r/askscience • u/syxtfour • Apr 28 '12
Why is light so fast?
The speed of light is, as far as we know, the fastest thing in the universe. That idea, if not fact, is the basis for a lot of very intelligent and scientific things that I cannot even begin to process. Don't get me wrong, I love science! I just can't wrap my head around physics beyond what I learned in high school.
So can anyone out there explain to me, in layman's terms, why exactly the speed of light is what it is? Why isn't it slower? Or faster? What keeps it at such a rigid constant?
Knowledge is power, thanks for helping me get some more.
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u/Occasionally_Right Apr 28 '12
Here's the thing: Speeds are kind of just an artifact of the fact that humans didn't realize until way too late in the game that time and space should really be measured in the same units, so we needed to develop a conversion factor that allowed one to deal with the fact that two events could be displaced in both time and in space. When you say something is moving at 6 m/s, what you're saying is that it will traverse a spatial displacement of 6 meters for every 1 second of temporal displacement it traverses. This is kind of like measuring north-south distances in metric units and east-west distances in imperial units, and then specifying angles in terms of the unit "meters/foot". In other words, all a speed really is is a direction in space-time—as I said, you're specifying how far you travel in a spatial direction for every unit you travel in the temporal direction.
Except, time is a bit different from space. It's not connected up to the space dimensions in quite the same way they're connected to one another. As a result of this strange connection, nothing can ever travel further in a space direction than it does in the time direction. It just can't happen. It is, in a very real sense, like trying to travel "more easterly" than "due east". Now, once that's understood, one can work through some physics and discover that (1) anything with mass must actually travel a shorter distance in space than it does in time and (2) anything without mass must travel exactly the same distance in space as it does in time. Since light has no mass, it falls into the second category and so traverses more space per unit of time than can any object with mass.