The theory of relativity describes the behaviour of large everyday objects in the world around us.At the level of atoms and sub-atomic particles objects behave very differently and quantum theory is an attempt to describe the behaviour of matter and energy at this sub atomic scale.

describes nature at ordinary (macroscopic) scale. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.

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Quantum mechanics (QM) is important because it plays a fundamental role in explaining how the world works. Physicists often says QM governs the behaviour of microscopic systems when in fact it governs the behaviour of all physical systems, regardless of their size.

QM tells us a lot about the structure of reality. For example, all physical systems exist in multiple versions. Information flows between different systems and under the circumstances we are familiar with in everyday life, the different versions sort themselves into approximately autonomous layers, each of which acts approximately like the world described by classical physics. The whole of physical reality in this context is called the multiverse. And information can flow in the multiverse in ways that can’t happen in classical physics, such as single particle interference and entanglement.

QM has some importance in technology as it is needed to design optical and electronic components, which often take advantage of distinctively quantum mechanical effects.

QM is also important for the theory of computation. According to QM it is possible to build a universal quantum computer that can simulate any physical system. This means we can test theories about physical systems by simulating them and then checking the results of the simulations against reality.

Source: A blog