Theory Of Relativity
The theory of relativity, or simply relativity in physics, usually encompasses two theories by Albert Einstein: special relativity and general relativity. (The word relativity can also be used in the context of 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 contracts and time dilates.
- 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 in 1906 by Max Planck, 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"
Effects of Relativity on Science:
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 When first published, relativity superseded a 200-year-old theory of mechanics created primarily by Isaac Newton.
In the field of physics, relativity improved 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.
Brief on Relativity:
The theory of relativity has been repeatedly contradicted by experiments, such as precise measurements of the advance of the perihelion of Mercury that show a shift greater than predicted by Relativity, well beyond the margin of error. It is unlikely tenure or a Ph.D would be awarded to any critic of the theory.
Relativity refers to two closely-related mathematical theories in physics:
- Special relativity (SR) is a theory which describes the laws of motion for non-accelerating bodies traveling at a significant fraction of the speed of light. As speeds approach zero, Special Relativity tends towards equivalence with Newton's Laws of Motion. Special Relativity was developed by Hendrik Lorentz, Henri Poincaré, and Hermann Minkowski, and Albert Einstein.
- General Relativity (GR) is a theory which explains the laws of motion as viewed from accelerating reference frames and includes a geometric explanation for gravity. This theory was developed by David Hilbert and Albert Einstein as an extension of the postulates of Special Relativity A dramatic but later discredited claim by Sir Arthur Eddington of experimental proof of General Relativity in 1919 made Einstein a household name.
These theories have augmented earlier approaches, such as Galilean Relativity.
Unlike most of physics, the theories of relativity have discontinuities whereby the limit of a physical quantity as a variable (such as mass or velocity) approaches a fixed value is not the same as the physical quantity at the fixed value. For example, the limit of momentum as mass approaches 0 and velocity approaches the speed of light is not equal to the momentum of (massless) light.
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More generally, and also unlike most of physics, the theories of relativity consist of complex mathematical equations relying on several hypotheses. For example, at Hofstra University general relativity is taught as part of an upperclass math course on differential geometry, based on three stated assumptions. The equations for special relativity assume that it is forever impossible to attain a velocity faster than the speed of light and that all inertial frames of reference are equivalent, hypotheses that can never be fully tested. Relativity rejects Newton's action at a distance, which is basic to Newtonian gravity and quantum mechanics. The mathematics of relativity assume no exceptions, yet in the time period immediately following the origin of the universe the relativity equations could not possibly have been valid.
Relativity has been met with much resistance in the scientific world. Louis Essen, the man credited with determining the speed of light, wrote many fiery papers against it such as The Special Theory of Relativity: A Critical Analysis. Relativity is in conflict with quantum mechanics, and although theories like string theory and quantum field theory have attempted to unify relativity and quantum mechanics, neither has been entirely successful or proven.
Unlike Newtonian physics, in which space and time intervals are each invariant as seen by all observers, in SR the only invariant quantity is a quadratic combination of space and time intervals (x2 - c2 t2). The (assumed) instantaneous transmission of Newtonian gravitational effects also contradicts special relativity.
In quantum mechanics, the uncertainty principle suggests that virtual particles can sometimes travel faster than the speed of light which would violate causality, but "[t]he only known way to resolve this tension involves introducing the idea of antiparticles." Consequently, in 1928 Paul Dirac derived the Dirac equation, one of the first quantum mechanical equations compatible with special relativity, by which Dirac predicted the existence of antimatter. Four years later, antimatter (the positron) was discovered by Carl Anderson, as successfully predicted by relativistic quantum mechanics. Quantum field theory, a generalization of quantum mechanics, is fully compatible with special relativity but not with general relativity, and still lacks a vital piece: evidence of the graviton.
Special Relativity:
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:
- The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
- The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the light source.
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.
- Relativistic mass
- 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. (Refer Maxwell's equations of electromagnetism).
General Theory Of 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 fallis 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 slower in deeper gravitational wells.[8] 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 space-time 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.
Everyday applications
The theory of relativity is used in many of our modern electronics such as the Global Positioning System (GPS). GPS systems are made up of three components, the control component, the space component, and the user component. The space component consists of satellites that are placed in specific orbits. The control component consists of a station to which all of the data from the space component is sent. Many relativistic effects occur in GPS systems. Since each of the components is in different reference frames, all of the relativistic effects need to be accounted for so that the GPS works with precision. The clocks used in the GPS systems need to be synchronized. In GPS systems, the gravitational field of the earth has to be accounted for. There are relativistic effects within the satellite that is in space that need to be accounted for too. GPS systems work with such precision because of the Theory of Relativity.
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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.
Although it is widely acknowledged that Einstein was the creator of relativity in its modern understanding, some believe that others deserve credit for it.