= Special relativity
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Explains how it is possible <invariance of the speed of light>[that everyone observes the same speed of light, even if they are moving towards or opposite to the light]!!!
This was first best observed by the <Michelson-Morley experiment>, which uses the movement of the Earth at different times of the year to try and detect differences in the speed of light.
This leads leads to the following conclusions:
* to <length contraction> and time dilation
* the speed of light is the maximum speed anything can reach
All of this goes of course completely against our daily Physics intuition.
The "special" in the name refers to the fact that it is a superset of <general relativity>, which also explains gravity in a single framework.
Since time and space get all messed up together, you have to be very careful to understand what it means to say "I observed this to happen over there at that time", otherwise you will go crazy. A good way to think about is this:
* use <Einstein synchronization> to setup a bunch of clocks for every position in your <frame of reference>
* on every point of space, you put a little detector which records events and the time of the event
* each detector can only detect events locally, i.e. events that happen very close to the detector
* then, after the event, the detectors can send a signal to you, who is sitting at the origin, telling you what they detected