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'''Physics''' is the branch of [[physical science]] that traditionally deals with [[matter]], [[energy]], [[force]], and [[motion]].  These concepts can be applied to virtually any area of the physical sciences; therefore, physics is often considered to be the most fundamental branch of science.  Indeed, according to [[reductionism|reductionist]] thought, all other branches of science are specialized subdivisions of physics.

'''Physics''' is the branch of [[physical science]] that traditionally deals with [[matter]], [[energy]], [[force]], and [[motion]].  These concepts can be applied to virtually any area of the physical sciences; therefore, physics is often considered to be the most fundamental branch of science.  Indeed, according to [[reductionism|reductionist]] thought, all other branches of science are specialized subdivisions of physics.

Physics can be broadly divided into two major categories:  [[classical physics]] and [[modern physics]]. Physicists generally take one of two approaches to their research: they either practice [[experimental physics]] or [[theoretical physics]].

Physics can be broadly divided into two major categories:  [[classical physics]] and modern physics. Physicists generally take one of two approaches to their research: they either practice experimental physics or theoretical physics.

== Classical and Modern Physics ==

== Classical and Modern Physics ==

[[Classical physics]] generally encompasses all areas of physics that were well-understood by the end of the 19th century (i.e., before the events that led to the advent of the theory of relativity and quantum mechanics).  It is applicable to problems on an "everyday" scale; that is, situations in which energies are large enough to permit one to neglect quantum effects, but small enough to neglect relativistic effects.  Areas of study within classical physics include

[[Classical physics]] generally encompasses all areas of physics that were well-understood by the end of the 19th century (i.e., before the events that led to the advent of the theory of relativity and quantum mechanics).  It is applicable to problems on an "everyday" scale; that is, situations in which energies are large enough to permit one to neglect quantum effects, but small enough to neglect relativistic effects.  Areas of study within classical physics include

*[[Mechanics]], the study of forces acting on bodies and the motion of them.

*[[Mechanics]], the study of [[force|forces]] acting on bodies and the motion of them

*[[Thermodynamics]], the study of energy, heat, work, entropy and its transformations.

*[[Thermodynamics]], the study of [[energy]], [[heat]], [[work]], [[entropy]] and its transformations

*[[Electricity and magnetism]], the study of the electric and magnetic phenomena.

*[[Electromagnetism]], the study of the electric and magnetic phenomena and [[light]]

Most kinds of [[wave]] behavior are also considered to lie within the classical domain.  Therefore, the studies of [[sound]] (which may be considered a subset of mechanics) and the wavelike nature of [[light]] (a subset of electricity and magnetism) are classical pursuits.

Most kinds of [[wave]] behavior are also considered to lie within the classical domain.  Therefore, the studies of [[sound]] (which may be considered a subset of mechanics) and the wavelike nature of [[light]] (a subset of electricity and magnetism) are classical pursuits.

[[Modern physics]] generally includes areas of study within physics that surfaced after 1900, the year in which [[Max Planck]] proposed a "quantum hypothesis" to explain the properties of light emitted by [[black body|hot, dark objects]].  The two major aspects of modern physics are  

Modern physics generally includes areas of study within physics that surfaced after 1900, the year in which [[Max Planck]] proposed a "quantum hypothesis" to explain the properties of light emitted by [[black body|hot, dark objects]].  The two major aspects of modern physics are  

*[[Relativity]], a new physical theory that stated that space and time are not absolute concepts

*[[Relativity]], a new physical theory based on all observers in [[Inertial frame of reference|inertial frames of reference]] measure the same value for the [[speed of light]]. This lead to space and time not being absolute concepts

*[[Quantum mechanics]], a probabilistic description of the discrete behavior of matter and energy at tiny length and energy scales

*[[Quantum mechanics]], a probabilistic description of the discrete behavior of [[matter]] and [[energy]] at tiny length and energy scales, such as this of [[subatomic particles]]

One trouble with this distinction is that electromagnetism of the 1860s is a relativistic theory. In modern terminology, magnetism is a relativistic effect of electricity.

One trouble with this distinction is that electromagnetism of the 1860s is a relativistic theory. In modern terminology, magnetism is a relativistic effect of electricity.

