Does Schrodinger's cat know whether it's dead?

And I wouldn't say (without great care) that decoherence "provides evidence that there aren't physical structures". It depends on how you define a physical structure.


I define 'physical structure' as in the physics sense -- the specific arrangment of constituent parts, that has observable attrubutes like Size, Mass, Position, Momentum and forces holding the structure together. Example:


http://img375.imageshack.us/img375/97/colorbuckyball.jpg [Broken]

Uploaded with ImageShack.us




Example of superposition of states(unobservable by definition):



















A physical structure that does not retain its physical defining characteristics at all times cannot unambiguously be regarded as a physical structure. The only definition that is not misleading is to call it 'event', isn't it?



You said I've missed the point. Have I? I clearly stated that superpositions are a huge conceptual issue. My only assertion that you quoted was that the emergence of the macro world as we perceive it is not a problem - one can easily see how the feeling of hardness, fullness, definiteness arises.

Okay, but it's not very obvious, as the exact underlying mechanism is still not there, but it's easier on the brain, than say a wave that sends signals ftl and directs particles(and fewer assumptions seem to be made).
 
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ZapperZ

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The problem with decoherence is that it doesn't say that matter(objects) are there all the time with properties in space. On the contrary, though it doesn't explain why we observe exatly what we do, it says that physical structures are not always there. If this doesn't blow one's mind, one must have rocks in his head.
Can you make an exact citation where decoherence claims such a thing, i.e. it doesn't say that matter are "there all the time with properties in space"?

You miss the point. Coherent and decoherent wave structures mean essentially - object/not-object, physical structure/non-physical structure, probable-actual, there--not-there, cat--non-cat, c60--no c60, matter--non matter.
Er... what exactly do you mean by "coherent" and "decoherent" here? This is not how it is defined in physics.

Zz.
 
I define 'physical structure' as in the physics sense -- the specific arrangment of constituent parts, that has observable attrubutes like Size, Mass, Position, Momentum and forces holding the structure together.
Ok yes by that definition you are 100% correct.

GeorgCantor said:
A physical structure that does not retain its physical defining characteristics at all times cannot unambiguously be regarded as a physical structure. The only definition that is not misleading is to call it 'event', isn't it?
I suppose, if you wish to be technically correct and you adhere to the definition of a physical structure that you adhere to.

GeorgCantor said:
Okay, but it's not very obvious, as the exact underlying mechanism is still not there, but it's easier on the brain, than say a wave that sends signals ftl and directs particles(and fewer assumptions seem to be made).
My only claim was that the feeling of objects as definite, hard and "solid" things is not difficult to envisage as arising from decoherence. If you push your hand onto the table, nothing in decoherence predicts anything other than an electric repulsion that your brain perceives as hardness. The macro world, as we perceive it, is supposed to be perceived that way. It's what one expects from decoherence.

I share your views on superpositions, coupling, etc.
 
Can you make an exact citation where decoherence claims such a thing, i.e. it doesn't say that matter are "there all the time with properties in space"?

"Quantum superposition refers to the quantum mechanical property which states that all particles exist in not one state but all possible states at once. Due to this property, to completely describe a particle one must include a description of every possible state and the probability of the particle being in that state."


http://en.wikipedia.org/wiki/Quantum_superposition





Matter(c60 molecules) causing interference pattern due to superposition of states:



"Recent studies have revealed that interference is not restricted solely to elementary particles such as protons, neutrons, and electrons. Specifically, it has been shown that large molecular structures like fullerene (C60) also produce interference patterns."


http://en.wikipedia.org/wiki/Double-slit_experiment





"Collisional decoherence observed in matter wave interferometry"


http://www.ncbi.nlm.nih.gov/pubmed/12731960




How is an object in superposition an object? That's what i said in the the text you quoted. Do you consider probability clouds to be objects?[/quote]


Er... what exactly do you mean by "coherent" and "decoherent" here? This is not how it is defined in physics.

Zz.


Gradual loss of quantum interference.
 
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"Quantum superposition refers to the quantum mechanical property which states that all particles exist in not one state but all possible states at once. Due to this property, to completely describe a particle one must include a description of every possible state and the probability of the particle being in that state."


http://en.wikipedia.org/wiki/Quantum_superposition





Matter(c60 molecules) causing interference pattern due to superposition of states:



"Recent studies have revealed that interference is not restricted solely to elementary particles such as protons, neutrons, and electrons. Specifically, it has been shown that large molecular structures like fullerene (C60) also produce interference patterns."


http://en.wikipedia.org/wiki/Double-slit_experiment





"Collisional decoherence observed in matter wave interferometry"


http://www.ncbi.nlm.nih.gov/pubmed/12731960
You've just given me a lesson on what I already know (I am a physicist). I need the exact source where decoherence says what you claimed it says. Decoherence is NOT superposition, even though superposition plays a role. All you've given me are stuff about superposition, which is not what I asked for.


Gradual loss of quantum interference.
And whose definition is this? Yours? It certainly isn't from physics/mathematics. If this is what you are using, but basing you continue to link it to physics papers and phenomena, then you are mixing things and changing the definition on the fly. This is not kosher.

Zz.
 
