Physics deals with the combination of matter and energy.
It also deals with a wide variety of systems, about which theories have been developed that
are used by physicists.
In general, theories are experimentally tested numerous times before they are accepted as
correct as a description of Nature.
For instance, the theory of classical mechanics accurately describes the motion of objects,
provided they are much larger than atoms and moving at much less than the speed of light.
These theories continue to be areas of active research: for instance, a remarkable aspect
of classical mechanics known as chaos was discovered in the 20th century, three centuries
after the original formulation of classical mechanics by Isaac Newton.
These "central theories" are important tools for research in more specialized topics, and
any physicist, regardless of his or her specialization, is expected to be literate in them.
Classical mechanics Classical mechanics is a model of the physics
of forces acting upon bodies.
It is often referred to as "Newtonian mechanics" after Isaac Newton and his laws of motion.
It also includes classical approach as give by Hamiltonian and Lagrange methods.
Thermodynamics and statistical mechanics
The first chapter of The Feynman Lectures on Physics is about the existence of atoms,
which Feynman considered to be the most compact statement of physics, from which science could
easily result even if all other knowledge was lost.
By modeling matter as collections of hard spheres, it is possible to describe the kinetic
theory of gases, upon which classical thermodynamics is based.
Thermodynamics studies the effects of changes in temperature, pressure, and volume on physical
systems on the macroscopic scale, and the transfer of energy as heat.
Historically, thermodynamics developed out of the desire to increase the efficiency of
early steam engines.
The starting point for most thermodynamic considerations is the laws of thermodynamics,
which postulate that energy can be exchanged between physical systems as heat or work.
They also postulate the existence of a quantity named entropy, which can be defined for any
system.
In thermodynamics, interactions between large ensembles of objects are studied and categorized.
Central to this are the concepts of system and surroundings.
A system is composed of particles, whose average motions define its properties, which in turn
are related to one another through equations of state.
Properties can be combined to express internal energy and thermodynamic potentials, which
are useful for determining conditions for equilibrium and spontaneous processes.
Electromagnetism
Relativity
The special theory of relativity enjoys a relationship with electromagnetism and mechanics;
that is, the principle of relativity and the principle of stationary action in mechanics
can be used to derive Maxwell's equations, and vice versa.
The theory of special relativity was proposed in 1905 by Albert Einstein in his article
"On the Electrodynamics of Moving Bodies".
The title of the article refers to the fact that special relativity resolves an inconsistency
between Maxwell's equations and classical mechanics.
The theory is based on two postulates: that the mathematical forms of the laws of physics
are invariant in all inertial systems; and that the speed of light in a vacuum is constant
and independent of the source or observer.
Reconciling the two postulates requires a unification of space and time into the frame-dependent
concept of spacetime.
General relativity is the geometrical theory of gravitation published by Albert Einstein
in 1915/16.
It unifies special relativity, Newton's law of universal gravitation, and the insight
that gravitation can be described by the curvature of space and time.
In general relativity, the curvature of spacetime is produced by the energy of matter and radiation.
Quantum mechanics
Quantum mechanics is the branch of physics treating atomic and subatomic systems and
their interaction with radiation.
It is based on the observation that all forms of energy are released in discrete units or
bundles called "quanta".
Remarkably, quantum theory typically permits only probable or statistical calculation of
the observed features of subatomic particles, understood in terms of wave functions.
The Schrödinger equation plays the role in quantum mechanics that Newton's laws and conservation
of energy serve in classical mechanics—i.e., it predicts the future behavior of a dynamic
system—and is a wave equation that is used to solve for wavefunctions.
For example, the light, or electromagnetic radiation emitted or absorbed by an atom has
only certain frequencies, as can be seen from the line spectrum associated with the chemical
element represented by that atom.
The quantum theory shows that those frequencies correspond to definite energies of the light
quanta, or photons, and result from the fact that the electrons of the atom can have only
certain allowed energy values, or levels; when an electron changes from one allowed
level to another, a quantum of energy is emitted or absorbed whose frequency is directly proportional
to the energy difference between the two levels.
The photoelectric effect further confirmed the quantization of light.
In 1924, Louis de Broglie proposed that not only do light waves sometimes exhibit particle-like
properties, but particles may also exhibit wave-like properties.
Two different formulations of quantum mechanics were presented following de Broglie's suggestion.
The wave mechanics of Erwin Schrödinger involves the use of a mathematical entity, the wave
function, which is related to the probability of finding a particle at a given point in
space.
The matrix mechanics of Werner Heisenberg makes no mention of wave functions or similar
concepts but was shown to be mathematically equivalent to Schrödinger's theory.
A particularly important discovery of the quantum theory is the uncertainty principle,
enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy
of certain measurements; as a result, the assumption by earlier scientists that the
physical state of a system could be measured exactly and used to predict future states
had to be abandoned.
Quantum mechanics was combined with the theory of relativity in the formulation of Paul Dirac.
Other developments include quantum statistics, quantum electrodynamics, concerned with interactions
between charged particles and electromagnetic fields; and its generalization, quantum field
theory.
Interdisciplinary fields To the interdisciplinary fields, which define
partially sciences of their own, belong e.g. the
astrophysics, the physics in the universe, including the properties and interactions
of celestial bodies in astronomy.
biophysics, studying the physical interactions of biological processes.
chemical physics, the science of physical relations in chemistry.
econophysics, dealing with physical processes and their relations in the science of economy.
geophysics, the sciences of physical relations on our planet.
medical physics, the application of physics to prevention, diagnosis, and treatment.
physical chemistry, dealing with physical processes and their relations in the science
of physical chemistry.
Summary The table below lists the core theories along
with many of the concepts they employ.
References