What is the fine-grained topic of the following text: Such simultaneity makes it impossible for each twin to observe the other as time-slowed: Heinlein settles for the more comfortable asymmetry of having the Earth-twin perceived by his brother as living faster (and, conversely, the space-twin slower), eliminating the paradox at some cost to the physics. Only return the topic.
Twins' perception of time in Heinlein's science fiction