Relation between Classical and Quantum Physics

Relation between Classical and Quantum Physics

To say that classical physics died around 1930 is a bit too dramatic. The physics of Newton and Maxwell still accurately and beautifully describes the macroscopic world. Knowing this, physicists developing quantum mechanics demanded that when applied to macroscopic systems, the new physics must reduce to the old physics. Thus, as the size of the system being studied increases, the quantum laws of motion must go over smoothly into those of Newtonian mechanics, and non-classical phenomena such as uncertainty and duality must become undetectable. Neils Bohr codified this requirement into his Correspondence Principle, the essential elements of which are sketched in Fig. 1.3. We shall return to this principle repeatedly, since the ability of quantum mechanics to satisfy it is an important theoretical justification of the new physics.

The experimental justification of quantum theory is that it works. The predictions, qualitative and quantitative, of quantum mechanics have been verified by a host of experiments on a diverse collection of systems. These verifications are of paramount importance to physics, for in some ways quantum theory is a fragile creation. It is an inferential theory, in which we devise a handful of postulates and from them derive equations, which can then be put to the test of experimental justification. If the equations pass this test, then we have confidence that the postulates, which cannot be directly tested, are correct. But it would take only one repeatable experiment whose results confounded the equations of quantum mechanics to bring the whole structure tumbling down. To date, the edifice stands. Quantum mechanics remains the only theory we have to explain how matter is constructed and how physical processes at the atomic level work.

The source:

Michael A. Morrison - Understanding Quantum Physics.

By. Fady Tarek