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.

durant35
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Hi guys,

just a quick question. This is an idea that came up to my mind while thinking about decoherence and stability of macroscopic objects in the world. As we know all superpositions and interference effects are destroyed in a enormously tiny timescale, and as Mfb mentioned it is meaningless to talk about a quantity that is in superposition in that short time interval. Now this may or may not imply bizzareness, but is it possible that all particles of a cat or a table can be in a superposition of different composition in that tiny instant, for instance a cat that is in a state of being in a normal state with its particles constitued as we see it and a state of let's say its neurons being in its legs, or whatever, just that those particles that make up her body are differently ordered. I hope somebody can clear up this bizzareness and explain. Thank you
 
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If macroscopic objects are in such a state for 10-bazillionths of a second or whatever
why worry? Once things get to the Planck scale, nobody knows what's happening anyway.
 
It is bizzare, just that
 
durant35 said:
and as Mfb mentioned it is meaningless to talk about a quantity that is in superposition in that short time interval.

Of course not. Macro objects do not come into existence instantaneously then decohere. They are there all the time constantly being decohered. As the object with localised position spreads it interacts with the environment and that stops the spreading keeping it in a definite position.

Thanks
Bill
 
I don't understand it, simply I don't. Logic says this - if decoherence is such a fast process any superposition that occurs is destroyed in a range of a smallest fragment of time. What it means to constantly decohere? Do superpositions of properties occur or not?
 
durant35 said:
I don't understand it, simply I don't. Logic says this - if decoherence is such a fast process any superposition that occurs is destroyed in a range of a smallest fragment of time. What it means to constantly decohere? Do superpositions of properties occur or not?
Dear durant35

Decoherence only causes apparent collapse. In principle, the superposition (macroscopic object entangled with the environment) still exists.
 
StevieTNZ said:
Dear durant35

Decoherence only causes apparent collapse. In principle, the superposition (macroscopic object entangled with the environment) still exists.

Yes, I know that. The thing I don't understand is the consequence of this for everyday objects. When we see or measure an object we always find in a definite state, so if we focus solely on that object and its properties does or does not the object have a definite state at each instant? Thats what I don't understand regarding decoherence.
 
durant35 said:
does or does not the object have a definite state at each instant? Thats what I don't understand regarding decoherence.

Yes - but it isn't not a pure state. Its a mixed state. Mixed states can be interpreted as being in a certain state with some probability. This is written as ∑pi |bi><bi| which says its in state |bi><bi| with probability p1. Trouble is that's not how it was necessarily prepared ie it was not necessarily done by some process randomly selecting some state |b1><ibi| with probability pi. If it was done that way its called a proper mixed state. But that not what happens in decoherence - it mathematically exactly the same - you can't tell the difference - but it wasn't prepared as a proper mixed state. That's why its only called apparent collapse. Can we assume it is a proper mixed state and actual collapse occurred. Of course you can - and if you do all problems in QM gone. Its called the ignorance ensemble interpretation (ensemble comes from the frequentest probability interpretation of the pi - its a slight variation on the ensemble interpretation). The thing is it an interpretive assumption - how does it become a proper one - blank out. There are all sorts of ways it could, BM, MW, Nelson Stochastics, primary state diffusion. It may even be nature is simply like that - nothing deeper going on at all. That's where ignorance comes in.

Thanks
Bill
 
durant35 said:
Yes, I know that. The thing I don't understand is the consequence of this for everyday objects. When we see or measure an object we always find in a definite state, so if we focus solely on that object and its properties does or does not the object have a definite state at each instant? Thats what I don't understand regarding decoherence.
To us we see it in a definite state; it's description by Quantum Mechanics has it being in a superposition. This is where we meet the measurement problem.
 
  • #10
StevieTNZ said:
it's description by Quantum Mechanics has it being in a superposition.

That's not true. After decoherence its not in a superposition - its in a mixed state. Note every pure state is a superposition of many other states and in many different ways. When referring to decoherence here in the macro world we are referring to superpositions of position because the usual radial symmetry of interactions decoheres into a mixed state of position ie the |bi><bi| of ∑pi |bi><bi| are position eigenstates. 100% for sure it is not in a superposition of position.

Thanks
Bill
 
  • #11
ut according to the MWI, it *is* a superposition (the mixed state is just an approximation). According to the MWI, unitary evolution is the only kind of evolution there is, and a unitary transformation can't turn a pure state into a mixed state.

(Bear in mind that I'm not necessarily agreeing with the MWI; I'm just trying to be clear about what it says.)

What about this old post from PeterDonis?
 
  • #12
durant35 said:
ut according to the MWI, it *is* a superposition (the mixed state is just an approximation).

That's wrong.

The off diagonal elements are not quite zero - but so low to be undetectable - but that does not mean its in a superposition. In fact since its entangled so it can't be in a pure state hence can't be in a superposition.

