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

[alweiss]

I know that so far as we know, it's impossible to figure this out, but is it possible that in a superposition, the atom(or whatever particle, in this case a cat) becomes two atoms once interaction with anything is blocked off, and then once a photon of light is sent some type of force pushed one cat away, leaving just the one cat in one state?

 

[ZapperZ]

I know that so far as we know, it's impossible to figure this out, but is it possible that in a superposition, the atom(or whatever particle, in this case a cat) becomes two atoms once interaction with anything is blocked off, and then once a photon of light is sent some type of force pushed one cat away, leaving just the one cat in one state?

Then you have misunderstood the superposition principle. What is in superposition is NOT the cat, or the atom, but rather a property of the cat or atom. This property, described by an observable, is the one that is in superposition, not the whole cat or atom.

By creating a duplicate system, you've created a whole slew of problems, including several conservation violations.

Zz.

 

[alweiss]

Thanks for the clarification !

 

[StarsRuler]

The cat is a macroscopical system with the precision for decide if he is alive. He is an observer, alive or dead, the system decohere in a very short time. There is no superposition with the cat. We can let an isolated closed box but the cat is alive or dead . It is question to do the calculations

 

[audioloop]

the pertinent question is if persist superposition (a linear principle, i.e. not change)
on macroscopic objects ?
to answer that, what is need is experimental testing.

 

[bahamagreen]

BM (Bohmian Mechanics) for example is not the product of 'sloppy theory, design, and analysis'. I don't hold to it but it's perfectly valid.

The issues with interpretation can not be resolved with throwaway lines like that.

Thanks
Bill

Why do I get the idea that you did not read my post? (#79) ;)

 

[bhobba]

Why do I get the idea that you did not read my post? (#79) ;)

Well let's see:

The measurement problem occurs because if you repeated kill the cat until all its lives are consumed, you will have counted and verified how many lives it had left, but it's no longer suitable to meet the initial conditions assumed in the experimental design.

All its lives consumed? Hmmmmm.

Maybe its not me reading it, which I did, maybe its what you are saying.

Thanks
Bill

 

[bhobba]

The cat is a macroscopical system with the precision for decide if he is alive. He is an observer, alive or dead, the system decohere in a very short time. There is no superposition with the cat. We can let an isolated closed box but the cat is alive or dead . It is question to do the calculations

In QM an observation does not require an observer. An observation is whenever a quantum process makes its mark here in the macro world, which in this setup occurs at the particle detector. Schrodinger's Cat never was an issue for Copenhagen - its solution within Copenhagen was utterly trivial. The issue with Schrodinger's Cat was, and always has been, pointing to the need of a fully quantum theory of measurement rather than the fundamental split Copenhagen makes between the quantum and the macro world.

I suspect you know that though and it was just the way you expressed it.

Your observations about decoherence are spot on - and is part of the modern view of Schrodengers Cat that reflects the work that has been done with developing that quantum theory of measurement since Schrodinger proposed his famous (infamous?) thought experiment.

Pop Sci accounts IMHO are responsible for much of the misunderstanding about this thought experiment.

Thanks
Bill

 

[bhobba]

the pertinent question is if persist superposition (a linear principle, i.e. not change) on macroscopic objects ? to answer that, what is need is experimental testing.

The cat is obviously not in a superposition of alive and dead.

However it is well known that once entanglement with the environment is removed, which is difficult, but not impossible, to do, (it usually means it must be kept ultra cold - most entanglement is via some kind of 'heat bath') quantum effects reassert themselves even for macro objects. And indeed some strange quantum effects have been observed with macro objects, and undoubtedly future experiments will reveal even more eg:
http://www.scientificamerican.com/article.cfm?id=quantum-microphone

Although I have a bone to pick with the article:
'As to how the day-to-day reality of objects that we observe, such as furniture and fruit, emerges from such a different and exotic quantum world, that remains a mystery'.

