Modern physics

Modern physics is the body of physical theories developed since the early twentieth century, especially quantum mechanics and relativity. It describes phenomena that classical physics cannot adequately explain, including atomic behavior and spacetime effects.

Quantum mechanics: A theory describing matter and energy through quantized states, probabilities, and measurement. Its account of atomic and subatomic phenomena forms one central pillar of modern physics.
Wave function: A mathematical object encoding the quantum state and probabilities of measurement outcomes. It is the central tool for predicting quantum-system behavior.
Black-body radiation: Electromagnetic radiation emitted by an idealized object in thermal equilibrium. Its spectrum challenged classical predictions and prompted Planck’s quantum hypothesis.
Semiconductor: A material whose electrical conductivity lies between that of a conductor and an insulator and can be controlled. Quantum theory of solids explains the electronic behavior used in modern devices.
Quantum gravity: A proposed framework for describing gravity within a quantum theory. No experimentally confirmed theory yet reconciles quantum mechanics with general relativity.
Special relativity: A theory relating space and time for inertial observers, with the speed of light invariant. It replaces classical mechanics when speeds approach the speed of light.
Schrödinger equation: An equation governing how a nonrelativistic quantum state changes over time. It gives quantum mechanics a precise rule for time evolution.
Photoelectric effect: The emission of electrons from a material when it absorbs light of sufficient frequency. Einstein’s explanation used light quanta, strengthening the case for quantum theory.
Laser: A device that produces coherent light through stimulated emission of radiation. Its operation depends on quantized energy levels and stimulated emission.
String theory: A theoretical framework in which fundamental entities are one-dimensional strings rather than point particles. It is one candidate approach to unifying quantum theory and gravity.

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