why does it have to be a natural source? If we can create them, would that not be enough to use them?

 

Because it takes at least as much energy to create them as you will get out of the annihilation. The same reasons you can't pump water uphill to run your hydroelectric plant: the first and second laws of thermodynamics.

I'd just like to know what are tachyons[i'm not making this up I promise!'], and why do they travel faster than the speed of light?

 

An addition to what YT said, they are a solution to the relativistic equation for energy of a particle for v>c. However, this solution requires imaginary (mathematically) mass, or if they have real mass, then imaginary (and negative) energy. As such, they would have to have very different properties than matter as we know it, and if there's no way to interact with them, we can't detect them.