Why quantum effects disappear at the classical level

[Maui]

But the classical world is not at variance with the quantum world. The reason it has properties when you are not observing it is because its virtually never not being observed - it is in constant entanglement with its environment which decoheres it and from the modern viewpoint is the reason for the emergence of the classical realm.
But remove that entanglement, at least partially, and quantum behaviors reassert themselves eg liquid helium and bucky balls. It is not a size issue - it is an entaglement and decoherence issue.

It seems to me there is some misunderstanding on your part. First, the buckyballs experiment, in the words of the experimenters, highlights that decoherence only happens when information about the system is potentially obtainable(as opposed to being environemtally induced, which is a another contradiction as the environment is also completely quantum).

"In quantum interference experiments, coherent superposition
only arises if no information whatsoever can be obtained, even in
principle, about which path the interfering particle took. Interaction
with the environment could therefore lead to decoherence.We
now analyse why decoherence has not occurred in our experiment
and how modifications of our experiment could allow studies of
decoherence using the rich internal structure of fullerenes.
In an experiment of the kind reported here, ‘which-path’ information
could be given by the molecules in scattering or emission
processes, resulting in entanglement with the environment and a
loss of interference. Among all possible processes, the following are
the most relevant: decay of vibrational excitations via emission of
infrared radiation, emission or absorption of thermal blackbody
radiation over a continuous spectrum, Rayleigh scattering, and
collisions.
When considering these effects, one should keep in mind that
only those scattering processes which allow us to determine the path
of a C60 molecule will completely destroy in a single event the
interference between paths through neighbouring slits. This
requires lpd; that is, the wavelength l of the incident or emitted
radiation has to be smaller than the distance d between neighbouring
slits, which amounts to 100nm in our experiment. When this
condition is not fulfilled decoherence is however also possible via
multi-photon scattering7,8,17.

http://qudev.ethz.ch/content/courses/phys4/studentspresentations/waveparticle/arndt_c60molecules.pdf

Anton Zeilinger et al.

I don't quite understand why this view is still prevalent - there are plenty of standard texts these days that give the correct view. The one I have is Schlosshauer's textbook that explains it very clearly:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Thanks
Bill

Second, there is no view yet that puts classical determinism at the crest and no one i know(and certainly no one I've seen here) understands emergent determinism of the type you are espousing. It's much less clear what and how takes place and if i am to take your view as the only correct, i'd have to subscribe to some of the quantum conspiracy theories, as the classical world and all of the history of the sciences REST completely on the idea that fundamental determinism cause what and how will happen and not emergent events that seem to conform to some form of apparent determinism after the fact. I am not even a physicist and will likely get a lot of deserved critiscism, but without the MWI, the decoherence theory makes no sense wrt to the outside world. Then again, most interpretations make no sense as well wrt the outside world :)

 

[jon4444]

without the MWI, the decoherence theory makes no sense wrt to the outside world. Then again, most interpretations make no sense as well wrt the outside world :)

Great. For those of us "on the sidelines," we're back to be completely confused about the current state of physics and how to think about QM.

 

[Jano L.]

Great. For those of us "on the sidelines," we're back to be completely confused about the current state of physics and how to think about QM.

I understand your confusion. One way out I can recommend is to study non-controversial (pre 20-th century) physics in more detail, build up mathematical and reasoning skills, and then try to salvage from the quantum theories what makes sense to you. There is much less confusion in fields like mechanics, electromagnetic theory, thermodynamics and statistical physics (although there are few deep-rooted misunderstandings popping up time to time as well).

 

[bhobba]

Second, there is no view yet that puts classical determinism at the crest and no one i know(and certainly no one I've seen here) understands emergent determinism of the type you are espousing.

Then read the reference from Schlosshauer's I gave - it does exactly what I said - namely gives the details of how the classical world emerges from entanglement and decoherence:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

I have not suggested that classical determinism is at the crest of anything. It clearly isn't since everything is Quantum.

