It's interesting to see how the ideas of interstellar civilization have evolved over the last almost 50 years.

I'm glad it was rejected that returns should be calculated based on Earth or Trantor time rather than perceived time. I mean if a ship disappears for 2N years, you need to consider the return over 2N years regardless of perceived time because you're comparing investments that locally would take 2N years.

This paper assumes essentially perfect information. It has to because otherwise it has to deal with the issue of how would an interstellar currency work? Or rather, how would an interstellar financial system work? This is a surprisingly difficult problem. You can't support a centralized currency over such distances. And metals like gold may be otherwise worthless as a store of value. You also can't maintain a blockchain over light years.

One thing that isn't touched on here, which also dovetails into my assertion that gold would be worthless, is the energy budget for interstellar travel.

If we assume reaction mass-less, infinite energy travel where you can accelerate to a high fraction of c for essentially zero cost then this is all fine but that's likely not the case.

Gemini tells me that the energy cost to accelerate to 0.1c, travel to Alpha Centauri and decelerate at 1g is ~10^15J/kg. That's a travel time of approximately 42 years each way. Time dilation doesn't really kick in until a high fraction of c. The energy budget from this is orders of magnitude higher and the drag of the interstellar medium actually becomes a real problem.

So your spaceship looks a lot like a colony. That means it has to be big. The building block for a space-based civilization in a Dyson Swarm is considered by many to be the O'Neil Cylinder, which is wide enough to have spin gravity. Think a diameter of about 8km and a length of 10-40km. A colony ship looks a lot like this. Such a cylinder weights 10-100 billion tons.

10B tons is 10^13kg so at 10^15J/kg that means the energy budget is 10^28J. The Sun's solar output is ~10^25W, which is 10^25J/s. So we're talking 300 seconds of the entire Sun's output for such a journey. Currently we receive about a billionth of that so it's about 10,000 years of the Sun's output hitting Earth.

Our civilization is estimated to use 10^11W of energy so if we devoted our entire energy budget towards this project, that's 10^17 seconds of output or ~3 billion years.

Each way. And that also assumes perfect energy-to-acceleration so it's likely 1-2 orders of magnitude higher.

This is partly why I consider a Dyson Swarm inevitable because interstellar travel is basically impossible without getting access to that amount of energy.

So, back to gold being worthless. Well, 1kg of anything is about 10^17J of energy. Going the other way, 10^28J of energy is 10^11kg of whatever element you want. That's 100 million tons.

So what exactly would you trade?

The Theory of Interstellar Trade [pdf] (1978)