Steve Jackson Games Forums - Gravity model of trade volumes

Steve Jackson Games Forums (https://forums.sjgames.com/index.php)

- GURPS (https://forums.sjgames.com/forumdisplay.php?f=13)

- - Gravity model of trade volumes (https://forums.sjgames.com/showthread.php?t=18304)

Re: Gravity model of trade volumes

Quote:

Originally Posted by Agemegos

I suggest 3.

I don't know if you know this, but you actually have to send an email to errata@sjgames.com.

Re: Gravity model of trade volumes

Quote:

Originally Posted by GoodGame

But still if you know better on a subject, it's hard to sometimes let the grossly unrealistic stuff just fly.

For some people it is. I know enough about physics to know that the idea of an FTL drive that doesn't play merry hell with cauality (people arriving before they leave etc.) makes no scientific sense, but can play in FTL sttings without blinking.

I can list at great length the utter absurdities not only physical, but social with a standard Super-Hero world, and yet supers is still one of my favorite genres.

Re: Gravity model of trade volumes

Quote:

Originally Posted by Agemegos

I didn't. Thanks for the pointer.

Is there a consensus for an exponent of three?[...]

For most applications, that's fair enough. It's just that "number of dimensions of the map, normally 3 for a space map" would be even more accurate.

Re: Gravity model of trade volumes

Quote:

Originally Posted by Agemegos

So do we go with '3' or 'the number of dimensions on the map'?

"Number of dimensions".

Re: Gravity model of trade volumes

Quote:

Originally Posted by DrTemp

It's just that "number of dimensions of the map, normally 3 for a space map" would be even more accurate.

Hear, hear.

Re: Gravity model of trade volumes

Okay, I have written to errata@sjgames.com as follows:

Quote:

In GURPS Space, on p. 95, a 'gravity trade model' for estimating trade flows is given as:

T = (K * V1 * V2)/D

I believe that this ought to be:

T = (K * V1 * V2)/(D^3)

or possibly

T = (K * V1 * V2)/(D^N), where N is the number of dimensions in which space is mapped.

The reason is that the version in which trade is proportional to 1/D produces "the economic equivalent of Olbers' paradox" (Jon F Zeigler), in which trade volumes would tend to infinity in large settings.

I note, further, that gravity models are appropriate only in cases in which transport costs are proportional to distance, and might not be appropriate for some settings with Jump drives or stargate networks. Logit models in terms of generalised transport cost would be more generally appropriate, but are probably too involved for use by most gamers.

Re: Gravity model of trade volumes

Quote:

Originally Posted by Brett (Post 273823)

In the otherwise good planetary design sequence in GURPS Space, there is a formula suggested for estimating the volume of trade (T) between any pair of planets with given economic outputs (V1 and V2), separated by a distance D. The formula is

T = k.V1.V2/D

where k is a constant set by the GM to reflect the specifics of his setting.

Anyone who attempts to map any reasonably large number of worlds and apply this formula pairwise is in for a nasty surprise. .

Well, not anyone. I for example, assumed that no numerically significant trade route was longer than about 15 light years in distance (with trade at longer ranges passing through economically significant planets and thus being subsumed into the trade of those planets) . Thus, it wasn't "any pair of planets". It was "any pair of planets between 15 light years". And, since there would be only about four economically significant planets at most in that range, the problem simply never arose.

Re: Gravity model of trade volumes

Holy thread necromancy.

Re: Gravity model of trade volumes

Quote:

Originally Posted by Anthony (Post 1133098)

Holy thread necromancy.

Seriously.

Re: Gravity model of trade volumes

Quote:

Originally Posted by Anthony (Post 1133098)

Holy thread necromancy.

Necromancy is traditionally considered unholy.