RobotRollCall comments on Why exactly can nothing go faster than the speed of light?
There are a lot of simple, intuitive explanations of this to be had out there … but I kind of hate them all. You might google around a bit and find discussion of something called "relativistic mass," and how it requires more force to accelerate an object that's already moving at a high velocity, stuff like that. That's a venerable way of interpreting the mathematics of special relativity, but I find it unnecessarily misleading, and confusing to the student who's just dipping her first toe into the ocean of modern physics. It makes the universe sound like a much different, and much less wonderful, place than it really is, and for that I kind of resent it.
When I talk about this subject, I do it in terms of the geometric interpretation that's consistent with general relativity. It's less straightforward, but it doesn't involve anything fundamentally more difficult than arrows on pieces of paper, and I think it offers a much better understanding of the universe we live in than hiding behind abstractions like "force" and outright falsehoods like "relativistic mass." Maybe it'll work for you, maybe it won't, but here it is in any case.
First, let's talk about directions, just to get ourselves oriented. "Downward" is a direction. It's defined as the direction in which things fall when you drop them. "Upward" is also a direction; it's the opposite of downward. If you have a compass handy, we can define additional directions: northward, southward, eastward and westward. These directions are all defined in terms of something — something that we in the business would call an "orthonormal basis" — but let's forget that right now. Let's pretend these six directions are absolute, because for what we're about to do, they might as well be. [...]
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