Editing Modern physics (section)

{{Short description|Physics developed since 1901}}
{{Use dmy dates|date=August 2021}}
{{merge|Classical physics|target=Classical and modern physics|discuss=Wikipedia_talk:WikiProject_Physics#Classical_and_modern_physics|date=February 2025}}
[[File:Modernphysicsfields.svg|thumb|430px|[[Classical physics]] is usually concerned with everyday conditions: speeds are much lower than the [[speed of light]], sizes are much greater than that of atoms, yet very small in astronomical terms. Modern physics, however, is concerned with high velocities, small distances, and very large energies.]]
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'''Modern physics''' is a branch of [[physics]] that developed in the early 20th century and onward or branches greatly influenced by early 20th century physics. Notable branches of modern physics include [[quantum mechanics]], [[special relativity]], and [[general relativity]].

''[[Classical physics]]'' is typically concerned with everyday conditions: speeds are much lower than the [[speed 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 the [[speed of light]] (special relativity), small distances comparable to the [[atomic 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 at [[human scale]]. While quantum mechanics is compatible with special relativity (See: [[Relativistic quantum mechanics]]), one of the [[List of unsolved problems in physics|unsolved problems in physics]] is the unification of quantum mechanics and general relativity, which the [[Standard Model]] of [[particle 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 term ''modern physics'' means up-to-date physics. In this sense, a significant portion of so-called ''[[classical physics]]'' is modern.<ref name=richtmyer>{{Cite book |author1= F. K. Richtmyer |author1-link= Floyd K. Richtmyer |author2=E. H. Kennard |author3= T. Lauristen |year=1955 |title=Introduction to Modern Physics |page=1 |edition=5th |publisher=[[McGraw-Hill]] |location=New York |lccn= 55006862}}</ref> However, since roughly 1890, new discoveries have caused significant [[paradigm shift]]s:<ref name=richtmyer /> especially the advent of [[quantum mechanics]] (QM) and [[Theory of relativity|relativity]] (ER). Physics that incorporates elements of either QM or ER (or both) is said to be ''modern physics''. It is in this latter sense that the term is generally used.<ref name=richtmyer />

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 at [[room temperature]], most phenomena will involve the (classical) [[Maxwell–Boltzmann distribution]]. However, near [[absolute zero]], the Maxwell–Boltzmann distribution fails to account for the observed behavior of the gas, and the (modern) [[Fermi–Dirac distribution|Fermi–Dirac]] or [[Bose–Einstein distribution|Bose–Einstein]] distributions have to be used instead.

{{multiple image|total_width=300
|image1 = Albert Einstein (Nobel).png
|image2 = Max Planck 1933.jpg
|footer = German physicists [[Albert Einstein]] (1879–1955), founder of the [[theory of relativity]], and [[Max Planck]] (1858–1947), who postulated the existence of [[Quantum|quanta]] leading to the development of [[quantum mechanics|quantum theory]].<ref>{{cite web | url=https://www.esa.int/Science_Exploration/Space_Science/Planck/Max_Planck_Originator_of_quantum_theory | title=Max Planck: Originator of quantum theory }}</ref><ref>{{cite web | url=https://www.britannica.com/biography/Max-Planck | title=Max Planck &#124; Biography, Discoveries, & Quantum Theory &#124; Britannica }}</ref>}}

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 a [[limit (mathematics)|limit]], or by making an [[approximation theory|approximation]]). When doing so, the result is called the ''[[classical limit]]''.

[[File:Black body.svg|thumb|250px|Classical physics ([[Rayleigh–Jeans law]], black line) failed to explain [[black-body radiation]] – the so-called [[ultraviolet catastrophe]]. The quantum description ([[Planck's law]], colored lines) is said to be ''modern physics''.]]