Would dwarves be naturally more resistant to long exposure to low-grav, due to higher bone density? What's the gravity like in their habs?
(With reference to this post here.)
I’m going to tackle this one as a separate post because it’s long and technical and the sort of thing that would be first on the cutting room floor if I was writing an actual sourcebook.
Anyway, the issue of bone density loss tends not to come up because most dwarven orbital habs have residential and recreational sections that can be spun up to provide artificial gravity. Oddly enough, this is another area where dwarves’ biology gives them an advantage.
In a nutshell, there are two major drawbacks associated with using rotating habitats to provide artificial gravity: tidal gradients, and Coriolis forces.
A tidal gradient is basically when gravity acts with different force on different parts of the same object. The most familiar example is (of course) Earth’s tides, where the position of the Moon causes its gravity to exert uneven force on the Earth’s oceans, raising and lowering the water level accordingly.
You see the same effect in rotating habitats: specifically, the artificial gravity is stronger the closer you are to the habitat’s rim, and weaker the closer you are to the habitat’s hub. If the habitat’s diameter is very large, this gradient will be difficult to notice on a human scale - but if it’s small, it may be discernible.
In fact, if the diameter of the rotating habitat is small enough, you can end up with a situation where the force of artificial gravity on your feet is significantly stronger than the force of artificial gravity on your head (provided that you’re standing up). This can cause blood to pool in your legs, inducing circulatory distress, oxygen and nutrient deprivation to the brain, and other nasty effects.
Coriolis forces, meanwhile, are virtual forces that act on moving objects in rotating reference frames. That’s really technical - the plain English version is that if the thing you’re standing on is spinning, you’re constantly experiencing a slight acceleration in order to keep you in sync with it, and that acceleration can do funky things to fluids and trajectories.
The most familiar example is, again, meteorological: hurricanes and other pressure systems consistently rotate in different directions depending on which hemisphere you’re in - counter-clockwise in the Northern hemisphere and clockwise in the Southern hemisphere - because the deviation induced by Coriolis forces is enough for them to favour one direction over the other.
If you’re into scientific trivia, you’re probably wondering why I used hurricanes instead of toilet bowls as my “familiar example” - after all, we’ve all seen far more of the latter than we have of the former. That’s actually a common misconception: the direction that water rotates in a toilet bowl is determined by the geometry of the bowl, not Coriolis forces. Counterintuitively, even though the Earth is spinning at breakneck speed, its rotational velocity relative to its diameter is small. Since the strength of the Coriolis forces associated with a given system is in proportion with that system’s rotational velocity, the Earth’s Coriolis forces are quite weak - far too weak to mess with localised systems like the water in a toilet bowl.
Some of you may have guessed where I’m going with this: the smaller your habitat, the faster it needs to rotate relative to its own diameter in order to produce useful artificial gravity. In practice, this means that for a given level of artificial gravity, the smaller the diameter of the habitat, the stronger the Coriolis forces upon everything inside it will be. If the habitat is small enough, those forces can be strong enough to screw with the fluids in your inner ear, which are responsible for your sense of balance. This essentially induces a permanent case of motion sickness - not a fun time for anyone!
The solution to both of these problems is the same: go big. For humans, a habitat that’s been spun up for artificial gravity needs a diameter on the order of hundreds of meters in order to be comfortably habitable; needless to say, this poses non-trivial engineering challenges.
For dwarves, it’s a different picture. Their short stature and robust circulatory systems help to moderate the effects of tidal gradients, both by making them better able to pump blood against gravity, and simply by reducing the distance between their feet and their heads. Meanwhile, the dwarven inner ear is relatively insensitive, perhaps because falling over is less dangerous for them; this insensitivity is usually a disadvantage, but in this specific situation it’s advantageous, because it renders them largely immune to motion sickness, including Coriolis-force-induced vertigo.
The upshot is that orbital habitats designed for dwarves can get away with much smaller diameters for their rotating sections, making them both simpler and cheaper to build.