Thanks to Onofrio Russo who aroused my interest in this by telling me what Dirac told him about light.
Thanks for online conversations with helpful comments and advice to (in alphabetical order):
Akram Ben Aissi, Steve Alvarez, Celso Antonio Almeida, Mehmet Ata, Alethea Black, Alasdair Broun, Gayle Dean, Cris Calude, Tom Campbell, David Chartrand, Jonathan Dickau, Andrew Eaglen, Kent Forbes, Robert Frot, Alan Grano, Carl Grove, Ken Hawick, Kevin Hyndman, Lucian Ionescu, Ben Iscatus, Tim Jones, Bogdan Lazar, Alex Lightman, Alexander Macris, Bruce Maier, Mason Mulholland, Paul Olivier, Ervin Olah, Kevin Player, Ross Rhodes, John Ringland, Paul Smith, Claudio Soprano, Andrew Thomas, Gunnar Jörgen Viggósson, Ram Vimal, Bryan Warner, Marty Wollner, Ian Wilson and Eden Yin.
Especial thanks to Matthew Raspanti and Belinda Sibly for detailed edits of the rough early chapters.
I also thank my son Alex who always helps me think more clearly. Still, the mistakes are mine alone.