Practical Use of knots in real world. I was to a place like exhibition and a guy was telling us the mathematical aspect about knots and knot theory and I was confused about the real practical reason for mathematicals to research about that. Can someone tell me the use at the real world of knots?
 A: Knots do appear in the real world.
1) Biology. DNA molecules frequently form knots. See for example
http://www.ams.org/publicoutreach/feature-column/fcarc-knots-dna
http://smf4.emath.fr/en/Publications/ExplosionDesMathematiques/pdf_en/smf-smai_explo-maths_47-50_en.pdf
https://link.springer.com/chapter/10.1007/978-3-642-15637-3_11
and many other publications you can find by internet search.
2) Physics. String theory (okay, that is only a theoretical approach to the quantum world and we do not know how "real" it is). See for example
https://www.ias.edu/ideas/2011/witten-knots-quantum-theory
A: Originally, Lord Kelvin proposed that matter was made of atoms that were vortices in the aether, prompting a study of the classification of knots in case this were true.
There have been studies of random knotting of physical objects, for example:
Raymer and Smith, "Spontaneous knotting of an agitated string."
http://www.pnas.org/content/104/42/16432
They used theoretical results to be able to easily identify which knots were created.
