A 9-line summary of textbook physics

What Griffiths lacks is an explanation of what a measurement is. He, like many other authors, explicitly avoids this because he says that measurement is an ineffable mystery, but it isn't. A measurement is a macroscopic system of mutually entangled particles. The only real mystery is why the outcomes obey the Born rule.

Decoherence does not solve the whole measurement problem. Like I said, it does not explain the Born rule. But it does solve parts of the measurement problem. Decoherence explains why measurements are not reversible (they are reversible in principle but not in practice because you would have to reverse O(10^23) entanglements). It explains why only one outcome is experienced (because you are part of the mutually entangled system of particles that constitutes the measurement, and all of the particles in the system are in classical correlation with each other). I don't know of any standard text that discusses this at all.

Whether or not Feynman is a "standard text" is quibbling over terminology. A lot of people learn QM from it (or at least try to).

U(1) symmetry (R also works in a classical setting: it's the Lie algebra that matters) is exactly how you get a "generic three-form" to drop out of the action.

I have a PhD in physics and left academia 25 years ago.

I would divide the learning of physics in four categories:

- the initial discovery part where you read about something for the first time (nutation, baryons, ...). It is best to have a shallow but interesting approach then, with some (or lots of) liberty on the precision.

- the "things are related" part which was extraordinary in my case. It is the part (about 2 or 3 years into the physics curriculum) where you discover that some things are closely related and you can reuse bits and pieces of what you knew from the previous part to see the "big picture". That part is the most fruitful because you can have your own internal discussions about physics and they are not difficult to confirm or not.

- the "let's dive into the details". That one is tough, very tough. You have a lot of details where you can miss what you are actually learning. There is a lot of math involved, up to voodoo math such as renormalisation. This is for the ones who want to make physics the main topic of their life (= academia).

- the "realization" part when you make peace with a lot of things you learned, where you trust math to drive some parts of physics, where you finally understand that there are some things that you will not understand (or that are not understandable with the current knowledge). This is a phase you get to sometimes after not having done physics for some time.

I would warm recommend to spend some time on part one with some "general public" books to get a hint of what is awaiting you. Then to go to step two with a introductory/mid-level book for students of physics, in the "introduction to physics" part. And then look further to selected areas if you want.

Physics is marvelous.

In physics, experiments are everything. That is why the page says: "No known observation contradicts these nine lines."

Indeed, it is a sport to boil down everything to a smallest set of assumptions. That is how the nine lines arose.

> how difficult it is to distinguish between a legitimate domain expert jargon meant to condense complexity and unnecessarily obfuscatory jargon

Yep. It's a problem at all levels. Sometimes it's also difficult to distinguish real science from well written crackotery

You can look at the webpages of a few universities. Many of the course have the list of official bibliography visible, and that's a good start. Try to follow the same order of courses, it's impossible to understand quantum mechanics without a good base of classic mechanics.

Sometimes the main book for a physic degree is too technical. You can also try to read the Schaum's Outline book for the topic. They have a lot of examples and exercise, but the theoretical part is shorter. I like them as a side book, but if you don't want to become a super expert, they are fine.

If you want books without math, that's a problem. Some are good, some are bad, and it's difficult to distinguish. There are a lot of fun topics that you can learn without too much math. For example there are a lot of things you can learn about particle physics imagining that quarks are just small balls, but some technical details are too difficult without math (for example why there a 8 gluons instead of 9). There are some good collections of divulgation of science that don't have too much math and are checked by a good editorial team.

> Noether's theorem

The main idea is that if you magically teletransport everything in the universe one mile to the right, then nobody will notice the teletransport. It's important that it's true if you choose any other direction (what does "to the right" mean?) or any other distances (like 1 feet, 1 light year, because "1 mile" is no special).

Obviously nobody has tried this experiment, but we as far as we know the laws of physics are the same everywhere, for example the mass of one proton is like 2000 bigger than the mass of an electron here and the mass of one proton is like 2000 bigger than the mass of an electron in a lab on the other side of the earth and the mass of one proton is like 2000 bigger than the mass of an electron in Andromeda. So this though experiment is just a good guess (if you ignore the curvature of the universe due to General Relativity).

But if this guess is correct, then the Noether's theorem says that momentum is conserved, that is a property that is verified in a lot of experiments. So it initially looks like a abstract magical though experiment, but the consequence is that there is an important number that is a constant in each real experiment.

This constant numbers sometimes simplify some calculations a lot. It's similar to the conservation of energy that in some cases is useful to prove that something is impossible or get the final result without looking at the nasty details of the experiment.

The magical translation of everything in the universe is a symmetry, and the idea is that you can discover other symmetries of the universe. Ignoring some technical details, then you can use the Noether's theorem to discover new numbers that are constant in all experiments.

Sometimes the symmetry is a symmetry of all the universe that is easy to see, sometimes it's a more abstract symmetry, sometimes is just a symmetry of the experiment and you ignore the rest of the universe. There are many applications of the Noether's theorem.