===Scientific Revolution===

===Scientific Revolution===

[[Image:Nicolaus Copernicus.jpg|thumb|[[Nicolaus Copernicus]].]]

[[Image:Nicolaus Copernicus.jpg|thumb|[[Nicolaus Copernicus]].]]

As [[scientific method]]ology became further developed, the Aristotelian view came under scrutiny. Europe became the leading centre of scientific research by the 16th century. Force and motion had become topics of interest among scientists of the day.  [[Galileo Galilei]] performed a series of experiments involving rolling balls down inclined planes, and [[Johannes Kepler]] found [[Kepler's laws|mathematical relations]] governing the motion of the [[planet]].  [[Isaac Newton]] was able to explain the results of Galileo and Kepler through the use of three succinct [[Newton's laws of motion|laws of motion]] and an expression that accounted for [[gravity|gravitational]] force.  Newton also made important contributions to the field of [[optics]] and suggested that light consisted of tiny corpuscles (particles).  

As [[scientific method]]ology became further developed, the Aristotelian view came under scrutiny. Europe became the leading centre of scientific research by the 16th century. Force and motion had become topics of interest among scientists of the day.  [[Galileo Galilei]] performed a series of experiments involving rolling balls down inclined planes, and [[Johannes Kepler]] found [[Kepler's laws of planetary motion|mathematical relations]] governing the motion of the [[planet]].  [[Isaac Newton]] was able to explain the results of Galileo and Kepler through the use of three succinct [[Newton's Laws of Motion|laws of motion]] and an expression that accounted for [[gravity|gravitational]] force.  Newton also made important contributions to the field of [[optics]] and suggested that light consisted of tiny corpuscles (particles).  

===Further Development of Classical Physics===

===Further Development of Classical Physics===

Newtonian mechanics was expanded by several other scientists, notably [[Pierre-Simon Laplace]] and [[Joseph-Louis Lagrange]]. As Newton's laws became the dominant tool in physical science, they began to be applied to phenomena other than gravitational interactions.  During the first half of the 19th century, several laws governing electricity and magnetism were discovered. In the mid-1860s, [[James Clerk Maxwell]] unified these individual laws into a single set of [[Maxwell's Equations|equations]].  Maxwell was able to derive a [[wave equation]] from his new set of equations, and found that the speed of the waves predicted by the equation was close to the measured [[speed of light]]. [[Thomas Young]] had [[double-slit experiment|demonstrated]] in 1801 that light had wavelike properties, but Maxwell's result provided the new insight that light waves were oscillating [[electric field|electric]] and [[magnetic field]]s.

Newtonian mechanics was expanded by several other scientists, notably Pierre-Simon Laplace and [[Joseph-Louis Lagrange]]. As Newton's laws became the dominant tool in physical science, they began to be applied to phenomena other than gravitational interactions.  During the first half of the 19th century, several laws governing electricity and magnetism were discovered. In the mid-1860s, [[James Clerk Maxwell]] unified these individual laws into a single set of [[Maxwell's Equations|equations]].  Maxwell was able to derive a [[wave equation]] from his new set of equations, and found that the speed of the waves predicted by the equation was close to the measured [[speed of light]]. Thomas Young had [[double-slit experiment|demonstrated]] in 1801 that light had wavelike properties, but Maxwell's result provided the new insight that light waves were oscillating [[electric field|electric]] and [[magnetic field]]s.

Thermodynamics also came of age during roughly the same era.  Advances by [[James Joule]], [[Sadi Carnot]], and [[Lord Kelvin]], among many others, led to a better understanding of heat and entropy, and the formulation of the laws of thermodynamics.  Later, attempts to relate the behavior of particles on the atomic and molecular level to the properties of large aggregates of such particles led to the development of [[statistical mechanics]].

Thermodynamics also came of age during roughly the same era.  Advances by [[James Joule]], Sadi Carnot, and [[Lord Kelvin]], among many others, led to a better understanding of heat and entropy, and the formulation of the laws of thermodynamics.  Later, attempts to relate the behavior of particles on the atomic and molecular level to the properties of large aggregates of such particles led to the development of [[statistical mechanics]].