That's not quite true - how classical behavior emerges is pretty well understood - environmental decoherence has shed a lot of light on it - but some issues do remain such as the so called factoring problem (there are others as well, and most good textbooks allude to them - but strangely, and I am not quite sure why this is, it may be for a deep reason or evidence of nothing at all, they do not talk about the factoring problem - I have only seen discussions here about it - interesting). I believe, and I think most physicists also believe, this is more along the lines of dotting the i's and crossing the t's so to speak and they will be resolved - but really who knows what the future will bring.

But that is not what is going on here. The cat and the rest of the paraphernalia of this thought experiment are entangled with each other and the environment - specifically its not done at an ultra cold temperature near absolute zero.

Thanks
Bill

 

[audioloop]

quantum effects reassert themselves even for macro objects.

That's not quite true - how classical behavior emerges is pretty well understood

valid to 430 atoms (experimental limit).

i am talking about 10 ²⁰ atoms.

 

[bhobba]

i am talking about 10 ²⁰ atoms.

Precisely what are you claiming that number of atoms demonstrate and exactly how does it deviate from the standard treatments found in virtually every QM textbook?

Thanks
Bill

 

[BruceW]

There are quantum systems (in the sense that they can only be described quantum-mechanically) of much more than just 430 particles. for example:
http://en.wikipedia.org/wiki/Macroscopic_quantum_phenomena
http://en.wikipedia.org/wiki/Bose-Einstein_condensation
In some of these situations, there are a very large number of particles (or atoms) in the same quantum state (i.e. all in superposition). And they are definitely macroscopic phenomena. So it is clear that QM can work for macroscopic phenomena. What is not clear is if QM works for a system like a cat, i.e. can we describe the entire cat as a single quantum state? No-one knows because such an experiment has not been done to find out. You would have to do something equivalent to diffracting a cat, which is experimentally going to be *very* difficult, definitely beyond current technology.

We can be fairly confident that the individual components of the cat (i.e. its atoms) can each be described approximately (i.e. considering each atom independently of the others). But this is an approximation. So I don't think it is justified to say a priori that we must be able to describe a cat as a single quantum state. Maybe we can. Maybe we can't. But we don't have the technology to test this. (I personally think that a cat could be diffracted if we had the technology to do such an experiment, but who knows).

 

[bhobba]

We can be fairly confident that the individual components of the cat (i.e. its atoms) can each be described approximately (i.e. considering each atom independently of the others). But this is an approximation. So I don't think it is justified to say a priori that we must be able to describe a cat as a single quantum state. Maybe we can. Maybe we can't. But we don't have the technology to test this. (I personally think that a cat could be diffracted if we had the technology to do such an experiment, but who knows).

All true. The point here however is the alive and dead states show no interference and each state to the limit of our current technology behaves entirely classically.

Thanks
Bill

 

[BruceW]

yeah, or I would word it as: we can't probe the (possible) interference between alive and dead states because a cat is darn complicated. (unlike a Bose-Einstein condensate which is relatively simple).

 

[audioloop]

So I don't think it is justified to say a priori that we must be able to describe a cat as a single quantum state. Maybe we can. Maybe we can't. But we don't have the technology to test this. (I personally think that a cat could be diffracted if we had the technology to do such an experiment, but who knows).

maybe not a cat

there are various schemes,
between them

http://eprints.gla.ac.uk/32707/1/ID32707.pdf
"We introduce a novel cooling technique capable of approaching the quantum ground state of a kilogram-scale system"


http://web.physics.ucsb.edu/~quopt/tow.pdf
"We have performed a detailed study of the experimental requirements for the creation and observation of quantum superposition states of a mirror consisting of 10¹⁴ atoms, approximately 9 orders of magnitude more massive than any superposition observed to date. Our analysis shows that, while very demanding, this goal appears to be within reach of current technology"


http://arxiv.org/pdf/1201.4756v2.pdf
http://link.springer.com/article/10.1007/s10686-012-9292-3
"Testing the predictions of quantum theory on macroscopic scales is one of today's outstanding challenges of modern physics and addresses fundamental questions on our understanding of the world. Specically: will the counterintuitive phenomena of quantum theory prevail on the scale of macroscopic objects? This is at the heart of the so-called \quantum measurement problem", also known as Schrodinger's cat paradox. Another question is whether quantum superposition states of massive macroscopic objects are consistent with our notion of space-time or whether quantum theory will break down in such situations"


.

