To illustrate the realities of the problem, let us perform an experiment in alternative history. Let's pretend that the United States government, flushed with hubris thanks to the success of the moon missions, decides to send a mission to Alpha Centauri in 1970. Alpha Centauri is 4.3 light years away, but let's say that state of the art technology will require a full decade to complete the trip.
Exactly what's involved in getting the colonists there in one piece is irrelevant - really good suspended animation, or a big ship loaded with all the Tang™ and TV dinners that anyone could want, along with an awfully efficient recycling system. Whatever it takes, NASA loads the ship up with a thousand of the best and the brightest of both sexes, along with a full quota of axes and fish hooks - well, chainsaws and drilling rigs, more likely - wishes them Godspeed, and sends them on their way. (Hopefully they'd actually send two or three ships - one ship seems like an awfully isolated basket for all those eggs, let's get a little redundancy going.)
Against all expectations from movies and television, everything goes flawlessly, and a thousand colonists land on Alpha Centauri Prime, cleverly name it New Earth*, and set up camp. They wait a few years to make sure that their beachhead on alien soil is going to thrive, then send the ships back to earth with the good news, along with a few tons of whatever resources seem appropriate for the purposes of trade (or debt reduction, depending on how the paperwork is set up), and a few people who have inevitably changed their minds about the whole colonial thing.
The ships make their triumphant return, and another thousand people jump on board. Well...maybe not right away. After all, science has had 25 years to march forward, and this is where things get interesting.
Going from 1970 to 1995 represents a huge leap in technology. Just look at the lifestyle of the average consumer: the jump from record albums to CDs, VHS to DVD, the introduction of microwave ovens, cell phones, and so on - not to mention computing technology!
So we sensibly take a few years to retrofit the spaceships with computers and DVD players, fill the freezers with people or Hungry Man Dinners™, based on whichever approach you picked originally, then send the ships on their way again. Ten years later the little fleet arrives and drops what is essentially a technology bomb on the nascent colony.
Repeat the process, and the next round trip provides the colony with MP3s**, iPads, LED screens, 64 bit computing, the Internet, and the complete Harry Potter series, both in book and movie form - or more accurately, as e-books and MP4 files.
This is just a 20 year round trip. In my previous post on this topic, the colonies had a 150 year circuit - what possible continuity of technology or culture could survive that kind of barrier? The same problem applies to whatever raw resources the colony is dutifully returning to the mother world.
"Hi, here's a spaceship full of crude oil, right on time."
"Of what? Gosh, no, ever since we switched to molecular valence fields, we don't use oil anymore."
"But what about gas?"
"About what - oh, GASOLINE! HAHAHAHAHAHA!!!"
However, there is a partial solution to this problem. If you start building more ships until you can launch one every six months or three months, you can create a continuum of communication, and start to get something that looks more like a unified society, with raw supplies going one way, and technology going in the other. I don't think the two groups ever end up at the same level, but when it comes right down to it, it probably doesn't matter.
And then, if you want to turn this into the plot for a book, one day the ships stop coming...
- Sid
* SF author Larry Niven has one group of interplanetary colonists name their new home We Made It. I think I prefer that approach over the whole "New Whatever" thing.
** Admittedly, this is based on the assumption that the colony isn't making
its own new inventions, but somehow I think that if you were busy
carving cities out of an alien wilderness, you probably wouldn't be wondering
about how to convert your Beatles albums into some kind of digital
format.