> the Planck length is presumed to be the smallest measurable distance before stuff turns into a black hole
This is not correct. The Planck length happens to be very very roughly the scale at which we expect effective theories that ignore quantum-gravitational effects (i.e. the Standard Model) to break down. It has no special physical significance.
> I'm guessing it's the one that says kinetic minus potential energy, but that seems arbitrary
It is arbitrary, in some sense: Lagrangians are specifications of a physical system in the same way that laws of motion are. Instead of directly describing a trajectory as the solution to some set of differential equations, the Lagrangian approach describes it as a stationary point (an extremum or a saddle point) of the action, which is the integral of the Lagrangian over time. The resulting equations of motion are then given by the Euler-Lagrange equation. L = K - V is (one formulation of) the Lagrangian for a system that obeys Newton's laws. Other systems have other Lagrangians, from which we get other laws of motion.
The other important formulation of classical mechanics is the Hamiltonian one, which very roughly speaking reparameterizes the Euler-Lagrange equation in terms of (generalized) positions and momenta, instead of just position. This turns your n-dimensional second order differential equation into a 2n-dimensional first order one, which is often more convenient to work with.
For simple "ideal billiard balls in a vacuum" situations it makes very little difference what approach you use, but Newtonian mechanics generalizes poorly. All of modern physics is based on the Lagrangian and Hamiltonian approaches.
- action is extremized or at a saddle point, not necessarily minimized
- Action is not quantized in QM (and quantum-mechanical action is not quite the same as classical action). There's another very indirectly related notion of "action" which shows up in historical semiclassical approximations and which is quantized. The author has likely confused the two.
- SU(2) is not the gauge group of the weak interaction, because there is no such thing. A purely weak gauge theory with no EM is not possible, on account of having massive force carriers. The SU(2) and SU(1) in SU(3) x SU(2) x U(1) corresponds to weak isospin and weak hypercharge respectively, not weak charge and (EM) charge.
But even if all the errors were corrected there's a huge amount of missing context. (For instance, the actual content of the standard model Lagrangian - the fact that there are 19 free parameters is in no way sufficient to determine it) And even if you had all of that, a full reduction of macrophysics to microphysics is likely computationally intractable, even in principle.
Just skimmed the EM section: I didn't catch anything egregiously wrong, per se (but again, I only skimmed it), but it's mostly fluff, and the actual physics content that does it exist is targeted at wildly different levels. For instance the author goes from "an electric field is like a tiny arrow attached to every point in space" (day 1 of intro physics for non-physics majors) in chapter 1 to assuming the reader knows what covariant derivatives are (advanced undergraduate) in chapter 2. I would strongly recommend against using these books.
> Shouldn’t physics focus much more on the underlying micro states and micro processes than the emergent phenomena? Obviously there needs to be a transition, but at some point you go from physics to engineering.
1. The boundaries between disciplines are where they are in part by historical accident, and in part because that's what the people working in them find useful - there is no actual fact of the matter.
2. We don't actually know the underlying microprocesses of anything. Effective theories are all we have, and there's no fundamental difference between an effective theory for the vacuum (if it is a vacuum) and one for, say, the bulk of a semiconductor.
Room temperature superconductors would not give us zero-cost energy any time soon. Even if one had a high enough critical current to be used in transmission lines (which is not a given), transmission losses are under 10% in modern grids.
This is not correct. The Planck length happens to be very very roughly the scale at which we expect effective theories that ignore quantum-gravitational effects (i.e. the Standard Model) to break down. It has no special physical significance.