Originally Posted by johnrain
How can any human build a robot like Ultron and make it powerful enough to challenge the Avengers? The technology does not exist.
Remember this Ultron evolves and escapes. The later Ultron robots in no way look like what Pym made. Every time they have thought he is gone , he comes back. Also you have to suspend your disbelief a little. This is after all fantasy. Ultron has a few basic goals. Survive, exterminate, evolve, and then there is that Oedipus complex.