All of it, to the users who haven't seen it posted here before.

It was mentioned in another comment thread[0] this morning. These things happen[1].

[0]: https://news.ycombinator.com/item?id=37199307

[1]: https://news.ycombinator.com/item?id=32795559

Undergraduate physics hasn't changed much in the past two years.

- insert relevant XKCD comic -

[comic]

https://xkcd.com/1053/

[/comic]