In proposing his theory Einstein postulated that the speed of light was a universal constant. However fast you may be traveling, you will always measure the speed of light relative to you to be the same—186,000 miles per second. You can never catch up with light. Even if you were traveling at 185,990 miles per second, light would still pass you by at 186,000 miles per second. Why should this be so? It seems totally counter-intuitive that the speed of light never varies. But this perplexing behavior takes on a rather different character when we distinguish our image of reality from the underlying reality. Space and time, and hence speed, are aspects of the phenomenal world; they have no meaning, it turns out, for light itself. According to the equations of Special Relativity, as an observer’s speed increases, time slows down, and length (in the direction of motion) contracts. At the speed of light, time has slowed to a standstill and length contracted to zero. Although no object with mass can ever attain the speed of light (the equations predict that it would then have an infinite mass), light itself does (by definition) travel at the speed of light. From light’s point of view—and this after all must be the most appropriate perspective from which to consider the nature of light, not our matter-bound mode of experience—it travels no distance and takes no time to do so. This reflects a unique property of light. In the spacetime continuum, the interval between the two ends of a light ray is always zero. How can we interpret this? We probably should not even try to interpret it. Any attempt to do so would make the mistake of applying concepts derived from our image of reality to the underlying reality. All we need to recognize is that, from light’s perspective, this zero interval manifests as zero space and a corresponding amount of zero time. However, when we in the world of sub-light speeds perceive light, we see a different manifestation of the zero interval. We observe a finite amount of space along with an “equal” amount of time. In our world, the light does travel through space and time. Since the total interval must be zero, the distance covered must exactly balance the time taken—that is, we must always observe 186,000 miles of space for every second of time. This we interpret as the speed of light. But this “speed” is not an intrinsic property of light itself; traveling no distance in no time, light has no need of speed. What we interpret as the speed of light is actually the ratio in which space and time manifest in our perception of reality. It is this ratio that is constant. And this is why all our measurements of the apparent speed of light are constant.
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