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Physics is the science of Nature in the broadest sense. Physicists study the
behaviour and interactions of matter and energy, which are referred to as
physical phenomena. Theories of physics are generally expressed as
mathematical relations. Well-established theories are often referred to as
physical laws or laws of physics; however, like all scientific theories,
they are ultimately provisional.

Physics is very closely related to the other natural sciences, particularly
chemistry, the science of molecules and the chemical compounds that they
form in bulk. Chemistry draws on many fields of physics, particularly
quantum mechanics, thermodynamics and electromagnetism. However, chemical
phenomena are sufficiently varied and complex that chemistry is usually
regarded as a separate discipline.

A Brief History of Physics

Note: The following is a cursory overview of the development of physics. For
a more detailed history, please refer to History of physics.

Since antiquity, people have tried to understand the behavior of matter: why
unsupported objects drop to the ground, why different materials have
different properties, and so forth. Also a mystery was the character of the
universe, such as the form of the Earth and the behavior of celestial
objects such as the Sun and the Moon. Several theories were proposed, most
of them were wrong. These theories were largely couched in philosophical
terms, and never verified by systematic experimental testing. There were
exceptions: for example, the Greek thinker Archimedes derived many correct
quantitative descriptions of mechanics and hydrostatics.

During the late 16th century, Galileo pioneered the use of experiment to
validate physical theories, which is the key idea in the scientific method.
Galileo formulated and successfully tested several results in dynamics, in
particular the Law of Inertia. In 1687, Newton published the Principia
Mathematica, detailing two comprehensive and successful physical theories:
Newton's laws of motion, from which arise classical mechanics; and Newton's
Law of Gravitation, which describes the fundamental force of gravity. Both
theories agreed well with experiment. Classical mechanics would be
exhaustively extended by Lagrange, Hamilton, and others, who produced new
formulations, principles, and results. The Law of Gravitation initiated the
field of astrophysics, which describes astronomical phenomena using physical theories.

From the 18th century onwards, thermodynamics was developed by Boyle, Young,
and many others. In 1733, Bernoulli used statistical arguments with
classical mechanics to derive thermodynamic results, initiating the field of
statistical mechanics. In 1798, Thompson demonstrated the conversion of
mechanical work into heat, and in 1847 Joule stated the law of conservation
of energy, in the form of heat as well as mechanical energy.

The behavior of electricity and magnetism was studied by Faraday, Ohm, and
others. In 1855, Maxwell unified the two phenomena into a single theory of
electromagnetism, described by Maxwell's equations. A prediction of this
theory was that light is an electromagnetic wave.

In 1895, Roentgen discovered X-rays, which turned out to be high-frequency
electromagnetic radiation. Radioactivity was discovered in 1896 by Henri
Becquerel, and further studied by Pierre Curie and Marie Curie and others.
This initiated the field of nuclear physics.

In 1897, Thomson discovered the electron, the elementary particle which
carries electrical current in circuits. In 1904, he proposed the first model
of the atom, known as the plum pudding model. (The existence of the atom had
been proposed in 1808 by Dalton.)

In 1905, Einstein formulated the theory of special relativity, unifying
space and time into a single entity, spacetime. Relativity prescribes a
different transformation between reference frames than classical mechanics;
this necessitated the development of relativistic mechanics as a replacement
for classical mechanics. In the regime of low (relative) velocities, the two
theories agree. In 1915, Einstein extended special relativity to explain
gravity with the general theory of relativity, which replaces Newton's law
of gravitation. In the regime of low masses and energies, the two theories agree.

In 1911, Rutherford deduced from scattering experiments the existence of a
compact atomic nucleus, with positively charged constituents dubbed protons.
Neutrons, the neutral nuclear constituents, were discovered in 1932 by Chadwick.

Beginning in 1900, Planck, Einstein, Bohr, and others developed quantum
theories to explain various anomalous experimental results by introducing
discrete energy levels. In 1925, Heisenberg and 1926, Schrdinger and Dirac
formulated quantum mechanics, which explained the preceding quantum
theories. In quantum mechanics, the outcomes of physical measurements are
inherently probabilistic; the theory describes the calculation of these
probabilities. It successfully describes the behavior of matter at small
distance scales.

Quantum mechanics also provided the theoretical tools for condensed matter
physics, which studies the physical behavior of solids and liquids,
including phenomena such as crystal structures, semiconductivity, and
superconductivity. The pioneers of condensed matter physics include Bloch,
who created a quantum mechanical description of the behavior of electrons in
crystal structures in 1928.

During World War II, research was conducted by each side into nuclear
physics, for the purpose of creating a nuclear bomb. The German effort, led
by Heisenberg, did not succeed, but the Allied Manhattan Project reached its
goal. In America, a team led by Fermi achieved the first man-made nuclear
chain reaction in 1942, and in 1945 the world's first nuclear explosive was
detonated in Alamagordo, New Mexico.

Quantum field theory was formulated in order to extend quantum mechanics to
be consistent with special relativity. It achieved its modern form in the
late 1940s with work by Feynman, Schwinger, Tomonaga, and Dyson. They
formulated the theory of quantum electrodynamics, which describes the
electromagnetic interaction.

Quantum field theory provided the framework for modern particle physics,
which studies fundamental forces and elementary particles. In 1954, Yang and
Mills developed a class of gauge theories, which provided the framework for
the Standard Model. The Standard Model, which was completed in the 1970s,
successfully describes almost all elementary particles observed to date.

Future directions

As of 2003, research is progressing on a large number of fields of physics.

In condensed matter physics, the biggest unsolved theoretical problem is the
explanation for high-temperature superconductivity. Strong efforts, largely
experimental, are being put into making workable spintronics and quantum

In particle physics, the first pieces of experimental evidence for physics
beyond the Standard Model have begun to appear. Foremost amongst this are
indications that neutrinos have non-zero mass. These experimental results
appear to have solved the long-standing solar neutrino problem in solar
physics. The physics of massive neutrinos is currently an area of active
theoretical and experimental research. In the next several years, particle
accelerators will begin probing energy scales in the TeV range, in which
experimentalists are hoping to find evidence for the higgs boson and
supersymmetric particles.

Theoretical attempts to unify quantum mechanics and general relativity into
a single theory of quantum gravity, a program ongoing for over half a
century, has yet to bear fruit. The current leading candidates are M-theory
and loop quantum gravity.

Many astronomical phenomena have yet to be explained, including the
existence of ultra-high energy cosmic rays and the anomalous rotation rates
of galaxies. Theories that have been proposed to resolve these problems
include doubly-special relativity, modified Newtonian dynamics, and the
existence of dark matter. In addition, the cosmological predictions of the
last several decades have been contradicted by recent evidence that the
expansion of the universe is accelerating.
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