You've just given me a lesson on what I already know (I am a physicist). I need the exact source where decoherence says what you claimed it says. Decoherence is NOT superposition, even though superposition plays a role. All you've given me are stuff about superposition, which is not what I asked for.


In the strict sense, decoherence is about transitioning from pure, coherent superpositional states to mixed states due to interaction with the environement. The issue i raised was more based towards the nature of superpositions(and what they say about matter), than the mechanism underlying decoherence.

I asked a question that you didn't see?/address in my previous post:

How is an object in superposition an object? That's what i said in the the text you quoted. Do you consider probability clouds to be objects?
 

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In the strict sense, decoherence is about transitioning from pure, coherent superpositional states to mixed states due to interaction with the environement. The issue i raised was more based towards the nature of superpositions(and what they say about matter), than the mechanism underlying decoherence.
So how does this ties in with YOUR interpretation that I questioned? You still haven't given me a proper citation to any sources that support such a statement. Do you need a reminder of what I was asking for?

I asked a question that you didn't see?/address in my previous post:
I didn't address it because I didn't make such a point. I didn't realize that I had to defend something that I never said. You are confusing the fact that it was you who made definitive statements about certain things. That is why I wanted to know what possible sources you could have used to arrive at such a conclusion, which, based on my understanding, is incorrect.

Zz.
 
So how does this ties in with YOUR interpretation that I questioned? You still haven't given me a proper citation to any sources that support such a statement. Do you need a reminder of what I was asking for?

If you read back into the thread, you'll see that the decoherence issue wasn't raised by me. My point, which is still valid, is that decoherence doesn't restore the realism Einstein was looking for - objects in space and time with definite properties. Einstein believed an underlying theory could eventually be uncovered, which after Bell-Aspect seems quite an untenable position.

What i said in response to the assertion that decoherence restores quantum theory to a theory that makes sense is(words by word):


GeorgCantor said:
"Any time you use a concept that involves or implies the term "superposition", you aren't talking of objects with properties in time and space(which happen to be the picture of reality that you appear to have made sense of).

A system that goes into a superposition, or that can be put in superposition, or that might EVER potentially be in superpositional states, is NOT something to be made sense of. The human mind DOESN'T and CANNOT comprehend de-localized, physical objects with indefinite properties. Anything that can be in superposition is not an object, it's an event, and there isn't anything in this 'universe' that cannot potentially be in superpositional state."

What exactly are you asking me to do? And what do you object to in the above passage?

I was talking of superpositions and what they mean for the nature and structure of matter. If you have something to contribute to what it means for matter to be in all possible states at once, by all means do so.


What i said was a logical extension of what Schrodinger's cat experiment was supposed to prove, that the cat doesn't exist in a definite state UNTIL after the lid is open. How is this different from what i said about superpositions prior to decoherence, i.e.:

Cat -- Not-cat

Fullerene-- Not fullerene (experimentally verified though still not fully understood, or rather simply ignored)

Object -- Not object


If you wish to say that superpositions have a reality of their own, i'd very much like to see the evidence for that. Until then, matter existing in all possible states at once belongs only to the configuration space, i.e. it isn't real.
 
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What i said in response to the assertion that decoherence restores quantum theory to a theory that makes sense is
I didn't assert this, but anyways, carry on. :)
 
I didn't assert this, but anyways, carry on. :)
No, that wasn't you. That was BruceG:

I was totally baffled by the cat problem for many years until I read about decoherence theory, then quantum theory all made sense to me.

Now if someone is willing to entertain the notion that decoherence explains what it means for a cat to have decohered and treat it as a common-sense object in space and time, i'd like to see it done in this thread. I'd like to see explantion on how single outomes are actualized, how mass/energy is conserved(it isn't in superpositions), how a decohered cat is somehow an object existing in space and time. The underlying mechanism behind decoherence isn't there yet.

The whole notion of objects, cats and so on, existing in space and time with definite properties in completely untenable and Zz knows this quite well. He's picking on definitions and semantics, which i am obviously not good at(English is not my native language).
 
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If you read back into the thread, you'll see that the decoherence issue wasn't raised by me. My point, which is still valid, is that decoherence doesn't restore the realism Einstein was looking for - objects in space and time with definite properties. Einstein believed an underlying theory could eventually be uncovered, which after Bell-Aspect seems quite an untenable position.

What i said in response to the assertion that decoherence restores quantum theory to a theory that makes sense is(words by word):







What exactly are you asking me to do? And what do you object to in the above passage?

I was talking of superpositions and what they mean for the nature and structure of matter. If you have something to contribute to what it means for matter to be in all possible states at once, by all means do so.


What i said was a logical extension of what Schrodinger's cat experiment was supposed to prove, that the cat doesn't exist in a definite state UNTIL after the lid is open. How is this different from what i said about superpositions prior to decoherence, i.e.:

Cat -- Not-cat

Fullerene-- Not fullerene (experimentally verified though still not fully understood, or rather simply ignored)

Object -- Not object


If you wish to say that superpositions have a reality of their own, i'd very much like to see the evidence for that. Until then, matter existing in all possible states at once belongs only to the configuration space, i.e. it isn't real.
There is a serious misunderstanding here on what, in physics, is meant by "realism". There is also a misunderstanding on what a superposition actually says, and I think you've taken it a step too far beyond that.

