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

[transcend_all]

I've read several descriptions of the Schrodinger's cat thought experiment, but I've never heard anyone discuss whether the cat qualifies as an observer that can collapse the superposition of states. This seems like too simple a question not to have been raised before, but I've never head anyone raise it.

If it is, how is the thought experiment valid? If the cat being aware of its own death or continued life can collapse the superposition, then the cat is either alive or dead and not in a superposition of states.

If it is not, that raises the question of what qualifies as an observer that can collapse the superposition. Do you have to be human to be able to observe a superposition of states such that the superposition will collapse? That seems like an absurdity.

 

[MarcStone]

Ill take a crack at this question...

Schrodinger's cat box...is...a horrible horrible analogy.

I think that half the reason people can't grasp high level physics is that physicists are terrible at creating analogies. I remember back in circa 1988 I read an old book on "relativity"...I was cross-eyed for a week. Anyways...

You can't ascribe the "cat" to having "consciousness" or have "observer" properties. In other words...there is no such thing as the damn cat! What this analogy is attempting to describe is a "unit of space" within the probability wave of an (electron/proton)...for a given "unit of space" here...the (electron/proton) "does" and "does not" exist...it is a "probability"...this (existence/non existence) per "unit space" for the electron dose not have whiskers...does not have a shiny coat..does not have sharp claws...and certainly does not have consciousness...incidentally it cannot be an observer either. Does that help?

 

[Eynstone]

The cat, as I see it, is a quantum object and not an observer in the experiment.
One doesn't have to be a 'living' being to be an observer either. ;)
The cat could always know whether it's dead by appealing to complimentarity!

 

[haael]

The answer for this question depends strongly on interpretation of QM. In fact, this is the very essence of "interpretation" - how microscopic QM laws interact with macroscopic classical world.

 

[Zarqon]

One major issue here is that for any object, macroscopic or otherwise, to be entangled, it has to be cooled close to it's ground state, such that the motion that comes from heat does not decohere the quantum state. In the case of the cat, the very brain functions that we would use to define the consciousness of the cat, is enough to cause decoherence, and one thus needs to cool below the limit where these occur.

Therefore I think one could never talk about the cat being conscious of it's own superposition, as a lack of consciousness seems to be a prerequisite for any object that is to be a part of a quantum state.

 

[Ken Natton]

The first point of reference with Schrödinger’s Cat is to understand that Schrödinger himself did not intend to create a philosophical monster that would leave Stephen Hawking reaching for his firearm. Schrödinger intended his thought experiment to highlight what he perceived as the absurdity of what is known as the Copenhagen Interpretation of quantum theory. But in point of fact, the thought experiment is useful in demonstrating the real point.

The second point of reference then, is to grasp that a great proportion of what is spoken about Schrödinger’s cat is pretentious nonsense, hence Mr Hawking’s eroding patience. A significant further proportion of the philosophising is perfectly earnest, but just as misguided. And the key thing is to realize that all of this is just philosophising, it is not science.

I am reading a book at the moment called ‘Quantum, A Guide for the Perplexed’, by Jim Al Khalili, a man who is beginning to emerge as one of the key conduits between cutting edge physical science and genuinely interested lay men and women such as myself. The book as a whole is a wonderful overview of exactly what is the source of some of the more challenging theories, such as the ’many worlds theory’. But he also includes an excellent demystification of Schrödinger’s cat. At risk of breaching copyright, I will mention that he suggests replacing the cat with a human being, and replacing the poisonous gas with some anaesthetic. The point not being sensibilities about killing a human being, the point being that the idea of a superposition of dead and alive is replaced by a superposition of being conscious and not conscious, and the key new element being, after the experiment is over, you can ask the experimental subject what happened. The point is, a bit like length contraction and time dilation being only relative to the stationary observer, the superposition only exists for the observer outside the box. As Al Khalili says, if we dig up some ancient rock and find a fault line caused by the release of an alpha particle when a radioactive nucleus decayed, the idea that that fault has existed in a superposition of having been there and not having been there until we dug it up and observed it is easy to dismiss. Clearly, at some point in geological history, the nucleus decayed, the alpha particle cut its fault trace, and it lay there unobserved until we dug it up.

