> The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
So I guess that means a meter is 30.66331898849836976219 cesium-133 transition periods.
Hope you can help me with a couple: why is there even a limit? how do we know there is an insuperable barrier nothing can cross for sure?
We have models that describe the world very well, but those models are never the whole truth (and even it were, we couldn't be sure of it).
Special and General Relativity are experimentally very well tested, and these theories predict that a particle that has a speed < c can never acquire a speed > c. There are solutions for speeds larger than c, but those solutions are quite weird, and can't interact with "normal" particles.
But the mere scientific approach of observing facts, modeling and verifying means that one can never be sure that something can't be - only in the limits of the models can we be sure that a thing can't be.
Someone else wrote my favorite answer: "Basically, the way to think of it is not that light is the fastest thing, but rather that there is a speed, c, which the geometry of space and time demands is the fastest possible speed.
One can also work out that anything without mass must travel at this fastest possible speed c. Light is one of those things, therefore light travels at c. It's only an accident of history that we call c "the speed of light": that's the context we discovered c's existence in.
As for why it's the speed it is, well, it's the speed in our universe. It's actually much more natural to say c=1 and all speeds are then unitless numbers between 0 and 1. From this point of view c is 300 Mm/s because of how we chose to define the meter and the second." http://www.reddit.com/r/askscience/comments/fjwkh/why_exactl...