OK, so say the bubble universe theory is correct (parallel universe etc) and somehow a human society from an alternate Earth in another universe (at our stage of advancement - i.e. 2013) has managed to come into contact with “transportation” technology (a cosmic fluke/malfunction, if you will).
Do you think we would readily establish relations with them? Trade? How do you think we as a society and humans would take the arrival and react to it? How would you react/deal with it?
You have left out quite a bit of detail.
Are they humanoids? An alternate earth could easily have developed intelligence using a different stem clade than ours. Or perhaps not. Just knowing that one thing would advance our understanding rather significantly.
Do they suddenly appear in an Iowa cornfield, Stuttgart or in some analog lab that is studying transdimensional portals?
Do they speak English? Or even any other Terran language? That would be rather surprising.
So, in summary, it would depend upon our commonalities in biology, culture and language. If they are similar enough then the probability of the explorers from Earth-2 would be accepted eventually is more likely than not, and trade would ensue.
If not, then they are going to have a hard time fighting off the military and angry villagers.
I would judge them on one thing and one thing only: How many Highlander movies did they make?
I watched this documentary when I was a kid. It didn’t end well for the professor or the pretty girl.
Are they Nazis? Television has taught me that about 60% of alternate Earths are full of Nazis.
And 30% are full of Confederates.
Relations? Like rishathra?
Maybe…
Do they have the ending to “Edmund Drood”?
In part, it depends on just how “alternate” the alternate Earth is. If the only difference is that Joe Harcourt of Poughkeepsie had a chicken salad sandwich for lunch one day instead of tuna salad, the implications would be different from an alternate Earth where the dominant species is a race of sapient dinosaurs. The sweet spot would probably be a culturally different Earth still dominated by humans. An exchange of art, music, and historical information would be highly interesting and probably very valuable, and it would likely be easier to negotiate with other humans.
The top priority would be the transport technology itself, regardless of other differences. The potential for opening up other alternate Earths–specifically, uninhabited ones–should not be dismissed. Consider a few scenarios:
Earths on which no sapient species evolved could provide an entire planet full of raw natural resources.
An uninhabitable Earth with no exploitable resources could at least be used as a dump for toxic and/or radioactive waste.
Even an Earth where the matter never coalesced into a planet could provide an effectively “free” launch point for space travel, if the tech worked right. Translate across, thrust gently away from any minimal gravity from the scattered rocks, then translate back once you’re far enough away to establish a high orbit.
Of course, the process of exploring all these worlds could be hideously dangerous, but the value would be high enough to justify the risk, as long as strict precautions are taken.
First contact would be through a new communication device. The researchers try to eliminate an “echo” in the equipment until they realize that this “echo” is caused by resonance from parallel researchers in a nearby alternative. This realization leads to a tuning to allow them to talk with more divergent alts elsewhen in n-space.
If it’s an “all Humans have Red hair” universe, I’m emigrating.
[Alternate Earth Guys]Highlander 2 is considered the peak of our artistic efforts.[/AEG]
Us: Well, I’m afraid that means war!
Now, don’t jump to conclusions. Maybe their version is actually…well, I won’t go far as to say “good”, though in an infinite multiverse we must admit the possibility. Mediocre, anyway. If so, we could bargain for a copy of it and plant it among those who believe there were multiple Highlander movies in this world, ameliorating the effect of their delusions. I bet if we label it “The REAL Director’s Cut”, they’ll go for it.
Let’s assume this is a world not unlike ours, but some things changed throughout history.
A different country is the world’s super power, something like that.
What we (and with that I mean the human race, not individuals) do is probably what we’d do if aliens show up.
We claim that we wish to talk and learn, but our first thought is “how fast can we kill them ‘just in case we have to’” followed by “Can I dominate and take over their world?”
Now since this is also what they think, we’re going to have a nice cold war.
I think this would be a wonderful thing. We’ve just instantly doubled both the size of the human race and the amount of resources available to it, allowing us to dramatically increase our mutual creative and scientific output. Think of all the time we’d save if a solution found on one Earth didn’t have to be laboriously re-engineered over here! Think of all the new art, literature, and culture that they’d bring to us, and we to them!
Did they cancel Firefly? If not, they may correctly regard us as barbarians.
Gilligan screwed things up again, I’ll bet. 
I’d say Rembrandt was closer to Thurston Howell than Gilligan. I guess Jerry O’Connell would have to be the Skipper because he’s basically the leader.
I don’t it would be quite as simple as this. There won’t be a cold war between ‘Earth 1’ and ‘Earth A’ because neither of those Earths would be monolithic. There’s no way that the US, Russia, and China are going to work together against Bellacastan, Rebootopia, and Federated Republic of Kappalappadingdong. It would just make international politics way more complicated by nearly doubling the number of players.
Depending on how they look I would first need to decide if I want to eat them or have sex with them.