Back in May there was a Starship Century Symposium in San Diego. I didn’t attend, but I later watched videos of most of the talks (here, here, book coming here). Many were about attempts by engineers and scientists to sketch out feasible designs for functioning starships. They’ve been at this for many decades, and have made some progress.

Most of us have seen many starships depicted in movies, and you might figure that since the future is so uncertain, fictional starships are our best guess to real future starships. Since no one can know anything, no one can beat fictional imagination. But that seems very wrong to me; we get a lot better insight into real starships from serious attempts to design them.

It is worth noting that these folks do futurism the way I say it should be done: making best combos.

Form best estimates on each variable one at a time, and then adjust each best estimate to take into account the others, until one has a reasonably coherent baseline combination: a set of variable values that each seem reasonable given the others. (more)

Even though this is the standard approach of historians, schedulers, and puzzle solvers, many express strong disapproval about doing futurism this way. Well at least when predicting social consequences. Starship designers don’t seem to get much flack. Why? I’d guess it is because they are high status. Starships and physicists are sexy enough that we forget to be politically correct, and just let experts do what seems best to them.

Some might say this is okay for engineering, because we know lots of engineering, but not ok for social things, because we know little there. But that is just wrong. Not only do we know lots about social things, this is still the right approach in areas of history and engineering where we know a lot less.

It is also worth noting that the usual starship vision mainly seem interesting if one expects familiar growth rates to continue for a while. A starship carrying humans would take about a decade or two in flight time, and a thousand times as much energy as the Apollo moon rockets. And today our economy doubles in about 15 years. So if energy capacity doubled with the economy, it would take about 150 years to get that capacity. Or since energy has doubled about every 25 years lately, it might take 250 years. But 150-250 years still seems culturally accessible to us; we feel we can relate to folks 200 years ago. Much more at least than to people 20,000 years ago.

But if growth rates either slow down or sleep up a lot, this doesn’t work. For example, if the economy doubled every thousand years, as it did during the farming era, then it would take ten thousand years to get enough capacity. And we feel much less related to people who will live ten thousand years in the future. We expect their culture to change so much that we are much less interested in stories about them, or in thinking about what they will do.

If growth rates instead speed up by the same factor, the economy would then double every three months. And then a decade long flight to another star would encompass forty doublings, or a factor of a trillion. At growth rates like that, a journey that  long just seems crazy. Before you’ve hardly left our system another much better ship is likely to whiz past you. And even if your ship gets there first, the civilization back home by then is likely to be culturally unrecognizable. A trillion size bigger economy is likely to be a very different place.

Of course fast growth can’t go on forever. So a fast growing economy will slow down eventually. And that is when it would make sense to take a decade long journey, when a decade doesn’t encompass that much cultural change back home. But such a post-fast-growth society will likely be so different from ours as to deflate most of our interest in thinking about their starships.

If our industry era growth rates continue on for several centuries, then we may have descendants capable of starlight, and culturally similar enough to us that we care a lot about them. But if growth rates either slow down or speed up a lot, the descendants who are finally willing and able to fly to the stars are likely to be so different from us that we are much less interested in them.