Backreaction: A newly proposed table-top experiment might be able to demonstrate that gravity is quantized
This is one of Sabine Hossenfelder's more readily accessible posts about a major issue in quantum physics.
[Just this weekend, while driving, I was trying to give a verbal description of the twin slit experiment to my son, but this post reminds me that I stopped at photons, and didn't continue to make the important point about electrons doing the same. No wonder he was underwhelmed. Tonight over dinner, perhaps!]