by Tarek Musslimani

Created on: October 02, 2010   Last Updated: October 03, 2010

The theory of relativity was formulated by Albert Einestein in the beginning of the 19th century.  It applies to systems travelling at speeds close to the speed of light.  It is a four dimensional theory that includes, in addition to the three cartesian coordinates x, y and z, the element of time (t) as a fourth variable.  The postulates of the special theory of relativity are that the speed of light is constant in all inertial reference systems and that all laws of physics are valid in all inertial reference systems. 

In classical physics, length, time and mass have constant values when measured in different inertial reference systems.  At speeds close to the speed of light the measurement of these quantities are affected by the choice of the reference frame in which the measurement is done. 

In the theory of special relativity, time measurement in different inertial systems gives different values for the time measured.  The time measured in one inertial system is related to that measured in another inertial reference system by the following: relation:

t' = t*/SQRT(1-(v/c)(v/c))  where t' is the time measured in the frame that is moving at a speed very close to the speed of light, and t is the time that is measured in the frame that is at rest.  Where SQRT means the square root. 

Analogously the distance that is measured in two different reference systems, one is at rest and the other travelling at a speed very close to the speed of light c, is not the same but is different by the factor 1/SQRT(1-(v/c)(v/c)).

This factor is the same as that one that relates measurements of time in different inertial systems one at rest and the other travelling at a speed v close to the speed of light c.

The equation that relates the measurements of length in two inertial systems, one is at rest and the other travelling at a speed of v close to the speed of light is as follows:

L' = l/SQRT(1-(v/c)(v/c)) where l' is the length measured at the travelling reference system and l is the length measured at the reference system at rest.

The third physical variable that its measurement is affected by applying the theory of special relativity is the mass of a body.  Measurement of the mass depends on the reference system whether it is at rest or it is moving at a speed of v close to the speed of light.  Travelling at speeds close to the speed of light leads to enlargement

• 1
• 2
• Next Page >>