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--- open_problems [2018/03/12 17:10]
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 ~~NOTOC~~
 ====== Open Problems ======
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 Our present understanding of nature is far from complete.
-In addition to **unexplained observed phenomena**, like dark matter or dark energy, there are also many **technical problems** in our existing [[theories|theories]]. An example for such a technical problem is the [[open_problems:hierarchy_problem|hierarchy problem]].
+Real problems are things related to **unexplained observed phenomena**, like dark matter or dark energy.
-Moreover, there are things that we can describe but do not understand. These are not really problems but rather **puzzles**. Examples are the still not understood masses and mixing angles of the elementary particles.
+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 5 Great 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 =====
+  * [[https://arxiv.org/abs/1804.08730|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
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 <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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