GeoGebra

The Postulates

Author:Timo Budarz
[url=https://pixabay.com/en/albert-einstein-scientist-physical-1100450/]"Einstein"[/url] by eugeniohansenofs is in the [url=http://creativecommons.org/publicdomain/zero/1.0/]Public Domain, CC0[/url]
"Einstein" by eugeniohansenofs is in the Public Domain, CC0
In 1905 a 26-year-old patent clerk named Albert Einstein published a paper with the (translated) title On the Electrodynamics of Moving Bodies. A copy of the paper (translated to English from German) is below. While the title might not sound revolutionary, the paper certainly sparked one. It was in this paper that Einstein caused the world to forever after question the nature of space and time. In the paper he showed that time and space are not absolute quantities, but rather ones that are subject to the observer's reference frame. What we will see in this chapter is that time ticks at different rates in different frames of reference and that sizes of objects, momenta of moving bodies, kinetic energy and what we call mass, all depend on the observer's frame.

On The Electrodynamics of Moving Bodies

specialrelativity.pdf

specialrelativity.pdf

Two Postulates

There are only two postulates on which all the results of special relativity depend. In Einstein's own words, they are:
  1. The same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good.
  2. Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.
The first postulate mentions frames of reference in which the laws of mechanics hold good. By this he means any non-accelerating frame of reference. If you were playing billiards on a moving train (or on a moving planet as we always do), the game acts as it should only until the train brakes or turns or accelerates in any way. The pool game on earth goes as expected only unless the earth accelerates as might happen during an earth quake. Keep in mind that orbital motion as our planet undergoes necessarily implies a perpetual centripetal acceleration, but being so small it doesn't mess up our game of pool. I should further mention that obviously we can still predict motions of objects in accelerated reference frames (as we did in the chapter on effective gravitation), but the basic laws from inertial frames don't hold. The second postulate implies something odd about light. It says that even if the source of light is coming toward us, we measure the emitted light approaching at the same speed c. This is true even if the source of light is traveling toward or away from us at nearly the speed of light itself while emitting the light in our direction. If this doesn't strike you as strange, maybe you didn't read it mindfully. Nothing in our experience behaves this way. The speed of light is the same whether you move toward or away from it at any speed, or whether it moves toward or away from you at any speed! If you haven't read before about the consequences of these two postulates, the discussions that will follow in this chapter will forever change your view of our world.

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