Can All Particles of a Cat Be in Superposition Simultaneously?

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    Decoherence
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Discussion Overview

The discussion revolves around the concept of superposition in quantum mechanics, particularly in relation to macroscopic objects like cats and tables. Participants explore the implications of decoherence and whether all particles of a macroscopic object can simultaneously exist in superposition, as well as the nature of states observed in everyday objects.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question whether it is possible for all particles of a macroscopic object to be in superposition for an extremely brief moment before decoherence occurs.
  • Others argue that decoherence is a continuous process that prevents macroscopic objects from existing in superposition for any meaningful duration.
  • There is a discussion about the nature of mixed states versus pure states, with some participants suggesting that decoherence leads to an apparent collapse of superpositions into mixed states.
  • One participant mentions the Many-Worlds Interpretation (MWI) and its implications for superposition, suggesting that mixed states may be approximations rather than definitive descriptions.
  • Another participant emphasizes that after decoherence, macroscopic objects are not in superposition but rather in a mixed state, particularly concerning their position.
  • There is a proposal that if one adopts the ignorance ensemble interpretation combined with decoherence, one could conclude that macroscopic objects have definite properties at each instant, despite being in a mixed state.

Areas of Agreement / Disagreement

Participants express differing views on the implications of decoherence and the nature of states in macroscopic objects. There is no consensus on whether macroscopic objects can be considered to exist in superposition or if they are definitively in mixed states after decoherence.

Contextual Notes

Participants note that the discussion involves complex interpretations of quantum mechanics, including the measurement problem and the implications of different interpretations like MWI and the ignorance ensemble approach. The nature of decoherence and its effects on the properties of macroscopic objects remain unresolved.

  • #31
StevieTNZ said:
That is correct.

Bill said it isn't correct.

Nugatory said:
Try posts #9 and #11 of this thread: https://www.physicsforums.com/threads/nature-of-quantum-mechanics.823160/

Now I understand what you meant, thank you. I still don't understand how does the macroscopic world emerge from quantum physics and how do all these states that seem definite occur. I can precisely describe the objects around me right now but I never get a simplification of anything and a common-sense description of the process.
 
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  • #33
So basically we would need a collapse to avoid bizzarity in macroscopic interferences, or a better quantum theory which explains the objective evolution of 'classical' objects?
 
  • #34
durant35 said:
Can you explain what do you mean when you say that all states are superpositions in some basis, with an example?

I am sorry but the jig is up here. You need to delve into the technicalities. One can only go so far in explaining this linguistically. Feynman commented on this. It doesn't matter how good you are at lay explaining you will always run into the difficulty the laws of physics are written in the language of math. You must learn that for any real understanding.

All you have to accept however is due to the nature of interactions here in the macro world we generally don't see superpositions of position.

Thanks
Bill
 
  • #35
durant35 said:
So basically we would need a collapse to avoid bizzarity in macroscopic interferences, or a better quantum theory which explains the objective evolution of 'classical' objects?

No. It is generally accepted that decoherence explains why we generally do not see interference effects.

Again if you want to go beyond simply accepting what we say and actually draw inferences you must learn the detail. Start here:
https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20
https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

Thanks
Bill
 
  • #36
durant35 said:
Bill said it isn't correct.

Just to be clear what I am saying. When objects are entangled with other objects and you observe one of those objects and not the other it is found to be in a mixed state. If the system as a whole is isolated they evolve by unitary evolution.

Suppose you have system 1 that can be in state |a> or |b> and system 2 can be in state |a> or |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b> and similarly if system 1 is in state |b> and system 2 in state |a> that is written as state |b>|a>. Now according to the principle of superposition you can have a superposition of these ie c1|a>|b> +c2|b>|a>. The systems are entangled. Each system is no longer in a pure state - they have lost their individuality. The interesting thing however is if you observe system 1 it acts as though its now in a mixed state. The entanglement has broken the unitary evolution. The two systems as a whole evolve unitarily - but the systems when observed behave differently. This is the origin of decoherence and since everything here in the macro world is entangled why we have classical properties.

