# The Theory of Special Relativity

The diagram below tries to set the scene for the following
discussion of special relativity, which will then be followed by a
discussion of general relativity. In part, Einstein’s publication of
the theory of special relativity, in 1905, established the
relativity of space and time, plus its implications on mass and
energy. Ten years later, in 1915, Einstein completed the general theory of relativity inclusive
of `*Einstein Field Equations`* which went onto describe the properties
of a gravitational field associated with a given mass. These equations
also help describe how an object curves space and how this curvature
can cause a spatial distortion of matter. However, as indicated. The
discussion of this aspect of relativity will be deferred to a separate
discussion specifically covering *`General
Relativity*`. In the context of special relativity, we shall
proceed on the initial assumption that space-time is flat.

** **

So, it was Einstein's special theory that forwarded the idea that both space and time are relative, rather than absolute concepts, in the absence of a gravitational field. This theory is based on two fundamental postulates:

- The laws of physics are the same in all reference frames that move uniformly and without rotation.
- The speed of light in a vacuum has the same constant value in all inertial systems.

However, the second postulate, which implies that nothing can travel
faster than the speed of light, was incompatible with Newton's Law of
Universal Gravitation. It was the implication of this postulate that
took Einstein a further 10 years to resolve in his *General Theory
of Relativity*. Initially, in the context of special relativity,
Einstein proposed that two observers in motion could have a different
perception of time and space, but in accordance with the 1^{st}
postulate, the laws of physics had to remain consistent to both observers.
As a consequence of the second postulate, i.e. the speed of light must
remain constant to all observers, and required the relative perception
of time and space to change as an object approaches the speed of light.
The implications of these observed changes can be described in terms
of a number of effects:

- Time dilation
- Length contraction
- Effective Mass & Energy
- Simultaneity & Causality

However, we will begin by outlining the basic principles behind the
first two effects, before proceeding to address some of the wider issues
associated with special relativity.