SDB writes about how xenophilic cultures prosper, while xenophobic cultures stagnate.
Cultural cross-pollination, like genetic crossing, leads to stronger cultures. The best of several cultures may combine. But many in times past and now have felt that this was evil, and have resisted it. ...I agree with him (of course), and so it's interesting to speculate on what the end result of this cross-polination will be. Our world has not reached an equilibrium cultural state; there are still many distinct cultures, even among developed nations. However, as communication becomes cheaper and travel becomes faster, the world will continue to shrink and it is inevitable that earth will eventually be culturally homogenous. It may take 1000 years, but frankly I think it will take less than 100.
This almost unique cultural pride in xenophilia and openness has been one of the great advantages of the people of the US and one of the major reasons why the US has become rich and powerful. It's the reason why we are more racially and culturally integrated than anyone else, and one of the big reasons why we're now the most powerful nation in the world.
If, as SDB argues, America owes its current rate of advancement to the exchange of ideas between various cultures, then it depends on a resource that will eventually be depleted. Once the world becomes homogenous, it will also become stagnant. Communication and travel will continue to get faster. What can be done?
On the planetary scale, I don't think there's any way to prevent this phenomenon, and so to preserve the advance of our civilization it is necessary to think beyond the mere planetary. If the speed of light truly is an insurmountable barrier, then physical distance can only thwart this effect when population centers are separated by years of travel that technology can never reduce or eliminate.
If humanity is able to spread to the stars, it is conceivable that cultural heterogeneity can be preserved indefinately, either through continual expansion to new star systems or simply due to the immense distances between settlements. The time required to pass people and information between systems would probably limit humanity to a slower rate of advance than we are currently experiencing, but that rate would still be much preferable to the inevitable stagnation that faces us if we remain concentrated solely on earth. Likewise, war under such a scenario would be nearly eliminated -- individual planets would be culturally homogenous, and star systems would be so far apart that interstellar warfare would be almost unthinkable.
With all these benefits, I'm astounded that the leftists of the world aren't pressing for greater space exploration. Environmental pollution would become a thing of the past, as would overcrowding, refugees, American imperialism, war, cultural homogeny -- all their favorite causes could be eliminated!
Then again, if none of their concerns are actually held in good faith but are actually mere scapegoats for their own ideological failures.... It's ironic that capitalistic space exploration will likely be the eventual solution to all the problems that the left holds so dear.
TMLucas at Flit responds that information overload will prevent cultural stagnation. I understand his point; there will always be too much culture for any one person to absorb. However, it isn't necessary for a single person to grok an entire body of understanding in order for that body to be considered a unified "culture".
Although there will remain a great many vibrant sub-cultures on earth for as long as the population is large enough to support them (and for as long as genetic variation produces people with varied interests), the underlying fabric of human existence will continue to mix and smooth. At some point, an arbitrary threshhold will be crossed and the whole earth will belong to a single, stable equilibrium culture.