Thanks
Bill

 

[bhobba]

I looked at QED, and took away something different from you. Feynman says something like (paraphrasing) "though we have methods to exactly calculate the phenomenon, there is no solid conceptual grounding for it."

I'm interested in there being an explanation at the conceptual level...

That's not quite what Feynman is saying - he is saying we do not have an explanation in terms of everyday pictures and anyone that has tried to develop one has gone down a black hole. If that is a lack of an explanation at a conceptual level is a matter of opinion - I believe there is a conceptual explanation.

What modern research has shown is that some very reasonable assumptions leads to basically two choices in modelling the world - QM and bog standard probability theory:
http://arxiv.org/pdf/0911.0695v1.pdf

The particular aspect QM has when modelling over normal probability theory is it has continuous transformations between the outcomes of your observations (they are called pure states) and entanglement. Either one says QM is what should be used.

The issue here is it requires advanced math to understand it. This turns most people off and because of that they don't get to really understand what's going on.

Great. For those of us "on the sidelines," we're back to be completely confused about the current state of physics and how to think about QM.

Everything I say can be checked in standard textbooks such as the link I gave to Schlosshauer's textbook:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

If you are getting confused by different views I can assure you what I have said in this thread is bog standard textbook stuff that can be checked by reading the literature. It can be found in the textbook above. However it does require quite a bit of background in QM which you may not have. If that is the case a better reference would be Omnes book I referred to previously:
https://www.amazon.com/dp/0691004358/?tag=pfamazon01-20

It is very difficult to get a sense of what is really going on from reading posts on this forum because of the myriad of competing views. There is really only one way out of that impasse - read the standard textbooks yourself.

Thanks
Bill

 

[bhobba]

It seems to me there is some misunderstanding on your part.

I am saying Bucky Balls show quantum aspects - that's it - that's all. They are large molecules and as such are still pretty small - but not as small as electrons which is the level most people seem to think you need to show quantum effects. I mentioned it purely to indicate size is not the pre-requisite for showing quantum effects.

I am claiming the classical world emerges from entanglement and decoherence. This is bog standard textbook stuff such as given in Omnes textbook:
https://www.amazon.com/dp/0691004358/?tag=pfamazon01-20

Or if you would like some video lectures check out Susskind's:
http://www.newpackettech.com/Resources/Susskind/PHY30/QuantumEntanglementPart1_Overview.htm

Thanks
Bill

 

[jon4444]

@bhobba: thanks for the references. I take your point that a textbook is probably a better place to start for someone looking to get up to speed on current thinking.

And thanks for introducing me to the phrase "bog standard."

 

[bhobba]

And thanks for introducing me to the phrase "bog standard."

Its Aussi idiom mate

We are a down to Earth bunch down under.

Thanks
Bill

 

[Dundeephysics]

Why are people continually thinking that QM behavior disappears at the macro level?

I didn't say that, on the contrary, I brought up the example of debroglie wavelength of a tennis ball, which is many orders of magnitudes smaller than quantum particles. In theory around us everything from sub atomic particles to galaxies have quantum mechanical behaviors

On my quoted sentence "This could help you think about why large objects don't exhibit good quantum mechanical behaviours like small ones." I said that they don't exhibit GOOD quantum mechanical behavior and I meant their quantum effect such as the tennis ball example become less apparent. Thanks

 

[audioloop]

I am unsure of the point you are trying to make. But the classical world is not at variance with the quantum world. The reason it has properties when you are not observing it is because its virtually never not being observed - it is in constant entanglement with its environment which decoheres it and from the modern viewpoint is the reason for the emergence of the classical realm. But remove that entanglement, at least partially, and quantum behaviors reassert themselves eg liquid helium and bucky balls. It is not a size issue - it is an entaglement and decoherence issue.

This really dates back to the Copenhagen interpretation. Modern research has not invalidated that interpretation but has clarified it. The issue of that interpretation is it divided the world into two realms - classical and quantum and postulated they behave differently. Quantum effects are known when they make their appearance via observations in that classical world in that interpretation. It is now known that distinction is a chimera - there is no difference - quantum and classical worlds are exactly the same - the difference is the classical world is entagled with its environment and decoherence rules. Removing that entaglement is difficult - but not impossible - and when its done quantum effects reassert themselves even for macro objects.

