When does quantum mechanics turn into Newtonian mechanics?

[conner.ubert]

Since quantum mechanics rules over the microscopic world and Newtonian mechanics rules over the macroscopic world, what is the greatest possible arrangement of atoms to where quantum mechanics still rules and anything more would lead to Newtonian mechanics? In other words where is the line and what touches it?

 

[PAllen]

Since quantum mechanics rules over the microscopic world and Newtonian mechanics rules over the macroscopic world, what is the greatest possible arrangement of atoms to where quantum mechanics still rules and anything more would lead to Newtonian mechanics? In other words where is the line and what touches it?

There is no boundary. For example, superconductivity and super-fluidity are large scale macroscopic phenomena that exhibit quantum behavior. Further, quantization of vibrational states (including groundstate motion) have been shown in small objects super cooled to near absolute zero. It is just that when such effects get swamped by decoherence, classical theories become an arbitrarily close approximation.

 

[Ivan92]

I was reading a very interesting article regarding QM and the Macro world a few days ago. It seems to highlight how QM is not just for the micro world, but also applies to the macro as well.

Here it is:
http://phy.ntnu.edu.tw/~chchang/Notes10b/0611038.pdf

 

[Ken G]

That is a very nice article, thank you for posting it.

 

[D H]

I was reading a very interesting article regarding QM and the Macro world a few days ago. It seems to highlight how QM is not just for the micro world, but also applies to the macro as well.

Here it is:
http://phy.ntnu.edu.tw/~chchang/Notes10b/0611038.pdf

Sometimes. The author is a quantum physicist, and one who is taking the rather extreme POV that his worldview is the only valid worldview.

A better point of view is that of layers of abstraction. For example, An ecology in biology is separated from the quantum world by multiple layers of abstraction. Ecologists need to see the forest for the trees. While quantum physics might be able to explain the iridescence of the chemicals that are a particular component of the wings of a particular species of butterfly that lives on a particular species of trees in a particular part of the forest, quantum physics in general does not help you see the forest for the trees. It is much more likely to hinder understanding, and that is why the sciences involve layer upon layer of levels of abstraction.

There is one key problem with abstractions: Any useful abstraction is inherently leaky. The underpinnings of the abstraction will leak through to the abstraction. Schrödinger's cat and a vat full of liquid helium, for example, demonstrate how the quantum world can leak through into a macroscopic Newtonian POV (the vatful of liquid helium literally so). Just because abstractions are inherently leaky does not mean they aren't valid.

IMO, the author of the cited paper is absolutely wrong in saying that "Thus, the fact that quantum mechanics applies on all scales forces us to confront the theory’s deepest mysteries. We cannot simply write them off as mere details that matter only on the very smallest scales." Sure we can. If we couldn't write them off we wouldn't be able to build bridges or airplanes, forecast the weather, describe an ecology, or explain the evolution of humanity. Abstractions are incredibly useful and powerful tools.

 

[xts]

It really depends what phenomena are you interested in. In many situations single electron behaves like a 'snooker ball' (relativistic rather than Newtonian, however) - e.g. particle tracks seen in high energy experiments.

For other phenomena - e.g. (mentioned already by DH) bucket of superfluid liquid Helium - 10²³ atoms is still 'quantum'.

 

[Fredrik]

It's not about size. It's about whether the system is sufficiently isolated from its environment. The more the system interacts with its environment (e.g. the surrounding air), the faster the quantum "weirdness" gets moved into that environment.

 

[Ken G]

IMO, the author of the cited paper is absolutely wrong in saying that "Thus, the fact that quantum mechanics applies on all scales forces us to confront the theory’s deepest mysteries. We cannot simply write them off as mere details that matter only on the very smallest scales." Sure we can. If we couldn't write them off we wouldn't be able to build bridges or airplanes, forecast the weather, describe an ecology, or explain the evolution of humanity. Abstractions are incredibly useful and powerful tools.

I agree with the point you're making about abstractions, and also this point here. I think what you are saying, if I understand you correctly, is a kind of "antidote for overzealous rationalism." The demand of the rationalist is the "theory of everything", the conceptual milieu that explains all mysteries via complete unification. I believe the article is written from that perspective, saying that if you want to understand everything, even at the macro level, you need to understand everything at the micro level. That case is made well, but the case that is not made at all is whether or not trying to understand everything at either level is really anything more than a pipe dream. If our goal is instead to generate a hierarchy of abstractions, then we get to pick and choose which level of understanding we seek, starting from the mindset that none of the levels can give us the complete picture. Maybe the closest we get to the complete picture is a capacity to maneuver among those levels, rather than complete mastery of the most fundamental level currently available.