From the beginning of physics, there have been those who imagined they would be the last generation to face the unknown. Physics has always seemed to its practitioners to be almost complete. This complacency is shattered only during revolutions, where honest people are forced to admit that they don't know the basics. But even revolutionaries still imagine that the big idea — the one that will tie it all up and end the search for knowledge — lies just around the corner.
We live in one of those revolutionary periods, and have for a century. The last such period was the Copernican revolution, beginning in the early sixteenth cxentury, during which the Aristotelian theories of space, time, motion, and cosmology were overthrown. The culmination was Isaac Newton's proposal of a new theory of ohysics, published in his Philosophiae Naturalis Principae Mathematica in 1687. The current revolution in physics began in 1900 with Max Planck's discovery of a formula describing the energy distribution of heat radiation, which demonstrated that the energy is not continous, but quantized. this revolution has yet to end. The problems that physicists must solve today are, to a large extent, questions that remain unanswered because of the incompleteness of the twentieth century's scientific revolution.
Problem 1: Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature.
This is called the problem of quantum gravity.
Problem 2: Resolve the problems in the foundation of quantum mechanics, either by making sense of the theory as it stands or by inventing a new theory that does make sense.
The whole issue goes under the name of the foundational problems of quantum mechanics.
Problem 3: determine whether or not the various particles and forces can be unified in a theory that explains them all as manifestations of a single, fundamental entity.
Let us call this problem the unification of the particles and forces.
Problem 4: Explain how the values of the free constants in the standard model of particle physics are chosen in nature.
Problem 5: Explain dark matter and dark energy. Or, if they don't exist, determine how and why energy is modified on large scales. more generally, explain why the constants of the standard model of cosmology, including dark energy, have the values they do.