What was the best question asked in 2016 by a user whose account was made in or before 2014?
Vote either by posting a link to your favorite qualifying question, with a brief description of why it's your favorite, or by upvoting an existing post. Voting lasts two weeks, i.e. until Jan 29 2017.
This is part of the Best-of PSE 2016. If you are interested in providing a prize bounty for the winner (mostly highly upvoted answer of this meta post), please comment.
I think this question, Do free-electron lasers actually lase?, is fantastic and was asked in Jan 2016 by a user who's account was created in 2012. The question accumulated over 30 up-votes.
I'll throw in my own question here:
What do the wave functions associated to the Fock states of each mode of a bound state system mean?
This question asks about the fundamental structure of quantum mechanics. The distinction between single particle wavefunctions and the wave functions of Fock states for a single mode of a distributed system confuses even seasoned physicists (i.e. most of my research group!). At the same time, because the question asks about the basic elements of quantum mechanics, it should be interesting to beginners.
I feel that after ten years in research questions like this one remind me that I'm never too old to learn.
