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Real problems are things related to **unexplained observed phenomena**, like dark matter or dark energy. | 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 still not understood masses and mixing angles of the elementary particles or also the [[open_problems:hierarchy_puzzle|hierarchy puzzle]]. | + | 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** | ||
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+ | - __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. | ||
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+ | 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 ===== | ===== 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 | * http://math.ucr.edu/home/baez/physics/General/open_questions.html | ||
* https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics | * https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics |