There are theoretical limits to the speed of computation. One limit is the minimum amount of energy it takes to flip a bit. For 256-bit encryption, you have to start saying things like "assume we can convert 100% of the energy from a supernova into a theoretically perfect computer with perfect efficiency". This is a round about way of saying "impossible".

We've been hammering AES and RSA for decades now, and we haven't been able to get significantly better than brute force against either one. Quantum computers will break RSA (if they can be made with enough qbits, but might be infeasible), but worst case scenario for AES is that we double the key length and we're good again.