Realism, as described by Tony Leggett, is nothing more than a description that the object has a definite property at all times. When you toss a coin and let it land, BUT, before you look at it, the coin has a definite property of being either UP or DOWN, but not both. It is just that you just don't know what it is, so you say it has a probability of being one or the other. Realism exists here.

This is not the same for a QM particle. The superposition principle describe a particle having BOTH orthogonal states simultaneously. In the SQUID experiments of Delft/Stony Brook, the superposition of the supercurrent in both directions gave rise to the coherence gap. The magnitude of the coherence gap directly tells us that this is a consequence of the supercurrent having both opposite direction simultaneously, and not simply a fraction going one way, while the rest going the opposite way. This observation is an indication that realism doesn't quite apply in this situation.

Note that these are NOT my definitions, nor something that I made up on my own to suit my needs. Throughout the years, I've given many references to the usage of these concepts (some are even listed in the Noteworthy papers thread in the General Physics forum).

For some odd reason, you have taken this to way beyond what it says. The cat DOES exist. The thought experiment is pointing out that the orthogonal states of {dead, alive} are in a superposition, i.e. it produces an absurd situation where the cat is BOTH dead AND alive simultaneously, as described by the wave function. How this superposition somehow implies that "... you aren't talking of objects with properties in time and space... " is beyond me, especially considering that the wavefunction contains both time and spatial description. Just because a system consisting more than one particle exists in "configuration space" doesn't mean that it doesn't exist in space and time! After all, the Hamiltonian is described using space and time! So that claim is very puzzling.

A system that goes into a superposition, or that can be put in superposition, or that might EVER potentially be in superpositional states, is NOT something to be made sense of. The human mind DOESN'T and CANNOT comprehend de-localized, physical objects with indefinite properties.
Common sense is nothing more than an accumulation of knowledge. I don't know which human you are claiming that cannot comprehend superposition, but I have no trouble with it, and people that I encounter professionally have no issues with it either. Take note that we have already accepted such a concept even BEFORE QM came along. Superposition of waves resulting in constructive and destructive interference was EASILY comprehended. No one claimed that those didn't make any "sense". Furthermore, we can certainly comprehend delocalized objects. A supercurrent is totally delocalized, which gives it the long-range coherence.

You need to separate what you are not capable of accepting/understanding, with the rest of the human population, especially with what physicists already know and understand.

Zz.
 
There is a serious misunderstanding here on what, in physics, is meant by "realism". There is also a misunderstanding on what a superposition actually says, and I think you've taken it a step too far beyond that.

Realism, as described by Tony Leggett, is nothing more than a description that the object has a definite property at all times. When you toss a coin and let it land, BUT, before you look at it, the coin has a definite property of being either UP or DOWN, but not both. It is just that you just don't know what it is, so you say it has a probability of being one or the other. Realism exists here.

This is not the same for a QM particle. The superposition principle describe a particle having BOTH orthogonal states simultaneously. In the SQUID experiments of Delft/Stony Brook, the superposition of the supercurrent in both directions gave rise to the coherence gap. The magnitude of the coherence gap directly tells us that this is a consequence of the supercurrent having both opposite direction simultaneously, and not simply a fraction going one way, while the rest going the opposite way. This observation is an indication that realism doesn't quite apply in this situation.

Note that these are NOT my definitions, nor something that I made up on my own to suit my needs. Throughout the years, I've given many references to the usage of these concepts (some are even listed in the Noteworthy papers thread in the General Physics forum).


Agreed, however...




Zz said:
For some odd reason, you have taken this to way beyond what it says.

Whether the cat exists while in a state of all possible superpositions is interpretation-dependent and after Bell, the options are quite limited. As you are well aware, you either toss relativity(or claim it's incomplete) in favor of non-local influences(this essentially would be just an assumption, not proof or evidence) or you learn to accept that systems in superpositions do not have definite properties and by extension - their own existence - until a measurement/decoherence takes place. This seems a very logical statement to make.



The cat DOES exist. The thought experiment is pointing out that the orthogonal states of {dead, alive} are in a superposition, i.e. it produces an absurd situation where the cat is BOTH dead AND alive simultaneously, as described by the wave function. How this superposition somehow implies that "... you aren't talking of objects with properties in time and space... " is beyond me, especially considering that the wavefunction contains both time and spatial description.

But the wave function doesn't live in classical 3D space but in Hilbert space. Cats in Hilbert space are not cats, agree? What would Bell say about cats in Hilbert space after his introduced his theorem? Is there an underlying reality where the cat exists prior to measurement/decoherence? And if there is, what kind of reality would that have to be?

Are you believing in the many world hypothesis? If you do, you could be justfied in saying that superpositions of physical matter make sense and are understood, and by extension physical objects exist in space and time, but it involves making the assumption that physical space is Hilbert space and the dramatic, sci-fi assumption that there are 8 billion trillion worlds. If this is the case, it's equally valid to say that making a number of 'appropriate' assumptions, one can prove anything, even that which is impossible.



Zz said:
Just because a system consisting more than one particle exists in "configuration space" doesn't mean that it doesn't exist in space and time! After all, the Hamiltonian is described using space and time! So that claim is very puzzling.


