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--- open_problems [2017/10/27 11:01]
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+{{indexmenu_n>10}}
+~~NOTOC~~
+====== Open Problems ======
+<WRAP group>
+<WRAP half column>
+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]].
+</WRAP>
+<WRAP half column><nspages open_problems -h1 -textPages=""></WRAP>
+</WRAP>
+----
+**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
+  * Chiral Fermions, see https://arxiv.org/pdf/hep-th/0102028.pdf
+----
+===== 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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