This is a book review of White Holes: Inside the Horizon by Carlo Rovelli.
White Holes: Inside the Horizon, by Carlo Rovelli (Allen Lane), 2023. Pp. 157, 19.8 x 11.8 cm. Price £14.99/¤19.49 (hardbound, ISBN 978 0 241 62897 3).
This book is a quick read, not only because of the small format (and not all that many pages), but because, like Rovelli's other books, it is very well written (more precisely, I can judge only the translation, by Simon Carnell, at least as far as the language goes; like his other popular-science books, the original is in Italian). Like many of his other popular-science books, it is a mixture of standard knowledge and his own work. The first part is mainly about _black_ holes, mostly standard stuff, though it would be difficult to find a better presentation of the basics. White holes are taken up in the next part. Most readers will probably have heard of them, but most also probably have some misconceptions, which Rovelli clears up (for example, their gravity is attractive; time reversal reverses the first derivative of spatial coordinates, not the second). Where Rovelli departs somewhat from standard lore is his idea that when the matter forming the black holes has been sufficiently compressed that quantum-gravity effects play a role, quantum tunneling can transform a black hole into a white hole. In the third part, Rovelli discusses his resolution of the black-hole information paradox as well as the concept of time and the relation between time-reversible microphysics and the macroscopic arrow of time. To the `extremely interesting if true' category belongs his idea that dark matter could consist of Planck-mass white holes, which is certainly compatible with observations. Unfortunately, such dark matter would be more difficult to detect directly than most other dark-matter candidates. The book is non-technical but takes care not to over-simplify things. This is a well written and interesting book accessible to a broad readership. Although one might not agree with his more speculative points (which might turn out to be wrong), most will probably learn something from it and might be inspired to follow up the references in order to learn more.