We are now ready to start learning the rules of the games! We have seen all the building blocks of Nature (see fig below). We can arrange them in pairs. First there is space and time, the tapestry on which everything moves.
We then have the "stuff" of Nature, which can either be particles or waves.
Finally, each particle/wave has a certain amount of energy and momentum, which dictates its motion through space and time. Energy and momentum are conserved in the whole Universe but they can both be transferred between different particles/waves.
Now that we know the basic structure, the next questions are
How does a particle's or wave's energy and momentum change?
How do things interact and how do they exchange energy and momentum?
Interactions between particles and waves occur via forces. Fundamentally, we know of four different forces in the world.
They are in order of best understood to least understood
and yes, gravity is the least understood force (in some ways). We will not discuss the weak nuclear force in much details in this course. We will spend most of our time on the first one: electromagnetism. This is the most important force in your daily life!