Why quantum effects disappear at the classical level

[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