# Reality check: multi-shell worlds and rotation

In the game Septerra Core, it is set on a shell world of 7 different world shells including a core. As per my Matryoshka world that I am building (this world but with full surface coverage on each world shell) I wanted to check on the reality of having multiple world shells with different rotations.

Assuming no magic and only advanced enough technology to create this world in the first place.

Is it possible to have a shell world with different rotations between world shells?

Images for reference. Image 2 details the diffferent rotational angles given on the various shells.

## 1 Answer

You can have differential rotation among concentric shells over geological times only if the space between the shell is vacuum.

Any fluid contained between them would, via shear stress, nullify the differential rotation over the above mentioned geological times.

A shear stress, often denoted by $$\tau$$, is the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section of the material. Normal stress, on the other hand, arises from the force vector component perpendicular to the material cross section on which it acts.

Shear stress arises from shear forces, which are pairs of equal and opposing forces acting on opposite sides of an object.

Unless you have an external source of energy that keeps adding energy to the system to maintain the differential.