Quantum mechanics is the branch of physics describing the behavior of matter and energy at atomic and subatomic scales, where classical physics breaks
down. Central to the theory is the idea that particles like electrons don’t have
definite positions or momenta until measured, but instead exist in superpositions of possible states described by a wavefunction. Measurement causes this
wavefunction to ”collapse” into one definite outcome, with probabilities governed by the Born rule. Other defining features include quantization of energy
levels (as in the discrete orbitals of an atom), wave-particle duality, and entanglement, where two particles can share correlated states even when separated by
large distances. Quantum mechanics underlies much of modern technology, from
semiconductors and lasers to more recent applications like quantum computing.
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