Erwin Schrödinger, a renowned physicist, won a Nobel Prize in 1933. By doing so, he proved that Quantum theory had been misinterpreted with the help of a cat and a box. This experiment illustrates an apparent paradox of quantum superposition. It can be suggested that a hypothetical cat may be considered simultaneously both alive and dead as a result of being linked to a random subatomic event that may or may not occur inside a box.

The experimental evidence shows conclusively that the particles of microscopic systems move according to the laws of some form of wave motion, and not according to the Newtonian laws of motion obeyed by the particles of macroscopic systems. Thus a microscopic particle acts as if certain aspects of its behaviour are administered by the behaviour of an associated de Broglie wave, or wave function. The experiments considered dealt only with simple cases such as free particles, or simple harmonic oscillators that can be analysed with simple procedures which involve direct applications of the de Broglie postulate, Planck's postulate. However, we certainly want to treat the more complicated cases that occur in nature because they are intriguing and significant.

To achieve this, we must have a more general procedure that can be utilised to treat the behaviour of the particles of any microscopic system. Schrödinger’s theory of quantum mechanics provides us with such a procedure.

The theory specifies the laws of wave motion that the particles of any microscopic system obey. This is done by specifying, for each system, the equation that controls the behaviour of the wave function, and also by specifying the connection between the behaviour of the wave function and the behaviour of the particle. The theory is an extension of the de Broglie postulate. Furthermore, there is a close relationship between it and Newton's theory of the motion of particles in macroscopic systems. Schrödinger’s theory is a generalization that includes Newton's theory as a special case (in the macroscopic limit), much as Einstein's theory of relativity is a generalization that includes Newton's theory as a special case (in the low-velocity limit).

We develop the essential points of the Schrödinger theory and use them to treat several important microscopic systems. For instance, we shall use the theory to obtain a detailed understanding of the properties of atoms. These properties form the basis of much chemistry and solid-state physics, and they are closely related to the properties of nuclei.

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