"Real", macroscopic, Newtonian objects do not seem to display quantum mechanical features such as superposition. There is a clear-cut dissonance between quantum mechanics and Newtonian physics, where only one configuration occurs(and by logical extension can be said to 'exist'). I have no idea what it means to assert that a bacteria, or a molecule or a human can exist in all possible states described by probability amplitudes at once in time and space. Can you elaborate? Seems like you are introducing a new meaning to the word "exist" that will hardly be found in regular dictionaries(unless they were written by particle physicists for particle physicists, who all agreed on a particular interpretation).



Common sense is nothing more than an accumulation of knowledge. I don't know which human you are claiming that cannot comprehend superposition, but I have no trouble with it, and people that I encounter professionally have no issues with it either.

Schoroedinger surely rings a bell(among others), since he was the author of what has turned to be called the Schroedinger's Cat paradox. And you could say, he was also a professional(and a Nobel prize winner).



Take note that we have already accepted such a concept even BEFORE QM came along. Superposition of waves resulting in constructive and destructive interference was EASILY comprehended. No one claimed that those didn't make any "sense". Furthermore, we can certainly comprehend delocalized objects. A supercurrent is totally delocalized, which gives it the long-range coherence.

You need to separate what you are not capable of accepting/understanding, with the rest of the human population, especially with what physicists already know and understand.

Zz.

My point was about the physical nature of matter and while constructive and destructive inteference of EM waves might be easier to imagine, it isn't so with matter waves. Unless you let go of realism, which more and more physicists after Bell seem to have no problem with.

If i were to sum up my point in one single sentence, it would be - "superpositions of quantum states pose a great challenge to our common-sense picture of physical matter(and by extension of all reality)". If you are denying this, you are denying the existence of the measurement problem and at least half of quantum theory(i.e. the "transition" from fields to single outcomes, aka "matter").


Here is a good question for you. The van der Waals radius of a C60 molecule is about 1 nanometer, the distance between the slits is 50nm( roughly 25 times the size of c60). It's very puzzling what would make someone claim that the unobserved 'entity' that went through both slits was a c60 molecule(that exists at all times)? Remember the c6o molecule is supposed to be a physical object(matter).

http://www.users.csbsju.edu/~frioux/two-slit/c60-slit.htm


The wave function of my body is gradually soaked in around me into space. Yet i claim that that wavefunction is not me, as there is no way to keep my physical processes functioning, and consequently i can't be alive and exist in all possible states at once...

How is this "taking it too far"?
 
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Whether the cat exists while in a state of all possible superpositions is interpretation-dependent and after Bell, the options are quite limited. As you are well aware, you either toss relativity(or claim it's incomplete) in favor of non-local influences(this essentially would be just an assumption, not proof or evidence) or you learn to accept that systems in superpositions do not have definite properties and by extension - their own existence - until a measurement/decoherence takes place. This seems a very logical statement to make.
This is a misrepresentation of quantum entanglement. If you look at all the experiments that reported on the Bell-type experiment, NONE OF THEM claim of violation of SR. Why do you think that is?

In a quantum entanglement measurement, no signal of any kind travels from one of the entangled entity to the other at a speed faster than c. This is a very crucial point. In fact, you can't use quantum entanglement to send communications faster than c!

Now, you can measure the responses made by, say, a bipartite pair, and show that they react at a rate faster than c. But in SR, it is the speed of a signal, or information, that is the limiting factor, and that cannot exceed c in the present-day formulation.

So no, no one other than you is tossing out SR based on all the Bell-type experiments. So here is an example of you extending something beyond what it says.

But the wave function doesn't live in classical 3D space but in Hilbert space.
The basis functions are in Hilbert space, but the basis themselves have spatial and temporal dependence. So how does this allows you to draw a conclusion that such a thing doesn't exist in space and time? This already doesn't make any sense since you have not only a spatial operator that measure the position of an object, but one can also transform from real space into momentum space and in reverse. When I construct, say, the Bloch wavefunction, what do you think defines the periodicity of the potential?

Are you believing in the many world hypothesis?
Er... don't jump to conclusions here. Again, you are bringing something and putting words into my mouth. I'm asking you to justify what you said. I have no desire to justify your imagination of what you THINK I said.

"Real", macroscopic, Newtonian objects do not seem to display quantum mechanical features such as superposition. There is a clear-cut dissonance between quantum mechanics and Newtonian physics, where only one configuration occurs(and by logical extension can be said to 'exist'). I have no idea what it means to assert that a bacteria, or a molecule or a human can exist in all possible states described by probability amplitudes at once in time and space. Can you elaborate? Seems like you are introducing a new meaning to the word "exist" that will hardly be found in regular dictionaries(unless they were written by particle physicists for particle physicists, who all agreed on a particular interpretation).
1. Tony Leggett is NOT a particle physicist. In fact, he is in the same field as I am, condensed matter physics. This is the physics of materials, i.e. the stuff that you are using NOW. So none of the stuff we are talking about is "esoteric", and in fact, many of the most amazing indication of quantum phenomena came out of condensed matter experiment. Don't believe me? Look at Carver Mead's article on "Collective Electrodynamics", where he flat-out claim that the most convincing and clearest evidence of quantum mechanics at the macroscopic scale is superconductivity, not some "particle physics" experiments!

