✎ Open Problems [The Physics Travel Guide]

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====== Open Problems ======

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Our present understanding of nature is far from complete.

Real problems are things related to **unexplained observed phenomena**, like dark matter or dark energy.

In addition, there are things that we can describe but do not understand. These are not really problems but rather **puzzles**. Examples are the [[open_problems:flavour_puzzle|still not understood masses and mixing angles]] of the elementary particles or also the [[open_problems:hierarchy_puzzle|hierarchy puzzle]] and the question why strong interactions [[open_problems:strong_cp_puzzle|do not violate]] [[advanced_notions:cp_symmetry|CP symmetry]].

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**The Biggest Problems**

- __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. 
  - __The problem of quantum theories.__ This is the problem of understanding quantum such that it makes sense and solve its fundamental problems. 
  -  __The unification of the particles and forces.__ Still no theory exists that unifies all known particles and interactions into a single fundamental entity. 
  -  __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.
  - __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.

===== Lists of Open Problems =====

* 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

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===== Quotes =====

<blockquote>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?

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

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