The law of heavyweights, a demographic shadow, a call for guardrails — Each Thursday, The Next Zeitgeist highlights reader articles on Medium, comments, and updates. The U.S.-Chinese showdown: Last week, Mathew Burrows and Julian Mueller-Kaler, both of the Atlantic Council, wrote that if the U.S. and China go to war, as seems more and more possible, subsequent trends could drive them into…