What does added mass mean for a spaceship travelling at the speed of light?

[richerrich]

Does that mean the spaceship has become bigger or denser up to the subatomic level? Or is it just more atoms have stuck to the spaceship? What happens when it slows down? The added mass just 'evaporates'?

[xts]

not at the speed of light but at the speed close to speed of light. There are no material objects travelling at the speed of light.

All its components (atoms, electrons, quarks building their nuclei, clockwheels and mice hiding in a food storage room) get heavier. All of them in the same proportion. No additional atoms appear.

[DaleSpam]

Note that the concept of relativistic mass is not generally used by modern physicists, and the extra relativistic mass is undetectable to the passengers on the ship.

[richerrich]

Thank you all.