An elementary particle such as a muon, when accelerated in a particle accelerator like the Stanford Linear Accelerator (or SLAC) to a speed approaching that of light, becomes many more times massive than it was when at rest.

This is an experimental test disproving the Theory of Relativity (both the Special and the General Theory), according to which there can be no such thing as absolute motion. As a consequence, according to the Theory of Relativity, acceleration and deceleration are supposed to be equivalent in every way — and that it can therefore be equally validly said that it was the muon before the experiment which was originally moving at a high velocity, and that during the experiment it was decelerated by the magnetic and electrical fields of the apparatus and thereby brought down in velocity.
 
 
 
 
 
 

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