# Special relativity

## Special relativity or special theory of relativity

Automatic translation Category: light and photons
Updated June 01, 2013

Special relativity or special theory of relativity revolutionized our world view, the movement causes a slowing of time.
Special relativity will question the existence of the ether and relativize the absolute nature of space and time. Galilean relativity says that the movement depends on the repository where it is measured, i.e. it is different depending on the movement of the observer. Indeed, one ground observer who sees a plane, has a different perception of the speed of the aircraft that a pilot flying alongside in another plane.
According to this theory, a man on the ground and a pilot should see the rays of light travel at different speeds. In extreme cases, an aircraft flying at the speed of light would see the light by the on-site.
A light that does not move! Einstein could not conceive. Light defined by Maxwell, is considered since 1840, as a wave, as the sound travels through the air. But if one removes the air, the sound disappears.
The physicists of the 19th century invented then ether, intangible substrate which supports in a vacuum to convey light. Special relativity will question the existence of the ether. In addition, for Albert Einstein, it is likely that the light is always the same speed, so he became interested in publications of Michelson and Morley. If the speed of light is always constant, two observers have different movements, must register for a photon, speed and distance the same. This is not what we observe. At the end of the 19th century, two physicists, Albert Abraham Michelson (1852-1931) and Edward Williams Morley (1838-1923) sought to determine the flow of the ether by measuring the speed of light.

The speed of light must be different considering the movement of the Earth in space. The two scientists were expecting to measure a change, however small this speed, but the result was surprising, all rays of light had the same speed. The light does not respect the law of composition of velocities. In the early 20th century, a new definition of that light is required, Albert Einstein took his chance in 1905, he finds that there is an incompatibility between Maxwell's equations and hypotheses of Planck. We had small grains (quanta) of light to that the hypotheses of Planck are correct, and he concludes by stating that light behaves as both a wave and a particle flux. Einstein will break with Newtonian physics. Einstein revise the postulate of a space and absolute time, as defined by Newtonian mechanics. Only when objects pass at high speed in a straight line next to the observer, they shrink and clocks slow. Time and space are thus relative.
Albert Einstein builds these two notions of space and time. Einstein redefined some laws of nature, but his theory has limits, that is why it is called special theory of relativity. Many historians of science recalls the work of predecessors, Hendrik Antoon Lorentz and Henri Poincare regarding special relativity.
The exact contribution of Einstein, compared to Henri Poincaré and some other physicists, was controversial.

nota: It was only in 1920 that the elementary particles of light "quanta" were called "photons."

« It is absolutely possible that beyond what our senses perceive, hide unsuspected worlds. »
Albert Einstein Nobel Prize in 1921.

### Image: The famous "soft watches" by Salvador Dali in his painting, The Persistence of Memory (1931). The soft watches represent his obsession with death, as the softness of the time they are the result of thinking that is was the Master of the "hard and soft". An evening of migraine, having finished dinner, he stayed to observe the soft cheese that was lying on his plate. Despite his headache he began his easel and painted this masterpiece in two hours only.

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