Thanks
Bill
 
  • #37
[Mentor's note: edited to remove quoted text from deleted posts]

As bhobba has (at least the way I've interpreted it) stated if you have system 1 + system 2 entangled, and you only look at system 1, then the Mathematics of system 1 is a mixed state. But the true reality is system 1 is still entangled with system 2, thus system 1 and 2 are in a superposition (Bill can correct me if I am wrong in summarising his point. I am unsure whether he is using the word 'observe' as theoretically looking at system 1, or experimentally measuring system 1).
 
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  • #38
StevieTNZ said:
As bhobba has (at least the way I've interpreted it) stated if you have system 1 + system 2 entangled, and you only look at system 1, then the Mathematics of system 1 is a mixed state. But the true reality is system 1 is still entangled with system 2, thus system 1 and 2 are in a superposition (Bill can correct me if I am wrong in summarising his point. I am unsure whether he is using the word 'observe' as theoretically looking at system 1, or experimentally measuring system 1).

That's it. The modern view of observation is to place the Von Neumann cut just after the systems become entangled which is equivalent to taking the mixed state as proper mixed state. Of course out there everything is always entangled so there is no pure state that evolves. Even in deep interstellar space objects are entangled with the CBMR and the quantum vacuum which is responsible for spontaneous emission.

Thanks
Bill
 
  • #39
bhobba said:
Just to be clear what I am saying. When objects are entangled with other objects and you observe one of those objects and not the other it is found to be in a mixed state. If the system as a whole is isolated they evolve by unitary evolution.

Suppose you have system 1 that can be in state |a> or |b> and system 2 can be in state |a> or |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b> and similarly if system 1 is in state |b> and system 2 in state |a> that is written as state |b>|a>. Now according to the principle of superposition you can have a superposition of these ie c1|a>|b> +c2|b>|a>. The systems are entangled. Each system is no longer in a pure state - they have lost their individuality. The interesting thing however is if you observe system 1 it acts as though its now in a mixed state. The entanglement has broken the unitary evolution. The two systems as a whole evolve unitarily - but the systems when observed behave differently. This is the origin of decoherence and since everything here in the macro world is entangled why we have classical properties.

Thanks
Bill

But what about other macroscopic properies and their interference? Can you say, reasonably, and for sure that you aren't both dead and alive in any fragment of time, that those two properties do not interfere like in the case of Schrödinger's cat?
 
  • #40
I think I understood it, when we talk about macroscopic objects we are talking about position basis and interference effects between different positions. In Schrödinger's cat the interference effects were created from a subatomic effect and that's why the interference properies were alive and dead. Since no microscopic/macroscopic sequence of events occurs in the real world we can safely assume that the only properties that are interfering are location properties, everything else is stable. Right?
 
  • #41
durant35 said:
Can you say, reasonably, and for sure that you aren't both dead and alive in any fragment of time,

I have answered that before - it never is - ever ever. Its impossible ie a live cat breaths, has a heart pumping - its constituent parts with definite position behave in a different way

Thanks
Bill
 
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  • #42
bhobba said:
I have answered that before - it never is - ever ever. Its impossible ie a live cat breaths, has a heart bumping - its constituent parts with definite position behave in a different way

Thanks
Bill

Ok. You said that decoherence leads to macroscopic position basis, so the only uncertainties or small undetectable interference effects are related to the position of the object, everything else evolves classicaly without interferences. So when we are sure that the object is well located the classical behavior emerges and there is no bizzareness.
 
  • #43
bhobba said:
I have answered that before - it never is - ever ever. Its impossible ie a live cat breaths, has a heart bumping - its constituent parts with definite position behave in a different way

Thanks
Bill

This was one of your older posts:
We know 100% for sure the cat is not in any kind of superposition that is detectable. In fact decoherence shows it decays to way way below our ability to detect very very quickly.

