[jennyp]

IMHO, SETI is our best bet for discovering if another tool building species is out there. You may have noticed I did not say intelligent. There may be lots of intelligent races, however, if they never build tools or use EM for communications, it will be nigh too impossible to discover they exist. Using spectroscopy, we can find the signature of life, however, if they don’t build tools we will never realize their intelligence.

OK, this brings up a question I've been pondering: Let's say an alien species exploded a 1 megaton (for instance) nuclear weapon in space within their solar system, as a "Hello World" signal. How far away could we be and still detect it, with our technology?

[RadioAstronomer]

OK, this brings up a question I've been pondering: Let's say an alien species exploded a 1 megaton (for instance) nuclear weapon in space within their solar system, as a "Hello World" signal. How far away could we be and still detect it, with our technology?

We would not even notice it I dont think. Most of the damage from a nudet is due to there being an atmosphere in the first place. The brief increase in gamma and other incidental emmisions would most likely be drowned out by the star.