Is the cat alive, dead, both or unknown

[atyy]

http://www.scholarpedia.org/article/Wightman_quantum_field_theory

If I understand correctly, the cluster decomposition does mean spacelike experiments have uncorrelated results - BUT the caveat is that it refers to ground state expectation values, and is derived from (1) no superluminal signalling (2) Poincare invariance of the ground state (3) uniqueness of the ground state.

The ground state is a property of the Hamiltonian, so this puts a constraint on the Hamiltonian, which provides some notion of "interactions". However, the "no superluminal signalling" assumption does enter, so cluster decomposition doesn't seem to provide a notion of "local interactions" that is distinct from "no superluminal signalling"?

 

[bhobba]

However, the "no superluminal signalling" assumption does enter, so cluster decomposition doesn't seem to provide a notion of "local interactions" that is distinct from "no superluminal signalling"?

Wienberg's definition is rather naive - yes even wrong - but regardless of how you look at it removing correlated systems is quite natural- if you don't you are asking for problems:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

As Bill_K said:
'But what he's talking about is a situation in which all of the in states (α1, α2,...), (αj,αj+1,...) are known and independent. In your pion example the in states α1 and αj are correlated and dependent.'

In QFT superluminal is even trickier because you need to include all sorts of paths in the path integral formulation - even superluminal ones. Even I got confused about this and I well remember a thread where I was corrected. Best to avoid it in the definition of locality.

Thanks
Bill

 

[Feeble Wonk]

Take the usual EPR setup with system A and B prepared in a maximally entangled pure state and allow another system C to be nearby. Take B off to Alpha Centauri and after some time make a measurement on every box and write down the results. We have now broken the entanglement between A and B, and can allow those systems to develop new entanglements, by say allowing them to interact with the environment.

Now at some fixed and agreed upon later time after those initial measurements but before the light travel time between Earth and Alpha Centauri, make a measurement on box B and C. Write the results down, and have your partner fly back to compare notes.

Here is the important thing. There will never be a correlation between the results in box B and C. Repeat the experiment however many times you want, you will always find the same result. The conclusion is obvious. B/c A and B were in a maximally entangled state, by monogamy of entanglement they could not be entangled with C. Further, once C was spacelike seperated, it could never create entanglement with B, even after the original entanglement was broken. This is a physical statement about the locality of the laws of physics and is not just about the transfer of information.

This part is confusing to me. IF A and B were maximally entangled before B took its trip to Alpha Centauri, and C subsequently became entangled with A, wouldn't C also be entangled with B by proxy? Wouldn't you have to consider there to be an A/B/C system? If you measure B at its Alpha Centauri address, it yields information about A, which would then yield information about C. What am I missing?

 

[jerromyjon]

http://arxiv.org/pdf/1211.0784v4.pdf

Very interesting, reminds me of a mobius strip.

 

[Xertese]

The cat is alive since the measurement or observation of said cat before during and after being placed in the box is alive so at a quantum level that cannot change as the possibility has already been determined.

The only way the cat will die without interaction is through dehydration (unlikely) or starvation (more likely)

Also which a lot of people seem to neglect is that the cat itself is making observations and measurements itself.

 

[jerromyjon]

The only way the cat will die without interaction is through dehydration (unlikely) or starvation (more likely)

I don't know about the cat, per se, but I would die of thirst, first, waiting for science to explain this.

Also which a lot of people seem to neglect is that the cat itself is making observations and measurements itself.

So would a camera, if it just took a picture of the atom. Until you look at the classical picture, is it a picture yet?

 

[Xertese]

Yes the camera in it's measurement since being created thus predetermining an outcome would take and make the picture without someone looking at it.

 

[atyy]

In QFT superluminal is even trickier because you need to include all sorts of paths in the path integral formulation - even superluminal ones. Even I got confused about this and I well remember a thread where I was corrected. Best to avoid it in the definition of locality.

If you avoid it, it is unclear that the statement of Bell's theorem is correct, since it isn't obvious that your definition corresponds to "classical relativistic causality" or to "no superluminal signalling".

Although it may be technically hard to implement, no superluminal siganalling for pairs of observables is straightforward - observables at spacelike separation commute.

The easy way to think about it is that a measurement will collapse the wave function, and so will change the probabilities of outcomes, and can be used to send a message.

