The twin who has left earth and travels close to the speed of light in his spaceship, lands back on Earth only to find everyone has died of old age thousands of years ago. This I'm told is because the perception of time and time itself, within the inertial frame of reference of the spaceship passes normally. But what about the cells in her body. Wouldn't the kidney for example be an inertial frame of reference for the kidney cells. From the cells' point of view they are aging and dying or multiplying with cancerous intent as if the body containing the organ were still on Earth because whether the body is moving or not, the organ containing the cells is stationary from the point of view of the cells. Therefore would it not be that while time would be perceived to be running normally for the person, that the internal components of the body would age the same way as her Earthbound brother.