(making sure everything below is correct)

It's interesting how modern physics has remained "modern" for at least half a century now. Ever since it was discovered that atoms have some sort of internal structure, we've been in the era of "modern physics." We discovered protons and neutrons, and then we developed quantum theory to explain the interactions between these and the more than 200 "elementary" particles that we've discovered since the early days of modern physics. Just before quantum theory came out, Einstein had formulated his theories of relativity, describing what happens to matter as it approaches the speed of light, that matter is just a frozen form of energy, and that gravity is a measure of the curvature of spacetime. Special and general relativity operate on a very large scale. When quantum theory came out, Einstein found it hard to accept, and tried everything he could to reconcile his theories of relativity with the new quantum theory, which operates at extremely tiny scales. No one has yet been able to do so.

While we've made progress within the last few decades and have managed to unify three of the four fundamental forces (unification means that one force becomes identical to another in a special set of conditions), gravity remains non-unified. The weak, strong, and electromagnetic can all be expressed in terms of one another at low or higher energies. Of all the four forces, we know the least about gravity. We know the equation describing the gravitational attraction between two objects, and we know what effects gravity has on our environment and on the universe. However, that's it. Gravity is the frontier of "modern physics."