 

[Hurkyl]

"The cat is alive" is a random true/false variable with some probability of being true and some probability of being false. (we don't have enough control of the environment to look at the experiment in another fashion)

We know the following true/false variables are true with near certainty:

• If the cat is alive, it knows its alive.
• If the cat is dead, it doesn't know it's dead.

(given, of course, the presumption that cats are self-aware, etc)

If the cat is in a superposition, that doesn't change anything.

• If we opened the box to see a live cat, then all future experiments we do will be consistent with that observation.
• If we opened the box to see a live cat, then all future experiments we do will be consistent with that observation.

These are also random variables that are true with near certainty. If the cat is in a superposition, that doesn't change anything.


It is an impossibility to observe multiple "branches" of a superposition. The experiments we do to measure superposition is not to directly confirm it, but prepare lots of identical experiments, and show the results could only be explained if there was a superposition involved.

Superpositions only seem weird when you don't realize there's a difference between what we, as hypothetical "godlike" observers with access to all of the mathematical details of a wave-function can see, versus what an observer stuck within the universe described by a wave-function is capable of seeing.

I.E. the cat is both dead and alive. No observer will see both. There is the hypothetical possibility of an incredibly complex (or unlikely) experiment whose results depend on the superposition -- but such an experiment will never see the cat as being both dead and alive: it will just be studying a different aspect of the problem that cannot be analyzed in terms of a dead branch and a living branch.

(Of course, in reality, collapse interpretations might turn out to be right and superpositions really don't propagate)

 

[GeorgCantor]

The double slit experiment done with fullerenes and the quantum eraser give support to the idea that there is no cat before you inquire about it, i.e. open the lid and look.

 

[BruceG]

One major issue here is that for any object, macroscopic or otherwise, to be entangled, it has to be cooled close to it's ground state, such that the motion that comes from heat does not decohere the quantum state. In the case of the cat, the very brain functions that we would use to define the consciousness of the cat, is enough to cause decoherence, and one thus needs to cool below the limit where these occur.

Therefore I think one could never talk about the cat being conscious of it's own superposition, as a lack of consciousness seems to be a prerequisite for any object that is to be a part of a quantum state.

I concur with this answer.
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.

The point is that if an entangled state of live-cat/dead-cat were ever to form, then on microscopic timescales it would immediately decohere into either a definite macroscopic dead cat or a definite macroscopic live cat. As such the language of macroscopic physics applies to the cat, and we can make an objective statement about the state of the cat.
It does not depend on subjective observation.

As such the question as posed does not apply.

 

[GeorgCantor]

I concur with this answer.
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.

If quantum theory makes sense to you, that's a very sure sign you still have a lot to learn. Or probably think and contemplate. Or a combination of both.

The point is that if an entangled state of live-cat/dead-cat were ever to form, then on microscopic timescales it would immediately decohere into either a definite macroscopic dead cat or a definite macroscopic live cat. As such the language of macroscopic physics applies to the cat, and we can make an objective statement about the state of the cat.
It does not depend on subjective observation.

As such the question as posed does not apply.

I'll cut right to the chase on this. There is no coming back to the old concepts of objects with properties in time and space, this is certain. We are in the mids of a paradigm shift and although it's not exactly clear how everything should fit in a coherent picture, it's clear how everything will NOT.

This common-sense view of reality that made sense to you is a total misconception.

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.

If you still think qt makes sense to you, you need to go back and re-think "the physical matter in superposition" issue. Today, tomorrow or in a month, when you have figured out what is really happeneing, you'll surely go down the drain into a blind alley from which nobody has escaped, as Feynman liked to say.

Realism, in the physics sense, is dead and so is the Newtonian universe of our perception. It's not to be made sense of, unless you are ready to believe the unbelievable.

 

[JoeDawg]

That seems like an absurdity.

The cat is either dead or asleep.

 

[BruceG]

If quantum theory makes sense to you, that's a very sure sign you still have a lot to learn. Or probably think and contemplate. Or a combination of both.