Before drawing conclusions like the above you need to study the detail so you have the background to make correct ones.

Thanks
Bill
 
  • #13
It wasn't my conclusion of course, I'm not competent enough to type down something like that. It was from another forum member and I just wanted your opinion about it because everything you say makes sense pretty much. So if we assume the ignorance ensemble approach combined with decoherence, can we conclude that each macroscopic object has a definite property at each instant on the timescale (but a mixed state ofc), nature doesn't really care and collapse has actually happened (there are no macroscopic superpositions in conditions we live in) ?
 
  • #14
durant35 said:
can we conclude that each macroscopic object has a definite property at each instant on the timescale (but a mixed state ofc), nature doesn't really care and collapse has actually happened (there are no macroscopic superpositions in conditions we live in) ?

Of course we can.

But again I stress we are dealing with the property of position.

As I have said don't get worried about the fact theoretically its not zero. It's the type of thing that happens in applied subjects all the time.

Thanks
Bill
 
  • #15
bhobba said:
Of course we can.

But again I stress we are dealing with the property of position.

As I have said don't get worried about the fact theoretically its not zero. It's the type of thing that happens in applied subjects all the time.

Thanks
Bill

I understood the thing about. So other properties are basically derived and the same as in classical world because the object is really-well localized?
The main thing I was stressing about (and you know it) are the Schrödinger cat-like properties (like dead or alive, or the shape of the object). So all objects are constantly decohering and the position basis or location is well defined at any instant, and from follows that the other properties of the object are definite (or classical) at any instant. Why does the decoherence time get mentioned so much then in textbooks so it leaves an impression that the objects properties in fact are in a superposition but then decoherence destroys the superposition?
 
  • #16
durant35 said:
Hi guys,

just a quick question. This is an idea that came up to my mind while thinking about decoherence and stability of macroscopic objects in the world. As we know all superpositions and interference effects are destroyed in a enormously tiny timescale, and as Mfb mentioned it is meaningless to talk about a quantity that is in superposition in that short time interval.

Just to clarify something: Every quantum state can be interpreted as a superposition of other quantum states. So it doesn't really make sense to say that decoherence gets rid of superpositions. What it does is prevent interference effects among macroscopically distinguishable states. So you can never observe interference effects between a dead cat and a live cat. But there are still superpositions involved.
 
  • #17
stevendaryl said:
Just to clarify something: Every quantum state can be interpreted as a superposition of other quantum states. So it doesn't really make sense to say that decoherence gets rid of superpositions. What it does is prevent interference effects among macroscopically distinguishable states. So you can never observe interference effects between a dead cat and a live cat. But there are still superpositions involved.

If we understand decoherence as a collapse, no as an apparent collapse, (like Bill mentioned), the superpositions go away, right? How would you describe the superpositions that are not macroscopically distinguishable, can you give me an example?
 
  • #18
durant35 said:
Why does the decoherence time get mentioned so much then in textbooks so it leaves an impression that the objects properties in fact are in a superposition but then decoherence destroys the superposition?

Decoherence isn't just for the quantum classical transition - it has practical applications as well. Now don't ask me about those because I am not into it, but its important for that, in particular they want to prevent it in quantum computing.

Thanks
Bill
 
  • #19
durant35 said:
ut according to the MWI, it *is* a superposition (the mixed state is just an approximation). According to the MWI, unitary evolution is the only kind of evolution there is, and a unitary transformation can't turn a pure state into a mixed state.

(Bear in mind that I'm not necessarily agreeing with the MWI; I'm just trying to be clear about what it says.)

What about this old post from PeterDonis?

Another point of clarification: When you ask whether some subsystem--such as an electron, or a molecule, or a cat--is in a superposition of states, that's a little ambiguous. In MWI, there is a single wave function for the entire universe, and that wave function is in a superposition of possibilities. But subsystems don't have a wave function, in general. If two subsystems are entangled, there can be a wave function for the composite system, but not for each subsystem.
 
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  • #20
durant35 said:
If we understand decoherence as a collapse, no as an apparent collapse, (like Bill mentioned), the superpositions go away, right?

I don't know if "go away" is the right phrase. The way I understand decoherence as applied to Schrödinger's cat is this: Originally, you have a uranium atom that is in a superposition of decayed/undecayed states. The superposition quickly "infects" the rest of the universe. You don't have a cat that is in a superposition of live and dead, instead, you quickly have the entire world in a superposition of a world with a dead cat and a world with a live cat. The implications of the uranium decay spread out at the speed of light. In one "branch", there is a decayed uranium atom, and there is a dead cat, and there is me, crying over my dead cat, and there is Schrödinger patting me on the back and telling me that my cat died for the good of science, etc. In another cat, the atom is undecayed and the cat is alive, and I'm not sad, and Schrödinger isn't patting me on the back.

