In physics, special relativity (**SR**, also known as *the special theory of relativity* or STR) is the generally accepted and experimentally well confirmed physical theory regarding the relationship between space and time. In Albert Einstein's original formulation, it is based on two postulates:

- the laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
- the speed of light in a vacuum (C) is the same for all observers, regardless of the motion of the light source.

#### Comment: Why is it ‘special’ ?

Normal relativity by itself is mostly just adding or subtracting speeds. The *special* in special relativity is really about C, the speed of light or what happens when there is a maximum speed, everything else needs to change to keep C constant no matter where it is.

It was originally proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies”.

**Why it is or was needed **

The inconsistency of Newtonian classical mechanics with Maxwell’s equations of electromagnetism and the lack of experimental confirmation for a hypothesized luminiferous aether led to the development of special relativity, which corrects mechanics to handle situations involving motions nearing the speed of light.

Special relativity is currently the most accurate model of motion at any speed. Even so, the Newtonian mechanics model is still useful (due to its simplicity and high accuracy) as an approximation at small velocities relative to the speed of light.

**Implications of SR are counter intuitive**

Special relativity implies a wide range of consequences, which have been experimentally verified, including length contraction, time dilation, relativistic mass, mass–energy equivalence, a universal speed limit and relativity of simultaneity.

It has replaced the conventional notion of an absolute universal time with the notion of a time that is dependent on reference frame and spatial position. Rather than an invariant time interval between two events, there is an invariant spacetime interval.

Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula E = mc^{2}, where c is the speed of light in a vacuum.

**Adapted from** Special relativity. (2016, April 23). In Wikipedia, The Free Encyclopedia. Retrieved 01:23, April 25, 2016, from https://en.wikipedia.org/w/index.php?title=Special_relativity&oldid=716666776

**Extra commentary**

Here is a a useful youtube video that explains most of these consequences