I'll cut right to the chase on this. There is no coming back to the old concepts of objects with properties in time and space, this is certain. We are in the mids of a paradigm shift and although it's not exactly clear how everything should fit in a coherent picture, it's clear how everything will NOT.

This common-sense view of reality that made sense to you is a total misconception.
...
Realism, in the physics sense, is dead and so is the Newtonian universe of our perception. It's not to be made sense of, unless you are ready to believe the unbelievable.

At the microscopic level I agree with you - quantum physics has revealed a form of reality totally remote from our intuitions. OK - I don't really 'understand' this - but I get how the maths works.

But we were talking about a macroscopic object (the cat). A necessary requirement of any interpretation of quantum mechanics is that it MUST recreate the "common-sense view of reality" at the macroscopic level with "objects with properties in time and space".

Decoherence theory seems to provide this missing link making quauntum theory more understandable. Feynman, for all his genius, was as far as I know unaware of this theory.

 

[GeorgCantor]

But we were talking about a macroscopic object (the cat). A necessary requirement of any interpretation of quantum mechanics is that it MUST recreate the "common-sense view of reality" at the macroscopic level with "objects with properties in time and space".

OK, but none of the interpretations do. We are used to thinking in terms of physical structues in space and time, whereas the whole notion of "physical structures" goes to trash assoon as you introduce superpositions. There are very obvious conceptual problems with the notions of space and time as well, and it will take nothing short of demolishing and re-bulding the entire 'universe' anew with wholly different basic constituents to restore the Newtonian universe of perception.

Decoherence theory seems to provide this missing link making quauntum theory more understandable. Feynman, for all his genius, was as far as I know unaware of this theory.

Only slightly more 'understandable', yes. Take a closer look and the common-sense reality is still not there, it's as far away as it can ever be. As i said earlier, "objects" that can be put in superpositions cannot be regarded as objects. They are registered events(detections).

 

[imiyakawa]

[on superpositions] A necessary requirement of any interpretation of quantum mechanics is that it MUST recreate the "common-sense view of reality" at the macroscopic level with "objects with properties in time and space".

Decoherence theory seems to provide this missing link making quauntum theory more understandable.

I've not read decoherence. In my short conversations with a physicist I asked what occurred during reduction to what seems to be discrete-state during measurement. He stated that what's occurring is the wavefunction is simply bunching up to a pointer like state during thermal interaction.

Doesn't this mean that there still isn't this nice, understandable point particle universe even at the large scale? And doesn't this fact actually support Georg's view of the conceptual confusion that should be here? Aren't there still large scale superpositions with decoherence, just not in the same way (at all) as a molecule may be superposed in near absolute zero temperature?

 

[JoeDawg]

demolishing and re-bulding the entire 'universe' anew with wholly different basic constituents to restore the Newtonian universe of perception.

Wow, you're such a drama queen. It means rebuilding the theory to include quantum weirdness and our everyday experience. That doesn't mean our everyday experience isn't real, or the way we describe it is no longer valid. This is what you call 'throwing out the baby with the bathwater'. A paradigm shift doesn't mean starting from scratch. It means... shifting to a different framework... changing some assumptions.

Gravity doesn't stop working just because we don't have a TOE.

QM can give us insights, but the Newtonian universe we experience isn't going anywhere, its just not the complete picture. In fact, what we experience is just as essential to understanding the complete picture.

And, Schrodinger's cat is an analogy, which means it can be useful, but is not definitive.

 

[GeorgCantor]

Wow, you're such a drama queen. It means rebuilding the theory to include quantum weirdness and our everyday experience. That doesn't mean our everyday experience isn't real, or the way we describe it is no longer valid. This is what you call 'throwing out the baby with the bathwater'. A paradigm shift doesn't mean starting from scratch. It means... shifting to a different framework... changing some assumptions.

If that theory is going to include physical structures with definite properties in space and time, that theory is going to be WRONG, as it won't be confirmed in experiments. As far as i am aware, there is no such theory in development, and there is no hint of motivation to pursue something that appears so patently absurd, given the experimental verification of qm and gr.

