There is no such thing as true perfection. Nevertheless, one can still say "perfect world" as a short-hand for a future where the core of what we seek for mankind is sought to the fullest, and the values that drive us are generalized. Discuss your perfect world, in this sense of the phrase, and think less in terms of what you wish to see attained, but in how things would move - i.e., the living, breathing, changing life of a world in which what you seek for humanity is embodied.
We can all say "I want a world at peace," but what is peace? A world without war could still be nightmarish in people's daily lives. "A free, happy world at peace" is likewise nebulous - free, happy people could just as easily sink into stagnation and dwindle away from lack of motivation. So think deeply, beyond platitudes, to that which is fundamental and alive in your dreams and aspirations. Further, tell us what problems this "perfect world" would (not quite paradoxically) confront, as every solution inevitably carries within it the seeds of further challenges. I share some of my own thoughts below.
I claim no special expertise in any subject that could justify these opinions in detail, but I offer them simply as impressions. The world, as I would have it arranged...
- Energy as the underlying organizing principle.
This already occurs by default in an ad hoc fashion, as energy is the source of life processes, but I would like to see humanity organize itself rationally around energy rather than in haphazard ways prone to conflict and disaster. For instance, choose locations for cities rationally, based on energy availability and the relative cost of environmental engineering to make them comfortable - other resources, such as water, can be reduced to an energy cost (e.g., energy of extraction and transport) that can be factored into these calculations.
Regions with a much greater amount of unused energy relative to the cost of expansion should expand faster than those with lower amounts, and all such calculations should be determined empirically and verified experimentally before decisions about infrastructure are made. This seems like it should happen automatically, but actually doesn't - people move where other people already are, so the city that now exists originally might have been catalyzed by a Neolithic settlement based on fishing in a nearby river. Thus we end up with huge cities powered by plants hundreds of miles away, and that are highly vulnerable to economic disruption - i.e., they're not self-sufficient.
If you demand self-sufficiency as a precondition for establishment, and also a precondition for a political entity to engage in export, you will have the most robust, efficient, and prosperous economies. Economic interdependency is not a good thing: It makes civilization highly vulnerable. Free trade theory functions on the concept of comparative advantage, which is only valid if capital is immobilized - if capital itself is mobile, all advantage is absolute, meaning that absolute minimum cost will win: A highly inefficient and unsustainable environment.
- Choice of culture and languages
Culture in the basic sense is determined by geographic proximity and lineal descent, and language is a product of these two factors. It, like the location of cities, is determined by energy only indirectly: It takes more energy to go far rather than near, and to learn another language than to speak with other speakers of your native language. This process, however, should not be systematized, but neutralized.
A universal language could either be designated or fashioned for maximum adaptability, and children around the world would obtain a culturally-neutral, universal education in this language. Later in their education, perhaps beginning in the upper grades of elementary school (or equivalent), they would learn broadly about the languages of mankind and the principles of human language generally. In addition to languages, they would learn about the cultural histories and traditions associated with them, and would choose a language/culture grouping to focus on in addition to learning about the language/culture grouping of where they live.
Once they have a proficient understanding of both broad groupings, they would focus further on a specific language and culture in the non-native one, and a second in the native one (e.g., if you're a native English-speaker, you could choose French, Spanish, German, etc. as your second native, and anything unrelated to English as your non-native). Fluency in the non-native language and deep knowledge of the culture(s) associated with it would be expected, and the student would ultimately be offered opportunities to relocate to that region and become a part of that society, or to continue where they are. Of course, anyone would be free to move anywhere, but as everyone would be given this kind of education, those who acquired the language and culture of the region would have profound competitive advantages.
This makes language and culture more of a choice than it is today, and would bring up the advantages and disadvantages among different languages and cultural mores. Furthermore, if we imagine space colonization to be occurring, which would indeed be part of my perfect "world," then people would even have the tools to construct their own "franchise" languages and cultures. The same might also be true of political systems: People would learn about (neutrally) and choose between any political system where the right to leave if they find it unsatisfactory is preserved unconditionally.
Political states that place substantial investments in their people could set conditions on entrance (e.g., admittance fees) to insure against people exploiting their benefits and then taking off when the bill comes due, but none could deny exit for any reason without the concurrence of an oversight body that the individual is a criminal seeking to escape penalty.
- Indefinite lifespans.
Population is not really the problem we think it is, especially with an energy-organized technocratic economy, and with ubiquitous space travel it becomes even less of an issue. An indefinite lifespan means not only that people can learn more, but they can plan over longer time scales and undertake projects that would span several conventional lifespans. Even a 150-year lifespan would have tremendous advantages. And if the long-lived began to cause society to stagnate, more adventurous people could just found their own society further out and impose an age limit for entrance.
- Logical, transparent information rating system.
People are inundated with information, but its value is often obscure. Mankind would benefit greatly from the development of a logical, transparently-constituted information rating system that tells people reliably what kind of information it is, to what degree it is objective, and whether it is formatted to inform or to deceive / obfuscate. The word "journalism" could be limited to arbitrarily-established high scores of objectivity and unbiased informativeness.
- Space exploration.
The same energy calculations that determine human growth on Earth should be applied to space environments, and settlements grown everywhere feasible.
- Ultimately, global unity
Eventually the "energy regions" would merge and a global energy strategy would appear. An external context would be provided by space settlement, so global unity would be a good thing, and would not then sink into stagnation.
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Challenges
The challenges faced by this type of civilization, once it had already been achieved, would be to deliver increasing efficiency without compromising flexibility too greatly. Too efficient in the short-term, and the system becomes vulnerable, and eventually untenable. The most robust systems have an efficiency that takes into account future contingency and systemic complexity. Discerning how to deal with that would be a very complicated, long-term undertaking, and there would probably be many grave disasters before a sufficient understanding had been achieved.
Humans would live like ants in warren-like cities that go on and on, but I don't see any theoretical reason why the Earth couldn't sustain a trillion people. Of course, by then it's questionable if that would even be necessary, given expansion into space and possible transformations in the physical nature of human beings. But that's too much speculation.