Relative velocity refers to the speed and direction of an object with respect to some other reference. There is no standard reference for a relative velocity; however, some references, such as the ground, are much more convenient than others. Due to this principle, it is possible to describe the same object as having multiple different velocities, each with a different reference frame. The speed of light is not a relative velocity in this sense, however.
Typically, all speeds must be relative to some inertial frame of reference. Any reference frame in space that is not accelerating is equally suitable. The surface of Earth is a good approximation for an inertial frame of reference when distances involved are not too great. This is because small areas of it appear to be flat and stationary; that is, objects appear to be at rest when they are moving at the same speed as the ground. When distances become too great, it no longer makes sense to give velocities relative to the ground—due to Earth’s rotation, different parts of the globe are moving in different directions.
For example, it is understood that a speed of 70 miles (112.7 kms) per hour on the highway is relative to the “stationary” ground. This is because the surface of Earth is rotating around its core, and Earth is traveling around the sun. The Solar System itself is revolving around the center of the Milky Way galaxy, and so on. Therefore, a velocity is only useful when it’s relative to some reference frame. A highway speed limit is actually a relative velocity limit.
Danish astronomer Ole Christensen Rømer first measured the speed of light in 1676. He compared the time it took light to travel from Jupiter’s moon Io when Earth was at various distances from it. When Earth was further from Jupiter, it took noticeably longer for the light to arrive. Unbeknownst to Rømer, however, light does not behave in the same way ordinary matter does. The speed of light, and of all electromagnetic radiation, is constant regardless of who is observing it.
In 1905, German physicist Albert Einstein proposed the theory that an observer’s motion does not affect the speed of light. This breakthrough served as the basis for the theory of special relativity. Its implications, although not commonly noticeable during everyday life, are far-reaching in the field of physics. In essence, the principle means that the speed of light is not a relative velocity in the previous sense. Rather, time itself is dependent on an observer’s motion.