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Theory of relativity
 
 
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projection of a three-dimensional
Three-dimensional space

Three-dimensional space is a geometric model of the physical universe in which we live. The three dimensions are commonly called length, width, and depth , although any three mutually perpendicular directions can serve as the three dimensions....
 analogy of space-time curvature described in General Relativity.]]

This page is about the scientific concept of relativity, for philosophical or sociological theories about relativity see Relativism
Relativism

Relativism is the idea that some elements or aspects of experience or culture are relative to, i.e., dependent on, other elements or aspects.Common statements that might be considered relativistic include...
.


The theory of relativity, or simply relativity, generally refers specifically to two theories of Albert Einstein
Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E = mc2....
: special relativity
Special relativity

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity"....
 and general relativity
General relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916....
. However, the word "relativity" is sometimes used in reference to Galilean invariance
Galilean invariance

Galilean invariance or Galilean relativity is a principle of relativity which states that the fundamental physical law are the same in all inertial frames....
.

The term "theory of relativity" was coined by Max Planck
Max Planck

Karl Ernst Ludwig Marx Planck, better known as Max Planck was a Germany physicist. He is considered to be the founder of the Quantum mechanics, and one of the most important physicists of the twentieth century....
 in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity
Principle of relativity

In physics, the principle of relativity is the requirement that the equations, describing the laws of physics, have the same form in all admissible frames of reference....
.

ial relativity is a theory of the structure of spacetime
Spacetime

In physics, spacetime is any mathematical model that combines space and Time in physics into a single continuum . Spacetime is usually interpreted with space being Three-dimensional space and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions....
.






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projection of a three-dimensional
Three-dimensional space

Three-dimensional space is a geometric model of the physical universe in which we live. The three dimensions are commonly called length, width, and depth , although any three mutually perpendicular directions can serve as the three dimensions....
 analogy of space-time curvature described in General Relativity.]]

This page is about the scientific concept of relativity, for philosophical or sociological theories about relativity see Relativism
Relativism

Relativism is the idea that some elements or aspects of experience or culture are relative to, i.e., dependent on, other elements or aspects.Common statements that might be considered relativistic include...
.


The theory of relativity, or simply relativity, generally refers specifically to two theories of Albert Einstein
Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E = mc2....
: special relativity
Special relativity

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity"....
 and general relativity
General relativity

General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916....
. However, the word "relativity" is sometimes used in reference to Galilean invariance
Galilean invariance

Galilean invariance or Galilean relativity is a principle of relativity which states that the fundamental physical law are the same in all inertial frames....
.

The term "theory of relativity" was coined by Max Planck
Max Planck

Karl Ernst Ludwig Marx Planck, better known as Max Planck was a Germany physicist. He is considered to be the founder of the Quantum mechanics, and one of the most important physicists of the twentieth century....
 in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity
Principle of relativity

In physics, the principle of relativity is the requirement that the equations, describing the laws of physics, have the same form in all admissible frames of reference....
.

Special relativity

Special relativity is a theory of the structure of spacetime
Spacetime

In physics, spacetime is any mathematical model that combines space and Time in physics into a single continuum . Spacetime is usually interpreted with space being Three-dimensional space and time playing the role of a fourth dimension that is of a different sort than the spatial dimensions....
. It was introduced in Albert Einstein
Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E = mc2....
's 1905 paper "On the Electrodynamics of Moving Bodies
Annus Mirabilis Papers

The Annus Mirabilis Papers are the papers of Albert Einstein published in the "Annalen der Physik" scientific journal in 1905. These four articles contributed substantially to the foundation of History of physics#Modern physics and changed views on space, time, and matter....
"; however, the term was first used by Galileo Galilei
Galileo Galilei

Galileo Galilei was a Grand Duchy of Tuscany physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution....
 in 1632 in his Dialogue concerning the World's Two Chief Systems. But Galileo's version was flawed: for example, he thought the spin of the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
 caused the tides. Special relativity is based on two postulates which are contradictory in classical mechanics
Classical mechanics

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies....
:
  1. The laws of physics
    Physics

    Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
     are the same for all observers in uniform motion
    Inertial frame of reference

    In physics, an inertial frame of reference is a frame of reference, tied to the state of motion of an Observer , with the property that each physical law portrays itself in the same form in every inertial frame....
     relative to one another (Galileo's
    Galileo Galilei

    Galileo Galilei was a Grand Duchy of Tuscany physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution....
     principle of relativity
    Principle of relativity

    In physics, the principle of relativity is the requirement that the equations, describing the laws of physics, have the same form in all admissible frames of reference....
    ),
  2. The speed of light
    Speed of light

    The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second....
     in a vacuum
    Vacuum

    A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty....
     is the same for all observers, regardless of their relative motion or of the motion of the source of the light
    Light

    Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
    .


