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{{main|Special relativity}} {{main|Special relativity}}
In the late 19th century, ] suggested that the principle of relativity holds for all laws of nature. ] and ] discovered that ], the cornerstone of ], were invariant only by a certain change of time and length units. This left quite a bit of confusion among physicists, many of whom thought that a ] is incompatible with the relativity principle. In the late 19th century, ] suggested that the principle of relativity holds for all laws of nature. ] and ] discovered that ], the cornerstone of ], were invariant only by a certain change of time and length units. This left quite a bit of confusion among physicists, many of whom thought that a ] is incompatible with the relativity principle.

In his 1902 book, Science and Hypothesis, Poincaré argued that there is no absolute space, that there is no absolute time, and that no physical experience can detect any inertial motion. In a famous 1904 lecture, he said:<ref>Congress of arts and science, universal exposition, St. Louis 1904, (1905), vol. 1, pp. 604-622, </ref> {{Quotation|The principle of relativity, according to which the laws of physical phenomena should be the same, whether for an observer fixed, or for an observer carried along in a uniform movement of translation; so that we have not and could not have any means of discerning whether or not we are carried along in such a motion.|Henri Poincaré,1904}}


In their 1905 papers on ], Henri Poincaré and ] explained that with the ] the relativity principle holds perfectly. Einstein elevated the (special) principle of relativity to a ] of the theory and derived the Lorentz transformations from this principle combined with the principle of the independence of the speed of light (in vacuum) from the motion of the source. These two principles were reconciled with each other (in Einstein's treatment, though not in Poincaré's) by a re-examination of the fundamental meanings of space and time intervals. In their 1905 papers on ], Henri Poincaré and ] explained that with the ] the relativity principle holds perfectly. Einstein elevated the (special) principle of relativity to a ] of the theory and derived the Lorentz transformations from this principle combined with the principle of the independence of the speed of light (in vacuum) from the motion of the source. These two principles were reconciled with each other (in Einstein's treatment, though not in Poincaré's) by a re-examination of the fundamental meanings of space and time intervals.

Revision as of 21:52, 24 August 2009

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.

For example, the Maxwell equations have the same form in all inertial frames of reference; the Einstein field equation has the same form in arbitrary frames of reference.

Several principles of relativity have been successfully applied throughout science, whether implicitly (as in Newtonian mechanics) or explicitly (as in Albert Einstein's special relativity and general relativity).

History of Relativity

Main article: History of special relativity

Basic relativity principles

Certain principles of relativity have been widely assumed in most scientific disciplines. One of the most widespread is the belief that any law of nature should be the same at all times; and scientific investigations generally assume that laws of nature are the same regardless of the person measuring them. These sorts of principles have been incorporated into scientific inquiry at the most fundamental of levels.

Any principle of relativity prescribes a symmetry in natural law: that is, the laws must look the same to one observer as they do to another. According to a deep theoretical result called Noether's theorem, any such symmetry will also imply a conservation law alongside. For example, if two observers at different times see the same laws, then a quantity called energy will be conserved. In this light, relativity principles are not just statements about how scientists should write laws: they make testable predictions about how nature behaves.

Special principle of relativity

See also: Inertial reference frame

According to the first postulate of the special theory of relativity:

Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K.

— Albert Einstein: The foundation of the general theory of relativity, Section A, §1

This postulate defines an inertial frame of reference.

The special principle of relativity states that physical laws should be the same in all inertial reference frames, but that they may vary across non-inertial ones. It has been used in both Newtonian mechanics and Special relativity; for the latter, its influence was so strong that Max Planck named the theory after the principle.

The principle forces physical laws to be the same in any vehicle moving at constant velocity as they are in a vehicle at rest. A consequence is that an observer in an inertial reference frame cannot determine an absolute speed or direction of their travel in space; they may only speak of their travel relative to some other object.

The principle does not extend this property to noninertial reference frames because those frames do not, in general experience, seem to abide by the same laws of physics -- these frames require fictitious forces to describe motion.

In Newtonian mechanics

Main article: Galilean relativity

The special principle of relativity was first explicitly enunciated by Galileo Galilei in 1632 in his Dialogue Concerning the Two Chief World Systems, using the metaphor of Galileo's ship.

Newtonian mechanics added to the special principle several other concepts--various laws and an assumption of an absolute time. When formulated in the context of these laws, the special principle of relativity states that the laws of physics are invariant under a Galilean transformation.