I don't quite understand why this view is still prevalent - there are plenty of standard texts these days that give the correct view. The one I have is Schlosshauer's textbook that explains it very clearly:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

Thanks
Bill

show a cat in superposition...

 

[bhobba]

show a cat in superposition...

That's the point - entanglement and decoherence prevents it.

Thanks
Bill

 

[DrDu]

Why are people continually thinking that QM behavior disappears at the macro level? That's simply not true. Consider liquid helium again.

Strange behaviour fine, but nevertheless all the observables necessary to describe liquid He are commuting. So it is clearly a classical object.

 

[bhobba]

Strange behaviour fine, but nevertheless all the observables necessary to describe liquid He are commuting. So it is clearly a classical object.

Not so sure about that - its seems only at sufficiently high temperatures do the kinetic and potential energies commute:
http://people.physics.illinois.edu/Ceperley/papers/036.pdf

You would expect something like this because something must prevent it from being governed by the classical Boltzmann distribution - if that was the case it would require absolute zero to be in its lowest energy state - see equation 2.2 and 2.3. And indeed it is QM that prevents absolute zero ever being reached for any object since it would mean its constituent parts were at rest ie have known position and momentum which QM does not allow.

Thanks
Bill

 

[mfb]

And indeed it is QM that prevents absolute zero ever being reached for any object since it would mean its constituent parts were at rest ie have known position and momentum which QM does not allow.

Be careful here. QM prevents a system from reaching the classical ground state - but it does not prevent a system from reaching its quantum-mechanical ground state (with zero temperature), with a different (larger) energy.

 

[bhobba]

Be careful here. QM prevents a system from reaching the classical ground state - but it does not prevent a system from reaching its quantum-mechanical ground state (with zero temperature), with a different (larger) energy.

Exactly

That's the precise point I am trying to make. At low temperatures ie with a quite a bit of environment entanglement removed - quantum effects of a decidedly non classical nature emerge. The reason for a classical macro domain is not size it is entanglement and decoherence.

I am really scratching my head why anyone doubts this - I thought it was pretty much a generally accepted position. I have done a separate post about it in relation to Bose-Einstein condensates that I believe demonstrates this most clearly, just in case I am missing something - but blowed if I can see what it is.

Thanks
Bill

 

[DrDu]

Not so sure about that - its seems only at sufficiently high temperatures do the kinetic and potential energies commute:
http://people.physics.illinois.edu/Ceperley/papers/036.pdf

Of course, but that is not what I meant. Ceperley calculates specific heat and the like and you always need quantum mechanics to calculate these properties. I am talking about the macroscopic degrees of freedom of liquid helium. Specifically one may describe liquid helium in terms of a "macroscopic wavefunction". Although it is clear that it is a result of the quantum mechanical behaviour of the bosons, the amplitude and phase of this wavefunction can be measured simultaneously without uncertainty. This is similar to classical electrodynamics where the electromagnetic field also becomes a classical field.

 

[bhobba]

Of course, but that is not what I meant.

One property of a classical object is you can continually subdivide it into distinguishable constituent parts - you can't do the with say a BEC - it is decidedly non classical.

The whole point of this discussion is the idea that for macro objects they always behave classically. Most of the time that's true - but most is not all - and I consider liquid helium and BEC's prime examples of those exceptions.

Thanks
Bill

 

[DrDu]

One property of a classical object is you can continually subdivide it into distinguishable constituent parts - you can't do the with say a BEC - it is decidedly non classical.

I don't even know what this should mean in the case of a classical electromagnetic field or even simpler for a container full of water.

 

[JK423]

We can extract entanglement from a BEC at T=0. Throw two distinguishable and localized spin one-half particles (call them probes) to locally interact with different regions of a BEC for a small amount of time. After the interaction, the two partiles become entangled, and hence they have extracted entanglement from the BEC. That's a purely quantum feature, hence a BEC is quantum mechanical. The entanglement comes from the fact that the bosons of the BEC are delocalized in space, so one boson simultaneously interacts with both probes.
You can also do that with a coherent state, hence a coherent state is not classical as well.

