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physics
Table of Contents
 * Introduction & Top Questions
   
 * The scope of physics
    * Mechanics
   
    * The study of gravitation
   
    * The study of heat, thermodynamics, and statistical mechanics
      * First law
      * Second law
      * Third law
      * Statistical mechanics
   
    * The study of electricity and magnetism
   
    * Optics
   
    * Atomic and chemical physics
   
    * Condensed-matter physics
   
    * Nuclear physics
   
    * Particle physics
   
    * Quantum mechanics
   
    * Relativistic mechanics
   
    * Conservation laws and symmetry
   
    * Fundamental forces and fields

 * 
   The methodology of physics
   
 * Relations between physics and other disciplines and society
    * Influence of physics on related disciplines
   
    * Influence of related disciplines on physics
   
    * The physicist in society

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 * Why does physics work in SI units?

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Contents Ask the Chatbot a Question

Science Physics


PHYSICS

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Written by
Richard Tilghman Weidner
Emeritus Professor of Physics, Rutgers University, New Brunswick, New Jersey.
Author of Physics and others.

Richard Tilghman Weidner,
Laurie M. Brown
Emeritus Professor of Physics and Astronomy, Northwestern University, Evanston,
Illinois. Coeditor of The Birth of Particle Physics.

Laurie M. Brown•All
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Last Updated: Aug 26, 2024 • Article History
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Top Questions
WHAT IS PHYSICS?

Physics is the branch of science that deals with the structure of matter and how
the fundamental constituents of the universe interact. It studies objects
ranging from the very small using quantum mechanics to the entire universe using
general relativity.

WHY DOES PHYSICS WORK IN SI UNITS?

Physicists and other scientists use the International System of Units (SI) in
their work because they wish to use a system that is agreed upon by scientists
worldwide. Since 2019 the SI units have been defined in terms of fundamental
physical constants, which means that scientists anywhere using SI can agree upon
the units they use to measure physical phenomena.


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physics, science that deals with the structure of matter and the interactions
between the fundamental constituents of the observable universe. In the broadest
sense, physics (from the Greek physikos) is concerned with all aspects of nature
on both the macroscopic and submicroscopic levels. Its scope of study
encompasses not only the behaviour of objects under the action of given forces
but also the nature and origin of gravitational, electromagnetic, and nuclear
force fields. Its ultimate objective is the formulation of a few comprehensive
principles that bring together and explain all such disparate phenomena.



(Read Einstein’s 1926 Britannica essay on space-time.)

Physics is the basic physical science. Until rather recent times physics and
natural philosophy were used interchangeably for the science whose aim is the
discovery and formulation of the fundamental laws of nature. As the modern
sciences developed and became increasingly specialized, physics came to denote
that part of physical science not included in astronomy, chemistry, geology, and
engineering. Physics plays an important role in all the natural sciences,
however, and all such fields have branches in which physical laws and
measurements receive special emphasis, bearing such names as astrophysics,
geophysics, biophysics, and even psychophysics. Physics can, at base, be defined
as the science of matter, motion, and energy. Its laws are typically expressed
with economy and precision in the language of mathematics.

Both experiment, the observation of phenomena under conditions that are
controlled as precisely as possible, and theory, the formulation of a unified
conceptual framework, play essential and complementary roles in the advancement
of physics. Physical experiments result in measurements, which are compared with
the outcome predicted by theory. A theory that reliably predicts the results of
experiments to which it is applicable is said to embody a law of physics.
However, a law is always subject to modification, replacement, or restriction to
a more limited domain, if a later experiment makes it necessary.

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The ultimate aim of physics is to find a unified set of laws governing matter,
motion, and energy at small (microscopic) subatomic distances, at the human
(macroscopic) scale of everyday life, and out to the largest distances (e.g.,
those on the extragalactic scale). This ambitious goal has been realized to a
notable extent. Although a completely unified theory of physical phenomena has
not yet been achieved (and possibly never will be), a remarkably small set of
fundamental physical laws appears able to account for all known phenomena. The
body of physics developed up to about the turn of the 20th century, known as
classical physics, can largely account for the motions of macroscopic objects
that move slowly with respect to the speed of light and for such phenomena as
heat, sound, electricity, magnetism, and light. The modern developments of
relativity and quantum mechanics modify these laws insofar as they apply to
higher speeds, very massive objects, and to the tiny elementary constituents of
matter, such as electrons, protons, and neutrons.





