Question

How should one REALISE Classical and Quantum ACTION.(Please explain both)
That is,what in PERFECT sense is its Physical or Mathematical interpretetion + intuition + need + use + origin?!

Answer 1

ORGIN /NEED OF QUANTUM MECHANICS

The discovery of quantum mechanics described the physical world in a way that was fundamentally new. By the early 1930s ,the application of quantum mechanics to problems involving nuclei, atoms, and matter in the solid state made it possible to understand a vast body of data and vital for any theory -led predictions of remarkable accuracy. Quantum mechanics has survived every experimental test thus far of even it's most unexpected conclusions such as photoelectric effect, black body radiation, stern gerlach experimentThe physicist Planck noticed a fatal flaw in our physics by demonstrating that the electron in orbit around the nucleus accelerates which means photons should be emitted. But, then the electron would lose energy and fall in to nucleus. To resolve this problem planck made an assumption that energy can only be transfered in a small packets called quanta.with this insight ,Planck's constant (h) introduced.

DIFFERENCE BETWEEN QUANTUM PHYSICS AND CLASSICAL PHYSICS

The fundamental difference between classical (Newtonian) physics and quantum physics lies in what they describe. In classical mechanics, the future history of a particle is completely determined by it's initial position and momentum together with the forces that act upon it. In quantum mechanics, the kind of certainty about the future characteristic of classical mechanics is impossible because the initial state of a particle cannot be established with sufficient accuracy. The quantities whose relationships quantum mechanics explores are probabilities.The quantity with which quantum mechanics is concerned is the wave function while wave function itself has no physical significance,the square of its absolute magnitude evaluated at a particular place at a particular time is proportional to the probability of finding the body there at that time.  For example, that the radius of the electrons orbit in a ground state hydrogen atom is always exactly 5.3*10 to the power -11meter,as the Bohr theory does, quantum mechanics states that this is the most probable radius. In a suitable experiment most trials will yield a different value, either large or smaller,but most likely to be found will be 5.3*10 to the power -11meter.

classical mechanics turns out to be an approximation of quantum mechanics. The certainties of classical mechanics are illusory, and their apparent agreement with experiment occurs because ordinary object consist of so many individual atoms that departures from average behavior are unnoticeable. Instead of two sets of physical principles,one for the macroworld and one for the microworld, there is only the single set included in quantum mechanics.

The quantisation of action as depicted in quantum mechanics differs sharply from classical mechanics which represented motion as smooth, continuous change. Quantisation limits the energy to be transfered to photons and resolves the UV catastrophe.