We're pretty good at physics. The g-factor for an electron is 2.0023193043622(15). That number is predicted by theory and measured experimentally, and both give that exact same result. The parentheses in the last 2-digits denote that we're not totally sure those last two numbers are a one and a five due to experimental error. There are very few other human endeavors where we have 12 or 13 decimal places worth of accuracy. While there's still a lot of interesting consequences to work out, and people are still working on getting quantum mechanics and general relativity to talk to each other, any new quantum physics is going to have to be hiding somewhere past the 15th decimal point.