In a post-apocalyptic world, a young adult studies learning. After finding a manual on how to call and use a blue orb through the use of his mind. Using the orb, he dominates his warring state and ends up conquering other nearby states.

The blue orb is activated by Einstein's E=mc^2 and can instantly convert mass into energy. He finds a spaceship only to find that aliens have come to Earth to conquer it. These aliens were defeated centuries ago by Earth's ultimate weapon (i.e. the blue orb). However, since then, something happened which lead to the post-apocalyptic world.

Using the orb, he defeats the aliens.

Does anyone remember the name of the book? Thanks, in advance.