===The Modern Era===

===The Modern Era===

By the close of the 19th century, the study of physics was widely thought to be essentially complete, with the exception of only a few "loose ends"—minor unsolved problems to be dealt with.{{cite-fact}}  As a solution to the so-called [[ultraviolet catastrophe]] problem, which involved light emission from [[black body|hot, dark objects]], Max Planck proposed in 1900 that the sources of light could be represented by small individual oscillators.  Planck referred to the individual oscillators as ''quanta''.  In 1905, [[Albert Einstein]] published a heuristic explanation for the [[photoelectric effect]] problem:  light could be thought of as being composed of very small, discrete packets (quanta), as opposed to the classical representation of light as a collection of waves. By the end of the 1930s, [[quantum mechanics]]—the name given to the new theory that applied to matter and energy on atomic scales—was well-established.  Other physicists began applying the theory to more specific aspects of physics, resulting in quantum mechanical descriptions of electricity and magnetism, the [[weak force|weak]] and [[strong force|strong]] atomic forces, [[condensed matter]] physics, and many other areas.  

By the close of the 19th century, the study of physics was widely thought to be essentially complete, with the exception of only a few "loose ends"—minor unsolved problems to be dealt with. As a solution to the so-called ultraviolet catastrophe problem, which involved light emission from [[black body|hot, dark objects]], Max Planck proposed in 1900 that the sources of light could be represented by small individual oscillators.  Planck referred to the individual oscillators as ''quanta''.  In 1905, [[Albert Einstein]] published a heuristic explanation for the [[photoelectric effect]] problem:  light could be thought of as being composed of very small, discrete packets (quanta), as opposed to the classical representation of light as a collection of waves. By the end of the 1930s, [[quantum mechanics]]—the name given to the new theory that applied to matter and energy on atomic scales—was well-established.  Other physicists began applying the theory to more specific aspects of physics, resulting in quantum mechanical descriptions of electricity and magnetism, the [[weak force|weak]] and [[strong force|strong]] atomic forces, condensed matter physics, and many other areas.  

Also in the early 1900s, [[Poincare]] published his discovery of what became known as the [[special relativity|special theory of relativity]], which gave new insights on the relation between [[space]] and [[time]].  Others later extended the concept to non-inertial [[reference frames]], which resulted in the [[general relativity|general theory of relativity]], an improved description of [[gravity|gravitational]] interactions.

Also in the early 1900s, [[Poincare]] published his discovery of what became known as the [[special relativity]], which gave new insights on the relation between [[space]] and [[time]].  Others later extended the concept to non-inertial [[Inertial frame of reference|reference frames]], which resulted in the [[general relativity|general theory of relativity]], an improved description of [[gravity|gravitational]] interactions.

=== Contemporary Physics ===

=== Contemporary Physics ===

*Evidence suggests that the matter in the universe that humans can currently detect directly is only a fraction of the matter that actually exists.  Attempts to discover the nature of the as-yet-undetectable [[dark matter]] are underway.

*Evidence suggests that the matter in the universe that humans can currently detect directly is only a fraction of the matter that actually exists.  Attempts to discover the nature of the as-yet-undetectable [[dark matter]] are underway.

*The universe is known to be expanding at an accelerated rate.  Such acceleration requires energy, but the form and source of this [[dark energy]] is not well-understood.

*The universe is known to be expanding at an accelerated rate.  Such acceleration requires energy, but the form and source of this [[dark energy]] is not well-understood.

*General relativity predicts the existence of gravitational waves, but none have been conclusively detected.  Due to the weakness of the gravitational interaction, extremely sensitive detectors are required.  The [[LIGO]] project is an example of current efforts to detect such waves.

*General relativity predicts the existence of gravitational waves and their discovery was announced in early 2016<ref>https://www.ligo.caltech.edu/news/ligo20160615</ref>.  Due to the weakness of the gravitational interaction, extremely sensitive detectors are required.  The [[LIGO]] project is an example of current efforts to detect such waves.

More applied areas of contemporany physical research include:

More applied areas of contemporany physical research include: