Is the cat alive, dead, both or unknown

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The discussion revolves around Schrödinger's cat thought experiment, questioning whether the cat should be considered in a state of "unknown" rather than "both alive and dead." The consensus is that while the cat is indeed in a superposition of states, the terms "unknown" and "superposition" are not synonymous. The cat's fate is tied to the decay of a radioactive atom, which introduces a probability of being alive or dead, but this does not imply the cat is in a mixed state. The conversation emphasizes the distinction between superpositions and mixed states in quantum mechanics, clarifying that observations affect the system's state. Ultimately, the discussion highlights the complexities of interpreting quantum states and the implications for understanding reality.
  • #151
Nugatory said:
The big bang is indeed in the causal past of everything, but any residual entanglement between two particles that have not interacted since then is near as no never mind zero. That is, the wave function of the two-particle system is for all practical purposes completely factorizable and the two particles can be treated separately.

(This might, however, be a good time to suggest googling for "quantum superdeterminism", as long as everyone promises not to prolong this thread based on what they find).
I wasn't saying it was realistic to be able to extract any useful information nowadays, just that it didn't "make no sense" to view them as entangled from the beginning, as I think this was a confusing and unnecessary complication to the argument. Thanks for chiming in.
 
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  • #152
Haelfix said:
So I wouldn't exactly call that entanglement, although I concede there is a subtle point there. I would instead say that it is a rather interesting statement about the form certain types of entanglement can take. Also, the anti-symmetrization of the wavefunction is really a consequence of the spin-statistics theorem, which crucially relies on the existence of local relativistic field theory.

Now I want to emphasize that this is not merely intuition, but rather the history of a long line of failed attempts. So, if someone thinks that they can write down a local theory that can 'create' entanglement at spacelike separation out of thin air, without using a local, and causal third party (like a messenger particle in the case of entanglement swapping), then write down that theory. The problem will become obvious the second you attempt to do that, as you will find that you need to write down an interaction Hamiltonian that will either involve fields that are evaluated at different spacetime points, or that will require higher derivatives. I think this is where the Bohmians run into issues. They have to be able to allow ftl communication between say EPR pairs, but not for anything else, which then requires imposition of extra rules that adds theoretical baggage. As I said, I don't really know how successful they are with that.

Nugatory said:
The big bang is indeed in the causal past of everything, but any residual entanglement between two particles that have not interacted since then is near as no never mind zero. That is, the wave function of the two-particle system is for all practical purposes completely factorizable and the two particles can be treated separately.

(This might, however, be a good time to suggest googling for "quantum superdeterminism", as long as everyone promises not to prolong this thread based on what they find).

@Haelfix, my intuition is closer to what Nugatory is saying - it can be present, but in general it should be very diluted (because of monogamy of entanglement). Also, originally, you mentioned it as analogous to the horizon problem. So would it be right to understand it not so much as "impossible", but more a question of fine tuning - ie. "dman odd" as you put it?
 
  • #153
Monogomy of entanglement? haven't they entangled as many as 5 particles intentionally? That's what I thought until google turned up th world record being 3000, but all the articles seemed to be pop-sci. Here's one on multiple entanglement.
http://m.phys.org/_news63037231.html
 
  • #154
BiGyElLoWhAt said:
Monogomy of entanglement? haven't they entangled as many as 5 particles intentionally? That's what I thought until google turned up th world record being 3000, but all the articles seemed to be pop-sci. Here's one on multiple entanglement.
http://m.phys.org/_news63037231.html

It refers to maximal entanglement.
 
  • #155
Hmmm... I'll have to look into that, would you care to elaborate a little bit?
 
  • #156
bhobba said:
Entanglement has nothing to do with anything like that - its simply applying the principle of superposition to systems. I gave a very careful explanation before - its really all there is to it. Nothing weird in the sense of being mystical etc etc is going on - it simply leads to a different type of correlation than occurs classically. The difference is classically you know it has properties all the time ie the green and red slips of paper are always green and red. In QM its more subtle as Bells theorem shows - but it's still just a correlation - its not some phenomena that needs further explanation. We know its explanation - systems can be in superposition and hence are correlated in a way different to classical correlations.