Your common-sense is generally the least reliable tool to understand reality, it's on par with the biblical account of creation, if not much worse(given that there are "common-sense" theories that the Earth is flat, or that the Sun orbits the Earth.)

Gravity doesn't stop working just because we don't have a TOE.

QM can give us insights, but the Newtonian universe we experience isn't going anywhere, its just not the complete picture. In fact, what we experience is just as essential to understanding the complete picture.

True, it isn't going anywhere, but the Newtonain picture of the universe is actually SPECTACULARLY WRONG, as long as we are talking about the God's veiw of the universe. Actually, the single most expensive and elaborate piece of equipment on Earth - the LHC - works because relativity is incorporated into it(very high speed and very high energies is not where relativity is simply visible, it's where relativistic effects are PREDOMINENT).

And, Schrodinger's cat is an analogy, which means it can be useful, but is not definitive.

You have not understood the cat analogy, and if you think i am a drama queen, you've hardly understood anything. The upcoming paradigm shift is not going to be about minor adjustments to current theories. But if you wish to believe you live in a Newtonian universe, so be it, there are people who believe in the flat Earth.

BTW, everybody is confused. Think about it.

A paradigm shift doesn't mean starting from scratch. It means... shifting to a different framework... changing some assumptions.

When you discard the realism assumption, it means starting from scratch. One of the hardest problems before string theorists(according to Greene and others) is to make ST background-independent and to account for the emergence of spacetime from ?.

Wow, you're such a drama queen. It means rebuilding the theory to include quantum weirdness and our everyday experience.

Would you want to "rebuild the theory" to also include yet another idea from our everyday experience - that the Earth is flat?

You can try here and browse the experimental evidence along with their Wiki and their newly opened flat-Earth shop:

http://theflatEarth'society.org/cms/


The 'drama part' or what might seem like strong wording is supposed to challenge those who believe in realism and objects with properties in space and time. I am genuinely curious what others have to say in defence of such a position.

 

[BruceG]

I've not read decoherence.

Doesn't this mean that there still isn't this nice, understandable point particle universe even at the large scale? And doesn't this fact actually support Georg's view of the conceptual confusion that should be here? Aren't there still large scale superpositions with decoherence, just not in the same way (at all) as a molecule may be superposed in near absolute zero temperature?

You seem to have grasped exacly the same issues out of decoherence as I have (despite claiming to not have read it!)

So yes, I think you, I and Georg can agree that decoherence makes things slightly better, but is still baffling.

This is as far as I got with it before I gave up:
- Decoherence does still leave superpositions at the macro-scale.
- The good news is though that you don't get the nasty coupling terms which makes quantum theory so weird (I rely on you understanding what I'm talking about here)
- What this does is make things look a bit more like standard statistical physics (there is a 50% chance the cat is alive, 50% chance the cat is dead -but then that is no different from any statistical physics when the inputs are unknown)
BUT
- we still need to try and make sense of where the macro-state which didn't happen went (ok so we later learn the cat is actually dead, so what of the live cat).
-it is so tempting at this point (and this is actually what I do to stay sane) to invoke the multi-universe interpretation : so I really believe another universe exists somewhere with a live cat (and then another me observing that and so on).
BUT
-I know I shouldn't : talk of such multiverses is metaphysical nonsense: how could I ever verify this. Popper would turn in his grave.

So that I where I leave it for other to enlighten someday.

 

[JoeDawg]

If that theory is going to include physical structures with definite properties in space and time, that theory is going to be WRONG, as it won't be confirmed in experiments.

You're missing the point. Newtonian mechanics describes the world enough for most people. Its not a common sense argument, its a utility argument.

as we are talking about the God's veiw of the universe.

Since when are we talking about god?? Or do you have a QM interpretation for gods?

Actually, the single most expensive and elaborate piece of equipment on Earth...

isn't useful if my car runs out of gas.

But if you wish to believe you live in a Newtonian universe, so be it, there are people who believe in the flat Earth.

Actually, it slopes down towards a lake where I live.

BTW, everybody is confused. Think about it.

