Re: Gravity model of trade volumes
 

I would bet it was based on the gravity trade model in GT:FT, which is using a 2d map where 1/D makes sense. It's unclear what the exponent should be on a 3d map, we don't have any real-world models to look at (the terrestrial case is generally close enough to 2d that it can be called 2d).

Re: Gravity model of trade volumes
 

Can't be, since the gravity trade model in GT:FT uses 1/D^2 (although it is concealed by the logarithms used in the actual tables).

It looks like a simple omission -- there should be another variable as an exponent on the distance factor.

When I asked Jim Maclean (the professional economist behind GT:FT, for those who don't know) he suggested that 1/D^3 was probably close enough.

Part of the problem with this subject is that the gravity trade model is an empirical observation, not a theoretical result. There are a number of competing explanations for why it works across such a wide range of applications -- and thus, at least as many possible ways to extend it to three dimensions.

The actual exponent for international trade is more like 1.9 than 2.0, by the way, but that's close enough to the square to make "gravity model" a reasonably accurate description.

Re: Gravity model of trade volumes
 

Good point. It might be simple reading on the gravity trade model, most of which seem to use an exponent fairly close to 1. I suspect the real problem is that remoteness is ignored and that most interstellar polities are rather limited in size.

Re: Gravity model of trade volumes
 

No, but I'm not talking about doing it in a game. I have in fact worked out interplanetary orbital trajectories for pleasure, and that's a lot like doing firearm ballistics. But doing it in a game would be a mistake, because the actual play of games is devoted to other sorts of fun.

Re: Gravity model of trade volumes
 

So... has anyone submitted this as an erratum yet?

Re: Gravity model of trade volumes
 

I'm over a thousand miles away from my notes, and I don't recall enough to know whether there's just a dropped exponent there, but I suspect there is. What we've really got here is the economic equivalent of Olbers' paradox - and just as the sky is dark at night, you really don't want your model predicting an infinite amount of trade coming in to every world in the galaxy.

If you lot can agree on a reasonable value for the exponent in a three-dimensional universe, and submit an erratum, I'd be happy to nod my head and say "yes" when the buck gets passed to my desk.

Re: Gravity model of trade volumes
 

The missing factor on trade models is the fact that the larger the network of potential trade partners, the lower the trade with any given partner. That's what would actually prevent Olber's Paradox in this case, though adjusting the exponent for range may not be a bad idea regardless.

Re: Gravity model of trade volumes
 

But still if you know better on a subject, it's hard to sometimes let the grossly unrealistic stuff just fly. Myself, I just picked up Business for Dummies, so this is all news to be.

Re: Gravity model of trade volumes
 

For 3D maps, that is. Maybe one could add "^(number of dimensions on the map)" to the formula in Space. That way, some Infinite Worlds in Space version with 3+1 dimenions would be covered, too, as would be Traveller-like settings with some kind of two-dimensional "jump space structure".

Re: Gravity model of trade volumes
 

A real man generates all the systems randomly, by hand. Then he dies of old age.