It seems we're been here before. More than once. The kilogram: a pesky unit of measure that hasn't been behaving itself.
Derek Muller goes deep into the explanation of the newly redefined kilogram. The task is not trivial. I may need to rewatch this robust explainer a couple of times before it all sinks in. Have a seat and prepare to get your geek on: there will be mathematics!
How We're Redefining the kg