THE SCOPE OF PHYSICS

The traditionally organized branches or fields of classical and modern physics
are delineated below.




MECHANICS

illustration of Robert Hooke's law of elasticity of materialsIllustration of
Hooke's law of elasticity of materials, showing the stretching of a spring in
proportion to the applied force, from Robert Hooke's Lectures de Potentia
Restitutiva (1678).(more)
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Mechanics is generally taken to mean the study of the motion of objects (or
their lack of motion) under the action of given forces. Classical mechanics is
sometimes considered a branch of applied mathematics. It consists of kinematics,
the description of motion, and dynamics, the study of the action of forces in
producing either motion or static equilibrium (the latter constituting the
science of statics). The 20th-century subjects of quantum mechanics, crucial to
treating the structure of matter, subatomic particles, superfluidity,
superconductivity, neutron stars, and other major phenomena, and relativistic
mechanics, important when speeds approach that of light, are forms of mechanics
that will be discussed later in this section.



In classical mechanics the laws are initially formulated for point particles in
which the dimensions, shapes, and other intrinsic properties of bodies are
ignored. Thus in the first approximation even objects as large as Earth and the
Sun are treated as pointlike—e.g., in calculating planetary orbital motion. In
rigid-body dynamics, the extension of bodies and their mass distributions are
considered as well, but they are imagined to be incapable of deformation. The
mechanics of deformable solids is elasticity; hydrostatics and hydrodynamics
treat, respectively, fluids at rest and in motion.



The three laws of motion set forth by Isaac Newton form the foundation of
classical mechanics, together with the recognition that forces are directed
quantities (vectors) and combine accordingly. The first law, also called the law
of inertia, states that, unless acted upon by an external force, an object at
rest remains at rest, or if in motion, it continues to move in a straight line
with constant speed. Uniform motion therefore does not require a cause.
Accordingly, mechanics concentrates not on motion as such but on the change in
the state of motion of an object that results from the net force acting upon it.
Newton’s second law equates the net force on an object to the rate of change of
its momentum, the latter being the product of the mass of a body and its
velocity. Newton’s third law, that of action and reaction, states that when two
particles interact, the forces each exerts on the other are equal in magnitude
and opposite in direction. Taken together, these mechanical laws in principle
permit the determination of the future motions of a set of particles, providing
their state of motion is known at some instant, as well as the forces that act
between them and upon them from the outside. From this deterministic character
of the laws of classical mechanics, profound (and probably incorrect)
philosophical conclusions have been drawn in the past and even applied to human
history.



Lying at the most basic level of physics, the laws of mechanics are
characterized by certain symmetry properties, as exemplified in the
aforementioned symmetry between action and reaction forces. Other symmetries,
such as the invariance (i.e., unchanging form) of the laws under reflections and
rotations carried out in space, reversal of time, or transformation to a
different part of space or to a different epoch of time, are present both in
classical mechanics and in relativistic mechanics, and with certain
restrictions, also in quantum mechanics. The symmetry properties of the theory
can be shown to have as mathematical consequences basic principles known as
conservation laws, which assert the constancy in time of the values of certain
physical quantities under prescribed conditions. The conserved quantities are
the most important ones in physics; included among them are mass and energy (in
relativity theory, mass and energy are equivalent and are conserved together),
momentum, angular momentum, and electric charge.


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THE STUDY OF GRAVITATION

Laser Interferometer Space Antenna (LISA)Laser Interferometer Space Antenna
(LISA), a Beyond Einstein Great Observatory, is scheduled for launch in 2035.
Funded by the European Space Agency, LISA will consist of three identical
spacecraft that will trail the Earth in its orbit by about 50 million km (30
million miles). The spacecraft will contain thrusters for maneuvering them into
an equilateral triangle, with sides of approximately 5 million km (3 million
miles), such that the triangle's center will be located along the Earth's orbit.
By measuring the transmission of laser signals between the spacecraft
(essentially a giant Michelson interferometer in space), scientists hope to
detect and accurately measure gravity waves. (more)

This field of inquiry has in the past been placed within classical mechanics for
historical reasons, because both fields were brought to a high state of
perfection by Newton and also because of its universal character. Newton’s
gravitational law states that every material particle in the universe attracts
every other one with a force that acts along the line joining them and whose
strength is directly proportional to the product of their masses and inversely
proportional to the square of their separation. Newton’s detailed accounting for
the orbits of the planets and the Moon, as well as for such subtle gravitational
effects as the tides and the precession of the equinoxes (a slow cyclical change
in direction of Earth’s axis of rotation), through this fundamental force was
the first triumph of classical mechanics. No further principles are required to
understand the principal aspects of rocketry and space flight (although, of
course, a formidable technology is needed to carry them out).