In special relativity

Main article: Special relativity

In the late 19th century, Henri Poincaré suggested that the principle of relativity holds for all laws of nature. Joseph Larmor and Hendrik Lorentz discovered that Maxwell's equations, the cornerstone of electromagnetism, were invariant only by a certain change of time and length units. This left quite a bit of confusion among physicists, many of whom thought that a luminiferous aether is incompatible with the relativity principle.

In his 1902 book, Science and Hypothesis, Poincaré argued that there is no absolute space, that there is no absolute time, and that no physical experience can detect any inertial motion. In a famous 1904 lecture, he said:

The principle of relativity, according to which the laws of physical phenomena should be the same, whether for an observer fixed, or for an observer carried along in a uniform movement of translation; so that we have not and could not have any means of discerning whether or not we are carried along in such a motion.

— Henri Poincaré,1904

In their 1905 papers on electrodynamics, Henri Poincaré and Albert Einstein explained that with the Lorentz transformations the relativity principle holds perfectly. Einstein elevated the (special) principle of relativity to a postulate of the theory and derived the Lorentz transformations from this principle combined with the principle of the independence of the speed of light (in vacuum) from the motion of the source. These two principles were reconciled with each other (in Einstein's treatment, though not in Poincaré's) by a re-examination of the fundamental meanings of space and time intervals.

The strength of special relativity lies in its derivation from simple, basic principles, including the invariance of the laws of physics under a shift of inertial reference frames and the invariance of the speed of light in a vacuum. (See also: Lorentz covariance.)

It is, in fact, possible to derive the Lorentz transformations from the principle of relativity alone and obtain the constancy of the speed of light as a consequence. Using only the isotropy of space and the symmetry implied by the principle of special relativity, one can show that the space-time transformations between inertial frames are either Galilean or Lorentzian. In the Lorentzian case, one can then obtain relativistic interval conservation and the constancy of the speed of light.

General principle of relativity

The general principle of relativity states:

All systems of reference are equivalent with respect to the formulation of the fundamental laws of physics.

— C. Møller The Theory of Relativity, p. 220

That is, physical laws are the same in all reference frames -- inertial or non-inertial. An accelerated charged particle might emit synchrotron radiation, though a particle at rest doesn't. If we consider now the same accelerated charged particle in its non-inertial rest frame, it emits radiation at rest.

Physics in non-inertial reference frames was historically treated by a coordinate transformation, first, to an inertial reference frame, performing the necessary calculations therein, and using another to return to the non-inertial reference frame. In most such situations, the same laws of physics can be used if certain predictable fictitious forces are added into consideration; an example is a uniformly rotating reference frame, which can be treated as an inertial reference frame if one adds a fictitious centrifugal force and Coriolis force into consideration.

The problems involved are not always so trivial. Special relativity predicts that an observer in an inertial reference frame doesn't see objects they'd describe as moving faster than the speed of light. However, in the non-inertial reference frame of Earth, treating a spot on the Earth as a fixed point, the stars are observed to move in the sky, circling once about the Earth per day. Since the stars are light years away, this observation means that, in the non-inertial reference frame of the Earth, anybody who looks at the stars is seeing objects which appear, to them, to be moving faster than the speed of light.

Since non-inertial reference frames do not abide by the special principle of relativity, such situations are not self-contradictory.

General relativity

Main article: General relativity

General relativity was developed by Einstein in the years 1907 - 1915. General relativity postulates that the global Lorentz covariance of special relativity becomes a local Lorentz covariance in the presence of matter. The presence of matter "curves" spacetime, and this curvature affects the path of free particles (and even the path of light). General relativity uses the mathematics of differential geometry and tensors in order to describe gravitation as an effect of the geometry of spacetime. Einstein based this new theory on the general principle of relativity, and he named the theory after the underlying principle.

See also

Further reading

See the special relativity references and the general relativity references.

Notes and references

  1. Einstein, A., Lorentz, H. A., Minkowski, H., & Weyl, H. (1952). The Principle of Relativity: a collection of original memoirs on the special and general theory of relativity. Courier Dover Publications. p. 111. ISBN 0486600815.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. Congress of arts and science, universal exposition, St. Louis 1904, (1905), vol. 1, pp. 604-622,
  3. Yaakov Friedman, Physical Applications of Homogeneous Balls, Progress in Mathematical Physics 40 Birkhäuser, Boston, 2004, pages 1-21.
  4. C. Møller (1952). The Theory of Relativity (2nd Edition ed.). Delhi: Oxford University Press. p. 220. ISBN 019560539X. {{cite book}}: |edition= has extra text (help)

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