Thanks
Bill

Indeed, and that is the point. People get very tripped up about spooky action at a distance, or how quantum mechanics can seemingly feel measurements that are very far apart and adjust accordingly etc. But at the end of the day, as long as you give up realism (counterfactual definitiveness to use the philosophical lingo) and simply accept that we don't have bits, but instead we have qubits, there is absolutely nothing bizarre about Bells inequalities being violated. We don't have to give up locality, relativity or anything like that. The correlations found in quantum mechanics are different than classical mechanics, but we already knew this 40 years before Bell formulated his thought experiment.
 
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  • #157
Sorry, I don't think my point has come across. I am not saying anything controversial here. The point is you can only create entanglement locally. Forget about the horizon problem, or anything like that, I regret bringing that up b/c it seems like it has caused confusion.

So how about this. 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.
 
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  • #158
Haelfix said:
Sorry, I don't think my point has come across. I am not saying anything controversial here. The point is you can only create entanglement locally. Forget about the horizon problem, or anything like that, I regret bringing that up b/c it seems like it has caused confusion.

So how about this. 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.

Is that the same thing as eg. a limitation on the states that say LOCC can prepare? Or more than that?

Edit: Or is it the same as eg. if B and C are in a product state, then local unitaries can never cause B and C to be entangled?
 
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  • #159
BiGyElLoWhAt said:
Hmmm... I'll have to look into that, would you care to elaborate a little bit?

Monogamy of entanglement is misleading but standard jargon. It certainly doesn't mean that one particle can only be entangled with another - that's considered so "obvious" that the misleading jargon is considered not that misleading. http://quantiki.org/wiki/Monogamy_of_entanglement
 
  • #160
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..
 
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  • #161
BiGyElLoWhAt said:
give me a dumbed down version of what entanglement actually means in a modern context? Because I was apparently unable to grasp it from what you've said...

See post 115. I will repeat it in summery - entanglement is the principle of superposition applied not just to the same system but composite systems.

BiGyElLoWhAt said:
If this is, in fact, how it works, then why are not ALL particles from the time of the big bang entangled, as nick666 has said?

Thats assuming they were all entangled to begin with - something those that know more cosmology than I do question:
http://physics.stackexchange.com/questions/55026/particles-entangled-after-the-big-bang

Thanks
Bill
 
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  • #162
rootone said:
and information does not have to travel any distance at all for this to be so. It is not spooky at all..

No it inst.

Its only spooky if you want to hold to counter-factual definiteness. That's the root cause of all this angst. And even if you still want to hold to it then pick an interpretation like Bohmian Mechanics and see what happens there. In that interpretation you can see exactly why you have locality broken:
http://arxiv.org/pdf/quant-ph/0611032v1.pdf

Also let's look at the definition of locality in the paper on Bell I linked to before:
'Let us define a “local” theory as a one where the outcomes of an experiment on a system are independent of the actions performed on a different system which has no causal connection with the first. For example, the temperature of this room is independent on whether I choose to wear purple socks today. Einstein’s relativity provides a stringent condition for causal connections: if two events are outside their respective light cones, there cannot be any causal connection among them.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

Thanks
Bill
 
  • #163
bhobba said:
Also let's look at the definition of locality in the paper on Bell I linked to before:
'Let us define a “local” theory as a one where the outcomes of an experiment on a system are independent of the actions performed on a different system which has no causal connection with the first. For example, the temperature of this room is independent on whether I choose to wear purple socks today. Einstein’s relativity provides a stringent condition for causal connections: if two events are outside their respective light cones, there cannot be any causal connection among them.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

But aren't Bertlmann's socks correlated systems?
 
  • #164
atyy said:
But aren't Bertlmann's socks correlated systems?

Of course. But its a classically correlated system and obeys Bell's inequalities. Quantum correlated systems do not and there is the rub. But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.

Thanks
Bill
 
  • #165
There are some who think entanglement is something that could exist at macroscopic scale, cause of a topological property of spacetime . They proposed some sort of macroscopic experiment based on an arvix paper, and that arvix paper was apparently reviewed by these guys . If anyone's interested I can post the paper.
 
  • #166
Nick666 said:
There are some who think entanglement is something that could exist at macroscopic scale,

Everything is quantum. Of course it exists at the macroscopic scale. The very reason for classical properties is classical objects are all the time being observed by its environment and entangled with it.