curved space-time The four dimensional space-time continuum itself is distorted
in the vicinity of any mass, with the amount of distortion depending on the mass
and the distance from the mass. Thus, relativity accounts for Newton's inverse
square law of gravity through geometry and thereby does away with the need for
any mysterious “action at a distance.” (more)

The modern theory of gravitation was formulated by Albert Einstein and is called
the general theory of relativity. From the long-known equality of the quantity
“mass” in Newton’s second law of motion and that in his gravitational law,
Einstein was struck by the fact that acceleration can locally annul a
gravitational force (as occurs in the so-called weightlessness of astronauts in
an Earth-orbiting spacecraft) and was led thereby to the concept of curved
space-time. Completed in 1915, the theory was valued for many years mainly for
its mathematical beauty and for correctly predicting a small number of
phenomena, such as the gravitational bending of light around a massive object.
Only in recent years, however, has it become a vital subject for both
theoretical and experimental research. (Relativistic mechanics refers to
Einstein’s special theory of relativity, which is not a theory of gravitation.)




THE STUDY OF HEAT, THERMODYNAMICS, AND STATISTICAL MECHANICS

temperature scalesStandard and absolute temperature scales.(more)

Heat is a form of internal energy associated with the random motion of the
molecular constituents of matter or with radiation. Temperature is an average of
a part of the internal energy present in a body (it does not include the energy
of molecular binding or of molecular rotation). The lowest possible energy state
of a substance is defined as the absolute zero (−273.15 °C, or −459.67 °F) of
temperature. An isolated body eventually reaches uniform temperature, a state
known as thermal equilibrium, as do two or more bodies placed in contact. The
formal study of states of matter at (or near) thermal equilibrium is called
thermodynamics; it is capable of analyzing a large variety of thermal systems
without considering their detailed microstructures.





FIRST LAW

The first law of thermodynamics is the energy conservation principle of
mechanics (i.e., for all changes in an isolated system, the energy remains
constant) generalized to include heat.




SECOND LAW

The second law of thermodynamics asserts that heat will not flow from a place of
lower temperature to one where it is higher without the intervention of an
external device (e.g., a refrigerator). The concept of entropy involves the
measurement of the state of disorder of the particles making up a system. For
example, if tossing a coin many times results in a random-appearing sequence of
heads and tails, the result has a higher entropy than if heads and tails tend to
appear in clusters. Another formulation of the second law is that the entropy of
an isolated system never decreases with time.




THIRD LAW

The third law of thermodynamics states that the entropy at the absolute zero of
temperature is zero, corresponding to the most ordered possible state.




STATISTICAL MECHANICS

Brownian particle(Left) Random motion of a Brownian particle and (right) random
discrepancy between the molecular pressures on different surfaces of the
particle that cause motion.(more)

The science of statistical mechanics derives bulk properties of systems from the
mechanical properties of their molecular constituents, assuming molecular chaos
and applying the laws of probability. Regarding each possible configuration of
the particles as equally likely, the chaotic state (the state of maximum
entropy) is so enormously more likely than ordered states that an isolated
system will evolve to it, as stated in the second law of thermodynamics. Such
reasoning, placed in mathematically precise form, is typical of statistical
mechanics, which is capable of deriving the laws of thermodynamics but goes
beyond them in describing fluctuations (i.e., temporary departures) from the
thermodynamic laws that describe only average behaviour. An example of a
fluctuation phenomenon is the random motion of small particles suspended in a
fluid, known as Brownian motion.



Quantum statistical mechanics plays a major role in many other modern fields of
science, as, for example, in plasma physics (the study of fully ionized gases),
in solid-state physics, and in the study of stellar structure. From a
microscopic point of view the laws of thermodynamics imply that, whereas the
total quantity of energy of any isolated system is constant, what might be
called the quality of this energy is degraded as the system moves inexorably,
through the operation of the laws of chance, to states of increasing disorder
until it finally reaches the state of maximum disorder (maximum entropy), in
which all parts of the system are at the same temperature, and none of the
state’s energy may be usefully employed. When applied to the universe as a
whole, considered as an isolated system, this ultimate chaotic condition has
been called the “heat death.”



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