your favorite theory in physics which you have learned about

[u/TadpoleFun1413]

For me, it was special relativity when i first learned about it in modern physics as an undergrad.

[u/ulam17]

Not technically a theory, but once I got the hang of being able to picture in my head what the solution spaces of PDEs with boundary conditions must look like was cool, and it helped a lot with intuition on how to approach problems.

[u/supermultiplet]

Ahh there's so many amazing things we learned in physics - how can anyone choose?!

Keeping it in the theme of standard courses, I remember being super impressed when we covered the Aharanov-Bohm effect in grad QM. I'm not sure I can articulate why, but it felt like a very profound result to me

[u/Shevcharles → u/supermultiplet]

We are so used to thinking of field theory as a local phenomenon with local degrees of freedom and local observables, especially given the foundational structure of special relativity, that the idea of nontrivial global (read: topological) observables seems positively counterintuitive by comparison. But they clearly exist and it's really cool that they do.

[u/Ok_Opportunity8008]

Chern-Simons Theory, very interesting in its play with the QHE. Also has one of the smartest billionaires as its co-discoverer.

[u/Shevcharles]

Probably GR for me. I know finding solutions to the beastly field equations is a different matter, but I just find the theory itself quite beautiful.

[u/TheBacon240]

Holographic duality between Chern Simons TQFT in 3D bulk and WZW 2D CFT on the boundary

[u/Ok_Opportunity8008 → u/TheBacon240]

great minds 

[u/Responsible_Ease_262]

The Hieb Conjecture.

It takes you down another level in the Fine Structure Constant rabbit hole.

[u/CB_lemon]

Not exactly physics but I love the combinatoric analog to Gauss-Bonnet theorem in differential geometry.

[u/One_Programmer6315]

GR, and QFT. The fact that the SM is deemed “the most successful scientific theory of all times” speaks for itself… GR because it is so elegant; I feel when I took the class everything flowed smoothly from the metrics and Euler-Lagrange equations.

[u/OhioAlien]

Heisenberg uncertainty principle was an absolute banger for me gang 😩🥀

[u/void1306]

If you don't move something, it will not move 🗿

[u/ZectronPositron]

Electromagnetics and Maxwell's (Heaviside) Equations - arguably the first "unifying theory" in physics.

[u/littleclaw6]

Very stupid answer, but it's funny: Nuclear Pasta

[u/NateTut]

Heisenberg's Uncertainty Principle

"The Universe is not only queerer than we suppose, but queerer than we can suppose."

J.B.S. Haldane, Possible Worlds

[u/Responsible_Ease_262 → u/NateTut]

It’s based on entropy…an aspect of thermodynamics of all things…Industrial Age science.

[u/kcl97]

Action equals reaction (Newton's 3rd Law,) and Conservation of Momentum (2nd Law due to Galileo).

1st Law is a definition of what force is so it is not a theory.

2nd law is deduced from Galelian Relativity (and Noether).

3rd law is an ontological statement by Newton. It is theory like Darwin's theory of evolution is a theory.

[u/Important_Adagio3824]

Constructor Theory

You can download the paper here. Here's an article that helps break it down.