Physics is the natural science that studies matter, motion, energy and force. It also studies the basic interactions between the fundamental constituents of the universe. Its subjects range in scale from subatomic particles to the large-scale structure of the cosmos. Physicists usually state their laws in mathematical form and test them by experiment and observation. Physics is often called the most fundamental of the natural sciences. Its principles underlie chemistry, large parts of biology, the Earth sciences and engineering. Many technologies of the modern world come from it, including electric power, electronics, lasers and nuclear energy. The word comes from the Greek physikē (epistēmē), "knowledge of nature", which derives from physis, "nature".
Origins and early history
People have studied regular patterns in nature since prehistoric times, most visibly the motions of the Sun, Moon and stars. In ancient Greece, philosophers tried to explain natural change through reasoning. Democritus proposed that matter is made of indivisible atoms. Aristotle developed a broad theory of motion and the elements that shaped Western and Islamic thought for nearly two thousand years. Archimedes worked out quantitative principles of levers and buoyancy, which were early examples of mathematical physics. Scholars in the medieval Islamic world, including those working under the Abbasid Caliphate, preserved and extended Greek learning. Ibn al-Haytham wrote the Book of Optics in the early 11th century, which joined experiment with geometrical analysis of light and vision. Ancient and medieval China also recorded observations in optics, magnetism and mechanics. The magnetic compass is one of its best-known results.
The Scientific Revolution and classical physics
Physics took its modern form during the Scientific Revolution of the 16th and 17th centuries. Nicolaus Copernicus proposed a heliocentric model of the solar system. Johannes Kepler derived empirical laws of planetary motion. Galileo Galilei combined controlled experiments with mathematical description and pointed the telescope at the sky. His approach helped establish the scientific method. Isaac Newton published his Principia in 1687. It set out three laws of motion and a law of universal gravitation, and it explained falling bodies on Earth and the orbits of planets with one set of principles. Newton and Gottfried Wilhelm Leibniz independently developed calculus, which became the main mathematical language of physics.
In the 18th and 19th centuries, mathematicians such as Euler, Lagrange and Hamilton reformulated classical mechanics in more general terms. Partly driven by the steam engine and the Industrial Revolution, scientists developed thermodynamics, the science of heat, work and energy conversion. Experiments by Michael Faraday and others revealed deep links between electricity and magnetism. In the 1860s, James Clerk Maxwell unified these phenomena in his equations of electromagnetism and showed that light is an electromagnetic wave. Ludwig Boltzmann and others founded statistical mechanics, which explains thermodynamic behaviour through the statistics of very large numbers of particles. The physics built up by about 1900 is called classical physics. It describes slow-moving, everyday-scale objects very accurately, along with heat, sound, electricity, magnetism and light.
Modern physics
Around 1900, several experimental results could not be explained by classical theory. These included blackbody radiation, the photoelectric effect and the failure to detect the "luminiferous ether". They led to two new frameworks. In 1905, Albert Einstein introduced special relativity, which changed the concepts of space and time and showed that mass and energy are equivalent. His general relativity (1915) describes gravity as the curvature of spacetime. Together these are known as the theory of relativity. Max Planck's quantum hypothesis of 1900 began a line of work that, through Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Paul Dirac and others, produced quantum mechanics in the 1920s. Quantum mechanics describes the behaviour of the atom, the electron and light at microscopic scales.
During the 20th century, Ernest Rutherford discovered the atomic nucleus. This opened the field of nuclear physics, which led to nuclear fission, nuclear power and nuclear weapons. Quantum field theory combined quantum mechanics with special relativity. It eventually produced the Standard Model of particle physics, which describes the known elementary particles and three of the four fundamental forces. The Higgs boson was the last particle the Standard Model predicted, and it was discovered at CERN's Large Hadron Collider in 2012. In 2015, the LIGO observatories directly detected gravitational waves, confirming a long-standing prediction of general relativity.
Major branches
Physics is usually divided by subject matter and scale:
- Classical mechanics: the motion of bodies under forces, including fluid mechanics and acoustics.
- Thermodynamics and statistical mechanics: heat, temperature, entropy and the collective behaviour of many particles.
- Electromagnetism and optics: electric and magnetic fields, electromagnetic radiation and light.
- Relativity: high speeds, strong gravity and the structure of spacetime.
- Quantum mechanics: matter and radiation at atomic and subatomic scales.
- Atomic, molecular and optical physics: atoms, molecules and their interaction with light.
- Nuclear and particle physics: the nucleus and elementary particles such as the quark, and the fundamental interactions between them.
- Condensed matter physics: solids and liquids, including superconductors, magnets and the semiconductor materials that underpin modern electronics. It is the largest subfield by number of researchers.
- Astrophysics and cosmology: stars, galaxies and the origin and evolution of the universe.
Interdisciplinary fields include biophysics, geophysics, chemical physics, medical physics and materials science.
Methods
Physics advances through the interplay of theory and experiment. Theoretical physicists build mathematical models that make quantitative predictions. Experimental physicists design measurements to test those predictions, often reaching extreme precision. Since the late 20th century, computational physics has become a third pillar of the field. It uses computers to simulate systems that are too complex to solve analytically. Physical theories are judged by their predictive power and range of validity. Older theories such as Newtonian mechanics remain valid as approximations within their domains, even after more general theories have replaced them.
Open problems
Major questions remain unanswered. General relativity and quantum mechanics have not been combined into a consistent theory of quantum gravity. Cosmological observations suggest that ordinary matter makes up only about 5 percent of the universe's energy content. The rest is attributed to dark matter and dark energy, whose nature is unknown. Other open problems include the origin of neutrino masses, the imbalance between matter and antimatter, and a full understanding of turbulence and high-temperature superconductivity.
References
- History of physicsen.wikipedia.org
- Physicsbritannica.com
- 1.1 Physics: Definitions and Applicationstexasgateway.org