On Day 16, we finished up complex numbers in polar form and introduced several new concepts:

- The
**circle group**T and**unitary group**U(1) **Infinite**vs**finite**groups**Continuous**vs**discrete**groups

We also started going through t’Hooft’s Scientific American article “Gauge Theories of the Forces between Elementary Particles“, which proved to be challenging but exciting. We discussed the Standard Model of particle physics and the distinction between **spacetime symmetries** and **internal** symmetries.