2. If you have looked at any of my writings on the quantum-classical boundary, you would have noticed that I claim that they are different from each other, and that the separation between the two at the mesoscopic scale is still unknown, i.e. we don't know if they are truly separated by something similar to a phase transition (i.e. many state variables change discontinuously through the transition), or that this is a smooth crossover. Decoherence is one way out, but it is NOT the only way out. Again, one of the papers I've highlighted in the Noteworthy thread is a paper that shows that our coarse-grained measurement of a quantum system can recover the classical state that we are familiar with. In other words, when we look at a cow from very far (i.e. our observation isn't very detailed), then we get back the sphere!

3. I used the term "real" to mean something to be physically meaningful/significant. See G. Giuliani, Eur. J. Phys. 31 871 (2010).

My point was about the physical nature of matter and while constructive and destructive inteference of EM waves might be easier to imagine, it isn't so with matter waves. Unless you let go of realism, which more and more physicists after Bell seem to have no problem with.
This automatically falsified your claim that the human brain can't fathom or understand with such a concept, unless, of course, you don't consider "more and more physicists" as belonging to the human specie.

Have you ever considered that familiarity breeds acceptance? We are more familiar with classical wave superposition. We are not familiar with quantum superposition. Is your difficulty in accepting the latter because you are not familiar with it? How often do you encounter it? How often do you do experiments in which these QM properties jump up at you? So the problem here may not be with the QM formalism, it could be YOU!

Here is a good question for you. The van der Waals radius of a C60 molecule is about 1 nanometer, the distance between the slits is 50nm( roughly 25 times the size of c60). It's very puzzling what would make someone claim that the unobserved 'entity' that went through both slits was a c60 molecule(that exists at all times)? Remember the c6o molecule is supposed to be a physical object(matter).

http://www.users.csbsju.edu/~frioux/two-slit/c60-slit.htm


The wave function of my body is gradually soaked in around me into space. Yet i claim that that wavefunction is not me, as there is no way to keep my physical processes functioning, and consequently i can't be alive and exist in all possible states at once...

How is this "taking it too far"?
Here's a question for you. Look at the experiment, and see what they have to prepare the C60 molecule to make it undergo such an experiment. Hint: if they do this at room temperature, they won't see the interference effect.

This is where you your misunderstanding of what "coherence" mean comes into play, because you do not realize that every part of the C60 molecules have to be in coherence with each other. It is why a soccer ball cannot undergo such a process. Think of how difficult it is to get each part of a macroscopic object to be in coherence with each other.

In the Delft/Stony Brook book experiment, they managed to make 10^11 particles to undergo the Schrodinger Cat state experiment. 10^11!!! Think about it. What this means is that 10^11 particles have to be in a complete coherent state! If they are not, the effect is gone!

What you are taking too far is the meaning of superposition at the quantum scale. I am not a fan of applying rules at a scale where they haven't been shown to work. Deepak Chopra is notorious for doing such a thing, and I'm hoping that you don't do the same thing. This isn't about quantum rules working at the classical scale. This is about misrepresenting quantum rules at the quantum scale!

Zz.
 
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This is a misrepresentation of quantum entanglement. If you look at all the experiments that reported on the Bell-type experiment, NONE OF THEM claim of violation of SR. Why do you think that is?

Yes, but Bell did make 2 common-sense assumptions - that of locality and realism that he proved one of them or both were inconsistent with qm. If you look back at my posts, you'll see that i always expressed great reservations about nonlocal influencing in pilot-wave theories. In this thread it was me who brought up the notion that the most elaborate piece of equipment the LHC works because SR is accurate, hence the universe is not newtonian. The issue that came up here centered around the assumption of realism, and whether objects with properties exist in space and time(to which i objected multiple times).




In a quantum entanglement measurement, no signal of any kind travels from one of the entangled entity to the other at a speed faster than c. This is a very crucial point. In fact, you can't use quantum entanglement to send communications faster than c!

Now, you can measure the responses made by, say, a bipartite pair, and show that they react at a rate faster than c. But in SR, it is the speed of a signal, or information, that is the limiting factor, and that cannot exceed c in the present-day formulation.


No doubt about it. They wouldn't spend 10 billion euro public money on a wrong theory.




So no, no one other than you is tossing out SR based on all the Bell-type experiments. So here is an example of you extending something beyond what it says.


Slow down, I said the opposite. I don't like making assumptions, like that of nonlocality. Nonlocal influences don't exist as far as i am concerned.




Zz said:
The basis functions are in Hilbert space, but the basis themselves have spatial and temporal dependence. So how does this allows you to draw a conclusion that such a thing doesn't exist in space and time?

Because as somebody already asked me in this thread, the standard definition of "physical structure" that i use doesn't fit the observed effect of superpositions of states.




This already doesn't make any sense since you have not only a spatial operator that measure the position of an object, but one can also transform from real space into momentum space and in reverse. When I construct, say, the Bloch wavefunction, what do you think defines the periodicity of the potential?

I don't question that there is clear correspondence between the formalism and the outcomes of measurements. I deny that probability amplitudes are objects(physical structures).





