Theory of relativity

This article is about the scientific concept. For philosophical or ontological theories about relativity, see Relativism. For the silent film, see The Einstein Theory of Relativity.

Two-dimensional projection of a three-dimensional analogy of spacetime curvature described in general relativity

The theory of relativity, or simply relativity, generally encompasses two theories of Albert Einstein: special relativity and general relativity. (The word relativity can also be used in the context an older theory, that of Galilean invariance).

Concepts introduced by the theories of relativity include:

• Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate.
• Spacetime: space and time should be considered together and in relation to each other.
• The speed of light is nonetheless invariant, the same for all observers.

The term "theory of relativity" was based on the expression "relative theory" (German: Relativtheorie) used by Max Planck in 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity" (German: Relativitätstheorie).

Scope

The theory of relativity transformed theoretical physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanics stated by Isaac Newton.

The theory of relativity overturned the concept of motion from Newton's day, by positing that all motion is relative. Time was no longer uniform and absolute. Physics could no longer be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved spacetime. Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds.

In the field of physics, relativity catalyzed and added an essential depth of knowledge to the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

Two-theory view

The theory of relativity was representative of more than a single new physical theory. There are some explanations for this. First, special relativity was published in 1905, and the final form of general relativity was published in 1916.

Second, special relativity fits with and solves for elementary particles and their interactions, whereas general relativity solves for the cosmological and astrophysical realm (including astronomy).

Third, special relativity was widely accepted in the physics community by 1920. This theory rapidly became a significant and necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, and quantum mechanics. Conversely, general relativity did not appear to be as useful. There appeared to be little applicability for experimentalists as most applications were for astronomical scales. It seemed limited to only making minor corrections to predictions of Newtonian gravitation theory. Its impact was not apparent until the 1930s.

Finally, the mathematics of general relativity appeared to be incomprehensibly dense. Consequently, only a small number of people in the world, at that time, could fully understand the theory in detail. This remained the case for the next 40 years. Then, at around 1960 a critical resurgence in interest occurred which has resulted in making general relativity central to physics and astronomy. New mathematical techniques applicable to the study of general relativity substantially streamlined calculations. From this, physically discernible concepts were isolated from the mathematical complexity. Also, the discovery of exotic astronomical phenomena in which general relativity was crucially relevant, helped to catalyze this resurgence. The astronomical phenomena included quasars (1963), the 3-kelvin microwave background radiation (1965), pulsars (1967), and the discovery of the first black hole candidates (1971).

On the theory of relativity

Einstein stated that the theory of relativity belongs to the class of "principle-theories". As such it employs an analytic method. This means that the elements which comprise this theory are not based on hypothesis but on empirical discovery. The empirical discovery leads to understanding the general characteristics of natural processes. Mathematical models are then developed which separate the natural processes into theoretical-mathematical descriptions. Therefore, by analytical means the necessary conditions that have to be satisfied are deduced. Separate events must satisfy these conditions. Experience should then match the conclusions.

The special theory of relativity and the general theory of relativity are connected. As stated below, special theory of relativity applies to all inertial physical phenomena except gravity. The general theory provides the law of gravitation, and its relation to other forces of nature.

Special relativity

Main article: Special relativity

USSR stamp dedicated to Albert Einstein

Special relativity is a theory of the structure of spacetime. It was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" (for the contributions of many other physicists see History of special relativity). Special relativity is based on two postulates which are contradictory in classical mechanics:

1. The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

The resultant theory copes with experiment better than classical mechanics, e.g. in the Michelson–Morley experiment that supports postulate 2, but also has many surprising consequences. Some of these are:

• Relativity of simultaneity: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
• Time dilation: Moving clocks are measured to tick more slowly than an observer's "stationary" clock.
• Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
• Mass–energy equivalence: E = mc², energy and mass are equivalent and transmutable.
• Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in a vacuum.

The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).

General relativity

Main article: 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, under which the states of accelerated motion 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; an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and 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. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:

• Clocks run more slowly in deeper gravitational wells. This is called gravitational time dilation.
• Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
• Rays of light bend in the presence of a gravitational field.
• Rotating masses "drag along" the spacetime around them; a phenomenon termed "frame-dragging".
• The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light.

Technically, general relativity is a theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.

Experimental evidence

Main articles: Tests of special relativity and Tests of general relativity

Relativistic effects such as

• two-way light-speed isotropy (Michelson–Morley experiment),
• velocity and gravitational time dilation (Ives–Stilwell experiment, Pound–Rebka experiment),
• relativistic energy and momentum increase at high velocities,

and many other effects have been tested and confirmed in various experiments.

History

Main articles: History of special relativity and History of general relativity

The history of special relativity consists of many theoretical results and empirical findings obtained by Albert Michelson, Hendrik Lorentz, Henri Poincaré and others. It culminated in the theory of special relativity proposed by Albert Einstein, and subsequent work of Max Planck, Hermann Minkowski and others.

General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915, with contributions by many others after 1915.

Minority views

Einstein's contemporaries did not all accept his new theories at once. However, the theory of relativity is now considered as a cornerstone of modern physics, see Criticism of relativity theory.

Although it is widely acknowledged that Einstein was the creator of relativity in its modern understanding, some believe that others deserve credit for it, see Relativity priority dispute.

See also

• General relativity references
• Special relativity references

Further reading

• Bergmann, Peter G. (1976). Introduction to the Theory of Relativity. Dover Publications. ISBN 0-486-63282-2.
• Brian, Denis (1995). Einstein: a life. New York: J. Wiley. ISBN 0471114596.
• Einstein, Albert (2005). Relativity: The Special and General Theory (The masterpiece science ed. ed.). New York: Pi Press. ISBN 0131862618. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Einstien, Albert (1979). Albert Einstein, Autobiographical Notes (A Centennial ed. ed.). La Salle, Ill.: Open Court Publishing Co. ISBN 0875483526. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Einstien, Albert (2009). Einstein's Essays in Science (Dover ed. ed.). Mineola, N.Y.: Dover Publications. ISBN 9780486470115. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Einstien, Albert (1956) [1922]. The Meaning of Relativity (5 ed.). Princeton University Press. {{cite book}}: Cite has empty unknown parameter: |1= (help)
• Ohanian, Hans C. (2008). Einstein's Mistakes: The Human Failings of Genius (1st ed. ed.). New York: W.W. Norton & Co. ISBN 9780393062939. {{cite book}}: |edition= has extra text (help)
• Pais, Abraham (1982). "Subtle is the Lord ...": The Science and the Life of Albert Einstein (1st ed. ed.). Oxford: Oxford Univ. Press. ISBN 019853907x. {{cite book}}: |edition= has extra text (help); Check |isbn= value: invalid character (help)
• Russell, Bertrand (1969). The ABC of Relativity (3rd rev. ed ed.). London: Allen & Unwin. ISBN 0045210012. {{cite book}}: |edition= has extra text (help)
• Stephen, Hawking (2005). A Briefer History of Time. New York, NY: Bantam Dell. ISBN 9780553804362. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Wheeler, John Archibald (1992). Spacetime Physics: Introduction to Special Relativity (2nd ed. ed.). New York: W.H. Freeman. ISBN 0716723271. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Brockman, John, ed. (2006). My Einstein (1. ed. ed.). New York: Pantheon Books. ISBN 0375423451. {{cite book}}: |edition= has extra text (help)

External links

• Theory of relativity at Curlie
• Relativity Milestones: Timeline of Notable Relativity Scientists and Contributions