I agree with the poster above - nuclear weapons send a signal across everything, time, space, what have you.

If aliens indeed were interstellar beings, watching over the development of others (as life is precious in this universe, hanging in the deep blackness of space, and powerful enough to keep the universe turning) then a nuclear explosion would be like a lightning bolt coming out of a baby monitor.

Also think we'll see something major with this higgs field stuff soon - I get the feeling it won't allow us to travel faster than light, but may let us build something with which we can communicate anywhere instantaneously. Just a weird hunch.