Zz said:
1. Tony Leggett is NOT a particle physicist. In fact, he is in the same field as I am, condensed matter physics. This is the physics of materials, i.e. the stuff that you are using NOW. So none of the stuff we are talking about is "esoteric", and in fact, many of the most amazing indication of quantum phenomena came out of condensed matter experiment. Don't believe me? Look at Carver Mead's article on "Collective Electrodynamics", where he flat-out claim that the most convincing and clearest evidence of quantum mechanics at the macroscopic scale is superconductivity, not some "particle physics" experiments!

2. If you have looked at any of my writings on the quantum-classical boundary, you would have noticed that I claim that they are different from each other, and that the separation between the two at the mesoscopic scale is still unknown, i.e. we don't know if they are truly separated by something similar to a phase transition (i.e. many state variables change discontinuously through the transition), or that this is a smooth crossover. Decoherence is one way out, but it is NOT the only way out. Again, one of the papers I've highlighted in the Noteworthy thread is a paper that shows that our coarse-grained measurement of a quantum system can recover the classical state that we are familiar with. In other words, when we look at a cow from very far (i.e. our observation isn't very detailed), then we get back the sphere!

3. I used the term "real" to mean something to be physically meaningful/significant. See G. Giuliani, Eur. J. Phys. 31 871 (2010).


Good. A recent poll indicated that the majority of the interviewed chose "none of the available interpretations". Seems like the most reasonable position to me.







GeorCantor said:
My point was about the physical nature of matter and while constructive and destructive inteference of EM waves might be easier to imagine, it isn't so with matter waves. Unless you let go of realism, which more and more physicists after Bell seem to have no problem with.

Zz said:
This automatically falsified your claim that the human brain can't fathom or understand with such a concept, unless, of course, you don't consider "more and more physicists" as belonging to the human specie.


This is the central point - that of the contextuality of GR and QM and the rejection of realism. As soon as we denounce realism, we have to accept the unacceptable, that objects are not objects with fixed properties existing in space and time, but that which we measure/collapse in our inertial frame, that which decoheres from all possible superpositional states, etc.

When you do a measurement, you get particles, atoms, molecules...

It takes a leap of faith to say there exist objects with fixed properties prior to a measurement and this is contradicted by SR as well. Your acceptance that realism might be wrong, suggests that we need a new definition of physical objects, that will depart the Newtonian picture. That picture might be already there among your collegues, but it's not there, out on the street. That the world may not be out there at all times with definite properties is not even remotely considered by the general population.






Zz said:
What you are taking too far is the meaning of superposition at the quantum scale. I am not a fan of applying rules at a scale where they haven't been shown to work. Deepak Chopra is notorious for doing such a thing, and I'm hoping that you don't do the same thing. This isn't about quantum rules working at the classical scale. This is about misrepresenting quantum rules at the quantum scale!

Zz.

I am not sure myself if macro objects can be shown to display interference effects but quantum field theory works just fine on the macro scale AFAIK(and it's not a theory that confirmes the Realism assumption of objects with properties existing at all times). It's only a conceptual difficulty in relating fields to uhhmm... 'observations' seems like the least misleading term.
 
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Now if someone is willing to entertain the notion that decoherence explains what it means for a cat to have decohered and treat it as a common-sense object in space and time, i'd like to see it done in this thread. I'd like to see explantion on how single outomes are actualized.
As I started this sub-thread off, I'd like another go at this. I've found this succinct account by Terry Gannon [Moonshine beyond the monster p.245]

"[Decoherence] alone doesn't resolve the measurement problem. At best decoherence can only explain why macroscopically distinct states in superposition don't 'see' each other. A (perhaps overly zealous) application of quantum mechanics insists that macroscopic superpositions must occur; from this, the 'Many-Worlds' interpretation is inevitable. The explanation for the mysterious wave-function collapse then would be that measurement entangles the microscopic quantum system with a macroscopic system.. Each coupled state ('world') in this superposition would decohere from the others, and so the various quantum states could no longer 'see' each other. It would be as if at the moment of measurement, the universe split into parallel universes, one for each possible experimental outcome."

I re-assert that this makes sense to me and allows me to continue being a realist. So its very tempting to take this interpretation.
 

apeiron

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I re-assert that this makes sense to me and allows me to continue being a realist. So its very tempting to take this interpretation.
Decoherence has the right feel for an interpretation because it does not seek to erradicate quantum weirdness but instead tames it, pushes it to the margin of the real.

There were three early reactions to the shocks of QM - denial, exaggeration and agnosticism. So either a search for a way out that preserved locality, crispness, and other properties deemed essential to Newtonian/Einsteinian mechanics, or - Georg's position - a call for complete ontological revolution where perhaps even human consciousness was needed to observe reality into existence, or a pragmatic "we can't understand, so shut up and calculate".

None of these three responses seem commonsensical. You have to respond to the experimental verification of QM, but in a way that preserves the well-supported formalisms of Newton, Einstein (and Boltzmann - never forget thermodynamics).

We already treat relativity as a "weirdness" that occurs at the global limit of Newtonian mechanics. It exists, but only at the margins of our "realistic" experience of the world - ie: our mesoscopic viewpoint standing bang in the middle of things.

