Classical from Quantum

To start with Nature does not have “two physics” . The underlying substrate of all physics is quantum mechanical. Depending on the dimensional regimes of measurement different mathematcal frameworks have been validated to work to the necessary accuracies.

The term ” two physics” should be modified to “different physical frameworks for observational data”, because they are not even two, they are many more.

classical mechanics to quantum mechanics

classical electrodynamics to quantum electrodynamics.

classical statistical mechanics to quantum statistical mechanics

and a number more esoteric formulations for special cases like fluids and superfluids, plasma, etc.

In some cases it is simple or doable to show how the classical emerges from the quantum level and in some cases harder or not yet done.

What an educated person with some knowledge of modern physics should keep in mind is that the underlying foundation of all is quantum mechanical. Heisenberg’s Uncertainty Principle defines the regions of validity


When distances times momenta are large enough, the quantum mechanical framework is irrelevant and the modeling of the data with the various classical mathematical tools is valid. In regions where the numbers are small, quantum mechanical models have to be used.

One has to keep in mind that all physical theories are mathematical models of our data, and are valid as long as they predict new phenomena and are not invalidated by any. Thermodynamics emerges from statistical mechanics and quantum statistical mechanics, the regions of overlap are defined and known. This does not mean that there are two physics. Just that there are two physical theories appropriate for the region of validity of the variables studied.

Currently physicists believe that the ultimate level is the quantum mechanical level because of various very strong consistency laws that would be violated if there existed a level below the QM level. All other theories are emergent and based on the QM substructure and laws.


In the macroscopic world, we apply a force and there is an “equal” reaction force. What’s actually going on here, though? Using the classic pushing against a wall example, the force being exterted upon the wall is causing compression in the material at the point of contact. Any compression will cause an artificial reduction in the gap between the material’s atoms. This in term will lead to a rise in atomic repulsive forces, which push back against the incoming force.The wall seems to be fighting back. There is no magical reflection of the force in Newton’s 3rd law, it’s all down to the atomic composition and state of the material in question. The curious part of this is that energy will be lost as heat through this interaction (work is being done to compress the material, after all), which kind of spoils the wonderful simplicity that every force produces an equal and opposite force. On a measurable scale, yes Newton’s 3rd works but delve deeper and it’s origins are in atomic forces and quantum behaviour.



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