The original post: /r/askscience by /u/tubby325 on 2025-02-17 18:59:56.

This is something that I’ve heard from a few different sources, but I can’t tell if it’s a dumbed down version of the truth. Does matter, when accelerated to nearly the speed of light, actually gain mass (functionally or literally) or is it just an illusion or something due to exponentially increasing inertia (that somehow wouldn’t be tied to mass, I guess?). For example, does its gravitational field strengthen, and does the force of gravity on it also increase? If so, are there equations that describe the mass increase?