The resultant theory has many surprising consequences. Some of these are:
  • Relativity of simultaneity
    Relativity of simultaneity

    The relativity of simultaneity is the concept that simultaneity is not absolute, but dependent on the observer. That is, according to the special theory of relativity formulated by Albert Einstein in 1905, it is impossible to say in an absolute sense whether two events occur at the same time if those events are separated in space....
    : Two events, simultaneous for some observer, may not be simultaneous for another observer if the observers are in relative motion.
  • Time dilation
    Time dilation

    Time dilation is the phenomenon whereby an observer finds that another's clock, which is physically identical to their own, is ticking at a slower rate as measured by their own clock....
    : Moving clock
    Clock

    A clock is an instrument used for indicating and maintaining the time and passage thereof. The word clock is derived ultimately from the Celtic languages words clagan and clocca meaning "bell"....
    s are measured to tick more slowly than an observer's "stationary" clock.
  • Length contraction
    Length contraction

    Length contraction, according to Hendrik Lorentz, is the physical phenomenon of a decrease in length detected by an observer in objects that travel at any non-zero velocity relative to that observer....
    : Objects are measured to be shortened in the direction that they are moving with respect to the observer.
  • Mass-energy equivalence
    Mass-energy equivalence

    In physics, mass?energy equivalence is the concept that any mass has an associated energy, and that any energy has an associated type of mass. In special relativity this relationship is expressed using the mass?energy equivalence formula...
    : , energy and mass are equivalent and transmutable.


The defining feature of special relativity is the replacement of the Galilean transformation
Galilean transformation

The Galilean transformation is used to transform between the coordinates of two reference frames which differ only by constant relative motion within the constructs of Newtonian physics....
s of classical mechanics by the Lorentz transformation
Lorentz transformation

In physics, the Lorentz transformation converts between two different observers' measurements of space and time, where one observer is in constant motion with respect to the other....
s. (See Maxwell's equations
Maxwell's equations

In electromagnetism, James Clerk Maxwell equations are a set of four partial differential equations that describe the properties of the electric field and magnetic field fields and relate them to their sources, charge density and current density....
 of electromagnetism
Electromagnetism

Electromagnetism is the physics of the electromagnetic field, a field which exerts a force on Elementary particles with the property of electric charge and which is reciprocally affected by the presence and motion of such particles....
 and introduction to special relativity
Introduction to special relativity

In physics, special relativity is a theory about the relations between space and time, developed by Albert Einstein in 1905. It abandoned the notion of absolute time, that is, that the temporal ordering of events and the duration of time intervals are independent from the observer....
).

General relativity

General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the equivalence principle
Equivalence principle

The equivalence principle is one of the fundamental background concepts of the General Theory of Relativity. For the overall context, see General relativity....
, under which the states of accelerated motion
Acceleration

File:Acceleration.JPGFile:Acceleration components.JPGIn physics, and more specifically kinematics, acceleration is the change in velocity over time....
 and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion
Inertia

File:192447main 017 law of inertia.oggInertia is the resistance of an object to a change in its state of motion. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the Motion of matter and how it is affected by applied forces....
: In other words an object in free fall is falling because that is how objects move when there is no force
Force

In physics, a force is that which can cause an object with mass to change its velocity. Force has both Euclidean_vector#Length of a vector and Direction , making it a Vector quantity....
 being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics
Classical mechanics

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies....
. This is incompatible with classical mechanics and special relativity
Special relativity

Special relativity is the physical theory of measurement in inertial frames of reference proposed in 1905 by Albert Einstein in the paper "Annus Mirabilis Papers#Special relativity"....
 because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved
Curvature

In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line , but this is defined in different ways depending on the context....
. In 1915, he devised the Einstein field equations
Einstein field equations

The Einstein field equations or Einstein's equations are a set of ten equations in Einstein's theory of general relativity in which the fundamental force of gravitation is described as a curved spacetime caused by matter and energy....
 which relate the curvature of spacetime with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:
  • Time goes more slowly in higher gravitational fields. This is called gravitational time dilation
    Gravitational time dilation

    Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the higher the local distortion of spacetime due to gravity, the more slowly time passes....
    .
  • Orbits precess
    Precession

    Precession refers to a change in the direction of the axis of a rotation object. In physics, there are two types of precession, torque-free and torque-induced, the latter being discussed here in more detail....
     in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury
    Mercury (planet)

    Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 88 days. The orbit of Mercury has the highest Orbital eccentricity of all the Solar System planets, and it has the smallest axial tilt....
     and in binary pulsar
    Binary pulsar

    A binary pulsar is a pulsar with a binary star, often another pulsar, white dwarf or neutron star. They are one of the few objects which allow physicists to test general relativity in the case of a strong gravitational field....
    s).
  • Rays of light
    Light

    Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
     bend in the presence of a gravitational field
    General relativity

    General relativity or the general theory of relativity is the Geometry Theoretical physics of gravitation published by Albert Einstein in 1916....
    .
  • Frame-dragging
    Frame-dragging

    Albert Einstein's theory of general relativity predicts that rotating bodies drag spacetime around themselves in a phenomenon referred to as frame-dragging....
    , in which a rotating mass "drags along" the space time around it.


Technically, general relativity is a metric
Metric (mathematics)

In mathematics, a metric or distance function is a function which defines a distance between elements of a Set . A set with a metric is called a metric space....
 theory of gravitation
Gravitation

Gravitation is a natural phenomenon that gives weight to objects. In everyday life, attraction due to gravity is the result of the presence of relatively large bodies, such as the Earth and the Moon....
 whose defining feature is its use of the Einstein field equations
Einstein field equations

The Einstein field equations or Einstein's equations are a set of ten equations in Einstein's theory of general relativity in which the fundamental force of gravitation is described as a curved spacetime caused by matter and energy....
. The solutions of the field equations are metric tensors
Metric tensor (general relativity)

In general relativity, the metric tensor is the fundamental object of study. It may loosely be thought of as a generalization of the gravitational field familiar from gravity....
 which define the topology
Topology

Topology is a major area of mathematics that has emerged through the development of concepts from geometry and set theory, such as those of space, dimension, shape, transformation and others....
 of the spacetime and how objects move inertially.

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