## Main Question or Discussion Point

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.

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How could the person who gets on a spaceship leave their cells on earth? when you get in a spaceship, a car, a train, go for a walk, your organs travel with you. Otherwise I think you would die, say, without a heart or lungs or a brain, if they did not stay in your body. So the organs travel with her on the spaceship, and are therefore in the same reference frame as her, not on earth's reference frame

you have misunderstood

...as if the body containing the organ were still on Earth...

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.
Everything accelerates inside the body as well when the body accelerates.

But from the cells pov with the kidney as the inertial frame of reference the cells are not accelerating. In the same way that we do not take into account the Earth's acceleration while orbiting the Sun, when we consider the Earth to be an inertial frame of reference with respect to physical laws taking place on the surface of the Earth.

But from the cells pov with the kidney as the inertial frame of reference the cells are not accelerating. In the same way that we do not take into account the Earth's acceleration while orbiting the Sun, when we consider the Earth to be an inertial from of reference with respect to physical laws taking place on the surface of the Earth.
Sorry what you say does not make any sense.

If you accelerate, your teeth will accelerate as well unless they fall out.