Speed (not the drug) is simply defined by physicists as distance (d) divided by time (t). How fast something travels is therefore defined by how far it travels and how long it takes to get there. Pretty simple.
The speed of sound, for instance, is 340.29 meters per second. What does this mean? It means that sound travels through the air at this speed. Note that the air is the medium through which the speed is determined. The speed of anything has to have something to be measured against or the very concept has no meaning.
Not so with light.
In the early twentieth century, a young Albert Einstein asked himself what a beam of light would look like if you could somehow catch up with one. What he envisioned was a light beam that, from the perspective of the observer, would appear to stand still, much as a car on an adjacent lane of the freeway will appear to be if you were traveling at the same speed beside it.
But he also knew this could not be the correct picture. Some fifty years earlier, the physicist James Clerk Maxwell had discovered, among other things, that light could never appear stationary. His equations forbade it.
The implications of this are subtle yet powerful. If light always appears to be moving at this cosmic speed limit, regardless of its relative motion to an observer, then what is it moving in respect to? What is the medium through which light travels that provides its standard of rest?
At the time, the consensus among the physics community was that light moved through an invisible and undetectable mystery substance called the “aether.” Einstein saw no need for the stuff. Maxwell’s equations certainly didn’t require it. So why use it?
This, in part, led to Einstein’s revolutionary realization that light moves at a constant speed, irrespective of the relative motion of an observer. This behavior is unlike anything else in the physical world.
Take, for example, a wave of water in the ocean. If you swim toward it, the speed of the wave will approach you more quickly. Similarly, if you swim away from it, it will take longer to reach you. This is because the water is moving in respect to the sea floor. This is a simple matter of distance over time. Speed, in other words.
In the case of light, though, the same thing will not occur. Even if you could somehow commandeer a spacecraft that flew at, say, 80% the speed of light and you headed toward a light source, the light beam would maintain its same speed. The same thing would happen if you flew away from it. It wouldn’t slow down, taking longer to reach you, as would a wave of anything else. If you were to clock its speed, you would find it the same value regardless of your relative motion to it. This is the value c, discovered by Maxwell to be nearly 300,000,000 meters per second.
When the definition of speed (distance over time) is applied to this strange property of light, things start getting really weird. This is where Einstein’s special theory of relativity entered the picture, proving conclusively that the universe is one heck of a lot more bizarre than people had previously thought.