
03172013, 11:25 PM  #1 
Join Date: Dec 2012
Location: Colorado, USA

[Space] GURPS
I did a search and couldn't find an answer, so I apologize if this was answered elsewhere.
In GURPS Space, there is a formula: L=1/G^(2/3) Where L is the linear dimension (height or length) and G is the local gravity. What unit is L in? How am I to utilize this formula? With Earthlike gravity, that gives an L of 1. The base unit for length in GURPS seems to be yards, but there are definitely things on Earth longer than 1 yard. I know it probably doesn't matter in the grand scheme of the book, but I'm really curious as to the intent of the formula. I suspect it means my randomly rolled 80 yard long critter should not be so large if I wish to be true to biology and physics as we know them, but I don't know what the cap should be on a world with Earthlike gravity. 
03172013, 11:32 PM  #2 
Join Date: Feb 2005
Location: Berkeley, CA

Re: [Space] GURPS
G^2/3, apparently. It's a multiplier on typical creature sizes  i.e. at 0.5G typical creature sizes are (0.5)^2/3 or about 1.58x larger. I'm not entirely sure where that value comes from, though, there's no obvious reason for an exponent of 2/3.

03182013, 12:13 AM  #3  
Join Date: Feb 2013

Re: [Space] GURPS
Quote:
http://en.wikipedia.org/wiki/Square_cube_law 

03182013, 12:48 AM  #4 
Join Date: Feb 2005
Location: Berkeley, CA

Re: [Space] GURPS

03182013, 01:02 AM  #5 
Dog of Lysdexics
Join Date: Oct 2004
Location: Melbourne FL, Formerly Wellington NZ

Re: [Space] GURPS
GURPS fantasy has a good section on how it works under it's scaling rules p.51

03182013, 07:25 AM  #6 
Join Date: Jun 2005
Location: Lawrence, KS

Re: [Space] GURPS
For scaling with regard to buckling strength, though, the length of a pillar varies as the fourth root of its mass (so its crosssectional area varies as the threefourths power of its mass). How much significance this has for biomechanics is debated, but some biomechanicists take it seriously. Using the twothirds power may be a gamable approximation rather than a physical reality.
Bill Stoddard 
03182013, 11:21 AM  #7 
Join Date: Feb 2005
Location: Berkeley, CA

Re: [Space] GURPS
Yes, but this produces an exponent of 1. Using very simple square/cube law proportions, a ST 10 creature (125 lb) in 1G has a BL of 16% of its weight. In 0.5G, a ST 20 creature (1,000 lb in 1G, so 500lb) has a BL of 16% of its weight. ST 10 vs 20 corresponds to a factor of 8 in mass and thus a factor of 2 in linear scale, if we keep proportions constant.
Now, there are a bunch of nongravitational reasons for creatures changing shape as size increases, so you wouldn't expect 0.5G creatures to look just like doublesized 1G creatures. This might result in a 2/3 power producing plausible results, but it's not an obvious outcome. Actually, they scale with weight, because desirable movement speed varies with gravity. 
03182013, 03:23 PM  #8 
Join Date: Feb 2005
Location: Berkeley, CA

Re: [Space] GURPS

03202013, 10:55 AM  #9 
Join Date: Dec 2012
Location: Colorado, USA

Re: [Space] GURPS
It appears you are all vastly more knowledgeable than me in a wide variety of matters!
So it appears it's a multiple for length in worlds of alternate gravity to Earth's? Is there a feasible cap to creature size at various levels of gravity, then? I very much appreciate the responses so far. 
03202013, 11:43 AM  #10 
Join Date: Feb 2005
Location: Berkeley, CA

Re: [Space] GURPS
Almost certainly. Figuring out what that cap is, however, is not trivial, because size limits are a function of both structure and metabolism, and those scale differently. It's possible that the 2/3 power is an adequate rule of thumb, it's just not immediately obvious why it would be correct.

Tags 
aliens, gurps 4e, space 
Thread Tools  
Display Modes  