Except you of course, you KNOW I'm wrong. Not that you have understood even a bit of what I said. You're too focused on being right.

When you discard the realism assumption, it means starting from scratch.

That's crap.

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

Scientific realism in classical (i.e. pre-quantum) physics has remained compatible with the naïve realism of everyday thinking on the whole; whereas it has proven impossible to find any consistent way to visualize the world underlying quantum theory in terms of our pictures in the everyday world. The general conclusion is that in quantum theory naïve realism, although necessary at the level of observations, fails at the microscopic level.

You're just an obsessive reductionist.

those who believe in realism and objects with properties in space and time.

Space and time are descriptive of observation, which means they are real enough. The fact observations on the quantum level require different descriptions should neither be surprising or problematic.

Even something as basic as atomic theory shows that everyday objects are not as they appear... that is... they are not 'solid' in the way people used to think of them. That doesn't mean they aren't solid in some fashion.

 

[GeorgCantor]

Even something as basic as atomic theory shows that everyday objects are not as they appear... that is... they are not 'solid' in the way people used to think of them. That doesn't mean they aren't solid in some fashion.

Yes, exactly. I don't deny that they seem solid in some fashion, it would have been ludicrious if i had. My point was about realism as defined in physics - that objects have definite properties at ALL times(the elements of reality as per the EPR argument). On the one hand we have much experimental evidence and a mathematical theorem that this is not the case(the cat is not always there), on the other hand we have common-sense and everyday experience that contradict this. It seems one has to leave one's sanity at the door, or make many new assumptions(though now is the least appropriate time for more controversial assumptions) - like the many world hypothesis, the ftl pilot-wave hypothesis(relativity theory wrong/incomplete), etc.

BTW, the God's view of the universe is the same as saying 'reality', as the term 'universe' is becoming increasingly misleading. Gotta love how there is always talk of 'reality' in the quantum physics forum and talk of 'universe' in the cosmology forum.

 

[JoeDawg]

Yes, exactly. I don't deny that they seem solid in some fashion, it would have been ludicrious if i had. My point was about realism as defined in physics - that objects have definite properties at ALL times(the elements of reality as per the EPR argument).

I think you are defining realism very narrowly.
Its philosophical history is much more complex.

and everyday experience that contradict this.

Which means our understanding of QM and everyday experience, and how they relate is lacking. Not that one obliterates the other.

When our understanding of matter shifted from particles to wave/particles, it didn't mean that describing matter as particles was no longer useful, it just means the some assumptions changed and our understanding became broader.

BTW, the God's view of the universe is the same as saying 'reality', as the term 'universe' is becoming increasingly misleading.

Again, 'reality' is another word loaded with history. Idealism and phenomenology have been around for quite a while. QM is really just part of an ongoing discussion.

Gotta love how there is always talk of 'reality' in the quantum physics forum and talk of 'universe' in the cosmology forum.

Its a paradigm shift, it happens often when you move from one science... one scientific framework... to another. Or really any field of study... to another.

 

[imiyakawa]

On the one hand we have much experimental evidence ... on the other hand we have common-sense and everyday experience that contradict this.

I don't see how everyday experience contradicts a viewpoint such as decoherence. The electric force of much of macro interaction is still there, i.e. the table is still "solid". Things are still hard (hand can't pass through them because of charge repulsion). The brain should still work as it is for practical purposes not structurally altered by not being composed of pure point particles. So the macro world's hardness and solidity isn't a problem under decoherence.

IF we look to decoherence,
-I see and recognize the conceptual block posed by QM (constant superpositions).
-I don't see the difficulty of macro hardness or the illusion of atomism arising from the perspective of the extremely large.

 

[GeorgCantor]

I don't see how everyday experience contradicts a viewpoint such as decoherence. The electric force of much of macro interaction is still there, i.e. the table is still "solid". Things are still hard (hand can't pass through them because of charge repulsion). The brain should still work as it is for practical purposes not structurally altered by not being composed of pure point particles. So the macro world's hardness and solidity isn't a problem under decoherence.