Thanks
Bill

So in the experiment is there a possibility for an undectetable interference?
 
  • #44
durant35 said:
So in the experiment is there a possibility for an undectetable interference?

I have already said theoretically its not actually zero. The other issue is the components of a cat were decoherted way way back even before the cat was born. The cat never ever ever is in a superpoition of position.

Thanks
Bill
 
  • #45
bhobba said:
I have already said theoretically its not actually zero.

Thanks
Bill

But that state can't be qualified as a relevant superposition, right?
 
  • #46
durant35 said:
But what about other macroscopic properies and their interference? Can you say, reasonably, and for sure that you aren't both dead and alive in any fragment of time, that those two properties do not interfere like in the case of Schrödinger's cat?

Yes, that's the whole point of decoherence is that there is no superposition of a live cat and a dead cat. Or rather, getting a superposition of a dead cat and a live cat is about as likely as getting a melted ice cube to reform on a hot day.
 
  • #47
stevendaryl said:
Yes, that's the whole point of decoherence is that there is no superposition of a live cat and a dead cat. Or rather, getting a superposition of a dead cat and a live cat is about as likely as getting a melted ice cube to reform on a hot day.

And of course, you would have to induce it with a microscopic event like in the experiment with the atom decay or like they induced an excited state with the photon in the diamonds experiment.
 
  • #48
bhobba said:
I have already said theoretically its not actually zero. The other issue is the components of a cat were decoherted way way back even before the cat was born. The cat never ever ever is in a superpoition of position.

Thanks
Bill

So let me get this straight, in the experiment there was a cause for the potential death of the cat (the nucleus decaying) and even though the decoherence happens at the particle detector when it reaches the cat there is still undetectable interference?

But without a cause like the nuclear decay, in everyday life, anything that is alive is decohered to the position basis and interference properties don't exist?
 
  • #49
It seems that if interference again occurs, that nature is forced to select again.

Consider this example, let'say the decoherence happened at the particle detector and the cat survived and it is alive. If the beyond detectable interference between alive and dead again occurs, the cat may die without a reason simply because nature chose a different outcome based on that moment of time. The same thing applies to any human if there occur interference terms between alive and dead at any instant. I know this sounds crazy, so please correct me when I'm wrong, if it's possible in common-sense terms.
 
  • #50
durant35 said:
The same thing applies to any human if there occur interference terms between alive and dead at any instant.

That's not possible as I have explained. I have zero idea why you want to pursue it.

You need to study the actual theory - you will never understand it the way you are trying to.

Thanks
Bill
 
  • #51
bhobba said:
That's not possible as I have explained. I have zero idea why you want to pursue it.

You need to study the actual theory - you will never understand it the way you are trying to.

Thanks
Bill

I don't want to pursue it, I want to get away from it, because it seems absurd. You said that theoretically the interference is not zero, that's why I'm asking.
 
  • #52
durant35 said:
I don't want to pursue it, I want to get away from it, because it seems absurd. You said that theoretically the interference is not zero, that's why I'm asking.

Did you read what else I said about applied stuff? For the last time - there is no superposition of alive and dead. As far as I am concerned that's the end of the matter and I will not discuss it any further.

Thanks
Bill
 
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  • #53
I'm sorry Bill, I didn't want to be annoying or anything like that. Believe me I read every letter that you write and try to analyze it but so many questions pop from my head and some things that you say then sound inconsistent, like the theoretical existence of interference and absence of superposition of alive and dead.
 
  • #54
And even reading some old posts on this forum implies even more confusion, for instance Nugatory said this:

"You have misunderstood the point of Schrödinger's thought experiment. There has never been any serious doubt that the cat is always either alive or dead (although we may not know which it is) with no funny 50% alive/50% dead states.

