Summary
Quantum mechanics is the study of matter and its interactions with energy on the scale of atomic and subatomic particles. By contrast, classical physics explains matter and energy only on a scale familiar to human experience, including the behavior of astronomical bodies such as the moon. Classical physics is still used in much of modern science and technology. However, towards the end of the 19th century, scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. The desire to resolve inconsistencies between observed phenomena and classical theory led to a revolution in physics, a shift in the original scientific paradigm: the development of quantum mechanics. Many aspects of quantum mechanics are counterintuitive and can seem paradoxical because they describe behavior quite different from that seen at larger scales. In the words of quantum physicist Richard Feynman, quantum mechanics deals with "nature as She is—absurd". One example of this is the uncertainty principle applied to particles, which implies that the more closely one pins down one measurement on a particle (such as the position of an electron), the less accurate another complementary measurement pertaining to the same particle (such as its speed) must become. The position and speed of a particle cannot both be measured with arbitrary precision, regardless of the quality of the measuring instruments. Another example is entanglement. In certain circumstances, two particles with a shared history may become mutually 'entangled', in which case a measurement made on one particle (such as an electron that is measured to have spin up) will provide full information about the outcome of a later equivalent measurement on the other particle (that the other will be found to have spin down). This applies even though the particles may be so far apart that it is impossible for the result of the first measurement to have been transmitted to the second particle before the second measurement takes place.
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