The simplest picture of light given by classical physics is of a wave or disturbance in the electromagnetic field. In a vacuum, Maxwell's equations predict that these disturbances will travel at a specific speed, denoted by the symbol . This well-known physical constant is commonly referred to as the speed of light. The postulate of the constancy of the speed of light in all inertial reference frames lies at the heart of special relativity and has given rise to a popular notion that the "speed of light is always the same". However, in many situations light is more than a disturbance in the electromagnetic field.