As Earth awaits the official debut of America's nightmare comedy, this might be a good time to talk about other planets. So here we go! (Again!)
One minor but durable trope in science fiction is the planetwide city. (For which the geek (and Greek) term is ecumenopolis, 'world city'.) For now - until the Star Wars prequels mercifully fade from popular memory - most people will associate this trope with Coruscant. But accept no substitutes: really it means Trantor, capital world of the Galactic Empire in Asimov's Foundation Trilogy.
Just for the record, this discussion obviously lies waaay beyond the Plausible Midfuture. Also a disclaimer that I am not trying to specifically reconstruct Asimov's Trantor, but a more broadly Trantor-esque world.
I bring up this trope because one of my interests, which has gotten oblique mention here before, is urban rapid transit. And while TERTA - the Trantor Ecumenopolitan Rapid Transit Agency - never got any mention in the books, it reasonably ought to be ... impressive.
As useful background for a transit ride, a few words - well, quite a few - about the overall cityscape, starting with its population. The Good Doctor A slipped up badly on this score. His canonical figure for Trantor - 40 billion - is laughably low, only a few times current world population. We want a global city, not a world of ten-acre exurban ranchettes.
Donald Kingsbury does much better in his unofficial Foundation sequel, Psychohistorical Crisis. His version of Trantor, called Splendid Wisdom, is home to a nice, round trillion people. Spread over the whole surface of an earthlike planet, even this comes out to merely suburban average density. But if we leave the oceans wet and only urbanize the land surface, we get roughly the population density of San Francisco.
Now we're talking. San Francisco, outside the Financial District, lacks the glass and steel canyons look, but cityscapes can vary considerably for a given density. Central Paris apparently has about the same population density as Manhattan, but a very different urban look. Likewise a modest urban density could still have an impressive skyline.
The image of Trantor (nominally Coruscant) above - click to embiggen - hints at one way of finessing this. Most of the city seems to be low-rise, or at least of roughly uniform height, but with monumental structures rising above the rest for added zip.
Another consideration is that an ecumenopolis surely cannot be like an ordinary habitable planet, where humans merely skim the cream off a self-sustaining natural ecosystem. It will need something more like a spacecraft life support system, on a suitably giga scale.
The details are far above my biology pay grade, but may well imply vast sublevels of, essentially, plumbing. The subways could thus run through what amount to basements rather than true tunnels. And the oceans may effectively be sewage treatment / oxygen regeneration ponds - nothing you would want to swim in, which at least would keep shorefront promenades from being impossibly crowded.
Parts of the life support system might rise to or above surface level, and some of the megastructures could well be the equivalent of rooftop air conditioning equipment.
Also, the population density need not be uniform all over town. If you really don't like urban living, Trantor is not the planet for you. But many neighborhoods (totalling millions of km2 and a couple of hundred billion people) might well be at suburban density, balanced by high-rise urban districts, thus accommodating people who want a yard as well as those who prefer living near good shopping and restaurants.
Kingsbury mentions one such commercial district on Splendid Wisdom that is suitably Trantorian in scale. I forget its name, but since Splendid Wisdom is a rebuilt Trantor of the Second Empire, I will call it Seldon Street. Though technically a pedestrian mall, functionally it is a suitably giga-scaled version of Market Street or Wilshire Boulevard, extending for some 3000 km.
That being a long, long stroll, expect some serious transit lines to run along the Seldon Street corridor.
But before we ride, a few more thoughts on the cityscape. First, a couple of annoying practicalities. Realism[TM] is not really a key issue in this exercise, but we should give it a superficial nod.
On a world without farmland, how do you feed a trillion people? Asimov's Trantor imports its food from 20 agricultural worlds, but they could only supply this Trantor with delicacies. For basic food supply you need a planetwide array of oscillating hands: something something hydroponics, something something algae.
And, of course, all this stuff should really be on the surface, with the city life below, but we want a planet that looks urban. We will delicately assume that technology originally developed for spacehabs and such will solve food supply along with the rest of the life support challenge.
Energy supply turns out to need less handwaving than food supply. An earthlike planet absorbs on order of 10^17 watts of insolation (instellation?) from its parent star. Current average US energy usage is about 10 kw/capita, scaling to 10^16 watts for a trillion people. So cover the rooftops with solar panels and you more or less get there. Waste heat disposal is not a problem, because you are merely using energy the planet would absorb anyway.
And energy consumption on Trantor can be relatively modest. It is not an industrial world; as the Imperial capital, its chief manufactured product is government. Large cities also tend to be energy efficient - not least because people ride the subway instead of driving.
From these material concerns we can turn to social considerations. An ecumenopolis must have substantial overall social stability to function at all, but Trantor presumably has its good neighborhoods and not so good, perhaps including slums the cops only enter in army-corps strength.
Or - another familiar urban SF trope - class stratification might be literal, with the down and out living among the plumbing sublevels, while the upper classes live on upper floors, the richest in penthouses. This lends itself to a transit subtrope that goes back to 1890: dismal subways for the poor, elegant els for the rich.
Note that the district shown in the image above must be served only by subways; there are no hints of elevated lines.
Given a world of a trillion people none of this needs be an either/or: Within vary broad limits, Trantor's cityscape and social life can be as varied - including the charmingly urbane and the dystopian - as you want them to be.
On yet another note, Isaac Asimov was famously agoraphobic, and his Trantorians rarely went up to the open surface. Depending on how the life support system operates, 'rarely' might be never, at least without a quasi space suit.
But this too is not a given. An ecumenopolis, or neighborhoods thereof, may have rooftop gardens and dining patios under the solar awnings, and even (shock!) open air streets instead of roofed over corridors. When I speak of the Seldon Street corridor I say nothing about its architecture, only that it is an elongated urban district.
Trantor might even have parks, though the only open space on Asimov's version was the Imperial palace grounds. But we have not come all this way to an ecumenopolis to visit a park. You can find those on any garden colony world. So in our next exciting episode we will head for Seldon Street.
And since it is already mostly written, you won't need to wait until the Galactic Era to read it.
Yes, knowing this blog's commenter community - if you have not all given up and deserted me - the discussion will work itself around to space battles.
The cityscape image comes from a blog review of Second Foundation. Alas, I know nothing of the artist who created it.