Einstein's theory of relativity isn't actually *a* theory, rather it is the sum of two theories: special relativity and general relativity.

In 1905 Albert Einstein published this theory on special relativity, arguing that the two quantities, space and time, are relative, not fixed.

However, special relativity was constructed for objects traveling at constant speeds and could not handle the addition of accelerating reference frames.

After about eight years of work, Einstein published his theory of general relativity in 1915, expanding the concepts of special relativity to include accelerating reference frames.

Despite the fact that the two theories are constructed on the same basic principle, that space and time are relative, there is no straight forward way of combining the theories into a single expression (see section on General Relativity below). Furthermore, the applications of the theories tend to be extended to different areas of physics. Special relativity is applied most often to particle and nuclear physics experiments, while general relativity is really the study of cosmology and astrophysics, such as studies of the intense gravitational fields around black holes and neutron stars.

### Special Relativity

While Einstein receives all of the credit for this theory, there were others that contributed to the development of its concept. However, it was Einstein who correctly formulated the ideas into a working mechanics.

The framework for special relativity was based on two concepts: That the laws of physics are the same for all observers (assuming uniform motion relative to one another), and the speed of light in a vacuum is the same for all observers *regardless* of their relative motion, or the motion of the light source.

There are several interesting consequences to the above framework. Namely:

- An observer measuring the length of an object moving near the speed of light would measure the object to be
*shorter*than if the object were at rest. (Length Contraction) - A consequence of this is that while one observer, moving near the speed of light, may observe a simultaneous event, while the stationary observer would not see the events as simultaneous.
- A clock traveling near the speed of light will pass time more
*slowly*relative to a stationary clock. (Time Dilation) - Mass and energy are equivalent (i.e. Energy equals mass times the square of the speed of light.)
- No mass can travel at the speed of light (or beyond), while light itself must always travel at the "speed of light" or slower in various media. (Interestingly, this does not preclude the possibility of a warp drive.)

After careful examination of the above statements and interesting consequence emerges: Nothing is really ever at rest. To measure something at rest requires a perspective; something to be at rest relative to.

### General Relativity

Once Einstein's theory of special relativity was published there was a glaring omission that troubled the young scientist: the theory was valid only for reference frames in uniform motion. What happens in a non-inertial (accelerating) reference frame?

(*In fact, this question dogged many of the great physics mind of the era, most notably David Hilbert who some believe should be acknowledged as a significant contributor to the theory and may have arrived at essentially an equivalent set of equations around the same time as Einstein.*)

Finally, in 1915 Einstein published his set of equations that completely describe the geometry of space and time (space-time). It is sometimes referred to as a theory of gravity.

The reason for this is that Einstein predicated his theory on, what has now become known as, the equivalence principle. This simply states that the cases of being at rest in a gravitational field and undergoing acceleration are equivalent.

In general relativity this concept is expressed by making shaping gravitational fields as curved space-time. In doing so, general relativity offers some interesting, and experimentally verifiable results:

- Like in special relativity, time can run slowly in some conditions, in this case by entering gravitational potentials. We actually have to account for this in our GPS satellites, if we didn't they would not work.
- Photons - packets of light - are bent when passing through a gravitational field. In cosmology, this phenomenon is the basis for gravitational lensing.
- The Universe is expanding, with distant objects receding from us faster than the speed of light. The observational evidence for this did not exist when Einstein formulated his equations in 1915, and he therefore added a "cosmological constant" to counteract the expansion. He later called this the greatest blunder of his career.