A sketch of twistor space: https://theportal.wiki/images/1/11/Penrose-Rindler-Clifford-parallels.jpg https://theportal.wiki/images/1/11/Penrose-Rindler-Cliffor

A sketch of twistor space: https://theportal.wiki/images/1/11/Penrose-Rindler-Clifford-parallels.jpg https://theportal.wiki/images/1/11/Penrose-Rindler-Clifford-... A Penrose slide pack: http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Penrose/ http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Pe... For example, slide 19: http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Penrose/Slides/s19.html http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Pe... An original manuscript with fully formed illustrations: https://math.ucr.edu/home/baez/penrose/Penrose-TheoryOfQuantizedDirections.pdf https://math.ucr.edu/home/baez/penrose/Penrose-TheoryOfQuant...