Physics
Physics is a natural science that seeks quantitative explanations of matter, energy, motion and their interactions, from subatomic particles to the universe. It combines observation and experiment with mathematical models, testing predictions and revising explanations when evidence requires. Classical physics describes many everyday systems; quantum theory and relativity extend the framework to very small scales, high speeds and strong gravity, while a complete unification of known interactions remains unresolved.
Scope and central questions
Physics investigates regularities in the physical world: how objects move, how forces and fields act, how energy changes form, and how matter is structured. Its reach spans scales. Mechanics can describe a falling object or a planet’s orbit; atomic and particle physics examine constituents too small to see directly; astrophysics and cosmology apply physical ideas to stars and the universe. The boundaries between physics and other sciences are porous: physical methods are used in chemistry, materials science, biology, Earth science and engineering.
Evidence, models and measurement
A physical explanation normally connects a model or theory to observations that can test its predictions. Experiments measure outcomes under specified conditions; agreement across repeated tests supports a theory within its tested range, while a reliable counterexample may require a revision or a narrower account of where it applies. In scientific usage, a theory is an evidence-supported explanatory framework, not simply a guess; a law often summarizes a regular pattern, and neither term guarantees that a statement can never be refined. Mathematics makes many relationships precise, while measurement gives them empirical meaning. The International System of Units (SI) provides an internationally agreed basis for expressing measurements; its seven base units are defined through fixed values of physical constants.
Major frameworks and fields
Classical mechanics studies motion and forces and remains highly effective for many systems moving far below the speed of light. Thermodynamics describes heat, work and energy transformations, while statistical mechanics connects large-scale properties such as temperature to the behavior of many microscopic constituents. Electromagnetism treats electric and magnetic phenomena and is closely linked to optics, the study of light. Quantum mechanics describes physical behavior at atomic and subatomic scales, where predictions are often probabilistic. Relativity revises concepts of space, time and gravity, especially at very high speeds or in strong gravitational fields. Other major fields include condensed-matter, nuclear, particle, plasma, atomic, biological and astrophysical physics; the categories often overlap.
Unification, limits and applications
A recurring aim is to explain different observations with a small number of connected principles. The Standard Model of particle physics describes elementary matter particles and three fundamental interactions—the electromagnetic, weak and strong forces—but does not incorporate gravity. General relativity describes gravity on large scales, and a fully tested framework that unites it with quantum physics has not been established. Physics is therefore both a mature body of knowledge and an active research enterprise. Its results underpin technologies and methods used in electronics, communications, energy systems and medical diagnosis and treatment, while new experiments continue to probe the limits of existing theories.
Key facts
- Subject
- Physical matter, motion, energy, fields and interactions across scales.
- Core approach
- Use mathematical models and theories to generate predictions, then compare them with measured evidence.
- Major frameworks
- Classical mechanics, thermodynamics and statistical mechanics, electromagnetism, quantum mechanics and relativity.
- Open unification problem
- The Standard Model does not include gravity, and a complete, experimentally established unification with general relativity remains unavailable.
- Shared measurement system
- The SI has seven base units defined using fixed values of physical constants.
Timeline
1687
Isaac Newton published the Principia, setting out laws of motion and universal gravitation that became foundations of classical mechanics.
1900
Max Planck introduced a quantum hypothesis in work on thermal radiation, an early step in the development of quantum theory.
1905 and 1915
Albert Einstein developed special relativity in 1905 and completed the general theory of relativity in 1915.
Early 1970s
The Standard Model of particle physics took shape as a framework for elementary particles and three fundamental interactions.
20 May 2019
The revised SI took effect, defining the base units through fixed numerical values of seven defining constants.
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References
- 1.Physics Definition, Types, Topics, Importance, & Facts
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 12 October 2026
Supports: The scope of physics, its major branches, the distinction between classical and modern physics, and the complementary roles of experiment and theory.
- 2.1.1 The Scope and Scale of Physics
OpenStax, Rice University · other · reliability: high · retrieved 12 October 2026
Supports: The range of physical scales, scientific models and theories, experimental testing, and applications of physics in biology and medicine.
- 3.The Standard Model
CERN · other · reliability: high · retrieved 12 October 2026
Supports: The particles and interactions described by the Standard Model, its development, and the fact that it remains incomplete.
- 4.SI Units
National Institute of Standards and Technology (NIST), U.S. Department of Commerce · other · reliability: high · retrieved 12 October 2026
Supports: The seven SI base units, their relationship to derived units, and the system’s use in scientific and technical work.
- 5.Definitions of SI Base Units
National Institute of Standards and Technology (NIST), U.S. Department of Commerce · other · reliability: high · retrieved 12 October 2026
Supports: The present definitions of SI base units in terms of fixed values of defining constants and the 2019 revision.
- 6.Principia
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 12 October 2026
Supports: Newton’s 1687 work and its role in presenting laws of motion and universal gravitation.
- 7.Quantum mechanics Definition, Development, & Equations
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 12 October 2026
Supports: The historical beginnings of quantum theory, including Planck’s work on radiation in 1900.
- 8.Relativity Definition, Equations, & Facts
Encyclopaedia Britannica · encyclopedia · reliability: high · retrieved 12 October 2026
Supports: Einstein’s theories of special relativity (1905) and general relativity (1915).
- 9.Resolution 1 of the 26th CGPM (2018): On the revision of the International System of Units (SI)
International Bureau of Weights and Measures (BIPM) · other · reliability: high · retrieved 12 October 2026
Supports: The revised SI took effect on 20 May 2019 and is based on seven defining constants.
References are suggested by the research engine and may be incomplete. Unverified URLs are omitted rather than guessed.
Article history
- Version 112 October 2026 · EditorCreated by Zapia