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.
One of the things you learn in Physics is that nothing is ever still or completely solid. All atoms vibrate and most(95+%) of their space is empty, where electrons spin. So everything you see is a vibrating mesh of molecules. Neutrinos are so small they can fly straight through everything, even planets! The only time anything stays completely still is at zero degrees Kelvin and it is theorized that time actually stands still at the this temperature.
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.
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).