Imagine there was another Earthlike planet in our solar system, say about where Mars is located. There is an intelligent species there, at approximately our level of technology.
Okay. This is major. All answers are pending what the species is like. I am assuming, for the moment, something not too far from "humans with funny foreheads", as opposed to, say hive-mind insects or aquatic species. I am assumign also that "level of technology" is similarly Trek-like, so that, for example, once they figure out how radio communication could be done, they actually *do* it...
How soon would we have begun communication with them?
We would have begun communication shortly after both species had radio communication of sufficient power to reach between the worlds. As soon as both species are putting out enough radio, detection within the same solar system is pretty much inevitable.
How soon would we begin interaction with them? What kind of interaction would we have with them?
Note that radio communication is a form of interaction. That probably comes first.
Would their existence be a prompt to advance our space industry faster?
Last question first - probably. We don't do lots with space now, because historically it has been difficult to justify the cost. If there were people out there, it becomes a whole other ballgame.
Would we have begun interplanetary trade of some kind by now? Would we be at war with them? ...
And on that last note, would discovering intelligent life elsewhere in the universe speed up our space exploration? Would we speed things up so we can interact with these others, or would we speed things up out of a sense of competition (or fear) like the US had with the USSR?
Well, here things get complicated. It depends on how that contact was made, and when. We are now in a realm of alternate history and speculation. We can only make such speculation with a set of assumptions...
Assume, for example, that we don't really know about the Martians until radio contact is made, between 1920 and 1940. We've had WWI, but contact with an entirely alien species comes before WWII. If WWII happens anyway (we can posit the socio-economic and political scene after WWI may have made it nigh-inevitable), we gain the technological basis for rocketry. From this point on, being able to reach Mars is merely a matter of time and motivation.
Even if we go full-bore on rocket development, I think war is unlikely. The physics and distances involved mean you might be able to move some very precious cargo, and do cultural exchanges, but chemical rocketry simply won't get you to the point where you can extend conventional military might over such distances. The supply lines are *years* long. Sure, you could pack nuclear warheads onto a rocket, but... what's the point? On Earth, there was a fundamental question of control of territory and resources that led to the cold war, and the nuclear threat. Between Earth and Mars, there is no real question of control of territory. There's just no point to blowing them up.
So, minor trade, some movement of small numbers of people back and forth. And, a fundamental change in how we view ourselves in the Universe.
