The 5 Great Problems

1. The problem of quantum gravity. This is the problem of combining general relativity and quantum theory into one theory. Such a theory would be able to describe all known interactions of nature.
2. The problem of quantum theories. This is the problem of understanding quantum such that it makes sense and solve its fundamental problems.
3. The unification of the particles and forces. Still no theory exists that unifies all known particles and interactions into a single fundamental entity.
4. Explain the constants of nature. The standard model of particle physics has 20+ free parameters that need to be inferred from experiments. It is an open problem to understand how these values are "chosen" by nature.
5. Explain dark matter and dark energy. If they really exists, as suggested by experiments, we need to understand what they consists of. In addition, the constants of the standard model of cosmology need the be explained.

For more information, see the chapter "The Five Great Problems in Theoretical Physics" in the book "Time Reborn" by Lee Smolin.

• Life, the universe, and everything - 42 fundamental questions by Roland E. Allen, Suzy Lidström
• http://math.ucr.edu/home/baez/physics/General/open_questions.html
• https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics
• Chiral Fermions, see https://arxiv.org/pdf/hep-th/0102028.pdf

The laws of physics seem to be composed out of five fundamental ingredients:

• 1 Identical particles.
• 2 Gauge interactions.
• 3 Fermi statistics.
• 4 Chiral fermions.
• 5 Gravity.

The question is whether one can find a “deeper structure” that gives rise to all five of these phenomena. In addition to being consistent with our current understanding of the universe, such a structure would be quite appealing from a theoretical point of view: it would unify and explain the origin of these seemingly mysterious and disconnected phenomena. The U(/1)xSU(2)xSU(3) Standard Model fails to provide such a complete story for even the first four phenomena. Although it describes identical particles, gauge interactions, Fermi statistics, and chiral fermions in a single theory, each of these components are introduced independently and by hand. For example, field theory is introduced to explain identical particles, vector gauge fields are introduced to describe gauge interactions Yang and Mills, 1954, and anticommuting fields are introduced to explain Fermi statistics. One wonders—where do these mysterious gauge symmetries and anticommuting fields come from? Why does nature choose such peculiar things as fermions and gauge bosons to describe itself?

Colloquium: Photons and electrons as emergent phenomena Michael Levin and Xiao-Gang Wen https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.77.871