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The history of quantum mechanics. This topic is considered to be one of the most difficult topics in Physics but we will try to understand it from the basics.
There is no doubt that the most outstanding development in modern science was the conception of quantum mechanics. It showed, better than anything else, the human capacity to comprehend the fundamental principles that underlie the world in which we live- even when these principles run contrary to our experience in dealing with our everyday environment. The French philosopher-mathematician Henri Poincaré said, “It is hardly necessary to point out how much quantum theory deviates from everything that one has imagined until now; it is, without doubt, the greatest and deepest revolution to which natural philosophy has been subjected since Newton.”

Much happened in physics between the time of Newton and the time of quantum mechanics. The discoveries and insights over the last three centuries characteristic feature: seemingly unconnected phenomena turned out to be manifestations of the same fundamental principle. It was a period of unification of disparate fields of experience. Here are some of the most important steps.

Newton showed that the motion of the planets is governed by the same law as the free fall of an object on earth. Thus, the unified terrestrial and celestial mechanics. In contrast to the belief of the ancients, he showed that the world of the earth and the heavens are governed by the same laws.

Scientists in earlier days believed that heat was some peculiar substance called caloric, which flowed from a hot object to a colder one. Physicists in the nineteenth century recognized that heat is the random motion or random vibration of the constituents of matter. Thus, thermodynamics and mechanics were unified. This feat is connected with the names of J.B. Mayer, B. Rumford, R.E. Clausius, L. Boltzmann, and J.W. Gibbs.

For a long time, the phenomena of electricity, magnetism, and light appeared to be unconnected. In the first half of the nineteenth century, one of the greatest unification of physics took place. Faraday and Maxwell, together with many others, were able to show that all three phenomena are manifestations of the electromagnetic field. And so the field concept entered into physics. The simplest example is the electric field of an electric charge that exerts a force on another charge when the latter falls within its range. An electric current produces a magnetic field that exerts a force on magnetic materials. Such fields may even propagate through space independently of any charges or magnets, in the form of electromagnetic waves, of which visible light is one example. The field concept is less directly connected to our everyday experience than the concept of a particle, but it can easily be realized by our senses. For example, if one feels the attraction of a piece of iron by a magnet, one obtains the immediate impression that something is surrounding the magnet that acts upon the iron. Finally, Einstein unified space, time, and gravity in his special and general theories of relativity.

Quantum mechanics also united two branches of science: physics and chemistry. But it did much more. In previous great developments in physics, fundamental concepts were not too different from those of our everyday experiences, such as particle, position, speed, mass, force, energy, and even field. We often refer to those concepts as classical. The world of atoms cannot be described and understood with those concepts. For atoms and molecules, the ideas and concepts formed in dealing with the objects in our immediate environment no longer suffice. Surprising forms of behavior were observed that not only needed a different language but required new concepts to understand the properties of atoms.

A small group of people conceived of and formulated these new ideas in the middle twenties of this century. The most important among them were W. Heisenberg, a German; E. Schrödinger, an Austrian, P.A.M. Dirac, an Englishman; W. Pauli, another Austrian; and M. Born, another German. They worked at different places, but the center of activities was in Copenhagen, where they met frequently under the leadership of the great Niels Bohr. Bohr was the ideal leader of such a group. Older than most of the others, who were then in their twenties or early thirties, he contributed enormously to the conception of the new ideas through his constant questioning, by his criticism, and by his enthusiasm. The crowd that assembled around him was a group of devoted forward-looking people who, free of the bonds of convention, attacked the deepest riddles of nature with a spirit of joy that can hardly be described. That joy of insight is a sense of involvement and awe, an elated state of mind akin to what you feel on top of a mountain after a hard climb, at the shore of the blue sea, or when you hear a great work of music. It comes not only after personal achievement but also after finally understanding an important new insight gained by the work of others. For every real scientist, it is great compensation for the hard work and trouble he must endure.

The quantum revolution changed our old concepts of reality in many respects. This drama is in five parts.
The first part will be a prologue in which we will discuss the riddles of Physics. The second part will have discussions on the famous wave-particle duality. In the third, we will know how this duality miraculously solved the riddles discussed in part I. The fourth part will be concerned with the significance of wave-particle duality and some new facts. Fifth and the last part will have the quantum ladder…i.e. the previous and future developments of quantum mechanics.

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