Relative speed is the speed and direction of an object relative to a reference point. The speed of light is not relative. Velocities should be relative to an inertial frame of reference. The speed of light is constant regardless of the observer and is the basis of the special theory of relativity.
Relative speed refers to the speed and direction of an object relative to some other reference. There is no standard reference for a relative speed; however, some references, such as terrain, are much more convenient than others. Thanks to this principle, it is possible to describe the same object as having several different speeds, each with a different frame of reference. However, the speed of light is not a relative speed in this sense.
Generally, all velocities should be relative to an inertial frame of reference. Any frame of reference in space that is not accelerating is equally suitable. The surface of the Earth is a good approximation for an inertial frame of reference when the distances involved are not too great. This is because small areas appear to be flat and stationary; that is, objects appear motionless when they move at the same speed as the ground. When the distances become too great, it no longer makes sense to give speed relative to the ground: due to the rotation of the earth, different parts of the globe move in different directions.
For example, a speed of 70 miles (112.7 km) per hour on a highway is understood to be relative to “still” ground. This is because the surface of the Earth rotates around its core and the Earth travels around the sun. The solar system itself revolves around the center of the Milky Way galaxy, and so on. Therefore, a velocity is only useful when it is relative to a frame of reference. A speed limit on a motorway is actually a relative speed limit.
Danish astronomer Ole Christensen Rømer first measured the speed of light in 1676. He compared the time it took for light to travel from Jupiter’s moon Io when Earth was at various distances from it. When Earth was farthest from Jupiter, it took considerably longer for light to arrive. Unbeknownst to Rømer, however, light does not behave the same way as ordinary matter. The speed of light, and of all electromagnetic radiation, is constant regardless of who observes it.
In 1905, German physicist Albert Einstein proposed the theory that the motion of an observer does not affect the speed of light. This breakthrough served as the basis for the special theory of relativity. The implications of him, while not commonly apparent in everyday life, are far-reaching in physics. In essence, the principle means that the speed of light is not a relative speed in the sense above. Rather, time itself depends on the motion of an observer.
Protect your devices with Threat Protection by NordVPN
