Chapter 21: Lorentz transformation
21.1 Einstein
https://www.youtube.com/watch?v=FvqutkaPmas
https://wap.hillpublisher.com/UpFile/202204/20220414165340.pdf
21.2 Bondi \(k\)-calculus
https://www.youtube.com/watch?v=Ghql2UNlWYA
21.3 wordline in Minkowski space
https://www.bilibili.com/video/BV1sV4y1Y7fX