Beam weapons do exist, and are a whooole lotta fun
Of course, what the truthers propose is more than a little outlandish, something to that scale and power would be rediculously inefficient, if not impossible to produce with current technology.
But still, as I said, existing one's can be a lot of fun! I'm thinking about building one of these to play with:
http://en.wikipedia.org/wiki/Electrolaser