Physics on Reddit

Reddit advertises itself as "the front page of the internet." It's an apt moniker: Reddit shows you what it's users find interesting because users can up-vote or down-vote any post. More specifically, it shows you what it's users find interesting within different subjects, called subreddits. The subreddits continue to break up fractally, such that you can find a group of people discussing just about anything. And if you find a sub-sub-sub-sub-topic that doesn't have a subreddit, you can make one.

Here's CGP Grey's explanation of Reddit and why it's an awesome place to learn new subjects:

So, for example, Reddit is a great place to learn physics, because physicists interested in discussing physics on-line tend to frequent the physics subreddit. Users post links to articles and pictures, and host "AMAs" (Ask Me Anything), in which a professional physicist hosts an open forum regarding their work. Here's a recent one regarding climate change, or one about earthquakes.

So, what interesting physics topics do you see on Reddit?