Why "lightness of being"?
"The Unbearable Lightness of Being" is the title of a famous novel by Milan Kundera. It is one of my favorite books. It is about many things, but perhaps above all about the struggle to find pattern and meaning in the seemingly random, strange and sometimes cruel world we live in.
Of course Kundera's approach to these problems, through story and art, is completely different from the one I've taken in this book, through science and (light) philosophy. For me at least, coming to understand the deep structure of reality has helped to make Being seem not merely bearable, but enchanted - and enchanting. Hence "The Unbearable Lightness of Being."
There's also a joke involved. A central theme of this book is that the ancient contrast between celestial Light and earthy Matter has been transcended in modern physics. There's only one thing, and it's more like the traditional idea of light than the traditional idea of matter. Hence "The Lightness of Being."