IF we look to decoherence,
-I see and recognize the conceptual block posed by QM (constant superpositions).
-I don't see the difficulty of macro hardness or the illusion of atomism arising from the perspective of the extremely large.

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.

Superpositions are an established experimental fact, there is no question about it. The remaining question is the realism that Einstein liked to believe in. The one that we now have solid evidence to be a misconception.

So the macro world's hardness and solidity isn't a problem under decoherence.

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.

Decoherence actually provide evidence that there are no physical structures in space and time, i.e. Einstein's idea of realism is wrong and hence matter(physical structures) are actually events, not objects with properties.

Outside decoherence, say the double slit experiment, you have fullerenes(physical structures) and de-localized wave-like entities(non-fullerenes).


You can't have a transistor made without superpositions, and although certainly useful, they do destroy the realism of perception. I would say that superpositions invite total insanity if you really give it much thought. The whole notion of physical structure is a misnomer and we need a considerable adjustment on the way we view reality.

I don't see how everyday experience contradicts a viewpoint such as decoherence.

Am i the only one that perceives a world of objects that there all the time, and that cannot magically vanish under certain circumstances, losing all their defining characteristics?

If you take relativity and superpositions seriously, you can only conclude that Einstein's realism is completely dead.

 

[GeorgCantor]

When our understanding of matter shifted from particles to wave/particles, it didn't mean that describing matter as particles was no longer useful, it just means the some assumptions changed and our understanding became broader.

The notion of "partciles" is a misnomer, particles that are not spatially extended cannot be spatially represented, except as a probability cloud. We basically don't know what we are talking about, as matter has dissolved into a ghostly substance. It's not that now "our understanding became broader" as you say, we basically have no understanding of what matter really is(and it's quite worthy of making a drama on it since we can no longer answer a simple question like "what the hell is really going on?").

 

[JoeDawg]

Am i the only one that perceives a world of objects that there all the time, and that cannot magically vanish under certain circumstances, losing all their defining characteristics?

But it is not magic. Newton didn't understand gravity, he just described it, and he was criticized for his claims of 'action at a distance'. It made no sense, until Einstein.

The fact that QM seems insane and magical to you doesn't mean that it, in fact, is. It just means you (most everyone I'd say) don't understand it.

http://en.wikipedia.org/wiki/Argume...ent_from_incredulity_.2F_Lack_of_imagination"

Changing the word from object to event may change the theory behind it, but it hardly makes objects disappear. It just more clearly defines what they are, or rather what they do.

Describing objects in Newtonian terms is valid, because its useful, even if its not complete or definitive.

 

[imiyakawa]

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.

It blows my mind as well.

It doesn't blow my mind as to why I have the illusion of atomism and point particles on the large scale. Decoherence explains that extremely nicely.

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.

Decoherence actually provide evidence that there are no physical structures in space and time, i.e. Einstein's idea of realism is wrong and hence matter(physical structures) are actually events, not objects with properties.

You quoted me:
"So the macro world's (perceived) hardness and solidity isn't a problem under decoherence."

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.

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.

 

[GeorgCantor]

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

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).

 

[ZapperZ]

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.

 

[imiyakawa]

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.

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.

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.

 

[GeorgCantor]

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.

 

[ZapperZ]

"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_superpositionMatter(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.

 

[GeorgCantor]

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?

 

[ZapperZ]

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.

 

[GeorgCantor]

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):

"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.

 

[imiyakawa]

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. :)

 

[GeorgCantor]

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).

 

[ZapperZ]

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.

 

[GeorgCantor]

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...

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.

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"?

 

[ZapperZ]

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.htmThe 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.

 

[GeorgCantor]

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.

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).

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.

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.

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.

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.

 

[BruceG]

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]

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".

 

[GeorgCantor]

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.

 

[BruceG]

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]

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

\psi = a_1|1> + a_2|2>

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

Please tell me what this is saying to you.

Zz.

 

[GeorgCantor]

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?



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)."

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

\psi = a_1|1> + a_2|2>

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.

 

[ZapperZ]

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?

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.

 

[GeorgCantor]

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.

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).

 

[Pythagorean]

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

 

[GeorgCantor]

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]

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