So, BEC is quantum mechanical!
I am sure there are numerous other quantum features but i am not an expert in the field.

 

[bhobba]

I don't even know what this should mean in the case of a classical electromagnetic field or even simpler for a container full of water.

I agree a field is more problematical and I will need to think that through a bit. But water is H2O molecules that are all distinguishable from each other - a BEC is not like that at all - indeed if it even has constituent parts is open to question.

I suspect what this argument boils down to is what you consider classical. I believe for an object to be classical it is more than if you can say it has a definite position and momentum.

Thanks
Bill

 

[DrDu]

We can extract entanglement from a BEC at T=0. Throw two distinguishable and localized spin one-half particles (call them probes) to locally interact with different regions of a BEC for a small amount of time. After the interaction, the two partiles become entangled, and hence they have extracted entanglement from the BEC. That's a purely quantum feature, hence a BEC is quantum mechanical. The entanglement comes from the fact that the bosons of the BEC are delocalized in space, so one boson simultaneously interacts with both probes.
You can also do that with a coherent state, hence a coherent state is not classical as well.

So, BEC is quantum mechanical!
I am sure there are numerous other quantum features but i am not an expert in the field.

Entanglement is not some kind of conserved quantity which can only be extracted from some other system. I'd rather say the product states evolved into an entangled state during the interaction with a classical field.

 

[DrDu]

I agree a field is more problematical and I will need to think that through a bit. But water is H2O molecules that are all distinguishable from each other.

Given that water is made up mainly of 1H and 16O, most water molecules are indistinguishable even in normal tap water.

 

[JK423]

Entanglement is not some kind of conserved quantity which can only be extracted from some other system. I'd rather say the product states evolved into an entangled state during the interaction with a classical field.

You cannot generate entanglement from nothing, entanglement satisfies some kind of 'conservation laws' and this behaviour is generally known as entanglement monogamy.
The entanglement of the probes came from the entanglement in the field and you can show that. A field with entangled degrees of freedom is quantum, not classical. And this quantum behaviour has nothing to do with statistics; it has to do solely with the fact that the BEC bosons are in spatial superposition, hence purely quantum.

 

[DrDu]

The entanglement of the probes came from the entanglement in the field and you can show that.

Ok, so after the interaction there may be some phonon like excitations being present in the BEC which may be called entangled when decomposed in some spatially localized states. This is possible in any system with some broken symmetry like a crystal. Clearly these excitations will be quantum objects. Nevertheless I would not like to speak of a crystal as a macroscopic quantum object only based on this reasoning.

 

[JK423]

Ok, so after the interaction there may be some phonon like excitations being present in the BEC which may be called entangled when decomposed in some spatially localized states. This is possible in any system with some broken symmetry like a crystal. Clearly these excitations will be quantum objects. Nevertheless I would not like to speak of a crystal as a macroscopic quantum object only based on this reasoning.

You say that BEC is just a classical wave. The existense of entanglement in it is just an example that this is not so. What do you mean "only based on this reasoning"? Some features may be explained by classical wave mechanics, others (like this one) cannot. If you regard a BEC as a quantum field you can explain everything.

 

[DrDu]

You say that BEC is just a classical wave. The existense of entanglement in it is just an example that this is not so. What do you mean "only based on this reasoning"? Some features may be explained by classical wave mechanics, others (like this one) cannot. If you regard a BEC as a quantum field you can explain everything.

That's not my point. I mean that these quantum effects won't influence the macroscopic variables of the system which behave classically. That some microscopic degrees of freedom, as those interacting with the two particles you mentioned, are QM is rather trivial.

 

[JK423]

That's not my point. I mean that these quantum effects won't influence the macroscopic variables of the system which behave classically. That some microscopic degrees of freedom, as those interacting with the two particles you mentioned, are QM is rather trivial.

