Description of modern physics -
The term modern physics refers to the post-Newtonian conception of physics. Modern physics deals with the underlying structure of the smallest particles in nature as well as a rigorous understanding of the fundamental interaction of particles, understood as forces. Small velocities and large distances is usually the realm of classical physics.
Modern physics often involves extreme conditions; quantum effects usually involve distances comparable to atoms (roughly 10−9 m), while relativistic effects usually involve velocities comparable to the speed of light (roughly 108 m/s).
The term "modern physics" implies that classical descriptions of phenomena are lacking, and that an accurate, "modern", description of reality requires theories to incorporate elements of quantum mechanics or Einsteinian relativity, or both. In general, the term is used to refer to any branch of physics either developed in the early 20th century and onwards, or branches greatly influenced by early 20th century physics.
Modern
physics began in the early 20th century with the work of Max Planck in
quantum theory and Albert Einstein's theory of relativity. Classical
mechanics predicted a varying speed of light, which could not be
resolved with the constant speed predicted by Maxwell's equations of
electromagnetism; this discrepancy was corrected by Einstein's theory of
special relativity, which replaced classical mechanics for fast-moving
bodies and allowed for a constant speed of light. Black body radiation
provided another problem for classical physics, which was corrected
when Planck proposed that light comes in individual packets known as
photons; this, along with the photoelectric effect and a complete theory
predicting discrete energy levels of electron orbitals, led to the
theory of quantum mechanics taking over from classical physics at very
small scales.
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