If observables commute, they can be measured in any order, and the wave function collapse will not change the probabilities of outcomes. So no superluminal signalling is implemented by observables at spacelike separation commuting.

 

[harrylin]

Excuse my naivety please. I want to understand entanglement.
Two objects exist, one is red, the other is blue.
These objects are then separated. they can be a few meters separated or maybe light years;
When we look at either object we instantaniously know what the state of the other object must be,
and information does not have to travel any distance at all for this to be so.
It is not spooky at all..

The "spooky" aspect of entanglement was discussed by Bell by means of an illustration with two socks of different colours, you can read it here (and if you want to understand that issue, you probably should read it): http://cds.cern.ch/record/142461?ln=en
His conclusion which is often called "Bell's theorem" has been the topic of many threads on this forum.

 

[rootone]

Thanks Bhobba and harrylin for pointing me in a useful direction (Bell's theorum.).

 

[Feeble Wonk]

This part is confusing to me. IF A and B were maximally entangled before B took its trip to Alpha Centauri, and C subsequently became entangled with A, wouldn't C also be entangled with B by proxy? Wouldn't you have to consider there to be an A/B/C system? If you measure B at its Alpha Centauri address, it yields information about A, which would then yield information about C. What am I missing?

Never mind. Sorry for the stupid mistake. The entanglement between A and B was broken upon the initial measurement of B at Alpha Centauri, so B couldn't be entangled with with C by proxy.

 

[Derek Potter]

I do not understand why entanglement is such a mystery. Entanglement is not a magic connection between two particles. Neither is it (pace bhobba) merely a correlation. It is plain old superposition. It doesn't arise unless there is superposition - which is, of course, always the case in one basis or another. But given superposition, entanglement is a direct result. With superposition there is no spooky action at a distance. No non-locality.

So what is the problem?

The problem is that people don't like superposition and want to get rid of it. It is just about psychologically acceptable to allow that a small system is in a weird quantum state until we look at it, but, as soon as we have observed it, surely the wierdness disappears? They say. So we have collapse of the wavefunction - or some similar ad hoc hypothesis - introduced to make our interpretation feel more natural. But when does the wavefunction collapse? When is an observation complete? In the case of an EPR experiment, Alice and Bob make measurements on their own photons. But Charles, who compares their results, has not yet made a measurement. Thus we must regard Alice and Bob, be they real people or mere detectors and recording devices, as being in superpositions *as far as Charles is concerned*. (Technically in his measurement basis.) And that doesn't feel right. You mean Alice sees both outcomes but only one Alice state is actualized and then only when Charles observes her? I don't think so! They say.

And yet, if Alice and Bob do enter an unambiguous state (of having seen a particular outcome - which is 100% common-sense!) then spooky action at a distance is inevitable unless you really bend over backwards to concoct weird superdeterministic theories. Suppose Alice's measurement is a tiny bit ahead of Bob's - nowhere near enough to alter their spacelike separation but enough to be able to say that, at a certain time, Alice and Bob have set their detectors; Alice has made her measurement but Bob has not. During this time, Bob's detector must alter its detection sensitivity to reflect the angle between the two detectors. It's as simple as that. Bob's detector presumably knows its own angle so the information about Alice's setting must get to it superluminally. It is actually possible to get quite good correlations if Bob's detector is allowed to know everything about how Alice's photon interacts with her detector (edit - without the information about her setting). This covers all sorts of hidden variables, pre-arrangements and so on and certainly covers the red/green sock case. However Bell's Theorem (the CHSH part) proves that, regardless of quantum mechanics, there are limits to what it can do unless Bob has the additional information of Alice's setting. QM predicts, and experiment confirms, that the correlations are the strongest that would be possible if Bob did have that information. Remember, Bob's sensitivity must adjust itself according to this information even though it cannot have reached him yet. That's spooky.

So the choice is yours:
1 Shelter behind "it's only a correlation" and "you can't send signals with it". Edit - Shut up and calculate!
2 Accept spooky action at a distance (edit - as well as superposition).
3 Accept that Alice and Bob, like Schrodinger's Cat, remain in superposition at least until Charles observes them (edit - and no spooky stuff needed).
4 Come up with an alternative theory involving time-travelling fairies or brains in a vat (edit - not spooky at all, oh no! ).

 

[Nick666]

spooky action at a distance

Maybe there's something inside the distance ( inside the space, a property of space) that determines the spookyness ?

 

[Derek Potter]

Maybe there's something inside the distance ( inside the space, a property of space) that determines the spookyness ?

Like I said, there is no spookiness other than superposition.

 

[jerromyjon]

https://en.wikipedia.org/wiki/File:Quantum_superposition_of_states_and_decoherence.ogv
Very nice 3 minute video, I finally know what "Rabi oscillations" are. But what I was looking for was the "birdhouse?" thing about how 2 birds must go through 1 hole? I have a mental block and can vaguely remember the concept but I forget the analogy that described it, can anyone point me to it, I would be extremely grateful.

 

[TrickyDicky]

So what is the problem?

The problem is that people don't like superposition and want to get rid of it.

Surely there must be a reason for this, because superpositions are nice and simple.

3 Accept that Alice and Bob, like Schrodinger's Cat, remain in superposition at least until Charles observes them (edit - and no spooky stuff needed).

This seems like your option, which leads to the question:why only until Charles observe them?, but that's precisely the reason most people is not satisfied with superposition so you're back to spot one.

In fact the problem with superposition in mathematical terms is sort of the opposite from the Schrodinger's cat kind of situation where one wonders why are not superpositions in the caricature sense that Schrodinger intended observed. Actually the problem with wave functions in the scattering case is that we only consider physical the superpositions of plane wave solutions, that are not normalizable and cannot be observed.

 

[Derek Potter]

Surely there must be a reason for this, because superpositions are nice and simple.
This seems like your option, which leads to the question:why only until Charles observe them?,

They can go on forever as far as I'm concerned. The point is, they must go on at least until Charles observes them. (There are loopholes like backward causality or superdeterminism but frankly they sound pretty contrived and, in any case the discussion, although it has veered off into EPR, is about Schrodinger's Cat and we should limit ourselves to the paradigm that Schrodinger used.) But as far as collapse is concerned it cannot occur before Charles makes his observation. Otherwise he would be observing a mixed state of Alice and Bob's independent observations (assuming no spooky action at a distance) and the correlations would not be seen.

but that's precisely the reason most people is not satisfied with superposition so you're back to spot one.

You mean they prefer spooky action at a distance plus a mysterious collapse of the wavefunction rather than plain wave mechanics which accounts for the appearence of both without needing either? You may be right. There's no accounting for taste.

 

[Buzz Bloom]

Hi JerromyJon:

https://en.wikipedia.org/wiki/File:Quantum_superposition_of_states_and_decoherence.ogv
Very nice 3 minute video

I found the video gave a good illustration of the physics of superpostion, but I have a problem with the language used to describe it:

a particle is in two energy states at the same time.​

That is not intended to be an exact quote. I would have to watch the video again to catch the exact quote, and I don't think it is important enough to do that.

To me the desciptive language is a philosophical interpretation about what is shown in the video.

QM is a collection of mathematical tools that can be used to make certain kinds of predictions that quite often and naively seem absurd. There is also a body of theory explaining what the ability to make such predictions means about the universe in which we live, and the theory can be helpful in developing new tools. But such a theory is not necessarily a complete and true description about how the universe really works. QM only predicts probabilities about possible measurement values.

With this in mind, the video particle having a superposition of two energy states simply means that if it's energy is measured the value will either be one specific value or the other. QM will predict the probability of which value it will be (with an error range depending on the measurement method). It does not mean it has both energy states at the same time.

I hope this is helpful,
Buzz

 

[jerromyjon]

It does not mean it has both energy states at the same time.

I agree to disagree... I certainly don't see it that way. I see it as "oscillatory interference" in a basic sense.

 

[bhobba]

I agree to disagree... I certainly don't see it that way. I see it as "oscillatory interference" in a basic sense.

You can view it any way you like, but I have to say I don't understand "oscillatory interference". The math however is quite clear - you can't say anything about what's going on without observing it - a superposition does NOT mean it has both properties at once.

Thanks
Bill

 

[jerromyjon]

I can't explain what is in my head any better than the term that just popped into my mind which was "oscillation interference", so I googled and found a similar biological term with pictures that kind of show what I mean but not quite.

I'll just solve any doubt by saying this. The cat is alive or dead. Period. That's obviously the point, you never get to see a cat die, or dying. Just the same as saying it is an atom in the ground state, or in the excited state. The "superimposed energy states" part specifically, I can't think of any way to depict it visually.

 

[atyy]

But what I was looking for was the "birdhouse?" thing about how 2 birds must go through 1 hole? I have a mental block and can vaguely remember the concept but I forget the analogy that described it, can anyone point me to it, I would be extremely grateful.

http://arxiv.org/abs/1407.3194

 

[jerromyjon]

Ahh pigeonhole! Thanks a million!

 

[atyy]

You can view it any way you like, but I have to say I don't understand "oscillatory interference".

Maybe:

Oscillatory: Φ(t) = e^iEtΦ

Interference: |Ψ(t)|² = |Σe^iEtΦ|²

 

[jerromyjon]

Yeah that's mostly greek to me. I just think the interference of the wave function needs to interfere exactly right to "bump it over the hump to flip the switch" so to speak. I already know the geometry of the universe so that part is simple. I'm just not quite good enough to piece it all together with the math. Actually it's even simpler than that now that I think deeper about it. You observe the nodes or the peaks of the interferences. Yeah, I think that pretty much sums it up right there.

 

[OCR]

Really, a very interesting thread... And, the cat is alive, however, he's getting... very pissed !

Carry on...

 

[jerromyjon]

How would we look at 3 cats in 2 boxes. We could only ever find 2 dead cats...

https://en.wikipedia.org/wiki/Bhabha_scattering
https://en.wikipedia.org/wiki/Møller_scattering

Those mean anything interesting?

 

[Buzz Bloom]

Hi JerromyJon:

The cat is alive or dead. Period.

Please read https://en.wikipedia.org/wiki/Schrödinger's_cat . It makes clear that Schrödinger's intention was to demonstrate by a paradox that the Copenhagen interpreation of QM was logically untenable. The way you have been discussing this thought experiment (TE) seems to be stuck in the paradox of the Copenhagen interpreation. Here is a quote:

Schrödinger coined the term Verschränkung (entanglement) in the course of developing the thought experiment.​

Entanglement interpretation has until recently also been caught in a similar paradox, which also has the same interpretive resolution I have recomended about the cat. You may want to look at

https://www.physicsforums.com/threads/entanglement-question.811193/​

and other threads about entaglement.

In the cat TE, the state of the particle which interacts with the detector is still unknown until the box is opened, but it has also had an unknown effect on the cat. Until the box is opened, the state of the partcle and the state of the cat are entangled.

Consider the TE elaborated a bit. Suppose the detector did not only conditionally emit a poison gas, it also set an electro-mechanical bit to 1 or 0 in a separate box. Suppose the bit is moved very far away from the cat's box. Now is is clear that the state of the cat and the state of the far away bit are entangled. If the bit box is looked at, it will instantly "predict" what will be seen when the cat box is opened, and vice versa. But there is no action at a distance faster than light message sending required.

Similarly, in the original TE, the past state of the particle is entangled with the state of the cat. The two possible states of the cat are exactly that, superimposed discrete possibilites. QM could predict the probability of each of these two possible cat states, but the reality of which of the the two states will be detected when the box is opened remains unknown until the box is opened. That is an interpretation of supersition of states that avoids paradox. The states are only contingent possibilities until a detector discovers the reality.

I hope this will be helpful,
Buzz

 

[bhobba]

It makes clear that Schrödinger's intention was to demonstrate by a paradox that the Copenhagen interpreation of QM was logically untenable.

If that was his intention (it wasn't) he failed.

Its purpose was to highlight a blemish with Copenhagen - namely where you put the classical quantum cut. The obvious place was at the particle detector - if you do that no issue arises. However the interpretation didn't force you to put it there. That's it, that's all.

In the cat TE, the state of the particle which interacts with the detector is still unknown until the box is opened, but it has also had an unknown effect on the cat. Until the box is opened, the state of the partcle and the state of the cat are entangled.

I have no idea where you are getting this from, but the the state of the cat has nothing to do with the opening or not of the box.

Thanks
Bill

 

[harrylin]

If that was his intention (it wasn't) he failed.

Its purpose was to highlight a blemish with Copenhagen - namely where you put the classical quantum cut. The obvious place was at the particle detector - if you do that no issue arises. However the interpretation didn't force you to put it there. That's it, that's all.

I have no idea where you are getting this from, but the the state of the cat has nothing to do with the opening or not of the box.

