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Physics can be broadly divided into two major categories:  [[Classical Physics]] and [[Modern Physics]].

Physics can be broadly divided into two major categories:  [[Classical Physics]] and [[Modern Physics]].

==Classical 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 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 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

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  

*[[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

Modern physics was distinguished from classical physics mainly because these two ideas were so radically different from the classical viewpoint that they produced a [[paradigm shift]] in the way that physicists interpret the natural world.

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.

==History==

==History==

===Physics in Antiquity===

=== Physics in Antiquity ===

 

Ancient [[Greek]] scholars, such as [[Democritus]] and [[Aristotle]], thought of the material [[universe]] as being composed four different [[elements]]--[[earth]], [[air]], [[fire]], and [[water]].  Motion and interactions between objects were explained by the concept of a "natural state" for each element.  Bodies tended to move towards their natural state, and remained motionless when they attained it.  For example, a rock fell to the ground after being tossed in the air because it was composed mostly of earth; as such, it moved toward its natural state at the center of the Earth as much as it was able.  Democritus proposed that matter was made of indivisible atoms. The Greeks also made advances in [[astronomy]]; Aristotle showed that the Earth was round;  [[Ptolemy]] developed a [[geocentric]] (Earth-centered) model of the solar system; and Aristarchus developed a [[heliocentric]] (Sun-centered) model.

Ancient [[Greek]] scholars, such as [[Democritus]] and [[Aristotle]], thought of the material [[universe]] as being composed four different [[elements]]--[[earth]], [[air]], [[fire]], and [[water]].  Motion and interactions between objects were explained by the concept of a "natural state" for each element.  Bodies tended to move towards their natural state, and remained motionless when they attained it.  For example, a rock fell to the ground after being tossed in the air because it was composed mostly of earth; as such, it moved toward its natural state at the center of the Earth as much as it was able.  The Greeks also made advances in [[astronomy]]; [[Ptolemy]] developed a geocentric (Earth-centered) model of the solar system.

 

The ideas of the Greeks formed a [[paradigm]] that lasted for over 1000 years, making it one of the most successful scientific theories ever in terms of length of time spent as the dominant viewpoint.

 

As [[scientific method|scientific methodology]] became further developed, the Aristotelian view came under scrutiny.  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|scientific methodology]] became further developed, the Aristotelian view came under scrutiny.  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).   

===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]]. The motions of

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.  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''.  At the time, Planck did not regard his finding as revolutionary; he thought that he was merely "fudging" the mathematics to arrive at a better solution.  However, in 1905, [[Albert Einstein]] published a solution to 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.  The discoveries of Planck and Einstein ushered in an entirely new era in physics.  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.

 

In another 1905 publication, Einstein announced his discovery of what became known as the [[special relativity|special theory of relativity]], which revolutionized the way physicists thought about [[space]] and [[time]].  Einstein 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, which was published in 1915.

 

The development of quantum mechanics and relativity induced a shift from the Newtonian "clockwork universe" paradigm.  According to quantum mechanics, the fundamental laws of the universe are not deterministic, but probabilistic; one can't predict events with certainty, but can only find the [[probability]] of an event taking place.  According to relativity, the universe has no preferred reference frame, and there is no such thing as absolute space and absolute time.

 

Like all other modern sciences, areas of interest in contemporary physics encompass an enormous variety of general and specialized subjects.  Today's professional physicists can generally be categorized into two types:  theoretical physicists, who are trained extensively in [[mathematics]] and work to develop or enhance physical theories; and experimental physicists, who obtain and analyze data in laboratory or laboratory-like settings.  Theorists and experimentalists often collaborate in attempts to reconcile theoretical predictions with experimental results.  Both types of physicists conduct research on a plethora of unsolved problems.   

Like all other modern sciences, areas of interest in contemporary physics encompass an enormous variety of general and specialized subjects.  Today's professional physicists can generally be categorized into two types:  theoretical physicists, who are trained extensively in [[mathematics]] and work to develop or enhance physical theories; and experimental physicists, who obtain and analyze data in laboratory or laboratory-like settings.  Theorists and experimentalists often collaborate in attempts to reconcile theoretical predictions with experimental results.  Both types of physicists conduct research on a plethora of unsolved problems.   

Some of the active areas of research on fundamental universal laws are listed below.

Some of the active areas of research on fundamental universal laws are listed below.

*The [[Standard Model]] of particle physics is known to be incomplete; particle physicists are working to predict and discover unknown constituents of matter

*The [[Standard Model]] of particle physics has been extremely successful insofar as it has been tested; particle physicists are working to test it at higher energy levels.

*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, 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.

[[Category: Science]]

[[Category: Science]]

[[Category: Physics]]

[[Category: Physics]]