 

[mathman]

For the last couple days the comic strip "Dilbert" has Schrodinger's cat as a character, having escaped from the box before it was inspected. The cat is half dead and half alive.

 

[JazzGuy]

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.

To interpret the Schrodinger's cat issue one first needs a basic primer in the orthodox postulates (the basics) of quantum mechanics. There is a very good presentation, for a general audience, at http://pirsa.org/displayFlash.php?id=12070006 This is one of the educational videos produced at the Perimeter Institute. It is well worth an hour and a half of watching as all the basic interpretations are mentioned. Rob Spekkens begins discussion of inconsistencies at 55:00 and goes on to Schrodinger's Cat. From this, one can glean the nature of the problem for the paradox of the cat being dead and alive. The heart of the issue is that we combine the history of a quantum level object with the history of a cat in a box. The unitary evolution of the quantum state is described in a way (linearly) that allows it to be summed with anything that is also linear. The assumption is that this can include the evolution of the cat. When you do this the cat is linked with the evolution of a quantum state. The defining function of the quantum state always includes the "square root of minus one" (i) which, itself is a paradoxical construction to the real world. We naively assume that some complex function (for linear evolution) should apply to the cat as well, simply by placing it in a box (so we can't see it). We also place a quantum-level particle in the box. We can't see its classical state either but ignore the fact that this is for an entirely different reason. By the way, it makes just as much sense to use cardboard for this box - which points out that there is something sadly lacking in our understanding of what constitutes quantum entanglement and our relationship to such a state. It is naive to assume a complex function applies to the cat because no one knows how to isolate the cat as such. Why this may not be possible is poorly understood. One obvious conclusion is that some sort of absolute boundary exists between the quantum state and the cat. In other words, between what is quantum and what is classical. Mathematics does not give any guidance on this issue which leads to the deeper question of whether or not mathematics is the final root of reality. Bell's inequality is universally interpreted to solve the issue of whether classical mathematics is the root because classical theory appears to fail the description of classical phenomena in its own basis of description. We are still missing something. Perhaps there is an absolute boundary across these paradoxical forms of description (classical and quantum). Then the universally accepted conclusion on Bell's inequality would also be wrong. We better call in the philosophers for this one - might as well include the musicians, the philosophers don't fair much better.

 

[alweiss]

This would be a good way to put it... except for a fact that having you be the observer changes the experiment too much. The cat as an observer in the original experiment knows whether its alive or dead. The cat is in a super position yet at some point it is dead. For the outside world, the cat is only dead when you send in a photon of light. So having you be "the cat" for this experiment is redundant, because it wouldn't help with the experiment

 

[Crazymechanic]

Well I don't want to sound too rude or whatever , but let's face it there is no half alive half dead cats or other creatures.All of this is just a physical limitation that we face in terms of knowing about the state of the cat at the very moment something happens beyond "our eyes".

Just like the guy buying a lottery ticket and later becoming the winner. He won the lottery actually in the moment he bought the "lucky" ticket because the right number combination was already in the computer but because it is a lottery he had no way of knowing that so it always feels like you won in the moment you get the information.

Now before you measure you cannot tell about the state of the cat but once you measure you have a great chance of setting the outcome and hence never knowing how long the cat would have been alive if you would have stayed aside and never touched the box.
That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.

 

[BruceW]

maybe not a cat

there are various schemes,
between them

Ah, nice links. thanks dude. They seem to keep putting larger and larger objects into superposition. Although I guess a cat is a lot more difficult. Another thing I remembered is the 'Penrose interpretation' which is pretty interesting. If I understand correctly, this would predict that due to general relativity, any superposition of states with an energy difference greater than the Planck mass would undergo an objective collapse. So really, it is more than just an interpretation, since it gives predictions which are different to that of the standard QM. And as I understand it, no experiment has been done yet which uses a superposition of states with an energy difference greater than the Planck mass.

