This article acts surprised about the possibility of changing an object's mass by varying its energy, but I thought that was common knowledge among relativists,

This article acts surprised about the possibility of changing an object's mass by varying its energy, but I thought that was common knowledge among relativists, as per [the classic "a compressed spring weighs more than a relaxed spring" meme][1]. [1]: https://news.ycombinator.com/item?id=4196182 https://news.ycombinator.com/item?id=4196182