Hey all!

I'm new to here, and I don't really know much about physics and maths, but I'm quite interested in knowing what relativity really is. Could anyone please explain it to me? :)

Hi Mithadon,

With the words "the theory of relativity" you refer to two theories, both attributed to Einstein. First there is the special theory of relativity (1905), or SR for short, which deals with high-energy phenomena and phenomena at high speeds. And you also have the general theory of relativity (1916), GR for short, which unites the ideas of the special theory of relativity with gravity.

SR, in its traditional form, is a mathematical framework which is an extension of classical (Newtonian) kinematics. It is build on two postulates:

1. The Galilean principle of relativity; also used in classical Newtonian mechanics. This principle basically defines what an observer is, through a mathematical definition of what is called "an inertial frame".
2. The postulate that the speed of light is constant for all observers (in the sense of postulate 1), regardless of their state of motion.

Using these two simple postulates, you can explain several experimentally known phenomena. First of all, it implies that the speed of light is an upper boundary for the speed of matter. Secondly, it predicts phenomena such as "time dilatation" and "length contraction" (sometimes called Lorentz contraction): the time and space (length) between two events can be perceived differently for different observers, depending on their relative motions with respect to the events. SR hence predicts that space and time (conveniently - and also necessarily - grouped into "spacetime") are relative notions.

The beauty of the theory consists in nicely combining classical electromagnetism and mechanics into one mathematical framework.


The general theory of relativity concerns gravity. It describes gravity not as a force, but as the geometry of spacetime. The equations involved are highly mathematical (differential geometry) and predict phenomena such as black holes, gravitational lensing, the perihelion precession of mercury, ...

Bye!

Crisp