Special theory of relativity

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Special Relativity (SRT) is a generalization of classical mechanics. In contrast to General Relativity, Special Relativity deals with processes observed in so called inertial frames (frames of observation without the influence of acceleration or gravity). It is based on two main observations from different experiments.

  1. The speed of light is constant for all (inertial) observers, regardless of their velocities relative to each other.
  2. The laws of physics are identical in all inertial reference frames.

It is not difficult to demonstrate that it is impossible to reconcile these conditions with a Newtonian mechanics, in which the coordinates are formed in a three-dimensional space. In SRT, each object moving in its own inertial frame has its own time, which constitutes a fourth coordinate describing its state.

The three most prominent SRT effects are time dilation, length contraction and the equivalence of mass and energy.

Four Vectors and relativistic metric

Experimental Proofs

  1. Michelson-Moreley experiment
  2. Blackbody radiation spectrum
  3. GPS clocks (general relativity)
  4. Lifetimes of fast traveling particles


History

Special Relativity (SR) was initially developed by Henri Poincaré and Hendrik Lorentz, working on problems in electrodynamics and the Michelson-Morley experiment, which had not found any sign of luminiferous aether, which was believed to be the substance which carried electromagnetic waves.


Relation between energy and mass

The famous equation attributed to Einstein, E=mc2, describes the relationship between energy and the minimum energy of a body in its own inertial frame, also called the rest mass. In nuclear physics, during fission or fusion reactions, the sum of the rest mass of the constituents changes, releasing energy in form of radiation or kinetic energy.

Classical mechanics as a limiting case

At low speeds (relative to light-speed), the Einstein-Lorentz relativity equations are equivalent to Newton's equations. Particles at low mass and low speed can be approximated by classical mechanics (Isaac Newton's laws of motion). If an objects energy is mainly constituted by it rest mass, the classic limit is valid. Relativity is essential for fast-moving bodies. Electromagnetism, including for light and gamma radiation, where the quanta (photons) have no rest mass, is always relativistic

Spin as a relativistic effect

The relativistic extension of Quantum Mechanics, described by the Dirac Equation allows, due to the symmetry of the equation in 4-space, an additional quantum number to exist, called spin. electron spin was known from chemistry before relativity arose.

Relation between energy and mass

The famous equation attributed to Einstein, E=mc2, describes the relationship between energy and the minimum energy of a body in its own inertial frame, also called the rest mass. In nuclear physics, during fission or fusion reactions, the sum of the rest mass of the constituents changes, releasing energy.


Gravitational lensing

(effect of general relativity)

Light passing through a region containing many massive bodies such as galaxies will be distorted. Telescopic observations confirm that galactic clusters distort the paths of the light passing through them, and the effect can be used to focus on Objects far behind the "gravitational lense" . classical mechanics, in which light travels in straight lines, can not explain this.

Interpretation and paradoxes

Some consequences of the SRT are:

  1. It is impossible ever to transmit information faster than the speed of light.[1]
  2. The laws of physics are identical, without any variation, in every location throughout the universe.
  3. The laws of physics are identical, without any variation, no matter how fast something is traveling (in the absence of acceleration).
Special relativity alters Isaac Newton's laws of motion by assuming that the speed of light will be the same for all observers, despite their relative velocities and the source of the light. (Therefore, if A sends a beam of light to B, and both measure the speed, it will be the same for both, no matter what the relative velocity of A and B. In Newtonian/Galilean mechanics, If A sends a physical object at a particular velocity towards B, and nothing slows it, the velocity of the object relative to B depends on the velocities of the object and of B relative to A.)
  1. This assumption is commonly restated in this manner. For example, a discussion of hypothetical tachyons talks "about using tachyons to transmit information faster than the speed of light, in violation of Special Relativity."[1] However, there is some question whether the Theory of Special Relativity really restricts faster-than-light communication of information.