Once a superposition has "infected" the whole world, the various macroscopically distinguishable branches no longer have any effect on each other. At this point, you may as well assume that the branch you're observing is the only one. Or you can go MWI and assume that the other branches exist, as well. It doesn't make any practical difference.

How would you describe the superpositions that are not macroscopically distinguishable, can you give me an example?

Sure. A molecule can be in two different configurations of the same atoms, or it can be in a superposition of those configurations. Such a superposition is not of macroscopically distinguishable states.
 
  • #21
stevendaryl said:
Sure. A molecule can be in two different configurations of the same atoms, or it can be in a superposition of those configurations. Such a superposition is not of macroscopically distinguishable states.

I thought so, but I was referring to small time intervals combined with macroscopic objects. So at any instant of time can we assign a definite macroscopically distinguishable state to an object?
 
  • #22
bhobba said:
Decoherence isn't just for the quantum classical transition - it has practical applications as well. Now don't ask me about those because I am not into it, but its important for that, in particular they want to prevent it in quantum computing.

Thanks
Bill

Zurek mentions it for neurons in our brain, so he leaves the impression that they are constantly jumping in and out of superposition.
 
  • #23
The thing that I still conceptualy grasp rightly is the 'evolution' of decoherence in time. So let's consider the example of my car and a period of half an hour of its existence. It is valid to say that it is undergoing process of decoherence, but my car existed before this interval, it wasn't brought to existence and than decohered for let's say 5 miliseconds and started behaving classically. During this period of an half an hour, what happens with the car? Is it decohered for the full interval (no presence of interference effects at any moment) due to constant interaction? The concept of 'decoherence time' is basically my biggest obstacle since it implies that an object jumps from a superposition to a definite state then again to a superposition and then again to a definite state, but that superpositions last shortly. So what is achieved by constant monitoring of an system like car, in everyday world, that wasn't in a superposition before focusing on the specific interval (unlike the Schordinger cat setup).
 
  • #24
durant35 said:
The concept of 'decoherence time' is basically my biggest obstacle since it implies that an object jumps from a superposition to a definite state then again to a superposition and then again to a definite state, but that superpositions last shortly.
Part of the problem here is that we're using this term "definite state" which doesn't have a clear definition. It sounds as if you're thinking that a "definite state" is one that is not a superposition, and that our definite measurement results indicate that we must have such a state after the measurement. However, "not a superposition" makes no sense because all states are superpositions (or mixtures of superpositions) in some basis - the question is whether the observables corresponding to this basis are macroscopically interesting. @stevendaryl posted a good example above: A chemically bonded molecule is just about always described by a superposition of states of the valence electrons, but a position measurement of the molecule doesn't care.
 
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  • #25
Nugatory said:
Part of the problem here is that we're using this term "definite state" which doesn't have a clear definition. It sounds as if you're thinking that a "definite state" is one that is not a superposition, and that our definite measurement results indicate that we must have such a state after the measurement. However, "not a superposition" makes no sense because all states are superpositions (or mixtures of superpositions) in some basis - the question is whether the observables corresponding to this basis are macroscopically interesting. @stevendaryl posted a good example above: A chemically bonded molecule is just about always described by a superposition of states of the valence electrons, but a position measurement of the molecule doesn't care.

You're right, the terminology is the problem, I'm not an expert and by my terminology I use common-sense phases.
By a definite state I mean a macroscopic property, like being dead or alive. In the Schrödinger's cat experiment the cat settles into a definite state before observation (for instance it survives and it's alive). We observe definite properties on everyday basis, my computer is white, rectangular and so on.. That's what I mean by definite state. Now you may understand it better.
 
  • #26
Nugatory said:
Part of the problem here is that we're using this term "definite state" which doesn't have a clear definition. It sounds as if you're thinking that a "definite state" is one that is not a superposition, and that our definite measurement results indicate that we must have such a state after the measurement. However, "not a superposition" makes no sense because all states are superpositions (or mixtures of superpositions) in some basis - the question is whether the observables corresponding to this basis are macroscopically interesting. @stevendaryl posted a good example above: A chemically bonded molecule is just about always described by a superposition of states of the valence electrons, but a position measurement of the molecule doesn't care.

Can you explain what do you mean when you say that all states are superpositions in some basis, with an example?
 
  • #27
durant35 said:
Can you explain what do you mean when you say that all states are superpositions in some basis, with an example?
E.g. the photon polarization |H> being a superposition of |45> - |135>, polarization |V> being |45> + |135>
 
  • #28
durant35 said:
and a unitary transformation can't turn a pure state into a mixed state.
That is correct.
 
  • #29
StevieTNZ said:
E.g. the photon polarization |H> being a superposition of |45> - |135>, polarization |V> being |45> + |135>

Okay, I get that. I was referring to the fact that let's say my computer is *rectangular*, that is not a superposition, we don't observe a superposition. That's what I mean when I say definite state.
 

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