Man, in his study of nature, has been able to codify mathematically many of her laws, and as a part of these mathematical expressions one frequently finds what are called "physical constants." These constants are numbers that are unchangeable. They may designate an actual absolute constant in nature or they may be inserted in order to achieve an equality. The gravitational constant is of the latter variety. It is in the equation for gravitational force for only one reason, to convert the units of mass and distance into force units. The speed of light, however, is of the first type, it is considered a constant quantity in nature. This was postulated as a part of Einstein's theory of relativity.
In contrast to constants there are also variables, and most of the quantities we deal with in everyday life are variables, quantities that change due to circumstances and viewpoint, especially those dealing with motion. Two people observing the same phenomenon will see it differently according to their relative positions and motion. For instance, if a boy is riding a bicycle at say 10 mph and throws a ball forward at 10 mph it will depart from the boy at 10 mph, but to an observer along the road the ball will appear to travel at 20 mph (speed of ball + speed of bicycle). Similarly, if the boy were to drop the ball, it would appear to him to drop straight down, but to the observer the ball would follow a curved path called a trajectory. If the boy threw the ball backward over his shoulder, it would travel toward the rear at 10 mph, but to the observer the ball would fall straight down.
This is called relativity, and simply stated, the path of a moving object depends on the point of observation, and the apparent speed of an object is relative to the observation point. The other day I was driving my car and I stopped at a red light. Another car stopped alongside in the lane next to me, and one stopped behind me. Suddenly I had the sensation of coasting backward, which was impossible because my foot was on the brake. I pressed harder but I still was moving, and I looked in the rear view mirror expecting to hear a loud honk or even a crash. But I wasn't any closer to the car behind me, and then I realized that I wasn't moving at all, but the car next to me was drifting forward. I had experienced a relativity attack. The same illusion can happen with two trains side-by-side, one moving and the other stationary.
According to Einstein's theory, the speed of light is not affected by relative motion. If the boy on the bicycle shined a flashlight beam forward or backward, it would not affect the speed of the light. In other words the speed of light is a physical constant. Christ is such a constant, He is the same yesterday, today and forever; with Him there is no variableness or shadow of turning. Jesus is the most reliable spiritual constant, "relatively speaking."
