You mean, a box that is immovable because it has infinite mass? Hmmm I don't get it. What boundaries, the box boundaries? How does it break translational symme

You mean, a box that is immovable because it has infinite mass? Hmmm I don't get it. What boundaries, the box boundaries? How does it break translational symmetry? Anyway in this case I think at least elastic collisions will still conserve momentum, eg this https://en.wikipedia.org/wiki/Elastic_collision#/media/File:Translational_motion.gif https://en.wikipedia.org/wiki/Elastic_collision#/media/File:... conserves momentum even if the box has infinite mass.