I’ve gotta give this little book a round of applause, but with a pretty significant caveat.
It’s always good to keep in mind whether you are reading a book written by a physicist (or other scientist) or one written by a science writer, especially when the book is meant to explain difficult and controversial subject matter. Here Rovelli is writing as a physicist taking his own position on a controversial topic.
In writing for a general audience, he doesn’t have room to present the details of the controversy, and we are in no position to evaluate the arguments and find our own position anyway. We just get Rovelli’s.
I’m not really faulting Rovelli for a one-sided presentation. I’m not sure what else he could do if he wants to write about the topic. He believes he has the correct position.
So what is the larger question behind Rovelli’s book, and his research?
It’s the vexing question of uniting general relativity, our best theory of gravity, with quantum theory, our best theory of microscopic reality. Rovelli is a proponent of loop quantum gravity, a true theory of quantum gravity.
I’m going to skip any details, ones I understand and the much greater number that I don’t understand. Rovelli doesn’t really present details here either. Part of the reason for that is no doubt that he’s avoiding difficult mathematics in a book for a general audience. In fact, there are no formulas or equations in the book.
The core of the book is his account of the fate of black holes. The idea that he sketches here is the result of an extended speculative conversation, in mathematical terms, that his colleague and partner in theory Hal Haggard initiated with him.
Rovelli takes us on an imaginative journey through the interior of a black hole, all the while casting our journey as analogous to Dante’s journey in The Divine Comedy. Just as Virgil was Dante’s guide through the Inferno, so relativity theory will be our guide through the black hole.
He dispels some common misconceptions about black holes involving time dilation and our subjective experience of time, what it means to “reverse time,” and what the internal structure of a black hole might be like.
The culmination, again not explained in detail, is the “leap” that a black hole may take at the end of its lifetime, as its mass has slowly evaporated, a leap across quantum time and space that reverses the evolution of the black hole, a true time reversal that produces the black hole’s time reversed opposite, a white hole.
Just as the black hole had collapsed to a state from which nothing could escape, in this time reversal a white hole expands from the now decayed black hole to a state from which nothing can enter. A black hole permits nothing to escape, only to enter. A white hole permits nothing to enter, only to escape.
If Rovelli is right, and if black holes have lived out their lives and given birth to white holes, our universe contains innumerable white holes to be discovered. None have been discovered yet. They are of course small, their black hole parents having lost much of their mass over their lifetimes.
If there are enough white holes in the universe today that result from this process, we might well have at least a partial solution to the dark matter problem as a bonus.
This account of white holes is not filled with hopes of interstellar travel and the like. It is a physicist’s account of what happens beyond the observable boundaries of a black hole, according to loop quantum gravity.
Hence my caveat. Loop quantum gravity is certainly not accepted as verified, settled theory. Its application to the evolution of black holes likewise.
Rovelli is a participant in this exciting and fast-moving controversy about quantum gravity and its consequences, and he could produce (and does in his professional activity) the math and details. He doesn’t here, and for good reason.
So this is in turn an exciting book about what might emerge from that controversial field, and a bit of a mind-bender at that.
My applause is for his bringing a general audience into a very controversial and technical subject. My caveat is that general audiences, including me, are in no position to assess the position that Cavelli takes.
I really want to like Carlo Rovelli. He does this kind of stuff. If you haven’t read his Reality Isn’t What It Seems (on quantum gravity) and The Order of Time, I’d recommend those over this one, at least in order of reading. Both are, like this one, speculative and aimed at a general audience, but are in some sense more fundamental to their topics.