When does quantum "take over" classical? There's seems to be a lot of information about how relativity "takes over" classical mechanics when certain speeds are reached, like at a certain % of speed of light, etc. I haven't found much information regarding when quantum mechanics takes over for atoms within a system. I have found the Quantum Realm from wikipedia but if I try to google that term produces a whole load of strange websites and I can't decipher what's garbage anymore. Any advice on where to start looking? Thanks.