In the same way, QM is the weirdness at we find down at the local scale, when things are very small or hot. Decoherence is a way of both accepting that QM is "everywhere" in the classically real world (just as relativity is), and yet also only nakedly apparent in all its glorious weirdness at the very margins.

Decoherence itself is not yet a complete answer as, in my understanding, the formalism smears the weirdness out rather successfully so that it is marginalised, but it does not yet do the same job for observation - the collapse part of the story.

On the other hand, achieving actual complete collapse would be an erradication of the weirdness, and so actually this should not be the aim anyway? If the QM weirdness is being pushed to the very edge of the field of view, then so is the final collapse issue. It too ceases to become a source of ontological concern. Everything can be vague at the margins, so long as it looks crisp and solid across the many orders of flat spatiotemporal scale that make up our "reality".
 
As I started this sub-thread off, I'd like another go at this. I've found this succinct account by Terry Gannon [Moonshine beyond the monster p.245]

"[Decoherence] alone doesn't resolve the measurement problem. At best decoherence can only explain why macroscopically distinct states in superposition don't 'see' each other. A (perhaps overly zealous) application of quantum mechanics insists that macroscopic superpositions must occur; from this, the 'Many-Worlds' interpretation is inevitable. The explanation for the mysterious wave-function collapse then would be that measurement entangles the microscopic quantum system with a macroscopic system.. Each coupled state ('world') in this superposition would decohere from the others, and so the various quantum states could no longer 'see' each other. It would be as if at the moment of measurement, the universe split into parallel universes, one for each possible experimental outcome."

I re-assert that this makes sense to me and allows me to continue being a realist. So its very tempting to take this interpretation.

Actually, you have to be a really weak realist, as under the MWI there are no objects with absolute properties in spacetime, PRIOR to a universe split. The best one can say is that there exists a wave "structure" of all possible events that become to look like "objects" only at the split. Three seconds into the future from now, there is not be a single object in existence(just a coherent wave structure). The notion that the universe splits instantaneously/nonlocally is another challenge to the idea of realism, if you really get down to it.

Zeilinger's position that what exists "out there" is information seems to be the only somewhat common-sensical explanation of reality. But take a second look at it, and it stops being common-sensical too.
 
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Actually, you have to be a really weak realist.
Yes, I'm happy being a weak realist in the general philosophical sense of not being an idealist: I can think about the evolution of the universe in a deterministic objective way existing independent of human observation of it. Decoherence + many-worlds at least does this, however crudely.
 

ZapperZ

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Yes, but Bell did make 2 common-sense assumptions - that of locality and realism that he proved one of them or both were inconsistent with qm. If you look back at my posts, you'll see that i always expressed great reservations about nonlocal influencing in pilot-wave theories. In this thread it was me who brought up the notion that the most elaborate piece of equipment the LHC works because SR is accurate, hence the universe is not newtonian. The issue that came up here centered around the assumption of realism, and whether objects with properties exist in space and time(to which i objected multiple times).
I work at a particle accelerator. I don't need to go to the LHC to show that SR is accurate.

Secondly, are you sure you know what is meant by "realism" as applied within this context? For example, is your definition of realism consistent with not only Bell, but also with the papers dealing with such issues? Read:

A.J. Leggett, Rep. Prog. Phys. v.71, p.022001 (2008)
S. Groeblacher et al., Nature v.446, p.871 (2007).

The problem here is the way you are "interpreting" what a "superposition of states" mean. I still don't think you know what it is, so I'm going to ask a very direct question via an example.

I have a system consisting of two orthogonal states |1> and |2>. The wavefunction describing the system is

[tex] \psi = a_1|1> + a_2|2>[/tex]

where the a's are the normalized amplitude for each state.

Please tell me what this is saying to you.

Zz.
 
Secondly, are you sure you know what is meant by "realism" as applied within this context? For example, is your definition of realism consistent with not only Bell, but also with the papers dealing with such issues? Read:

A.J. Leggett, Rep. Prog. Phys. v.71, p.022001 (2008)
S. Groeblacher et al., Nature v.446, p.871 (2007).

I haven't finished reading yet but i wonder what you will say ontologically(how it pertains to the real world) about the cat from this paragraph:


"This state clearly exhibits several quantum phase-space interference fringes between the 'dead' and 'alive' components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory."


Does the cat exist in both states at once and how does the notion of coarse-graininess solve the dead/alive cat paradox? :bugeye:



EDIT: I could only find abstracts from those papers, but found this article referenced by you as a summation of the points(supposedly) raised in the paper:

http://physicsworld.com/cws/article/news/27640


which seems to be consistent with what i stated in the beginning in this thread about realism.

"Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism -- giving the uneasy consequence that reality does not exist when we are not observing it (Nature 446 871)."




Zz said:
The problem here is the way you are "interpreting" what a "superposition of states" mean. I still don't think you know what it is, so I'm going to ask a very direct question via an example.

I have a system consisting of two orthogonal states |1> and |2>. The wavefunction describing the system is

[tex] \psi = a_1|1> + a_2|2>[/tex]

where the a's are the normalized amplitude for each state.

Please tell me what this is saying to you.

Zz.


