# 10: Einstein's Postulates

- Page ID
- 17430

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- 10.1: An Old and a New Axiom
- The theory of special relativity is built on two postulates (our axioms for this chapter). The first one also applies to classical mechanics and simply states that: (1) The laws of physics are identical in every inertial reference frame and (2) The speed of light in vacuum is the same in all inertial reference frames.

- 10.2: Consequences of Einstein's Postulates
- The combination of Einstein's Postulates leads to a number of consequences that appear to be at odds with everyday experience. They imply that there are no such things as universal measures of time and length, nor even agreement on whether events are simultaneous or not. The reason why we do not observe these consequences all the time is that their effects are very small for objects which are moving slowly (as compared to the speed of light).

Thumbnail: Einstein cross: four images of the same astronomical object, produced by a gravitational lens. (Public Domain; NASA and ESA).