What physical principle explains why the twin paradox occurs?

The twin paradox is primarily explained by the concept of time dilation, a consequence of Einstein's theory of relativity. In the scenario, one twin travels at a high velocity into space while the other remains on Earth. Due to time dilation, the traveling twin experiences time at a slower rate compared to the twin on Earth. As a result, when the traveling twin returns, they find that less time has passed for them than for their sibling who stayed behind.

Inertia refers to an object’s resistance to changes in its motion, while non-inertial frames of reference involve acceleration, which can cause confusion in relativity but do not directly explain the time differential experienced by the twins. Universal gravitation relates to the attractive force between masses and is not the primary factor in the twin paradox context. The core of the paradox is rooted in the relativistic effects of time dilation as described by special relativity.