Schrödinger proposed the thought experiment to point out a problem with the 1920's vintage understanding of quantum mechanics - the formalism as it was then understood suggested that the cat could be in one of these funny states even though no one believed that it could be.

The resolution came with the discovery of quantum decoherence some decades later. The half-dead/half-alive state very rapidly evolves into a state in which the cat is either alive or dead - we may not know which, but it is one or the other as surely as a tossed coin is heads or tails but not some funny mixture of the two."e

Everything is fine until this sentence: The half-dead/half-alive state very rapidly evolves into a state in which the cat is either alive or dead. I understand the second part of the sentence but the first clearly implies that the half-dead and half-alive state exists. And you said that the cut is at the particle detector so the cat sets in a definite state long before opening the box.
 
  • #55
durant35 said:
like the theoretical existence of interference and absence of superposition of alive and dead.

Your issue is you may read what is said but do not take it on board. For example you have had explained to you in another thread that some really small number like say 1/googleplex is the same as zero. It one of the basics of applied math used, often without explicitly starting it because its so obvious, to make sense of things. Then you turn around and start carrying on about some theoretical existence of interference. Any theoretical interference terms are zero because they are so small.

If you want to avoid confusion you must start thinking more clearly about this.

Thanks
Bill
 
  • #56
durant35 said:
The half-dead/half-alive state very rapidly evolves into a state in which the cat is either alive or dead. I understand the second part of the sentence but the first clearly implies that the half-dead and half-alive state exists. And you said that the cut is at the particle detector so the cat sets in a definite state long before opening the box.

All that is saying is if you analyse it as an entangled system (ie the cat is entangled with the radioactive source) rather than the classical parts already decohered by the environment then that's what the analysis shows. But in reality the cat and everything in the box is decohered by the environment. This is the type of thing that's done in mathematical modelling all the time - we have slightly different models giving slightly different results. But again we have the 1/googleplex issue - the time is so small its zero.

Once and for all the cat is a macro object decohered into parts with definite position. An alive cat has a totally different arrangement of those parts than a dead cat. The alive cat has a beating heart - the dead cat doesn't. Things with definite position can not move and be still. They can never, ever, ever be in superposition.

Thanks
Bill
 
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  • #57
I've read a bit of the Where does the weirdness go and it's great, combined with your help it increases my understanding. Thanks.

The only barriere is the fact that Zurek and other authors mention decoherence time so often, like superpositions occur all the time and they get destroyed which isn't the same like the cat that is decohered since its birth and alive. What do they mean really? I'm sure you have insight.
 
  • #58
durant35 said:
I'm sure you have insight.

No need to post here - you already posted it in another thread.

Thanks
Bill
 
  • #59
bhobba said:
No need to post here - you already posted it in another thread.

Thanks
Bill

I'm sorry, I accidentally posted it there without noticing that it's another thread. So basically Zurek and others are referring to the decoherence time as a reason for localization of the object and that it stops it from spreading. They aren't referring to the fact that we as humans enter some weird dead and alive state that gets destroyed in quick time. Right? So macro objects are decohered from the beginning, there is no coherence to begin with.

[QUOTE="bhobba, post: 5383592, member: 366323" But in reality the cat and everything in the box is decohered by the environment. This is the type of thing that's done in mathematical modelling all the time - we have slightly different models giving slightly different results. But again we have the 1/googleplex issue - the time is so small its zero.
Thanks
Bill[/QUOTE]

What do you exactly mean or on what did you refer when you mentioned this amount of time?
 
  • #60
durant35 said:
So basically Zurek and others are referring to the decoherence time as a reason for localization of the object and that it stops it from spreading.

No - they have their own arguments that I am not into nor particularly interested in. You are on your own there. Start a thread on it.

I am explaining why decoherence time is still a consideration in what I am explaining to you that all macro objects are constantly decohered into an actual position and why it remains like that.

Thanks
Bill
 

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