Gauge theory Updated 2025-07-16
The term and idea was first introduced initialized by Hermann Weyl when he was working on combining electromagnetism and general relativity to formulate Maxwell's equations in curved spacetime in 1918 and published as Gravity and electricity by Hermann Weyl (1918). Based on perception that symmetry implies charge conservation. The same idea was later adapted for quantum electrodynamics, a context in which is has even more impact.
Fock space Updated 2025-07-16
Yup, this one Focks you up.
Second quantization Updated 2025-07-16
Second quantization also appears to be useful not only for relativistic quantum mechanics, but also for condensed matter physics. The reason is that the basis idea is to use the number occupation basis. This basis is:
Bibliography:
Path integral formulation Updated 2025-07-16
This one might actually be understandable! It is what Richard Feynman starts to explain at: Richard Feynman Quantum Electrodynamics Lecture at University of Auckland (1979).
The difficulty is then proving that the total probability remains at 1, and maybe causality is hard too.
The path integral formulation can be seen as a generalization of the double-slit experiment to infinitely many slits.
Feynman first stared working it out for non-relativistic quantum mechanics, with the relativistic goal in mind, and only later on he attained the relativistic goal.
TODO why intuitively did he take that approach? Likely is makes it easier to add special relativity.
This approach more directly suggests the idea that quantum particles take all possible paths.
Renormalization Updated 2025-07-16
Video 1.
The Biggest Ideas in the Universe | 11. Renormalization by Sean Carroll (2020)
Source. Gives a very quick and high level overview of renormalization. It is not enough to satisfy Ciro Santilli as usual for other Sean Carroll videos, but it goes some way.
Graphomania Updated 2025-07-16
Coulomb gauge Updated 2025-07-16
Alternative to the Lorentz gauge, but less used in general as it is not as nice for relativity invariance.
Last mile problem Updated 2025-07-16
The exact same problem appears over and over, e.g.:
Ciro Santilli also identified knowledge version of this problem: the missing link between basic and advanced.
Existence of the matrix logarithm Updated 2025-07-16
en.wikipedia.org/wiki/Logarithm_of_a_matrix#Existence mentions it always exists for all invertible complex matrices. But the real condition is more complicated. Notable counter example: -1 cannot be reached by any real .
The Lie algebra exponential covering problem can be seen as a generalized version of this problem, because

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