To those that don't know what an RKV is: a projectile (could be a spaceship, could be a bullet) that moves with a significant fraction of lightspeed. Maybe even close to lightspeed. Anything it hits will get vaporized just as well as in a nuclear explosion.
So far so good, but how big would one need to be? Obviously, the larger the mass, the larger the impact. Twice the mass, twice the kaboom. But also... the closer we get to lightspeed, the bigger the kaboom. To get to lightspeed would take infinite energy, so this implies that we can pack in arbitrary much energy by simply speeding up ever more.
So, to take this to the extreme, would it be possible to accelerate a single particle (say, an electron or maybe a muon since it's a bit heavier) so fast, that it could destroy a planet?
Or would there be some other laws of physics that would start to get in the way? Or maybe at those speeds it would just pass through harmlessly, creating a microscopical hole along the way?
If it's the latter case, then how big would a relativistic projectile have to be in order to deliver devastation to a planet? What equation governs this?
Let's assume that expense and technology are no barrier, but the laws of physics are as we know them.