 

[audioloop]

Ah, nice links. thanks dude. They seem to keep putting larger and larger objects into superposition. Although I guess a cat is a lot more difficult. Another thing I remembered is the 'Penrose interpretation' which is pretty interesting. If I understand correctly, this would predict that due to general relativity, any superposition of states with an energy difference greater than the Planck mass would undergo an objective collapse. So really, it is more than just an interpretation, since it gives predictions which are different to that of the standard QM. And as I understand it, no experiment has been done yet which uses a superposition of states with an energy difference greater than the Planck mass.

the second link, from penrose.

 

[bhobba]

That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.

I have noticed that to. Beats me as well.

The solution to this within Copenhagen, the Ensemble interpretation and others is utterly trivial - the quantum weirdness happens at the detector - from then on everything is classical.

Although I have mentioned it many times I think with the way this thread keeps on continuing it needs to be said again - the import of Schrodinger's Cat is it points to the necessity of a fully quantum theory of measurement. That was a valid observation when it was first promulgated - but that has now largely been accomplished.

Thanks
Bill

 

[BruceW]

the second link, from penrose.

aha! oh yeah, I didn't even look at the authors. I should really get into the habit of that. Although, there is not much discussion about the possible implications for the 'Penrose interpretation', I guess probably because the energy difference would still not nearly be great enough to cause the 'objective collapse'.

Although I have mentioned it many times I think with the way this thread keeps on continuing it needs to be said again - the import of Schrodinger's Cat is it points to the necessity of a fully quantum theory of measurement. That was a valid observation when it was first promulgated - but that has now largely been accomplished.

I'm curious, but I don't really know what you mean.

 

[bhobba]

I'm curious, but I don't really know what you mean.

Ok - here it is again.

Schrodinger's Cat is trivial in Copenhagen that divides the world into two bits - classical and quantum. In Copenhagen we know about the quantum world when it makes its appearance here in the classical world which in Schrodinger's Cat is when the detector detects a particle - from that point on there is no mystery - every thing is honkey dorey.

But this dividing of the world into classical and quantum is a bit fishy - everything is really quantum. And if you look at it that way that's when you get issues - what we need is a fully quantum theory of observation. It was not available when Schrodinger proposed his thought experiment but a lot of work has been done on it since then and its pretty much solved.

Rather than me going over the decoherence explanation again (that's what in modern times we know a lot more about) you can either do an internet search on it or have a look at Susskinds Lectures:
http://www.newpackettech.com/Resources/Susskind/PHY30/QuantumEntanglementPart1_Overview.htm

I forget exactly which lecture he examines Schrodinger's Cat in but its probably a good idea to have a look at them all.

Thanks
Bill

 

[JazzGuy]

Well I don't want to sound too rude or whatever , but let's face it there is no half alive half dead cats or other creatures.All of this is just a physical limitation that we face in terms of knowing about the state of the cat at the very moment something happens beyond "our eyes".

Just like the guy buying a lottery ticket and later becoming the winner. He won the lottery actually in the moment he bought the "lucky" ticket because the right number combination was already in the computer but because it is a lottery he had no way of knowing that so it always feels like you won in the moment you get the information.

Now before you measure you cannot tell about the state of the cat but once you measure you have a great chance of setting the outcome and hence never knowing how long the cat would have been alive if you would have stayed aside and never touched the box.
That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.

Thanks Crazymechanic. Here is my take on the difference between "not knowing" in quantum and then classical terms (for two possible outcomes). In quantum terms, "not knowing" means the outcomes are paradoxically simultaneous (before collapse occurs which destroys the quantum structure). In classical terms, it means that some action in time has simply not occurred yet (such as we are going to flip a coin to realize the outcomes in sequence). The quantum version structures "not knowing" paradoxically as simultaneous outcomes of events. Action in time does not apply. In the quantum structure, time is imaginary (and without direction) since it does not flow. In classical structure we will eventually see both outcomes but never simultaneously. My conclusion is that the two forms of "not knowing" are categorically different.