The system's probability to be in state one is a1 and a2 to be in state 2. Their sum describes the superposition of the two states(the simultaneous existence of both states).

It's the standard procedure to consider that that which goes through both slits at the double slit, is a probability wave, since the interference pattern is perfectly accounted for by solving the Schroedinger equation. From here a probability wave cannot be regarded as an object, unless we make a few assumptions, or make up a new definition of what we mean by object.
 
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ZapperZ

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I haven't finished reading yet but i wonder what you will say ontologically(how it pertains to the real world) about the cat from this paragraph:


"This state clearly exhibits several quantum phase-space interference fringes between the 'dead' and 'alive' components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory."


Does the cat exist in both states at once and how does the notion of coarse-graininess solve the dead/alive cat paradox? :bugeye:
We have a failure to communicate here. I've been telling you since the first time I ventured into this thread on why your argument that things just don't exist, especially in space and time, is false simply based on the superposition principle. The Schrodinger Cat experiments are showing that the "orthogonal" states are there simultaneously! Again, look at what I've written, especially when I was discussing the Delft/Stony Brook experiment. Remember, I said that the supercurrent exists in BOTH OPPOSITE DIRECTIONS SIMULTANEOUSLY.

EDIT: I could only find abstracts from those papers, but found this article referenced by you as a summation of the points(supposedly) raised in the paper:

http://physicsworld.com/cws/article/news/27640


which seems to be consistent with what i stated in the beginning in this thread about realism.
No way!

First of all, you are quoting a popular article for the general public that is now a 2nd hand source. Secondly, how could you use such sources and phrase as if they are biblical quotes? If this is a reflection of how you acquire your information (and I'm beginning to suspect that it is), no wonder you have such a view!

The system's probability to be in state one is a1 and a2 to be in state 2. Their sum describes the superposition of the two states(the simultaneous existence of both states).

It's the standard procedure to consider that that which goes through both slits at the double slit, is a probability wave, since the interference pattern is perfectly accounted for by solving the Schroedinger equation. From here a probability wave cannot be regarded as an object, unless we make a few assumptions, or make up a new definition of what we mean by object.
I would not nitpick the fact that the "probability" is the square of the amplitudes, rather than the amplitudes themselves. But you never answered my question. I asked:

"Please tell me what this is saying to you."

Zz.
 
I would not nitpick the fact that the "probability" is the square of the amplitudes, rather than the amplitudes themselves. But you never answered my question. I asked:

"Please tell me what this is saying to you."

Zz.

That the system can be said to exist in both a1 and a2 states simultaneously. Interference is the result of superpositions of two or more waves(say passing through the double slit experiment). When performing a certain measurement on a quantum state, the result is described by a probability distribution, however we don't observe superpositions, objects in superpositions are not real(do not exist) until the state vector reduction that describes the evolution of the system.



Zz said:
We have a failure to communicate here. I've been telling you since the first time I ventured into this thread on why your argument that things just don't exist, especially in space and time, is false simply based on the superposition principle. The Schrodinger Cat experiments are showing that the "orthogonal" states are there simultaneously!

Perhaps there really is miscommunication. What do you mean by 'Cat'? Is this supposed to be a quantum cat existing in all possible states simultaneously? If so, what is your definition of 'exist'? By exist i mean the act of being in space and time with particular properties as in:


1.to have actual being; be: The world exists, whether you like it or not.
2.to have life or animation; live.
3.to continue to be or live: Belief in magic still exists.
4.to have being in a specified place or under certain conditions; be found; occur: Hunger exists in many parts of the world.
5.to achieve the basic needs of existence, as food and shelter: He's not living, he's merely existing.


http://dictionary.reference.com/browse/exist



Mind you, the definition of 'to exist' calls for Actual being, not Potential.



A cat that is dead and alive simultaneously is not a cat. How is this statement supposed to sound weird?



Here is the standard definition for 'cat':

"The cat (Felis catus), also known as the domestic cat or housecat[5] to distinguish it from other felines and felids, is a small domesticated carnivorous mammal that is valued by humans for its companionship and ability to hunt vermin and household pests. Cats have been associated with humans for at least 9,500 years,[6] and are currently the most popular pet in the world."

http://en.wikipedia.org/wiki/Cat



Nowhere will you find a mention of cats in superposition of states. There doesn't exist such a cat, if by cat you mean what wikipedia says.



What makes you say both the Potential and the Actual 'exist'? Is probability an object?

Cats exist in multiple possibilities of realities known as superpositions(until whatever it is you believe in). That doesn't mean they are real or exist in the traditional sense.


And since superposition of possible positions for an electron can be demonstrated by the observed phenomenon quantum tunneling, also in transistors, etc. superpositions are an observed effect. However what they mathematically represent cannot be real, unless you believe in the many world hypothesis(but that has its realism problems too).
 
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Pythagorean

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Perhaps we should derive the superposition principle as a consequence of indistinguishable particles for Georg.
 
Perhaps we should derive the superposition principle as a consequence of indistinguishable particles for Georg.


The MOST striking feature of quantum mechanics is the existence of superposition states, where an object appears to be in different situations at the same time.


Are you disputing this statement?
 

Pythagorean

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No, because it was all subjectively stated with words like striking and appears
 

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