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Modern physics is a branch ofphysics that developed in the early 20th century and onward or branches greatly influenced by early 20th century physics. Notable branches of modern physics includequantum mechanics,special relativity, andgeneral relativity.
Classical physics is typically concerned with everyday conditions: speeds are much lower than thespeed of light, sizes are much greater than that of atoms, and energies are relatively small. Modern physics, however, is concerned with more extreme conditions, such as high velocities that are comparable to thespeed of light (special relativity), small distances comparable to theatomic radius (quantum mechanics), and very high energies (relativity). In general, quantum and relativistic effects are believed to exist across all scales, although these effects may be very small athuman scale. While quantum mechanics is compatible with special relativity (See:Relativistic quantum mechanics), one of theunsolved problems in physics is the unification of quantum mechanics and general relativity, which theStandard Model ofparticle physics currently cannot account for.
Modern physics is an effort to understand the underlying processes of the interactions of matter using the tools of science and engineering. In a literal sense, the termmodern physics means up-to-date physics. In this sense, a significant portion of so-calledclassical physics is modern.[1] However, since roughly 1890, new discoveries have caused significantparadigm shifts:[1] especially the advent ofquantum mechanics (QM) andrelativity (ER). Physics that incorporates elements of either QM or ER (or both) is said to bemodern physics. It is in this latter sense that the term is generally used.[1]
Modern physics is often encountered when dealing with extreme conditions. Quantum mechanical effects tend to appear when dealing with "lows" (low temperatures, small distances), while relativistic effects tend to appear when dealing with "highs" (high velocities, large distances), the "middles" being classical behavior. For example, when analyzing the behavior of a gas atroom temperature, most phenomena will involve the (classical)Maxwell–Boltzmann distribution. However, nearabsolute zero, the Maxwell–Boltzmann distribution fails to account for the observed behavior of the gas, and the (modern)Fermi–Dirac orBose–Einstein distributions have to be used instead.
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Very often, it is possible to find – or "retrieve" – the classical behavior from the modern description by analyzing the modern description at low speeds and large distances (by taking alimit, or by making anapproximation). When doing so, the result is called theclassical limit.
These are generally considered to be experiments regarded leading to the foundation of modern physics:
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