 

[BruceW]

@ bhobba - ah, OK. So the thing that has 'been accomplished' is the use of 'quantum decoherence' to explain how quantum effects become negligible when the original system becomes entangled with the 'environment'. And of course, most detectors can be classed as 'environment' in this context (since most detectors will have a huge amount of degrees of freedom). So really, as soon as the original quantum system gets entangled with the detector, quantum effects are then negligible.

On the surface, this is kindof obvious, since it is not practically possible to diffract something like a Geiger counter. But I know that there is also a lot of useful research into stuff like decoherence times, and how to try to reduce interaction of quantum systems with environment.

I agree about the Copenhagen interpretation. Using the Copenhagen interpretation, we impose a 'subjective collapse' of the state, at the boundary between the quantum and classical worlds. And this separation of the quantum and classical worlds is an arbitrary choice.

decoherence does not help us with this choice. It tells us that for an environment with a large number of degrees of freedom, there will be negligible quantum effects. But the key is the word negligible. There is no exact cut-off which we might use to define the quantum and classical worlds. We are still stuck with the problem that the separation between quantum and classical according to the Copenhagen picture is essentially an arbitrary choice.

That's why the Copenhagen interpretation doesn't really answer the Schrodinger's cat problem. We can use the 'subjective collapse', but we have no definitive way to say when we are allowed to use it and when we cannot.

edit: p.s. thanks for the link. everyone seems to love this Susskind guy. so I will be interested to hear him talking about QM.

 

[bhobba]

There is no exact cut-off which we might use to define the quantum and classical worlds.

Errrrr. So you think a cat isn't classical?

Thanks
Bill

 

[akhmeteli]

Errrrr. So you think a cat isn't classical?

So you think a cat isn't quantum?

 

[stevendaryl]

Well I don't want to sound too rude or whatever , but let's face it there is no half alive half dead cats or other creatures.All of this is just a physical limitation that we face in terms of knowing about the state of the cat at the very moment something happens beyond "our eyes".

Just like the guy buying a lottery ticket and later becoming the winner. He won the lottery actually in the moment he bought the "lucky" ticket because the right number combination was already in the computer but because it is a lottery he had no way of knowing that so it always feels like you won in the moment you get the information.

Now before you measure you cannot tell about the state of the cat but once you measure you have a great chance of setting the outcome and hence never knowing how long the cat would have been alive if you would have stayed aside and never touched the box.
That's that and that's all , sometimes I really don't get why people like to make this so mystical or sometimes the other way around , so complicated.

Yes, there is nothing at all mysterious about quantum mechanics if you believe that the questions you might ask, such as "Is the cat alive, or dead?" all have definite answers, regardless of whether we know those answers, and that measurement merely allows us to know about a pre-existing fact. But it's hard to see how to make this idea consistent with what we know about quantum mechanics. For example, if we prepare a electron in a state that is an eigenfunction of the x-component of spin, then the z-component of the spin is simply undefined, according to our current understanding of quantum mechanics. It's not that we don't know what it is, but that it just doesn't have a value.

There is no reason, in principle, that this particularly quantum type of uncertainty can't scale up to the level of cats and people.

 

[Crazymechanic]

Well I could agree that for a single electron there really is a "guess what" thing involved but as you just said not for a cat or a human , not at that level , so the main thought of the cat in the box thing is correct but if we really speak about a cat then no there is no superposition there is just a lack of knowledge or rather I should say a lack of a physical way to get the information without "writing history"

Now as for the single quantum particle , here we can speak about superposition and still then the next discussion appears , is it a fundamental rule of nature to let us or any conscious observer know any of the two variables while hiding the other or is it again our lack of knowledge or lack of a way to access that knowledge even if it's there somewhere to which i want to believe it is.