What is energy?

There's no satisfying definition beyond "the quantity that is conserved over time." This may sound arbitrary and ad hoc but it emerges from this deep mathematical principal called Noether's theorem that states that for each symmetry (in this case, staying the same while moving forward or backwards in time), there is something that is conserved. In this context, momentum is the thing that is conserved over distance, and angular momentum is the thing that is conserved through rotations.

The less rigorous explanation is that it's essentially the currency used by physical systems to undergo change.

Emergent complexity: complexity arising from simple rules is interesting. One example of it is cellular automata and [Conway's game of life](https://en.wikipedia.org/wiki/Conway's_Game_of_Life).

The theoretical minimum - Has pretty amazing courses.

Principles of Optics - Electromagnetic theory of propagation, interference and diffraction of light.

Physics Travel Guide - Expository wiki which explains concepts in three levels of difficulty. We call these levels: intuitive, concrete and abstract. (HN)

Gears (2020) (HN)

Einstein Papers Project - Provides the first complete picture of Einstein’s massive written legacy.

HEPML Resources - Listing of useful learning resources for machine learning applications in high energy physics (HEPML).

Explorable Physics - Course notes for algebra based physics with explorable explanations. (Code)

The Wolfram Physics Project (2020) (HN) (Video) (Article) (Code)

DIY Particle Detector - Mobile low-cost spectrometer for measuring radioactivity and the energy of ionising radiation like alpha particles and electrons. (HN)

Symmetry Magazine - Dimensions of particle physics.

Mathematics for Physics - Illustrated Handbook.

Elements of Physics - Interactive simulation which describes different elements of physics like Gravity, Acceleration and Force.

Spacetime Physics book (1992) - By Taylor and Wheeler (Feynman's PhD advisor).

HyperPhysics - Physics Exploration Environment.

Theoretical Physics Reference - Open source theoretical physics book. (Web) (Code)