Thanks
Bill

Interesting discussion about the myth of single "Copenhagen interpretation":

"The Copenhagen interpretation is often taken to subscribe to a solution to the measurement problem that has been offered in terms of John von Neumann's projection postulate. In 1932 he suggested that the entangled state of the object and the instrument collapses to a determinate state whenever a measurement takes place. This measurement process (a type 1-process as he called it) could not be described by quantum mechanics; quantum mechanics could only described type-2 processes (i.e., the development of a quantum system in terms of Schrödinger's equation). According to von Neumann, the shift from a type 2-process to a type 1-process takes place only in the presence of the observer's consciousness. So what causes such a collapse seems to be the mind of the observer. But von Neumann never explained how it was possible for something mental to produce a material effect like the collapse of a quantum system. This led to the famous paradox of Schrödinger's cat. Although von Neumann's position is usually associated with the Copenhagen Interpretation, such a view was definitely not Bohr's [..]
[..]
The Copenhagen interpretation is not a homogenous view. This insight has begun to emerge among historians and philosophers of science over the last ten to fifteen years."
- http://plato.stanford.edu/entries/qm-copenhagen/

 

[bhobba]

The Copenhagen interpretation is not a homogenous view.

It isn't.

But it overall has a number of features discussed here, as well as its blemishes, and resolutions:
http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

Thanks
Bill

 

[KiNGGeexD]

The cat is in a superposition of states and when you observe the cat it will collapse into the dead state, or the alive state! It's a thought experiment simply to highlight that until a measurement is made there is no way to predict the state of the cat. Determinism disappears on a quantum level and we can only calculate probabilities. Hence the power of a qubit which like a bit can have two states, 0 or 1, or a superposition of both of those states, hence more processing power. To highlight what I'm trying to say :)Regards

 

[bhobba]

The cat is in a superposition of states and when you observe the cat it will collapse into the dead state, or the alive state!

That's incorrect.

I explained carefully earlier on with the math that is exactly what is NOT going on. Because the cat is entangled with the emitted particle and you just observe the cat it is in a mixed state - not a superposition. In fact you can push it all the way back to the particle detector that is entangled with the emitted particle and is really the clearest way of looking at it. But people are so fixated with the cat its what I used.

Thanks
Bill

 

[Derek Potter]

The cat is in a superposition of states and when you observe the cat it will collapse into the dead state, or the alive state! It's a thought experiment simply to highlight that until a measurement is made there is no way to predict the state of the cat. Determinism disappears on a quantum level and we can only calculate probabilities. Hence the power of a qubit which like a bit can have two states, 0 or 1, or a superposition of both of those states, hence more processing power. To highlight what I'm trying to say :)

Your assertion about superposition, determinism and collapse is an interpretation. It also presupposes there is such a thing as "the quantum level".

 

[Buzz Bloom]

Hi Bill:

I have no idea where you are getting this from, but the the state of the cat has nothing to do with the opening or not of the box.

I hope you will excuse an small attempt at humor in my response.

I understand that there are controversies about the philosophical spectrum of possibilities for interpreting QM phenomena, in particular concerning when a particle's state changes from a superposition of possibilites to a specific single state. At one extreme, the answer is an interaction with another particle. At the other extreme, it's when a conscious mind becomes aware of a measurement. In between, there are many possible criteria that are plausible in different contexts. One such middle-of-the-road criteria is the detector. On weekends, I prefer the detector. On Monday and Tuesday I prefer the interaction, On Thursday and Friday, I prefer the mind. On Wednesday, I make up something new in the middle. Today is Thursday, so opening the box does make a difference.

A friend with a PhD in particle physics once explained a very complicated TE to me. A particle with a superposed state of discrete possibilities has a state detected, and the detection puts the particle into a different superposed state with different probabilites. If the information in the detector is destroyed before the a state of the changed particle is detected, are the probabilities of it's possible states different than if it had not been destroyed? As I vaguely remember the math of his argument, (paradoxically?) there would be a difference.

What I learned from my friend's TE (and some Zen study) is that any philosophical choice for when the superposed state changes to a discrete state can make a reasonable interpretaion of QM that works until one can find an implied paradox. One can then accept that: The true nature of reality is fundementally paradoxical, or one can choose an alternative for which a paradox has not yet been found.

Thanks for your discussion,
Buzz