What are the macroscopic variables that you are referring to?

 

[DrDu]

E.g. particle densities and velocities averaged over small but macroscopic volumina. Eventually also the similarly coarse grained macroscopic wavefunction (or correlation functions of the latter as it is not a direct observable).

 

[JK423]

Ok. You understand that what you describe is only an approximation of the real thing, and this approximation cannot describe a process of entanglement extraction for example, and in general interactions with other quantum systems.
Why do you baptise a BEC classical when your definition relies only on an approximation which gives wrong results in some circumstances? If your approximation gave always the correct results then i would agree with you. But in the specific case it's not just that it gives the wrong value for something, it cannot even predict phenomena like the extraction of entanglement.

 

[DrDu]

I tried to distinguish between the microscopic and the macroscopic behaviour. On the microscopic level, we always observe quantum mechanical effects but seldomly on a macroscopic aka classical level as whas the question of this thread. I don't deny that there are quantum effects observable in a BEC, however it's macroscopic properties can well be described by a classical field theory.

 

[audioloop]

Be careful here. QM prevents a system from reaching the classical ground state - but it does not prevent a system from reaching its quantum-mechanical ground state (with zero temperature).

right and loses the superposition.

Quantum Upsizing
http://www.fqxi.org/community/articles/display/103
"To investigate where quantum mechanics breaks down and classical mechanics begins, the team is investigating two weird quantum properties: entanglement and superposition. When two particles become entangled"

Phys. Rev. Lett. 107, 020405 (2011)
Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects
http://prl.aps.org/abstract/PRL/v107/i2/e020405
http://arxiv.org/abs/1103.4081

"more general tests of quantum theory against full classes of macrorealistic theories"

 

[audioloop]

from Schwab, Aspelmeyer, Romero-Isart...
experimental testingMacroscopic quantum resonators
http://link.springer.com/article/10.1007/s10686-012-9292-3
http://arxiv.org/pdf/1201.4756v2.pdf..."Testing the predictions of quantum theory on macroscopic scales is one of today's outstanding challenges of modern physics and addresses fundamental questions on our understanding of the world. Speci cally: will the counterintuitive phenomena of quantum theory prevail on the scale of macroscopic objects? This is at the heart of the so-called \quantum measurement problem", also known as Schrodinger's cat paradox. Another question is whether quantum superposition states of massive macroscopic objects are consistent with our notion of space-time or whether quantum theory will break down in such situations."...

 

[StarsRuler]

All objects are quantum. But if your precission in measurements is t.q ΔpΔx>$\hbar$, ( but only neccesary that it be > but in the order of$\hbar$), the system is classical because you can´t distingish the probability cloud and the object resembles a unique position in time. This is a basic learning of QM. ¿ What book did you study with it.

 

[bhobba]

All objects are quantum. But if your precission in measurements is t.q ΔpΔx>$\hbar$, ( but only neccesary that it be > but in the order of$\hbar$), the system is classical because you can´t distingish the probability cloud and the object resembles a unique position in time. This is a basic learning of QM. ¿ What book did you study with it.

He is referring to some experiments that demonstrate quantum effects in macroscopic objects eg:
http://www.scientificamerican.com/article.cfm?id=quantum-microphone

What he doesn't seem to understand however is none of this contradicts anything in the standard treatments found in QM textbooks. For example in the above experiment the effect is caused by the atoms being in a superposition of position - but well within the Heisenberg uncertainly principle - so rather than contradicting what textbooks say instead confirms it. Indeed if it didn't that would be Earth shattering news indeed, rather than simply a very intriguing and interesting phenomena - newsworthy - yes - but far from Earth shattering. Its simply due to deocherence being removed (its normally largely a result of entanglement with an objects surroundings that manifests itself in the form of heat - these effects require nearly absolute zero for them to manifest) - we don't normally notice them. Its very similar to the behavior of liquid helium near absolute zero. Very weird - yes - and many call it QM laws writ large - which it is - but nothing is going on that contradicts any standard textbook stuff.

Thanks
Bill