I am unaware of any relation to the topology of space-time. But it sounds highly speculative and probably best in its own thread.

Thanks
Bill
 
  • #167
Yeah the proposer of the experiment says the macroscopic things will also be corelated just like the entangled photons, so there would be nothing special about the quantum realm when it comes to entanglement, the correlations being consequences of the topological properties of space. And the experiment being macroscopic it would manifestly be local and realistic.
 
  • #168
bhobba said:
Of course. But its a classically correlated system and obeys Bell's inequalities. Quantum correlated systems do not and there is the rub. But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.

I don't know if your definition of cluster decomposition is the same as Wikipedia's https://en.wikipedia.org/wiki/Cluster_decomposition_theorem, which makes use of the vacuum state. I also don't think that Wikipedia's definition of cluster decomposition is the same as that used in Bell's theorem, because Bell's theorem is independent of QM, whereas cluster decomposition is defined with respect to QM, since it uses the concept of a vacuum state.
 
  • #169
  • #170
atyy said:
I don't know if your definition of cluster decomposition is the same as Wikipedia's

I gave a link earlier that addressed that:
https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

Even Weinberg is a bit loose in his bible textbook. The key point is it doesn't apply to correlated systems in QFT.

The definition of locality I gave from my linked paper on Bell is very similar to the cluster decomposition property.

Thanks
Bill
 
  • #171
bhobba said:
But if you remove correlated systems from the definition of locality, and you are pretty well forced to in the cluster decomposition property for it to be reasonable, then its of no concern as far as locality goes. If however you want counter-factual definiteness then it must be included and you get something like: 'experiments that are sufficiently separated in space have unrelated results'. It immediately jumps out - what if they are correlated to begin with. So you say - of course - that needs to be removed and you get 'Uncorrelated experiments that are sufficiently separated in space have unrelated results'. But if you do that you can't have counter-factual definiteness because in order for Bells inequality to be broken correlated systems need to break locality. In a sense you are led to something quite unnatural.
You are fooling with definitions. You are bringing in irrelevant definition of "locality" while there is common and widely used relevant definition of "locality".
Your irrelevant definition - Uncorrelated experiments that are sufficiently separated in space have unrelated results
Relevant definition - There can't be any influence between spacelike separated events.
 
  • #172
@bhobba, here is a different explanation of cluster decomposition - the underlying principle would then be no superluminal signalling of classical information.

http://relativity.livingreviews.org/Articles/lrr-2005-5/ (section 2.7.1)

Yes, I do think Weinberg's explanation is wrong.
 
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  • #174
zonde said:
You are fooling with definitions. You are bringing in irrelevant definition of "locality" while there is common and widely used relevant definition of "locality".

I gave the common and widely used one as found in my linked paper on Bell.

Thanks
Bill
 
  • #175
bhobba said:
Also let's look at the definition of locality in the paper on Bell I linked to before:
'Let us define a “local” theory as a one where the outcomes of an experiment on a system are independent of the actions performed on a different system which has no causal connection with the first. For example, the temperature of this room is independent on whether I choose to wear purple socks today. Einstein’s relativity provides a stringent condition for causal connections: if two events are outside their respective light cones, there cannot be any causal connection among them.'

That is a slightly less clearly stated version of the cluster decomposition property - it doesn't state a necessary caveat - namely - it can't apply to correlated systems. Entangled systems are correlated so - it isn't even an applicable definition. Only if you want counter-factual definiteness is it even a worry. If you do then you have to allow locality to apply to correlated systems.

I think the definition of "local" is ok without the caveat that about correlated systems. It means that if two systems are spacelike separated, then an action on one cannot affect the probabilities of local observables on the other. This prevents superluminal signalling, and that is the basis of the cluster decomposition.

Weinberg's informal statement is just so wrong, I don't think it is worth rescuing by adding caveats. If we use "no superluminal signalling", we get cluster decomposition, and an acceptable definition of locality used in some derivations of Bell's theorem.
 
  • #176
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"?
 
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  • #177
atyy said:
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
 
  • #178
Haelfix said:
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?
 
  • #180
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.
 

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