In Einstein's theory of relativity everything is relative to an observer. For example, if two observers are moving relative to each other then their measurements of the time between two events will be different. (Aside: they have to be moving at some speed which is a substantial fraction of the speed of light for the difference to be significant). The theory is then sometimes used to claim a "scientific" basis for relativism:
"it all depends on your perspective""there is no absolute truth""there is no right answer".
However, such conclusions are based on mis-understanding the theory. It actually describes the relationship between the physical measurements of time and length of two different observers. In particular, this relationship defines an absolute time (a space-time invariant) which is the same for all observers.
Einstein's special theory of relativity (1905) is based on two postulates:
- The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity),
- The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.
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