Each of the letters of E = mc² stands for a particular physical quantity. Writing them out in full we get:
In other words:
* E = energy (measured in joules, J)
* m = mass (measured in kilograms, Kg)
* c = the speed of light (measured in metres per second, ms-1)
Note that the case of each letter is important and it would be incorrect to show the equation as, for example, e = MC². This is because physicists use the case of letters as well as the letters themselves to denote particular physical entities, quantities and constants in equations.
In order for the equation to be correct we need to "square" the term c (the speed of light), i.e. we multiply the speed of light by itself: hence c² is the same as c times c. This allows us to be write the equation in another, slightly unusual, but equally correct way:
As a matter of interest, the equals sign was only invented during the 16th century, by the Welsh mathematician Robert Recorde. Apparently he was fed up having to write out "is equal to" in his work. He could of chosen any number of symbols but chose two parallel lines because, as he himself put it "noe 2 thynges can be moare equalle".
We will now examine each term (letter) in the equation in turn before addressing the question of what the equation means.