Introduction to Quantum mechanics is an introductory version of Quantum mechanics. It describes Quantum mechanics as the set of scientific principles describing the behavior of energy and matter on the atomic and subatomatic scale. Much like the universe on the large and very vast scale (i.e., general relativity), so the universe on the small scale (i.e., quantum mechanics) does not neatly conform to the rules of classical physics. As such, it presents a set of rules that is counterintuitive and difficult to understand for the human mind, as humans are accustomed to the world on a scale dominated by classical physics. In other words, as stated by Richard Feynman: quantum mechanics deals with "Nature as She is—absurd." (see biography below) [1]
Many elementary parts of the universe, such as photons (discrete units of light) have some behaviours which resemble a particle but other behaviours that resemble a wave. Radiators of photons such as neon lights have spectra, but the spectra are chopped up instead of being continuous. The energies carried by photons form a discontinuous and colour coded series. The energies, the colours, and the spectral intensities of electromagnetic radiation produced are all interconnected by laws. But the same laws ordain that the more closely one pins down one measure (such as the position of a particle) the more wildly another measure relating to the same thing (such as momentum) must fluctuate. Put another way, measuring position first and then measuring momentum is not the same as measuring momentum first and then measuring position. Even more disconcerting, particles can be created as twins and therefore as entangled entities -- which means that doing something that pins down one characteristic of one particle will determine something about its entangled twin even if it is millions and millions of miles away.
Around the turn of the twentieth century, it became clear that classical physics was unable to explain several phenomena. As Thomas Kuhn explains in his analysis of the philosophy of science, The Structure of Scientific Revolutions, understanding these limitations of classical physics led to a revolution in physics and resulted in a shift of the original scientific paradigm: the development of quantum mechanics in the early decades of the last century.[2]