quantum mechanics applies to macro objects.

Of course, we don't even need quantum mechanics for macro objects, we only need it for micro objects. What's more, the whole value of quantum mechanics is to be able to treat "the wave function" of an individual micro object, which above I said does not actually even exist. This is the core contradiction of Many Worlds, and I see it as a pretty severe flaw, but many rationalist physicists (Weinberg was mentioned) seem fine with it. When we say an electron "has a wave function", what we really mean is that if we prepare the electron by a particular observation (say in a momentum state), then the projection of the universal wave function onto that electron is a mixed state of a bunch of correlates in the rest of the universe, and a bunch of different "electron wave functions". We then interrogate the universe to establish which correlates prevail in our "branch" of the Many Worlds (if you take that view-- for Copenhagen we just say we interrogate the actual reality), and we manually throw out all the other wave functions because they don't represent a "consistent history" with the observation we are trying to predict or understand. There's no evolutionary equation for that in QM, it's purely a manual process. Only then can we talk about the "wave function of the electron."

Again, my own problem with Many Worlds is that the whole concept of a wave function, which the universe as a whole is supposed to have, is derived from the concept of the wave function of an individual particle. Ontologically speaking, there is no such thing in the Many Worlds view, but that's the view that says the wave function of the universe is an ontologically valid entity! Of course we never observe the wave function of the universe, nor use it to make any predictions, nor check it in any way-- all that is done for individual particles. But those particles do not actually have their own wave function in the Many Worlds ontology, so we have the curious case of an interpretation of a physics theory that says we started with an ontological structure called a single particle wave function, used it to check and support that theory, then used the theory to establish that the single-particle wavefunction was ontologically incomplete-- it has to be made complete by embedding it in a universal wave function that we have no way to test or use in our physics.

Put differently, if you adopt the Many Worlds view, you are constantly doing your physics by treating wave functions that you avow are not ontologically real, yet your fundamental stance is that a wave function, that of the whole universe, is ontologically real. Meanwhile, the Copenhagen folks are constantly treating wave functions that they view as mere computational devices, and hence have no reason to appeal to any universal wave function, and face no internal contradictions in their scientific effort.
Last edited by Ken G; 2009-Oct-30 at 09:34 PM.

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Why the angry smiley in your title, Ken? It doesn't seem to match the content.

Grant Hutchison

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Originally Posted by grant hutchison
Why the angry smiley in your title, Ken? It doesn't seem to match the content.

Grant Hutchison

28. Originally Posted by Neverfly
The reason being: You almost seem to be taking interpretation to the level of faith or belief.
Of course-- that's exactly what it is. It is a belief in whatever it is that you are actually doing when you do physics. The motions you go through are clear enough, the ways you test your predictions and use them for various purposes are quite cut-and-dried. But what you think it all means is extremely personal, and is clearly in the realm of philosophy-- which is always a pretty thin line from other faith-based pursuits like religion.
The line there is fuzzy enough to make it understandable and yet, I think one must take great care on treading that line.
Indeed, yet pretending the line is not there is no kind of solution to a scientist. What I mean is, any conclusions that stem from the tested theory of quantum mechanics that also rely crucially on some interpretation of quantum mechanics, so would be a valid conclusion for Many Worlds but not Copenhagen for example, are fundamentally not conclusions that stem from the tested aspects of quantum mechanics. In stronger terms, they are not conclusions that carry the authority of scientific progress.

29. Originally Posted by grant hutchison
Why the angry smiley in your title, Ken? It doesn't seem to match the content.
Oops, thanks for pointing that out, I have no idea how that got there. Just a typo of some kind, I'll edit it out.

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