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Physics
Physics (from Greek from φυσικός (phusikos):
natural, from φύσις (fysis): Nature) is the science of Nature
in the broadest sense.
Physicists study the behaviour and interactions of
matter and
radiation. 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.
Below is an overview of the major subfields and concepts in
physics, followed by a brief outline of the history of physics and its
subfields.
Note: The following is a cursory overview of the
development of physics. For a more detailed history, please refer to the main
article on this subject,
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 and there are
anachronisms: 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,
Schrödinger 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 at
Trinity site, near
Alamogordo,
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.
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 computers.
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.
See
unsolved problems in physics for a fuller treatment of this subject.
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