Yes, there is a fundamental atomic-scale explanation for the time dilation experienced by a fast traveler returning younger than their twin. It arises from the theory of special relativity, which was developed by Albert Einstein.
According to special relativity, time dilation occurs when an object moves relative to another object at a significant fraction of the speed of light. When an object travels at high speeds, such as a spaceship approaching the speed of light, time appears to pass more slowly for the moving object compared to a stationary observer.
At the atomic scale, this effect can be understood by considering the behavior of clocks and the fundamental properties of matter. In special relativity, time is intimately connected with space, forming a four-dimensional continuum known as spacetime. The motion of an object through spacetime affects the passage of time.
At high speeds, the behavior of clocks is influenced by the relativistic effects of time dilation. For example, the ticking rate of a clock on a fast-moving spaceship will be measured to be slower by an observer in a stationary reference frame. This slowdown occurs because the relative motion of the spaceship and the observer introduces a discrepancy in the way time is experienced by each of them.
Since matter is composed of atoms, which contain electrons and atomic nuclei, the behavior of these atomic constituents is subject to the effects of time dilation as well. The interactions between electrons and atomic nuclei, the energies of atomic transitions, and other atomic processes are influenced by the relativistic effects of time dilation.
When a traveler undertakes a journey at a high velocity relative to a stationary observer, their onboard clocks and atomic processes are affected by time dilation. As a result, the traveler experiences a slower passage of time compared to the observer on Earth. When the traveler returns to Earth, they find that less time has passed for them compared to their twin who remained stationary on Earth, leading to the phenomenon of the twin paradox.
In summary, the time dilation experienced by a fast traveler is a consequence of the fundamental principles of special relativity, which affect the behavior of clocks and atomic processes due to relative motion through spacetime.